Document and Entity Information Document Information [Table] Class of Stock [Axis] Class of Stock [Domain] Common Stock [Member] Common Shares Common Stock Warrant [Member] Redeemable Warrants Warrants (public and private) Document Information [Line Items] Document Type Document Annual Report Current Fiscal Year End Date Document Fiscal Year Focus Document Period End Date Document Transition Report Entity File Number Entity Registrant Name Entity Incorporation, State or Country Code Entity Tax Identification Number Entity Address, Address Line One Entity Address, Adress Line Two Entity Address, City or Town Entity Address State Or Province Entity Address, Postal Zip Code City Area Code Local Phone Number Title of 12(b) Security Trading Symbol Security Exchange Name Entity Well-known Seasoned Issuer Entity Voluntary Filers Entity Current Reporting Status Entity Interactive Data Current Entity Filer Category Entity Small Business Entity Emerging Growth Company ICFR Auditor Attestation Flag Document Financial Statement Error Correction [Flag] Entity Ex Transition Period Entity Shell Company Entity Public Float Entity Common Stock, Shares Outstanding Documents Incorporated by Reference [Text Block] Auditor Name Auditor Location Auditor Firm ID Entity Central Index Key Document Fiscal Period Focus Amendment Flag Consolidated Balance Sheets Statement [Table] Related and Nonrelated Parties [Axis] Related and Nonrelated Parties [Domain] Nonrelated Party [Member] Nonrelated party Nonrelated party Related Party [Member] Related party Related party Statement [Line Items] Assets [Abstract] ASSETS Assets, Current [Abstract] Current assets Cash and Cash Equivalent Cash and cash equivalents Cash and cash equivalents Short-Term Investments Short-Term Investments Prepaid Expense and Other Assets, Current Prepaid expenses and other current assets Total Assets, Current Total current assets Property, Plant and Equipment, Net Property and equipment, net Total Intangible Assets, Net (Excluding Goodwill) Intangible assets, net Intangible assets recorded Operating Lease, Right-of-Use Asset Right-of-use assets Other Assets, Noncurrent Other assets Assets Total Assets Liabilities and Equity [Abstract] LIABILITIES AND STOCKHOLDERS' EQUITY (DEFICIT) Liabilities, Current [Abstract] Current liabilities Accounts Payable and Accrued Liabilities, Current Accounts payable and accrued expenses Total Operating Lease, Liability, Current Less current portion Operating lease liabilities, current Finance Lease, Liability, Current Less current portion Finance lease liabilities, current Related Party Deposit Liabilities Related party advance (Note 12) Liabilities, Current Total current liabilities Convertible Notes Payable, Noncurrent Balance at end Balance at beginning Convertible notes, net of debt discount Carrying value as of the balance sheet date of [accrued] interest payable on all forms of debt, including trade payables, that has been incurred and is unpaid, classified as noncurrent. Interest Payable, Noncurrent Accrued interest on convertible notes Operating Lease, Liability, Noncurrent Operating lease liabilities, noncurrent Noncurrent portion Finance Lease, Liability, Noncurrent Finance lease liabilities, noncurrent Noncurrent portion Deferred Income Tax Liabilities, Net Deferred tax liabilities, net Liabilities Total liabilities Commitments and Contingencies. Commitments and Contingencies (Note 18) Equity, Attributable to Parent [Abstract] Stockholders' Equity (Deficit) Common Stock, Value, Issued Common shares, $0.0001 par value; 500,000,000 authorized shares; 81,771,422 and 39,159,901 shares issued and outstanding as of December 31, 2025 and 2024, respectively Amount of aggregate par or stated value of issued nonredeemable exchangeable stock (or redeemable solely at the option of the issuer). Exchangeable Stock, Value, Issued Exchangeable shares, $0.0001 par value; 24,011,017 shares issued and outstanding as of December 31, 2025 and December 31, 2024 Additional Paid in Capital Additional paid-in-capital Retained Earnings (Accumulated Deficit) Accumulated deficit Accumulated deficit Accumulated Other Comprehensive Income (Loss), Net of Tax Accumulated other comprehensive income Equity, Including Portion Attributable to Noncontrolling Interest Ending balance Beginning balance Total stockholders' equity (deficit) Liabilities and Equity Total liabilities and stockholders' equity (deficit) Common Stock, Par or Stated Value Per Share Common shares, par value (in dollars per share) Common Stock, Shares Authorized Common stock, number of shares authorized Common Stock, Shares, Issued Common stock, shares issued Common shares, Issued (in shares) Common Stock, Shares, Outstanding Common stock, shares outstanding Face amount or stated value per share of exchangeable stock. Exchangeable Stock, Par or Stated Value Per Share Exchangeable shares, par value (in dollars per share) Number of exchangeable shares of an entity that have been sold or granted to shareholders. Exchangeable Stock, Shares, Issued Exchangeable shares, shares issued Total recapitalized exchangeable shares at December 31, 2025 Number of shares of exchangeable stock outstanding. Exchangeable Stock, Shares, Outstanding Exchangeable shares, share outstanding Consolidated Statements