Loans - Credit Card Loan Portfolio, Other Credit Quality Indicators (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Loans and Leases Receivable Disclosure [Line Items]
Retained loans	$ 960,506	$ 945,601	$ 969,415
Credit card
Loans and Leases Receivable Disclosure [Line Items]
Retained loans	$ 143,432	$ 168,924
Percentage of portfolio based on carrying value with estimated refreshed FICO scores, Equal to or greater than 660	85.90%	84.00%
Percentage of portfolio based on carrying value with estimated refreshed FICO scores, Less than 660	13.90%	15.40%
Percentage of portfolio based on carrying value with estimated refreshed FICO scores, No FICO available	0.20%	0.60%
Credit card | California
Loans and Leases Receivable Disclosure [Line Items]
Retained loans	$ 20,921	$ 25,783
Credit card | Texas
Loans and Leases Receivable Disclosure [Line Items]
Retained loans	14,544	16,728
Credit card | New York
Loans and Leases Receivable Disclosure [Line Items]
Retained loans	11,919	14,544
Credit card | Florida
Loans and Leases Receivable Disclosure [Line Items]
Retained loans	9,562	10,830
Credit card | Illinois
Loans and Leases Receivable Disclosure [Line Items]
Retained loans	8,006	9,579
Credit card | New Jersey
Loans and Leases Receivable Disclosure [Line Items]
Retained loans	5,927	7,165
Credit card | Ohio
Loans and Leases Receivable Disclosure [Line Items]
Retained loans	4,673	5,406
Credit card | Pennsylvania
Loans and Leases Receivable Disclosure [Line Items]
Retained loans	4,476	5,245
Credit card | Colorado
Loans and Leases Receivable Disclosure [Line Items]
Retained loans	4,092	4,763
Credit card | Michigan
Loans and Leases Receivable Disclosure [Line Items]
Retained loans	3,553	4,164
Credit card | All other
Loans and Leases Receivable Disclosure [Line Items]
Retained loans	$ 55,759	$ 64,717