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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 (Details) - Residential mortgage loans - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Aggregate  Fair Value    
Loans measured at fair value $ 1,642 $ 1,447
Past due loans of 90 days or more 3 2
Nonaccrual loans 0 1
Aggregate Unpaid Principal Balance    
Loans measured at fair value 1,567 1,410
Past due loans of 90 days or more 3 2
Nonaccrual loans 0 1
Difference    
Loans measured at fair value 75 37
Past due loans of 90 days or more 0 0
Nonaccrual loans $ 0 $ 0