v3.26.1
Document and Entity Information - shares
 6 Months Ended
Jun. 30, 2026
Aug. 11, 2026
Document Information [Line Items]    
Document Type 10-Q  
Document Quarterly Report true  
Current Fiscal Year End Date --12-31  
Document Fiscal Year Focus 2026  
Document Period End Date Jun. 30, 2026  
Document Transition Report false  
Entity File Number 001-42252  
Entity Registrant Name TERRESTRIAL ENERGY INC.  
Entity Incorporation, State or Country Code DE  
Entity Tax Identification Number 98-1785406  
Entity Address, Address Line One 2730 W. Tyvola Road  
Entity Address, Adress Line Two Suite 100  
Entity Address, City or Town Charlotte  
Entity Address State Or Province NC  
Entity Address, Postal Zip Code 28217  
City Area Code 646  
Local Phone Number 687-8212  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Non-accelerated Filer  
Entity Small Business true  
Entity Emerging Growth Company true  
Entity Ex Transition Period false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   105,935,254
Entity Central Index Key 0002019804  
Document Fiscal Period Focus Q2  
Amendment Flag false  
Common Stock    
Document Information [Line Items]    
Title of 12(b) Security Common Stock, par value $0.0001 per share  
Trading Symbol IMSR  
Security Exchange Name NASDAQ  
Redeemable Warrants    
Document Information [Line Items]    
Title of 12(b) Security Redeemable Warrants, each whole warrant exercisable for one Common Stock at a price of $11.50 per share  
Trading Symbol IMSRW  
Security Exchange Name NASDAQ