Composition of the Company's Loan Portfolio (Additional Information) (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other Real Estate	[1]	$ 53		$ 54
Letters of Credit Outstanding, Amount		4,200		5,800
Loans and Leases Receivable, Gross	[2]	155,233		151,839
Loans Receivable, Fair Value Disclosure		134		163
Loans Held-for-sale, Fair Value Disclosure		1,100		1,200
Transfer of Portfolio Loans and Leases to Held-for-sale		614	$ 204
Transfer of Loans Held-for-sale to Portfolio Loans		4	6
Transfer to Other Real Estate		10	19
Consumer Indirect [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross		13,268		12,419
Finance Leases Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross		4,100		4,059
Installment Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross		786		796
Consumer Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross		71,504		70,899
Loans Receivable, Fair Value Disclosure		134		163
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other Real Estate		2		2
Loans and Leases Receivable, Gross		83,729		80,940
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		30.00%		27.00%
Loans and Leases Receivable, Gross		$ 467		$ 459
Federal Family Education Loan Program (FFELP) Guaranteed Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of Loan Portfolio Current		73.00%		72.00%
Loans and Leases Receivable, Gross		$ 7,300		$ 7,200
Credit Concentration Risk [Member] | Home Equity Line of Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unused Commitments to Extend Credit		10,500		10,300
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Significant Sales	[3],[4]	40	36
Federal Reserve Bank Advances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Pledged as Collateral		30,200		28,100
Line of Credit Facility, Remaining Borrowing Capacity		22,600		21,300
Federal Home Loan Bank Advances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Pledged as Collateral		39,800		39,200
Line of Credit Facility, Remaining Borrowing Capacity		32,100		$ 31,000
Non-recurring loan purchase [Member] | Consumer Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Significant Purchases	[5],[6]	173	0
Recurring loan purchase [Member] | Consumer Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Significant Purchases	[5],[7]	$ 445	$ 475

[1] Does not include foreclosed real estate related to loans insured by the FHA or guaranteed by the VA. Proceeds due from the FHA and the VA are recorded as a receivable in Other assets in the Consolidated Balance Sheets until the property is conveyed and the funds are received. The receivable related to proceeds due from the FHA and the VA totaled $50 million at both March 31, 2019 and December 31, 2018.

[2] Includes loans of consolidated VIEs of $147 million and $153 million at March 31, 2019 and December 31, 2018, respectively.

[3] Net gain on loan sales was immaterial for the three months ended March 31, 2019 and 2018.

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

[5] Represents UPB of loans purchased.

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

[7] Purchases are routine in nature and are conducted in the normal course of business.