Loans - Credit Card Loan Portfolio (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	$ 947,651	$ 924,838	$ 909,182
Credit card
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	$ 147,856	$ 149,387
% of 30 plus days past due to total retained loans	1.75%	1.80%
% of 90 plus days past due to total retained loans	0.85%	0.92%
Percentage of portfolio based on carrying value with estimated refreshed FICO scores, Equal to or greater than 660	83.70%	84.00%
Percentage of portfolio based on carrying value with estimated refreshed FICO scores, Less than 660	14.90%	14.60%
Percentage of portfolio based on carrying value with estimated refreshed FICO scores, No FICO available	1.40%	1.40%
Credit card | California
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	$ 22,166	$ 22,245
Credit card | Texas
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	14,171	14,200
Credit card | New York
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	12,908	13,021
Credit card | Florida
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	9,064	9,138
Credit card | Illinois
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	8,482	8,585
Credit card | New Jersey
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	6,345	6,506
Credit card | Ohio
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	4,803	4,997
Credit card | Pennsylvania
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	4,677	4,883
Credit card | Colorado
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	4,090	4,006
Credit card | Michigan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	3,710	3,826
Credit card | All other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	57,440	57,980
Credit card | Current and less than 30 days past due and still accruing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	145,271	146,704
Credit card | 30–89 days past due and still accruing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	1,323	1,305
Credit card | 90 or more days past due and still accruing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	$ 1,262	$ 1,378