Loans - Credit Card Loan Portfolio, Other Credit Quality Indicators (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	$ 963,665	$ 960,506	$ 965,448
Credit card
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	$ 141,079	$ 143,432
Percentage of portfolio based on carrying value with estimated refreshed FICO scores, Equal to or greater than 660	88.40%	85.90%
Percentage of portfolio based on carrying value with estimated refreshed FICO scores, Less than 660	11.40%	13.90%
Percentage of portfolio based on carrying value with estimated refreshed FICO scores, No FICO available	0.20%	0.20%
Credit card | California
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	$ 20,857	$ 20,921
Credit card | Texas
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	14,535	14,544
Credit card | New York
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	11,692	11,919
Credit card | Florida
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	9,299	9,562
Credit card | Illinois
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	7,910	8,006
Credit card | New Jersey
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	5,860	5,927
Credit card | Ohio
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	4,541	4,673
Credit card | Pennsylvania
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	4,306	4,476
Credit card | Colorado
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	4,272	4,092
Credit card | Michigan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	3,426	3,553
Credit card | All other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retained loans	$ 54,381	$ 55,759