Composition of the Company's Loan Portfolio (Additional Information) (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other Real Estate	[1]	$ 55		$ 55		$ 54
Letters of Credit Outstanding, Amount		4,200		4,200		5,800
Loans and Leases Receivable, Gross		156,589		156,589		151,839
Loans Receivable, Fair Value Disclosure		127		127		163
Loans and Leases Receivable, Gain (Loss) on Sales, Net		45		45
Transfer of Portfolio Loans and Leases to Held-for-sale				713	$ 327
Transfer of Loans Held-for-sale to Portfolio Loans				12	18
Transfer to Other Real Estate				23	33
Consumer Indirect [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross		13,598		13,598		12,419
Finance Leases Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross		4,100		4,100		4,059
Installment Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross		862		862		796
Consumer Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross		72,598		72,598		70,899
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other Real Estate		1		1		2
Loans and Leases Receivable, Gross		83,991		83,991		80,940
Loans Receivable, Fair Value Disclosure		$ 0		$ 0		$ 0
Federal National Mortgage Association (FNMA) Insured Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of Loan Portfolio Current		28.00%		28.00%		27.00%
Loans and Leases Receivable, Gross		$ 439		$ 439		$ 459
Federal Family Education Loan Program (FFELP) Guaranteed Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of Loan Portfolio Current		76.00%		76.00%		72.00%
Loans and Leases Receivable, Gross		$ 7,200		$ 7,200		$ 7,229
Credit Concentration Risk [Member] | Home Equity Line of Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unused Commitments to Extend Credit		10,600		10,600		10,300
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Significant Sales	[2],[3]	177	$ 37	217	72
Consumer Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Significant Sales	[2],[3]	432	100	432	100
Federal Reserve Bank Advances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Pledged as Collateral		32,200		32,200		28,100
Line of Credit Facility, Remaining Borrowing Capacity		24,100		24,100		21,300
Federal Home Loan Bank Advances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Pledged as Collateral		39,400		39,400		39,200
Line of Credit Facility, Remaining Borrowing Capacity		31,900		31,900		$ 31,000
Non-recurring loan purchase [Member] | Consumer Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Significant Purchases	[4],[5]	85	0	258	0
Recurring loan purchase [Member] | Consumer Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Significant Purchases	[5],[6]	$ 471	$ 548	$ 916	$ 1,023

[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 $51 million and $50 million at June 30, 2019 and December 31, 2018, respectively.

[2] Excludes sales of loans originated for sale and loans recorded at fair value conducted in the normal course of business.

[3] Net gain on loan sales totaled $45 million for both the three and six months ended June 30, 2019, and was immaterial for the three and six months ended June 30, 2018.

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

[5] Represents UPB of loans purchased.

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