of Operations and Comprehensive Loss Revenues [Abstract] REVENUE Revenue from Contract with Customer, Excluding Assessed Tax Engineering services revenue Revenue Revenue from Contract with Customer, Product and Service [Extensible Enumeration] Operating Expenses [Abstract] OPERATING EXPENSES Research and Development Expense Research and development costs Research and development expenses paid to companies controlled by officers included in general and administrative General and Administrative Expense General and administrative General and administrative expenses Depreciation, Depletion and Amortization Depreciation and amortization Operating Expenses Total Operating Expenses Operating Income (Loss) Operating losses OPERATING LOSS Nonoperating Income (Expense) [Abstract] OTHER (EXPENSE) INCOME Government Assistance, Nonoperating Income, Increase (Decrease) Government grants Interest Expense, Nonoperating Interest expense Gain (Loss) on Extinguishment of Debt Loss on extinguishment of debt Loss on extinguishment of debt Investment Income, Interest and Dividend Interest and dividend income Gain (Loss), Foreign Currency Transaction, before Tax Foreign exchange gain (loss) Nonoperating Income (Expense) OTHER (EXPENSE) INCOME Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Net loss before income tax Earnings/(loss) before income taxes Income Tax Expense (Benefit) Income tax expense Total income tax expense Net loss Earnings Per Share, Basic Loss per common share, basic (in dollars per share) Net loss per share, basic (in dollars per share) Earnings Per Share, Diluted Loss per common share, diluted (in dollars per share) Net loss per share, diluted )in dollars per share) Weighted Average Number of Shares Outstanding, Basic Weighted-Average Shares of Common Shares Outstanding, Basic (in shares) Weighted-average shares outstanding, basic (in shares) Weighted Average Number of Shares Outstanding, Diluted Weighted-Average Shares of Common Shares Outstanding, Diluted (in shares) Weighted-average shares outstanding, diluted (in shares) Other Comprehensive Income (Loss), Net of Tax [Abstract] Other comprehensive (loss) income net of tax: Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax Foreign currency translation adjustments Currency translation adjustments Other Comprehensive Income (Loss), Increase Or Decrease in Unrealized Gains Or Losses on Short Term Investments Change in unrealized gains on short term investments Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Comprehensive loss Consolidated Statements of Changes in Stockholders' Equity (Deficit) Equity Components [Axis] Equity Component [Domain] Parent [Member] Total Terrestrial Energy Inc. Stockholders' Equity (Deficit) Preferred Stock [Member] Preferred Shares Represents information pertaining to preferred exchangeable stock. Preferred Exchangeable Stock [Member] Preferred Exchangeable Shares Represents information pertaining to common exchangeable stock. Common Exchangeable Stock [Member] Common Exchangeable Shares Represents information pertaining to exchangeable stock. Exchangeable Stock [Member] Exchangeable Shares Additional Paid-in Capital [Member] Additional Paid-In-Capital AOCI Attributable to Parent [Member] Accumulated Other Comprehensive Income (Loss) Retained Earnings [Member] Accumulated Deficit Noncontrolling Interest [Member] Attributable to Non-Controlling Interest Revision of Prior Period [Axis] Revision of Prior Period [Domain] Previously Reported [Member] Previously Reported Revision of Prior Period, Adjustment [Member] Retrospective application of recapitalization Sale of Stock [Axis] Sale of Stock [Domain] Offering of securities for cash. Cash Issuance [Member] Cash issuance Private Placement [Member] PIPE Financing Increase (Decrease) in Stockholders' Equity [Roll Forward] Consolidated Statements of Changes in Stockholders' Equity (Deficit) Shares, Issued Ending balance (in shares) Beginning balance (in shares) APIC, Share-Based Payment Arrangement, Increase for Cost Recognition Stock-based compensation Stock Issued During Period, Value, Acquisitions Acquisition of non-controlling interest Stock Issued During Period, Shares, Acquisitions Acquisition of non-controlling interest (in shares) Issuance of shares to HCM II Stock Issued During Period, Value, New Issues Issuance of shares Issuance of shares Issuance of shares to PIPE investors Issuance of shares (in shares) Adjustments to Additional Paid in Capital, Warrant Issued Issuance of warrants in connection with convertible notes, net of tax Amount of Increase (decrease) in additional paid in capital (APIC) resulting from gain (loss) on extinguishment of debt. Adjustments to Additional Paid in Capital, Gain (Loss) On Extinguishment Of Debt Loss on extinguishment of debt from related parties Number of shares issued during the period as a result of the conversion of preferred stock. Stock Issued During Period, Shares, Conversion of Preferred Stock Conversion of preferred stock into common stock (in shares) The gross value of stock issued during the period upon the exercise of warrants. Stock Issued During Period, Value, Warrant Exercises Shares issued upon exercise of warrants Number of shares issued during the period as a result of the exercise of warrants. Stock