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Financial Guarantee Insurance Contracts - Estimated Future Amortization Expense for Insurance Intangible Asset (Detail) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Dec. 31, 2020
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Amortization Of Intangible Assets [Line Items]                      
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life                 7 years 6 months    
Amortization of insurance intangible assets $ 16 $ 14 $ 14 $ 13 $ 15 $ 17 $ 226 $ 36 $ 57 $ 295 $ 107
Intangible Assets, Gross (Excluding Goodwill) 1,281       1,273       1,281 1,273  
Finite-Lived Intangible Assets, Accumulated Amortization 908       847       908 847  
Intangible assets 409       $ 427       409 $ 427  
Insurance Intangible Asset [Member]                      
Amortization Of Intangible Assets [Line Items]                      
2018 39               39    
2019 35               35    
2020 32               32    
2021 29               29    
2022 26               26    
Thereafter $ 213               $ 213