Loans - Credit Card Loan Portfolio (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	$ 947,728	$ 969,415	$ 940,440
Credit card
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	$ 157,568	$ 156,616
% of 30 plus days past due to total retained loans	1.71%	1.83%
% of 90 plus days past due to total retained loans	0.87%	0.92%
Percentage of portfolio based on carrying value with estimated refreshed FICO scores, Equal to or greater than 660	84.00%	84.20%
Percentage of portfolio based on carrying value with estimated refreshed FICO scores, Less than 660	14.70%	15.00%
Percentage of portfolio based on carrying value with estimated refreshed FICO scores, No FICO available	1.30%	0.80%
Credit card | California
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	$ 24,047	$ 23,757
Credit card | Texas
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	15,495	15,085
Credit card | New York
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	13,676	13,601
Credit card | Florida
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	9,867	9,770
Credit card | Illinois
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	9,038	8,938
Credit card | New Jersey
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	6,721	6,739
Credit card | Ohio
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	5,060	5,094
Credit card | Pennsylvania
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	4,900	4,996
Credit card | Colorado
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	4,477	4,309
Credit card | Michigan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	3,885	3,912
Credit card | All other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	60,402	60,415
Credit card | Current and less than 30 days past due and still accruing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	154,876	153,746
Credit card | 30–89 days past due and still accruing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	1,323	1,426
Credit card | 90 or more days past due and still accruing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	$ 1,369	$ 1,444