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Fair Value (Fair Value Option for Residential Mortgage Loans) (Details) - Residential mortgage loans - FVO - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
Fair Value, Option, Quantitative Disclosures [Line Items]    
Unpaid principal balance $ 401 $ 650
Difference between estimated fair value and unpaid principal balance (78) (130)
Carrying value at estimated fair value 323 520
Loans in nonaccrual status 104 198
Loans more than 90 Days past due 49 94
Loans in nonaccrual status or more than 90 days past due, or both $ (56) $ (102)