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 Quarterly 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 Current Reporting Status Entity Interactive Data Current Entity Filer Category Entity Small Business Entity Emerging Growth Company Entity Ex Transition Period Entity Shell Company Entity Common Stock, Shares Outstanding Entity Central Index Key Document Fiscal Period Focus Amendment Flag Condensed Consolidated Balance Sheets Statement [Table] Statement [Line Items] Assets [Abstract] ASSETS Assets, Current [Abstract] Current assets Cash and Cash Equivalent Cash and cash equivalents Cash and Cash Equivalents Debt Securities, Available-for-Sale, Excluding Accrued Interest, Current Short-term investments Prepaid Expense and Other Assets, Current Prepaid expenses and other current assets Assets, Current Total current assets Property, Plant and Equipment, Net Property and equipment, net Debt Securities, Available-for-Sale, Excluding Accrued Interest, Noncurrent Long-term investments Intangible Assets, Net (Excluding Goodwill) Intangible assets, net Operating Lease, Right-of-Use Asset Right-of-use assets Operating lease right-of-use assets Other Assets, Noncurrent Other assets Assets Total Assets Liabilities and Equity [Abstract] LIABILITIES AND STOCKHOLDERS' EQUITY Liabilities, Current [Abstract] Current liabilities Accounts Payable and Accrued Liabilities, Current Accounts payable and accrued expenses Operating Lease, Liability, Current Operating lease liabilities, current Finance Lease, Liability, Current Finance lease liabilities, current Liabilities, Current Total current liabilities Operating Lease, Liability, Noncurrent Operating lease liabilities, noncurrent Finance Lease, Liability, Noncurrent Finance lease liabilities, noncurrent Liabilities Total liabilities Commitments and Contingencies. Commitments and Contingencies (Note 11) Equity, Attributable to Parent [Abstract] Stockholders' Equity Common Stock, Value, Issued Common shares, $0.0001 par value; 500,000,000 authorized shares; 82,718,567 and 81,771,422 shares issued and outstanding as of June 30, 2026 and December 31, 2025, 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; 23,216,687 and 24,011,017 shares issued and outstanding as of June 30, 2026 and December 31, 2025, respectively 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, Attributable to Parent Total stockholders' equity Liabilities and Equity Total liabilities and stockholders' equity 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 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 Number of shares of exchangeable stock outstanding. Exchangeable Stock, Shares, Outstanding Exchangeable shares, share outstanding Condensed Consolidated Statements of Operations and Comprehensive Loss Related and Nonrelated Parties [Axis] Related and Nonrelated Parties [Domain] Nonrelated Party [Member] Nonrelated party Related Party [Member] Related party 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 INCOME (EXPENSE) Government Assistance, Nonoperating Income, Increase (Decrease) Government grants Interest Expense, Nonoperating Interest expense Investment Income, Interest and Dividend Interest and dividend income Gain (Loss), Foreign Currency Transaction, before Tax Foreign exchange (loss) gain Nonoperating Income (Expense) OTHER INCOME (EXPENSE) Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Net loss before income tax Income Tax Expense (Benefit) Income tax expense Income tax (expense) benefit 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, Portion Attributable to Parent [Abstract] Other comprehensive (loss) income net of tax: Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax, Portion Attributable to Parent Foreign currency translation adjustments Currency translation adjustments Other Comprehensive Income (Loss), Available-for-Sale Securities Adjustment, Net of Tax, Portion Attributable to Parent Change in net unrealized gains on short-term and long-term investments Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Comprehensive loss Condensed Consolidated Statements of Changes in Stockholders' Equity (Deficit) Equity Components [Axis] Equity Component [Domain] Parent [Member] Total Terrestrial Energy Inc. Stockholders' Equity (Deficit) 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 Increase (Decrease) in Stockholders' Equity [Roll Forward] Consolidated Statements of Changes in Stockholders' Equity (Deficit) Equity, Including Portion Attributable to Noncontrolling Interest Ending balance Beginning balance 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, Stock Options Exercised Shares issued upon exercise of options Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Exercises in Period Shares issued upon exercise of options (in shares) Number of common shares or preference shares exchanged for exchangeable shares during the period. Number Of Shares Exchanged During the Period Number of shares