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Loans - Credit Card Portfolio - Impaired Loans (Details) - Credit card - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Financing Receivable, Impaired [Line Items]          
Impaired credit card loans with an allowance $ 1,252   $ 1,252   $ 1,215
Allowance for loan losses related to impaired credit card loans 402   402   383
Average impaired credit card loans 1,244 $ 1,212 1,234 $ 1,220  
Interest income on impaired credit card loans 16 $ 15 31 $ 29  
Credit card loans with modified payment terms          
Financing Receivable, Impaired [Line Items]          
Impaired credit card loans with an allowance 1,191   1,191   1,135
Modified credit card loans that have reverted to pre-modification payment terms          
Financing Receivable, Impaired [Line Items]          
Impaired credit card loans with an allowance 61   61   80
Noncompliance with Modified Terms          
Financing Receivable, Impaired [Line Items]          
Impaired credit card loans with an allowance 29   29   43
Completion of Short Term Modification          
Financing Receivable, Impaired [Line Items]          
Impaired credit card loans with an allowance $ 32   $ 32   $ 37