Loans - Credit Card Loan Portfolio (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	$ 943,841	$ 969,415	$ 925,611
Credit card
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	$ 150,515	$ 156,616
% of 30 plus days past due to total retained loans	1.85%	1.83%
% of 90 plus days past due to total retained loans	0.97%	0.92%
Percentage of portfolio based on carrying value with estimated refreshed FICO scores, Equal to or greater than 660	83.10%	84.20%
Percentage of portfolio based on carrying value with estimated refreshed FICO scores, Less than 660	15.60%	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	$ 22,935	$ 23,757
Credit card | Texas
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	14,784	15,085
Credit card | New York
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	13,054	13,601
Credit card | Florida
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	9,527	9,770
Credit card | Illinois
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	8,591	8,938
Credit card | New Jersey
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	6,402	6,739
Credit card | Ohio
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	4,830	5,094
Credit card | Pennsylvania
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	4,693	4,996
Credit card | Colorado
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	4,185	4,309
Credit card | Michigan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	3,713	3,912
Credit card | All other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	57,801	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	147,726	153,746
Credit card | 30–89 days past due and still accruing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	1,334	1,426
Credit card | 90 or more days past due and still accruing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	$ 1,455	$ 1,444