Loans - Consumer Banking: Net Charge-Offs and Nonperforming Loans (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Loans and Leases Receivable Disclosure [Line Items]
Nonperforming Loans	$ 899	$ 1,599
Financing Receivable, Nonaccrual, Percent Past Due	0.35%	0.65%
Consumer Portfolio Segment [Member]
Loans and Leases Receivable Disclosure [Line Items]
Net charge-offs	$ 1,038	$ 820	$ 731
Percentage annualized net charge-off by average loans held for investment	1.39%	1.15%
Nonperforming Loans	$ 587	$ 527
Financing Receivable, Nonaccrual, Percent Past Due	0.78%	0.72%
Percentage Annualized Net Charge-Offs by Average Loans Held for Investment, Excluding Loans Acquired Impact	1.65%	1.49%
Nonperforming loans, excluding acquired loans (as percent)	0.91%	0.90%
Consumer Portfolio Segment [Member] | Auto
Loans and Leases Receivable Disclosure [Line Items]
Net charge-offs	$ 957	$ 752
Percentage annualized net charge-off by average loans held for investment	1.86%	1.69%
Nonperforming Loans	$ 376	$ 223
Financing Receivable, Nonaccrual, Percent Past Due	0.70%	0.47%
Consumer Portfolio Segment [Member] | Home loan
Loans and Leases Receivable Disclosure [Line Items]
Net charge-offs	$ 15	$ 14
Percentage annualized net charge-off by average loans held for investment	0.08%	0.06%
Nonperforming Loans	$ 176	$ 273
Financing Receivable, Nonaccrual, Percent Past Due	1.00%	1.26%
Percentage Annualized Net Charge-Offs by Average Loans Held for Investment, Excluding Loans Acquired Impact	0.07%	0.20%
Nonperforming loans, excluding acquired loans (as percent)	2.39%	3.81%
Consumer Portfolio Segment [Member] | Retail banking
Loans and Leases Receivable Disclosure [Line Items]
Net charge-offs	$ 66	$ 54
Percentage annualized net charge-off by average loans held for investment	1.92%	1.53%
Nonperforming Loans	$ 35	$ 31
Financing Receivable, Nonaccrual, Percent Past Due	1.00%	0.86%