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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, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Amortization Of Intangible Assets [Line Items]                      
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life                 7 years 7 months 6 days    
Amortization of insurance intangible assets $ 15 $ 17 $ 226 $ 36 $ 29 $ 26 $ 23 $ 29 $ 295 $ 107 $ 151
Intangible Assets, Gross (Excluding Goodwill) 1,273       $ 1,552       1,273 1,552  
Insurance Asset [Member]                      
Amortization Of Intangible Assets [Line Items]                      
Amortization of insurance intangible assets                 295 $ 107 $ 151
Insurance Intangible Asset [Member]                      
Amortization Of Intangible Assets [Line Items]                      
2018 45               45    
2019 39               39    
2020 36               36    
2021 33               33    
2022 30               30    
Thereafter $ 244               $ 244