Issued During Period, Shares, Warrant Exercises Shares issued upon exercise of warrants (in shares) Stock Issued During Period, Value, Stock Options Exercised Shares issued upon exercise of options Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Exercises in Period Exercised Shares issued upon exercise of options (in shares) Value of stock issued pursuant to merger financing during the period. Stock Issued During Period, Value, Merger Financing Merger financing Number of shares of stock issued during the period pursuant to merger financing.. Stock Issued During Period, Shares, Merger Financing Merger financing (in shares) Amount of decrease in additional paid in capital (APIC) resulting from merger and recapitalization transaction costs. Adjustments to Additional Paid in Capital, Stock Issued, Merger And Recapitalization Transaction Costs Transaction costs Stock Issued During Period, Value, Conversion of Convertible Securities Conversion of convertible notes to Common Shares Stock Issued During Period, Shares, Conversion of Convertible Securities Conversion of convertible notes to Common Shares (in shares) Conversion of convertible notes to common shares Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Net loss Net loss Net loss Consolidated Statements of Cash Flows Cash Provided by (Used in) Operating Activity, Including Discontinued Operation [Abstract] Cash flows from operating activities Adjustment to Reconcile Net Income to Cash Provided by (Used in) Operating Activity [Abstract] Adjustments to reconcile net loss to net cash used in operating activities: Amortization of Debt Issuance Costs Amortization of debt discount Share-Based Payment Arrangement, Noncash Expense Stock-based compensation Unrealized Gain (Loss), Foreign Currency Transaction, before Tax Unrealized foreign currency transaction gain Operating Lease, Right-of-Use Asset, Periodic Reduction Noncash lease expense Deferred Income Tax Expense (Benefit) Deferred income taxes Total deferred tax Adjustment to Reconcile Net Income to Cash Provided by (Used in) Operating Activity, Increase (Decrease) in Operating Capital [Abstract] Changes in operating assets and liabilities Increase (Decrease) in Accounts Receivable Accounts receivable Increase (Decrease) in Prepaid Expense and Other Assets Prepaid expenses and other current assets Increase (Decrease) in Accounts Payable and Accrued Liabilities Accounts payable and accrued expenses The increase (decrease) during the reporting period in interest payable, which represents the amount owed to note holders, bond holders, and other parties for interest earned on loans or credit extended to the reporting entity. Increase (Decrease) in Interest Payable, Non-Current Accrued interest Increase (Decrease) in Operating Lease Liability Operating lease payments Cash Provided by (Used in) Operating Activity, Including Discontinued Operation Negative cash flow from operations Net cash used in operating activities Cash Provided by (Used in) Investing Activity, Including Discontinued Operation [Abstract] Cash flows from investing activities Payments to Acquire Intangible Assets Purchases of intangible assets Payments to Acquire Property, Plant, and Equipment Purchases of property and equipment Payments to Acquire Short-Term Investments Purchase of short-term investments Cash Provided by (Used in) Investing Activity, Including Discontinued Operation Net cash used in investing activities Cash Provided by (Used in) Financing Activity, Including Discontinued Operation [Abstract] Cash flows from financing activities Proceeds from Notes Payable Proceeds from issuance of convertible notes Proceeds from Issuance of Preferred Stock and Preference Stock Proceeds from Series A-1 preferred shares issuance Proceeds from Stock Options Exercised Proceeds from the exercise of stock options for common shares Proceeds from Warrant Exercises Proceeds from warrant exercise for common shares Proceeds from Issuance of Private Placement Proceeds from issuance of shares to PIPE investors Add: proceeds from issuance of shares from PIPE investors The cash inflow associated with merger financing. Proceeds From Merger Financing Proceeds from merger financing Amount of cash payments made as a result of merger and recapitalization related transaction costs. Payment of merger and recapitalization related transaction costs Payment of merger and recapitalization related transaction costs Proceeds from Related Party Debt Proceeds from related party advances Finance Lease, Principal Payments Repayment of finance lease liabilities Cash Provided by (Used in) Financing Activity, Including Discontinued Operation Net cash provided by financing activities Effect of Exchange Rate on Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Continuing Operation Effect of exchange rate changes on cash and cash equivalents Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Period Increase (Decrease), Including Exchange Rate Effect and Discontinued Operation Increase (decrease) in cash and cash equivalents during the year Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Continuing Operation Cash and cash equivalents, end of year Cash and cash equivalents, beginning of year Supplemental Cash Flow Information [Abstract] Supplemental cash flow information