v3.26.1
Document and Entity Information - USD ($)
 4 Months Ended
Dec. 31, 2025
Mar. 31, 2026
Jun. 30, 2025
Document Information [Line Items]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2025    
Document Transition Report false    
Entity File Number 001-42493    
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 Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
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    
ICFR Auditor Attestation Flag false    
Document Financial Statement Error Correction [Flag] false    
Entity Ex Transition Period false    
Entity Shell Company true    
Entity Public Float     $ 0
Entity Central Index Key 0002090452    
Current Fiscal Year End Date --12-31    
Document Fiscal Year Focus 2025    
Document Fiscal Period Focus FY    
Amendment Flag false    
Auditor Name Fruci & Associates II, PLLC    
Auditor Location Spokane, Washington    
Auditor Firm ID 5525    
Inception Date Sep. 16, 2025    
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 Class A Ordinary Shares    
Trading Symbol FGII    
Security Exchange Name NASDAQ    
Entity Common Stock, Shares Outstanding   23,025,000  
Class B ordinary shares      
Document Information [Line Items]      
Entity Common Stock, Shares Outstanding   5,687,500  
Warrants      
Document Information [Line Items]      
Title of 12(b) Security Warrants    
Trading Symbol FGII.W    
Security Exchange Name NASDAQ