Subsequent Events	9 Months Ended	12 Months Ended
Sep. 30, 2025	Dec. 31, 2024	Dec. 31, 2024
Subsequent Events (Details) [Line Items]
SUBSEQUENT EVENTS	9. Subsequent Events The Company evaluated subsequent events from September 30, 2025, the date of these condensed consolidated financial statements, through November 14, 2025, the date these condensed consolidated financial statements were available to be issued, for events requiring recording or disclosure in the condensed consolidated financial statements. The Company concluded that no events have occurred that would require recognition or disclosure in the condensed consolidated financial statements, except as described below. The Business Combination closed on October 28, 2025, subject to the satisfaction or waiver of all closing conditions, with trading commencing on the Nasdaq Stock Market LLC (“Nasdaq”) on October 29, 2025. Upon closing of the transaction, the combined company became known as Terrestrial Energy Inc. and its securities were listed on Nasdaq under the symbols “IMSR” and “IMSRW”, respectively.		17. Subsequent Events The Company evaluated subsequent events from December 31, 2024, the date of these financial statements, through May 30, 2025, the date these consolidated financial statements were available to be issued, for events requiring recording or disclosure in the consolidated financial statements as of and for the year ended December 31, 2024. The Company concluded that no events have occurred that would require recognition or disclosure in the financial statements, except as described below. In February 2025, the Company completed another closing of the Bridge Round Offering and raised additional units totaling approximately $11.0 million. On March 26, 2025, the Company entered into a Business Combination Agreement (the “Business Combination Agreement”) with HCM II Acquisition Corp., a Cayman Islands exempted company (which will transfer by way of continuation and domesticate as a Delaware corporation prior to the Closing (“HCM II”), and HCM II Merger Sub Inc., a Delaware corporation and a direct wholly owned subsidiary of HCM II (“Merger Sub”), pursuant to which, among other things and subject to the terms and conditions contained therein, Merger Sub will merge with and into the Company (the “Merger”), with the Company continuing as the surviving company. Under the terms of the Merger Agreement, the Company’s outstanding shares and convertible notes will be exchanged for shares in HCM II at an exchange ratio specified in the Business Combination Agreement. The closing of the Merger is subject to the receipt of the required approvals by the Company’s and HCM II’s shareholders and the fulfilment of other customary closing conditions set forth in the Business Combination Agreement. In connection with the closing of the Merger, HCM II will change its name to Terrestrial Energy Inc.
HCM II Acquisition Corp
Subsequent Events (Details) [Line Items]
SUBSEQUENT EVENTS	NOTE 10. SUBSEQUENT EVENTS The Company evaluated subsequent events and transactions that occurred after the condensed consolidated balance sheet date up to the date that the unaudited condensed consolidated financial statements were issued. Based upon this review, other than as described below, the Company did not identify any subsequent events that would have required adjustment or disclosure in the unaudited condensed consolidated financial statements. On October 28, 2025, the Company completed its business combination with Terrestrial Energy Inc. (“Terrestrial Energy”), a developer of advanced nuclear power technology. In connection with the closing, the Company changed its name from HCM II Acquisition Corp. to Terrestrial Energy Inc. and its securities began trading on the Nasdaq Stock Market under the symbols “IMSR” and “IMSRW” on October 29, 2025. The transaction generated gross proceeds exceeding $292 million before expenses, including a previously placed $50 million PIPE investment and a negligible level of redemptions by public shareholders. The proceeds are expected to support the Company’s ongoing operations and accelerate the commercial deployment of its proprietary Integral Molten Salt Reactor (IMSR) nuclear technology.	NOTE 11. SUBSEQUENT EVENTS The Company evaluated subsequent events and transactions that occurred after the balance sheet date up to the date that the financial statements were issued. Based upon this review, other than as described below, the Company did not identify any subsequent events that would have required adjustment or disclosure in the accompanying financial statements. On March 26, 2025, the Company entered into a Business Combination Agreement (as it may be amended, supplemented or otherwise modified from time to time in accordance with its terms, the “Business Combination Agreement”) by and among the Company, Terrestrial Energy Inc., a Delaware corporation (“Terrestrial Energy”), and HCM II Merger Sub Inc., a Delaware corporation and a direct wholly owned subsidiary of Company (“Merger Sub”), pursuant to which, among other things and subject to the terms and conditions contained therein, Merger Sub will merge with and into Terrestrial Energy (the “Merger”), with Terrestrial Energy continuing as the surviving entity (the “Surviving Company”). The transactions contemplated by the Business Combination Agreement are referred to herein as the “Business Combination.” The combined company’s business will continue to operate through Terrestrial Energy and its subsidiaries. NOTE 11. SUBSEQUENT EVENTS (Cont.) The Business Combination Agreement and the Business Combination were unanimously approved by the board of directors of the Company and the board of directors of Terrestrial Energy. The Business Combination is expected to close in the fourth quarter of 2025, subject to the receipt of the required approvals by Company’s shareholders and the fulfilment of other customary closing conditions. In addition to the Merger, the Company will, subject to obtaining the required shareholder approvals and at least one (1) day prior to the date of the Closing, change its jurisdiction of incorporation by deregistering as a Cayman Islands exempted company and continuing and domesticating as a corporation incorporated under the laws of the State of Delaware. The Company will provide its public shareholders the opportunity to elect, at least two (2) Business Days prior to the Company shareholder’s meeting, to redeem their shares on the terms and conditions set forth in the Business Combination Agreement and the Company’s governing documents (the “Redemption”). Subject to the receipt of approval from shareholders of the Company, and at least one (1) day prior to the Domestication, the Company will carry out the Redemption. By virtue of the Domestication and subject to the satisfaction or waiver of the conditions of the Business Combination Agreement, including approval of the Company’s shareholders: (i) immediately prior to the Domestication, each of the then issued and outstanding Class B Ordinary Shares of the Company will convert automatically, on a one-for-one basis, into one (1) Class A Ordinary Share, par value of $0.0001 per share, of the Company; and (ii) immediately following the Sponsor Share Conversion, in connection with the Domestication, (x) each then issued and outstanding Class A Ordinary Share (other than any Class A Ordinary Share included in the Cayman Purchaser Units (as defined in the Business Combination Agreement)) will convert automatically, on a one-for-one basis, into one (1) share of common stock, par value $0.0001 per share, of the Company (after the Domestication) (the “New Terrestrial Common Share”); (y) each of the then issued and outstanding warrants (other than any Cayman Purchaser Public Warrants (as defined in the Business Combination Agreement) included in the Cayman Purchaser Units) representing the right to purchase one (1) Class A Ordinary Share will convert automatically into a warrant to acquire one (1) share of New Terrestrial Common Share (each a “New Terrestrial Warrant”); and (z) each of the then issued and outstanding Cayman Purchaser Units will be cancelled and each holder thereof will be entitled to one share of New Terrestrial Common Share and one-half (1/2) of one (1) New Terrestrial Warrant. The Company has also entered into the PIPE Subscription Agreements, each dated as of March 26, 2025, with the PIPE Investors, pursuant to which, among other things, the Company has agreed to issue and sell, in private placements to close immediately prior to or substantially concurrently with the Closing, the PIPE Financing. The PIPE Investors are permitted, under the PIPE Subscription Agreements, to satisfy their commitments thereunder if they hold shares of New Terrestrial Common Share that qualify as Non-Redeemed Shares (as defined herein), subject to certain conditions and restrictions set forth in the PIPE Subscription Agreements.

