Loans - Consumer Banking: 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
Consumer Banking
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 59,248	$ 59,205
Percentage of portfolio	100.00%	100.00%
Consumer Banking | Auto
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 56,444	$ 56,341
Consumer Banking | Auto | Loans receivable | Geographic concentration risk
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 56,444	$ 56,341
Percentage of portfolio	95.30%	95.20%
Consumer Banking | Auto | Loans receivable | Geographic concentration risk | Texas
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 7,256	$ 7,264
Percentage of portfolio	12.20%	12.30%
Consumer Banking | Auto | Loans receivable | Geographic concentration risk | California
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 6,416	$ 6,352
Percentage of portfolio	10.80%	10.70%
Consumer Banking | Auto | Loans receivable | Geographic concentration risk | Florida
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 4,645	$ 4,623
Percentage of portfolio	7.80%	7.80%
Consumer Banking | Auto | Loans receivable | Geographic concentration risk | Georgia
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 2,638	$ 2,665
Percentage of portfolio	4.50%	4.50%
Consumer Banking | Auto | Loans receivable | Geographic concentration risk | Ohio
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 2,521	$ 2,502
Percentage of portfolio	4.30%	4.20%
Consumer Banking | Auto | Loans receivable | Geographic concentration risk | Louisiana
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 2,134	$ 2,174
Percentage of portfolio	3.60%	3.70%
Consumer Banking | Auto | Loans receivable | Geographic concentration risk | Illinois
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 2,156	$ 2,171
Percentage of portfolio	3.60%	3.70%
Consumer Banking | Auto | Loans receivable | Geographic concentration risk | Pennsylvania
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 2,161	$ 2,167
Percentage of portfolio	3.60%	3.70%
Consumer Banking | Auto | Loans receivable | Geographic concentration risk | Other
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 26,517	$ 26,423
Percentage of portfolio	44.90%	44.60%
Consumer Banking | Retail banking
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 2,804	$ 2,864
Consumer Banking | Retail banking | Loans receivable | Geographic concentration risk
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 2,804	$ 2,864
Percentage of portfolio	4.70%	4.80%
Consumer Banking | Retail banking | Loans receivable | Geographic concentration risk | Texas
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 628	$ 647
Percentage of portfolio	1.00%	1.10%
Consumer Banking | Retail banking | Loans receivable | Geographic concentration risk | Louisiana
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 753	$ 772
Percentage of portfolio	1.30%	1.30%
Consumer Banking | Retail banking | Loans receivable | Geographic concentration risk | Other
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 110	$ 109
Percentage of portfolio	0.20%	0.20%
Consumer Banking | Retail banking | Loans receivable | Geographic concentration risk | New York
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 827	$ 837
Percentage of portfolio	1.40%	1.40%
Consumer Banking | Retail banking | Loans receivable | Geographic concentration risk | New Jersey
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 193	$ 201
Percentage of portfolio	0.30%	0.30%
Consumer Banking | Retail banking | Loans receivable | Geographic concentration risk | Maryland
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 159	$ 161
Percentage of portfolio	0.30%	0.30%
Consumer Banking | Retail banking | Loans receivable | Geographic concentration risk | Virginia
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 134	$ 137
Percentage of portfolio	0.20%	0.20%