v3.26.1
Document and Entity Information - shares
 3 Months Ended
Mar. 31, 2026
May 14, 2026
Document Information [Line Items]    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Mar. 31, 2026  
Document Transition Report false  
Entity File Number 001-43056  
Entity Registrant Name FG IMPERII ACQUISITION CORP.  
Entity Tax Identification Number 98-1884449  
Entity Address State Or Province IL  
Entity Address, Address Line One 104 S. Walnut Street  
Entity Address, Adress Line Two Unit 1A  
Entity Address, City or Town Itasca  
Entity Incorporation, State or Country Code E9  
Entity Address, Postal Zip Code 60143  
City Area Code 847  
Local Phone Number 791-6817  
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 true  
Entity Common Stock, Shares Outstanding   28,940,000
Entity Central Index Key 0002090452  
Current Fiscal Year End Date --12-31  
Document Fiscal Year Focus 2026  
Document Fiscal Period Focus Q1  
Amendment Flag false  
Units    
Document Information [Line Items]    
Title of 12(b) Security Units  
Trading Symbol FGII-U  
Security Exchange Name NASDAQ  
Class A Ordinary Shares    
Document Information [Line Items]    
Title of 12(b) Security Ordinary Share  
Trading Symbol FGII  
Security Exchange Name NASDAQ  
Warrants    
Document Information [Line Items]    
Title of 12(b) Security Warrants  
Trading Symbol FGII-W  
Security Exchange Name NASDAQ