Subsequent Events

9 Months Ended

12 Months Ended

Sep. 30, 2025

Dec. 31, 2024

Subsequent Events (Details) [Line Items]

9. Subsequent Events

The Company evaluated subsequent events from September 30, 2025, the date of these condensed consolidated financial statements, through November 14, 2025, the date these condensed consolidated financial statements were available to be issued, for events requiring recording or disclosure in the condensed consolidated financial statements. The Company concluded that no events have occurred that would require recognition or disclosure in the condensed consolidated financial statements, except as described below.

The Business Combination closed on October 28, 2025, subject to the satisfaction or waiver of all closing conditions, with trading commencing on the Nasdaq Stock Market LLC (“Nasdaq”) on October 29, 2025. Upon closing of the transaction, the combined company became known as Terrestrial Energy Inc. and its securities were listed on Nasdaq under the symbols “IMSR” and “IMSRW”, respectively.

17. Subsequent Events

The Company evaluated subsequent events from December 31, 2024, the date of these financial statements, through May 30, 2025, the date these consolidated financial statements were available to be issued, for events requiring recording or disclosure in the consolidated financial statements as of and for the year ended December 31, 2024. The Company concluded that no events have occurred that would require recognition or disclosure in the financial statements, except as described below.

In February 2025, the Company completed another closing of the Bridge Round Offering and raised additional units totaling approximately $11.0 million.

On March 26, 2025, the Company entered into a Business Combination Agreement (the “Business Combination Agreement”) with HCM II Acquisition Corp., a Cayman Islands exempted company (which will transfer by way of continuation and domesticate as a Delaware corporation prior to the Closing (“HCM II”), and HCM II Merger Sub Inc., a Delaware corporation and a direct wholly owned subsidiary of HCM II (“Merger Sub”), pursuant to which, among other things and subject to the terms and conditions contained therein, Merger Sub will merge with and into the Company (the “Merger”), with the Company continuing as the surviving company. Under the terms of the Merger Agreement, the Company’s outstanding shares and convertible notes will be exchanged for shares in HCM II at an exchange ratio specified in the Business Combination Agreement. The closing of the Merger is subject to the receipt of the required approvals by the Company’s and HCM II’s shareholders and the fulfilment of other customary closing conditions set forth in the Business Combination Agreement. In connection with the closing of the Merger, HCM II will change its name to Terrestrial Energy Inc.

