Loans - Consumer Banking: Risk Profile by Geographic Region, Delinquency Status and Performing Status (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 245,899	$ 254,473	$ 245,586
Consumer Portfolio Segment
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 59,205	$ 75,078
Percentage of portfolio	100.00%	100.00%
Consumer Portfolio Segment | Auto
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 56,341	$ 53,991
Consumer Portfolio Segment | Auto | Loans Receivable | Geographic Concentration Risk
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 56,341	$ 53,991
Percentage of portfolio	95.20%	71.90%
Consumer Portfolio Segment | Auto | Loans Receivable | Geographic Concentration Risk | Texas
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 7,264	$ 7,040
Percentage of portfolio	12.30%	9.40%
Consumer Portfolio Segment | Auto | Loans Receivable | Geographic Concentration Risk | California
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 6,352	$ 6,099
Percentage of portfolio	10.70%	8.10%
Consumer Portfolio Segment | Auto | Loans Receivable | Geographic Concentration Risk | Florida
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 4,623	$ 4,486
Percentage of portfolio	7.80%	6.00%
Consumer Portfolio Segment | Auto | Loans Receivable | Geographic Concentration Risk | Georgia
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 2,665	$ 2,726
Percentage of portfolio	4.50%	3.60%
Consumer Portfolio Segment | Auto | Loans Receivable | Geographic Concentration Risk | Ohio
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 2,502	$ 2,318
Percentage of portfolio	4.20%	3.10%
Consumer Portfolio Segment | Auto | Loans Receivable | Geographic Concentration Risk | Louisiana
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 2,174	$ 2,236
Percentage of portfolio	3.70%	3.00%
Consumer Portfolio Segment | Auto | Loans Receivable | Geographic Concentration Risk | Illinois
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 2,171	$ 2,181
Percentage of portfolio	3.70%	2.90%
Consumer Portfolio Segment | Auto | Loans Receivable | Geographic Concentration Risk | Pennsylvania
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 2,167	$ 2,014
Percentage of portfolio	3.70%	2.70%
Consumer Portfolio Segment | Auto | Loans Receivable | Geographic Concentration Risk | Other
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 26,423	$ 24,891
Percentage of portfolio	44.60%	33.10%
Consumer Portfolio Segment | Retail banking
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 2,864	$ 3,454
Consumer Portfolio Segment | Retail banking | Loans Receivable | Geographic Concentration Risk
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 2,864	$ 3,454
Percentage of portfolio	4.80%	4.60%
Consumer Portfolio Segment | Retail banking | Loans Receivable | Geographic Concentration Risk | Texas
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 647	$ 717
Percentage of portfolio	1.10%	0.90%
Consumer Portfolio Segment | Retail banking | Loans Receivable | Geographic Concentration Risk | Louisiana
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 772	$ 953
Percentage of portfolio	1.30%	1.30%
Consumer Portfolio Segment | Retail banking | Loans Receivable | Geographic Concentration Risk | Other
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 109	$ 267
Percentage of portfolio	0.20%	0.40%
Consumer Portfolio Segment | Retail banking | Loans Receivable | Geographic Concentration Risk | New York
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 837	$ 955
Percentage of portfolio	1.40%	1.30%
Consumer Portfolio Segment | Retail banking | Loans Receivable | Geographic Concentration Risk | New Jersey
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 201	$ 221
Percentage of portfolio	0.30%	0.30%
Consumer Portfolio Segment | Retail banking | Loans Receivable | Geographic Concentration Risk | Maryland
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 161	$ 187
Percentage of portfolio	0.30%	0.20%
Consumer Portfolio Segment | Retail banking | Loans Receivable | Geographic Concentration Risk | Virginia
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 137	$ 154
Percentage of portfolio	0.20%	0.20%
Consumer Portfolio Segment | Home loan
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment		$ 17,633
Consumer Portfolio Segment | Home loan | Loans Receivable | Geographic Concentration Risk
Loans and Leases Receivable Disclosure [Line Items]
Percentage of portfolio	0.00%	23.50%
Consumer Portfolio Segment | Home loan | Home Loans Receivable | Geographic Concentration Risk
Loans and Leases Receivable Disclosure [Line Items]
Loans held for investment	$ 0	$ 17,633