Allowance for Loan and Lease Losses - Components of Allowance for Loan and Lease Losses by Impairment Methodology (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Allowance for loan and lease losses by impairment methodology:
Collectively evaluated	$ 7,044	$ 6,007
Asset-specific	421	465
Total allowance for loan and lease losses	7,502	6,503
Loans held for investment by impairment methodology:
Collectively evaluated	241,323	227,555
Asset-specific	2,375	2,960
Loans held for investment	$ 254,473	$ 245,586	$ 229,851
Allowance as a percentage of period-end loans held for investment	2.95%	2.65%
Credit Card Portfolio Segment [Member]
Allowance for loan and lease losses by impairment methodology:
Collectively evaluated	$ 5,356	$ 4,367
Asset-specific	292	239
Total allowance for loan and lease losses	5,648	4,606	3,654	$ 3,204
Loans held for investment by impairment methodology:
Collectively evaluated	113,948	104,835
Asset-specific	812	715
Loans held for investment	$ 114,762	$ 105,552
Allowance as a percentage of period-end loans held for investment	4.92%	4.36%
Consumer Portfolio Segment [Member]
Allowance for loan and lease losses by impairment methodology:
Collectively evaluated	$ 1,158	$ 1,016
Asset-specific	53	57
Total allowance for loan and lease losses	1,242	1,102	868	779
Loans held for investment by impairment methodology:
Collectively evaluated	64,080	57,862
Asset-specific	705	736
Loans held for investment	$ 75,078	$ 73,054
Allowance as a percentage of period-end loans held for investment	1.65%	1.51%
Commercial Banking
Allowance for loan and lease losses by impairment methodology:
Collectively evaluated	$ 529	$ 622
Asset-specific	76	169
Total allowance for loan and lease losses	611	793	604	395
Loans held for investment by impairment methodology:
Collectively evaluated	63,237	64,794
Asset-specific	858	1,509
Loans held for investment	$ 64,575	$ 66,916
Allowance as a percentage of period-end loans held for investment	0.95%	1.19%
Other
Allowance for loan and lease losses by impairment methodology:
Collectively evaluated	$ 1	$ 2
Asset-specific	0	0
Total allowance for loan and lease losses	1	2	$ 4	$ 5
Loans held for investment by impairment methodology:
Collectively evaluated	58	64
Asset-specific	0	0
Loans held for investment	$ 58	$ 64
Allowance as a percentage of period-end loans held for investment	1.72%	3.13%
Receivables Acquired with Deteriorated Credit Quality [Member]
Allowance for loan and lease losses by impairment methodology:
Financing Receivable, Allowance for Credit Losses	$ 37	$ 31
Loans held for investment by impairment methodology:
Loans held for investment	10,775	15,071
Receivables Acquired with Deteriorated Credit Quality [Member] | Credit Card Portfolio Segment [Member]
Allowance for loan and lease losses by impairment methodology:
Financing Receivable, Allowance for Credit Losses	0	0
Loans held for investment by impairment methodology:
Loans held for investment	2	2
Receivables Acquired with Deteriorated Credit Quality [Member] | Consumer Portfolio Segment [Member]
Allowance for loan and lease losses by impairment methodology:
Financing Receivable, Allowance for Credit Losses	31	29
Loans held for investment by impairment methodology:
Loans held for investment	10,293	14,456
Receivables Acquired with Deteriorated Credit Quality [Member] | Commercial Banking
Allowance for loan and lease losses by impairment methodology:
Financing Receivable, Allowance for Credit Losses	6	2
Loans held for investment by impairment methodology:
Loans held for investment	480	613
Receivables Acquired with Deteriorated Credit Quality [Member] | Other
Allowance for loan and lease losses by impairment methodology:
Financing Receivable, Allowance for Credit Losses	0	0
Loans held for investment by impairment methodology:
Loans held for investment	$ 0	$ 0