exchanged during the period Conversion of exchangeable shares to common shares Issuance of shares Issuance of shares for private placement (in shares) Adjustments to Additional Paid in Capital, Warrant Issued Issuance of warrants in connection with convertible notes, net of tax Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Net loss Net loss Condensed 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 Investment Income, Net Interest income and accretion of discount on investments, net Share-Based Payment Arrangement, Noncash Expense Stock-based compensation Unrealized Gain (Loss), Foreign Currency Transaction, before Tax Unrealized foreign currency transaction gains Operating Lease, Right-of-Use Asset, Periodic Reduction Noncash lease expense 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 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 Debt Securities, Available-for-Sale Purchase of investments Proceeds from Sale and Maturity of Debt Securities, Available-for-Sale Proceeds from redemptions of investments Cash Provided by (Used in) Investing Activity, Including Discontinued Operation Net cash provided by (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 preferred stock subscription payable Proceeds from Stock Options Exercised Proceeds from the exercise of stock options for common shares 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 in cash and cash equivalents during the period Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Continuing Operation Cash and cash equivalents, end of period Cash and cash equivalents, beginning of period Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract] Supplemental noncash investing and financing activities Amount of warrants recognized in connection with convertible notes net of tax during the period. Recognition Of Warrants In Connection With Convertible notes, Net Of Tax Recognition of warrants in connection with convertible notes, net of tax Right-of-Use Asset Obtained in Exchange for Operating Lease Liability Operating lease liabilities obtained in exchange for operating lease assets Right of use asset and corresponding lease liability Organization and Description of Business Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] Organization and Description of Business Significant Accounting Policies Significant Accounting Policies [Text Block] Significant Accounting Policies No definition available. Financial Instruments Financial Instruments Disclosure [Text Block] Financial Instruments Related Party Balances and Transactions Related Party Transactions Disclosure [Text Block] Related Party Balances and Transactions Leases Lessee, Operating Leases [Text Block] Leases Stockholders' Equity (Deficit) Equity [Text Block] Stockholders' Equity (Deficit) Net Loss Per Share of Common Share Earnings Per Share [Text Block] Net Loss Per Share of Common Share Stock-Based Compensation Share-Based Payment Arrangement [Text Block] Stock-Based Compensation Income Taxes Income Tax Disclosure [Text Block] Income Taxes Segment Information Segment Reporting Disclosure [Text Block] Segment Information Commitments and Contingencies Commitments and Contingencies Disclosure [Text Block] Commitments and Contingencies Subsequent Events Subsequent Events [Text Block] Subsequent Events Basis of Accounting, Policy [Policy Text Block] Basis of Presentation and Principles of Consolidation Use of Estimates, Policy [Policy Text Block] Use of Estimates Foreign Currency Transactions and Translations Policy [Policy Text Block] Foreign Currency Concentration Risk, Credit Risk, Policy [Policy Text Block] Concentration of Credit Risks Fair Value Measurement, Policy [Policy Text Block] Fair Value Measurements Disclosure of accounting policy for warrants. Warrants, Policy [Policy Text Block] Warrants Share-Based Payment Arrangement [Policy Text Block] Stock-Based Compensation Government Assistance [Policy Text Block] Government Grants Earnings Per Share, Policy [Policy Text Block] Net Loss Per Share Segment Reporting, Policy [Policy Text Block] Segment reporting Disclosure of accounting policy for emerging growth company status. Emerging Growth Company Status, Policy [Policy Text Block] Emerging Growth Company Status New Accounting Pronouncements, Policy [Policy Text Block] Recent Accounting Pronouncements Fair Value, by Balance Sheet Grouping [Table Text Block] Schedule of financial instruments Investments Classified by Contractual Maturity Date [Table Text Block] Schedule of investment by contractual maturity Schedule of Related Party Transactions [Table Text Block] Schedule of related party transactions Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Schedule of computation of basic and dilutive net loss per share Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table Text Block] Schedule of potentially dilutive securities Schedule of Segment Reporting Information, by Segment [Table Text Block] Schedule of segment information