Composition of the Company's Loan Portfolio (Additional Information) (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	[1]	$ 144,935		$ 144,935		$ 143,181
Loans Receivable, Fair Value Disclosure		177		177		196
Loans Held-for-sale, Fair Value Disclosure		2,005		2,005		1,577
Transfer of Portfolio Loans and Leases to Held-for-sale		123	$ 67	327	$ 127
Transfer of Loans Held-for-sale to Portfolio Loans		12	3	18	10
Loans held for investment sold		137	110	172	228
Loans held for investment purchased		532	$ 493
Consumer Indirect [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross		11,712		11,712		12,140
Loans held for investment purchased				16	$ 99
Finance Leases Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross		3,755		3,755		3,693
Installment Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross		800		800		778
Consumer Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross		67,834		67,834		67,704
Loans Receivable, Fair Value Disclosure		$ 177		$ 177		$ 196

[1] Includes loans of consolidated VIEs of $165 million and $179 million at June 30, 2018 and December 31, 2017, respectively.