Insurance in Force (Narrative) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Guarantor Obligations [Line Items]
Transactions guaranteed by MBIA on behalf of affiliated companies	$ 1,700
Aggregate amount of insurance in force ceded to reinsurers	3,000	$ 3,500
Aggregate amount of insured par outstanding ceded to reinsurers	1,480	$ 1,800
U S Public Finance Insurance [Member]
Guarantor Obligations [Line Items]
Aggregate amount of insured par outstanding ceded to reinsurers	1,100
Structured Finance And International Insurance [Member]
Guarantor Obligations [Line Items]
Aggregate amount of insured par outstanding ceded to reinsurers	$ 413
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