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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
Dec. 31, 2018
Dec. 31, 2017
Residential mortgage loans    
Aggregate fair value    
Loans measured at fair value $ 716 $ 536
Past due loans of 90 days or more 2 5
Nonaccrual loans 2 1
Aggregate unpaid principal balance    
Loans measured at fair value 696 522
Past due loans of 90 days or more 2 5
Nonaccrual loans 2 1
Difference    
Loans measured at fair value 20 14
Past due loans of 90 days or more 0 0
Nonaccrual loans 0 $ 0
Commercial loans    
Aggregate fair value    
Loans measured at fair value 7  
Aggregate unpaid principal balance    
Loans measured at fair value 7  
Difference    
Loans measured at fair value $ 0