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Fair Value (Fair Value Option for Residential Mortgage Loans) (Details) - Residential mortgage loans - FVO - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Fair Value, Option, Quantitative Disclosures [Line Items]    
Unpaid principal balance $ 496 $ 650
Difference between estimated fair value and unpaid principal balance (91) (130)
Carrying value at estimated fair value 405 520
Loans in nonaccrual status 129 198
Loans more than 90 Days past due 64 94
Loans in nonaccrual status or more than 90 days past due, or both $ (66) $ (102)