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Fair Value (Fair Value Option for Certain Assets and Liabilities) (Details) - Residential mortgage loans - FVO - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Fair Value, Option, Quantitative Disclosures [Line Items]    
Unpaid principal balance $ 172 $ 209
Difference between estimated fair value and unpaid principal balance (7) (21)
Carrying value at estimated fair value 165 188
Loans in nonaccrual status 45 47
Loans more than 90 days past due 27 18
Loans in nonaccrual status or more than 90 days past due, or both - difference between aggregate estimate fair value and unpaid principal balance $ (13) $ (19)