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Insurance in Force (Narrative) (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Guarantor Obligations [Line Items]    
Transactions guaranteed by MBIA on behalf of affiliated companies $ 900  
Aggregate amount of insurance in force ceded to reinsurers 1,500 $ 1,700
Aggregate amount of insured par outstanding ceded to reinsurers 671 $ 782
US Public Finance Insurance [Member]    
Guarantor Obligations [Line Items]    
Aggregate amount of insured par outstanding ceded to reinsurers 539  
Structured Finance And International Insurance [Member]    
Guarantor Obligations [Line Items]    
Aggregate amount of insured par outstanding ceded to reinsurers $ 132  
Financial Guaranty Insurance Company [Member]    
Guarantor Obligations [Line Items]    
Financial guaranty maturity year 2058  
Investment agreements, MTNs and other affiliated contracts [Member]    
Guarantor Obligations [Line Items]    
Financial guaranty maturity year 2037