HCM II Acquisition Corp

Subsequent Events (Details) [Line Items]

SUBSEQUENT EVENTS

NOTE 10. SUBSEQUENT EVENTS

The Company evaluated subsequent events and transactions that occurred after the condensed consolidated balance sheet date up to the date that the unaudited condensed consolidated financial statements were issued. Based upon this review, other than as described below, the Company did not identify any subsequent events that would have required adjustment or disclosure in the unaudited condensed consolidated financial statements.

On October 28, 2025, the Company completed its business combination with Terrestrial Energy Inc. (“Terrestrial Energy”), a developer of advanced nuclear power technology. In connection with the closing, the Company changed its name from HCM II Acquisition Corp. to Terrestrial Energy Inc. and its securities began trading on the Nasdaq Stock Market under the symbols “IMSR” and “IMSRW” on October 29, 2025. The transaction generated gross proceeds exceeding $292 million before expenses, including a previously placed $50 million PIPE investment and a negligible level of redemptions by public shareholders. The proceeds are expected to support the Company’s ongoing operations and accelerate the commercial deployment of its proprietary Integral Molten Salt Reactor (IMSR) nuclear technology.

NOTE 11. SUBSEQUENT EVENTS

The Company evaluated subsequent events and transactions that occurred after the balance sheet date up to the date that the financial statements were issued. Based upon this review, other than as described below, the Company did not identify any subsequent events that would have required adjustment or disclosure in the accompanying financial statements.

On March 26, 2025, the Company entered into a Business Combination Agreement (as it may be amended, supplemented or otherwise modified from time to time in accordance with its terms, the “Business Combination Agreement”) by and among the Company, Terrestrial Energy Inc., a Delaware corporation (“Terrestrial Energy”), and HCM II Merger Sub Inc., a Delaware corporation and a direct wholly owned subsidiary of Company (“Merger Sub”), pursuant to which, among other things and subject to the terms and conditions contained therein, Merger Sub will merge with and into Terrestrial Energy (the “Merger”), with Terrestrial Energy continuing as the surviving entity (the “Surviving Company”). The transactions contemplated by the Business Combination Agreement are referred to herein as the “Business Combination.” The combined company’s business will continue to operate through Terrestrial Energy and its subsidiaries.

NOTE 11. SUBSEQUENT EVENTS (Cont.)

The Business Combination Agreement and the Business Combination were unanimously approved by the board of directors of the Company and the board of directors of Terrestrial Energy.

The Business Combination is expected to close in the fourth quarter of 2025, subject to the receipt of the required approvals by Company’s shareholders and the fulfilment of other customary closing conditions.

In addition to the Merger, the Company will, subject to obtaining the required shareholder approvals and at least one (1) day prior to the date of the Closing, change its jurisdiction of incorporation by deregistering as a Cayman Islands exempted company and continuing and domesticating as a corporation incorporated under the laws of the State of Delaware. The Company will provide its public shareholders the opportunity to elect, at least two (2) Business Days prior to the Company shareholder’s meeting, to redeem their shares on the terms and conditions set forth in the Business Combination Agreement and the Company’s governing documents (the “Redemption”). Subject to the receipt of approval from shareholders of the Company, and at least one (1) day prior to the Domestication, the Company will carry out the Redemption.

By virtue of the Domestication and subject to the satisfaction or waiver of the conditions of the Business Combination Agreement, including approval of the Company’s shareholders: (i) immediately prior to the Domestication, each of the then issued and outstanding Class B Ordinary Shares of the Company will convert automatically, on a one-for-one basis, into one (1) Class A Ordinary Share, par value of $0.0001 per share, of the Company; and (ii) immediately following the Sponsor Share Conversion, in connection with the Domestication, (x) each then issued and outstanding Class A Ordinary Share (other than any Class A Ordinary Share included in the Cayman Purchaser Units (as defined in the Business Combination Agreement)) will convert automatically, on a one-for-one basis, into one (1) share of common stock, par value $0.0001 per share, of the Company (after the Domestication) (the “New Terrestrial Common Share”); (y) each of the then issued and outstanding warrants (other than any Cayman Purchaser Public Warrants (as defined in the Business Combination Agreement) included in the Cayman Purchaser Units) representing the right to purchase one (1) Class A Ordinary Share will convert automatically into a warrant to acquire one (1) share of New Terrestrial Common Share (each a “New Terrestrial Warrant”); and (z) each of the then issued and outstanding Cayman Purchaser Units will be cancelled and each holder thereof will be entitled to one share of New Terrestrial Common Share and one-half (1/2) of one (1) New Terrestrial Warrant.

The Company has also entered into the PIPE Subscription Agreements, each dated as of March 26, 2025, with the PIPE Investors, pursuant to which, among other things, the Company has agreed to issue and sell, in private placements to close immediately prior to or substantially concurrently with the Closing, the PIPE Financing. The PIPE Investors are permitted, under the PIPE Subscription Agreements, to satisfy their commitments thereunder if they hold shares of New Terrestrial Common Share that qualify as Non-Redeemed Shares (as defined herein), subject to certain conditions and restrictions set forth in the PIPE Subscription Agreements.