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Fair Value Measurements (Difference Between the Aggregate Fair Value and the Aggregate Unpaid Principal Balance for Residential Mortgage Loans Measured at Fair Value) (Detail) - USD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Residential mortgage loans    
Aggregate fair value    
Loans measured at fair value $ 816 $ 716
Past due loans of 90 days or more 2 2
Nonaccrual loans 1 2
Aggregate unpaid principal balance    
Loans measured at fair value 793 696
Past due loans of 90 days or more 2 2
Nonaccrual loans 1 2
Difference    
Loans measured at fair value 23 20
Past due loans of 90 days or more 0 0
Nonaccrual loans 0 0
Commercial loans    
Aggregate fair value    
Loans measured at fair value 22 7
Aggregate unpaid principal balance    
Loans measured at fair value 22 7
Difference    
Loans measured at fair value $ 0 $ 0