Composition of the Company's Loan Portfolio (Additional Information) (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Letters of Credit Outstanding, Amount		$ 5,800	$ 6,700
Loans and Leases Receivable, Gross	[1]	151,839	143,181
Loans Receivable, Fair Value Disclosure		163	196
Loans Held-for-sale, Fair Value Disclosure		1,200	1,600
Transfer of Portfolio Loans and Leases to Held-for-sale		532	288	$ 360
Transfer of Loans Held-for-sale to Portfolio Loans		28	19	30
Transfer to Other Real Estate		62	57	$ 59
Consumer Indirect [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross		12,419	12,140
Finance Leases Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross		4,100	3,700
Installment Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross		796	778
Consumer Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross		70,899	67,704
Loans Receivable, Fair Value Disclosure		163	196
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross		80,940	75,477
Loans Receivable, Fair Value Disclosure		$ 0	$ 0
Federal National Mortgage Association (FNMA) Insured Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of Loan Portfolio Current		27.00%	28.00%
Loans and Leases Receivable, Gross		$ 459	$ 560
Federal Family Education Loan Program (FFELP) Guaranteed Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of Loan Portfolio Current		72.00%	75.00%
Loans and Leases Receivable, Gross		$ 7,229	$ 6,600
Credit Concentration Risk [Member] | Home Equity Line of Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unused Commitments to Extend Credit		10,300	10,100
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Significant Sales	[2],[3]	170	703
Consumer Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Significant Sales	[2],[3]	99	2
Federal Reserve Bank Advances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Pledged as Collateral		28,100	24,300
Line of Credit Facility, Remaining Borrowing Capacity		21,300	18,200
Federal Home Loan Bank Advances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Pledged as Collateral		39,200	38,000
Line of Credit Facility, Remaining Borrowing Capacity		31,000	30,500
Non-recurring loan purchase [Member] | Consumer Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Significant Purchases	[4]	101	233
Recurring loan purchase [Member] | Consumer Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Significant Purchases	[5]	$ 2,122	$ 1,729

[1] Includes loans of consolidated VIEs of $153 million and $179 million at December?31, 2018 and December?31, 2017, respectively.

[2] Net gain on loan sales were immaterial during the years ended December 31, 2018 and 2017.

[3] Excludes sales of residential and commercial mortgage LHFS conducted in the normal course of business.

[4] Purchases are episodic in nature and are conducted based on specific business strategies.

[5] Purchases are episodic in nature and are conducted based on specific business strategies.