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Loans - Credit Card Portfolio - Impaired Loans (Details) - Credit card - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Financing Receivable, Impaired [Line Items]          
Impaired credit card loans with an allowance $ 1,284   $ 1,284   $ 1,215
Allowance for loan losses related to impaired credit card loans 421   421   383
Average impaired credit card loans 1,267 $ 1,205 1,245 $ 1,215  
Interest income on impaired credit card loans 17 $ 15 48 $ 44  
Credit card loans with modified payment terms          
Financing Receivable, Impaired [Line Items]          
Impaired credit card loans with an allowance 1,228   1,228   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 56   56   80
Noncompliance with Modified Terms          
Financing Receivable, Impaired [Line Items]          
Impaired credit card loans with an allowance 26   26   43
Completion of Short Term Modification          
Financing Receivable, Impaired [Line Items]          
Impaired credit card loans with an allowance $ 30   $ 30   $ 37