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Debt (Narrative) (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Debt Instrument [Line Items]    
Net gains (losses) on extinguishment of debt $ (1)  
Long-term debt principal amount purchased   $ 44
MBIA Inc. Senior Notes [Member]    
Debt Instrument [Line Items]    
Debt instrument interest rate 5.70%  
Long-term debt, maturity year 2034  
MBIA Corp Surplus Notes [Member]    
Debt Instrument [Line Items]    
Callable date for fifth anniversary of initial callable date Jan. 15, 2023  
Mbia Inc. Debentures [Member]    
Debt Instrument [Line Items]    
Debt instrument interest rate 7.00%  
Long-term debt, maturity year 2025  
6.400% Senior Notes due 2022 [Member]    
Debt Instrument [Line Items]    
Percentage of debt redemption 100.00%  
Debt instrument interest rate 6.40%  
Long-term debt, maturity year 2022  
Long-term debt principal amount purchased $ 150  
Long-term debt principal amount owned [1] $ 115 $ 265
Medium-term notes [Member]    
Debt Instrument [Line Items]    
Weighted average interest rate 3.07% 3.23%
Investment Agreement Obligations [Member]    
Debt Instrument [Line Items]    
Weighted average interest rate 5.86% 5.83%
Refinanced Facility [Member]    
Debt Instrument [Line Items]    
Proceeds from issuance of senior notes $ 54  
Debt issuance cost paid 6  
Refinanced Facility [Member] | Expenses Of Consolidated Variable Interest Entities [Member]    
Debt Instrument [Line Items]    
Debt issuance cost expensed $ 3  
MBIA Corp Financing Facility [Member]    
Debt Instrument [Line Items]    
Debt instrument interest rate 12.00%  
Long-term debt, maturity date Jan. 20, 2022  
MBIA Corp Financing Facility [Member] | Initial [Member]    
Debt Instrument [Line Items]    
Long-term debt principal amount owned $ 278  
MBIA Corp Financing Facility [Member] | Outstanding [Member]    
Debt Instrument [Line Items]    
Long-term debt principal amount owned 246 $ 373
Mbia Inc [Member] | MBIA Corp Surplus Notes [Member]    
Debt Instrument [Line Items]    
Long-term debt principal amount owned 13  
National [Member] | MBIA Inc. Senior Notes [Member]    
Debt Instrument [Line Items]    
Long-term debt principal amount owned 308  
National [Member] | Mbia Inc. Debentures [Member]    
Debt Instrument [Line Items]    
Long-term debt principal amount purchased 10 $ 10
Long-term debt principal amount owned $ 10  
Variable Interest Entity [Member]    
Debt Instrument [Line Items]    
Weighted average interest rate 5.57% 6.91%
Variable Interest Entity Primary Beneficiary [Member]    
Debt Instrument [Line Items]    
Long-term debt principal amount owned $ 2,300 $ 2,600
MBIA Insured Variable Interest Entity Primary Beneficiary [Member]    
Debt Instrument [Line Items]    
Long-term debt principal amount owned $ 1,600 $ 1,800
Maximum [Member] | Medium-term notes [Member]    
Debt Instrument [Line Items]    
Debt instrument interest rate 6.00% 6.00%
Maximum [Member] | Investment Agreement Obligations [Member]    
Debt Instrument [Line Items]    
Debt instrument interest rate 6.88% 6.88%
Maximum [Member] | Variable Interest Entity [Member]    
Debt Instrument [Line Items]    
Debt instrument interest rate 12.00% 14.00%
Minimum [Member] | Medium-term notes [Member]    
Debt Instrument [Line Items]    
Debt instrument interest rate 0.00% 0.00%
Minimum [Member] | Investment Agreement Obligations [Member]    
Debt Instrument [Line Items]    
Debt instrument interest rate 4.78% 4.78%
Minimum [Member] | Variable Interest Entity [Member]    
Debt Instrument [Line Items]    
Debt instrument interest rate 3.71% 2.81%
[1] Callable on or after August 15, 2006 at par.