Loans - Credit Card: Risk Profile by Geographic Region (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 240,273	$ 245,899	$ 248,256
Credit Card
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 109,836	$ 116,361
Percentage of portfolio	100.00%	100.00%
Credit Card | California
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 10,978	$ 11,591
Percentage of portfolio	10.00%	10.00%
Credit Card | Texas
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 7,782	$ 8,173
Percentage of portfolio	7.10%	7.00%
Credit Card | New York
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 6,948	$ 7,400
Percentage of portfolio	6.30%	6.40%
Credit Card | Florida
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 6,755	$ 7,086
Percentage of portfolio	6.20%	6.10%
Credit Card | Illinois
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 4,443	$ 4,761
Percentage of portfolio	4.00%	4.10%
Credit Card | Pennsylvania
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 4,256	$ 4,575
Percentage of portfolio	3.90%	3.90%
Credit Card | Ohio
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 3,676	$ 3,967
Percentage of portfolio	3.30%	3.40%
Credit Card | New Jersey
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 3,420	$ 3,641
Percentage of portfolio	3.10%	3.10%
Credit Card | Michigan
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 3,303	$ 3,544
Percentage of portfolio	3.00%	3.00%
Credit Card | Other
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 49,491	$ 52,612
Percentage of portfolio	45.10%	45.30%
Credit Card | Canada
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 5,832	$ 6,023
Percentage of portfolio	5.30%	5.10%
Credit Card | United Kingdom
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 2,952	$ 2,988
Percentage of portfolio	2.70%	2.60%
Credit Card | Domestic credit card
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 101,052	$ 107,350
Percentage of portfolio	92.00%	92.30%
Credit Card | International card business
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 8,784	$ 9,011
Percentage of portfolio	8.00%	7.70%