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Fair Value (Fair Value Option for Certain Assets and Liabilities) (Details) - Residential mortgage loans - FVO - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Fair Value, Option, Quantitative Disclosures [Line Items]    
Unpaid principal balance $ 209 $ 344
Difference between estimated fair value and unpaid principal balance (21) (45)
Carrying value at estimated fair value 188 299
Loans in nonaccrual status 47 89
Loans more than 90 days past due 18 41
Loans in nonaccrual status or more than 90 days past due, or both - difference between aggregate estimate fair value and unpaid principal balance $ (19) $ (36)