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Fair Value (Fair Value Option for Residential Mortgage Loans) (Details) - Residential mortgage loans - FVO - USD ($)
$ in Millions
Sep. 30, 2021
Dec. 31, 2020
Fair Value, Option, Quantitative Disclosures [Line Items]    
Unpaid principal balance $ 135 $ 172
Difference between estimated fair value and unpaid principal balance (1) (7)
Carrying value at estimated fair value 134 165
Loans in nonaccrual status 32 45
Loans more than 90 days past due 16 27
Loans in nonaccrual status or more than 90 days past due, or both — difference between aggregate estimated fair value and unpaid principal balance $ (6) $ (13)