Composition of the Company's Loan Portfolio (Additional Information) (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other Real Estate	[1]	$ 52		$ 52		$ 54
Letters of Credit Outstanding, Amount		4,200		4,200		5,800
Loans and Leases Receivable, Gross	[2]	158,455		158,455		151,839
Loans Receivable, Fair Value Disclosure		124		124		163
Loans and Leases Receivable, Gain (Loss) on Sales, Net		45		47
Transfer of Portfolio Loans and Leases to Held-for-sale				812	$ 449
Transfer of Loans Held-for-sale to Portfolio Loans				17	23
Transfer to Other Real Estate				33	44
Consumer Indirect [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross		14,060		14,060		12,419
Finance Leases Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross		4,000		4,000		4,100
Installment Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross		817		817		796
Consumer Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross		73,448		73,448		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		85,007		85,007		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		29.00%		29.00%		27.00%
Loans and Leases Receivable, Gross		$ 457		$ 457		$ 459
Federal Family Education Loan Program (FFELP) Guaranteed Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of Loan Portfolio Current		78.00%		78.00%		72.00%
Loans and Leases Receivable, Gross		$ 7,146		$ 7,146		$ 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, Sale	[3],[4]	171	$ 14	387	87
Consumer Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Sale	[3],[4]	0	0	432	100
Federal Reserve Bank Advances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Pledged as Collateral		33,300		33,300		28,100
Line of Credit Facility, Remaining Borrowing Capacity		24,400		24,400		21,300
Federal Home Loan Bank Advances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Pledged as Collateral		39,800		39,800		39,200
Line of Credit Facility, Remaining Borrowing Capacity		32,500		32,500		$ 31,000
Non-recurring loan purchase [Member] | Consumer Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Purchase	[5],[6]	160	101	418	101
Recurring loan purchase [Member] | Consumer Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Purchase	[6],[7]	$ 517	$ 545	$ 1,433	$ 1,568

[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 $43 million and $50 million at September 30, 2019 and December 31, 2018, respectively.

[2] Includes LHFI of consolidated VIEs of $136 million and $153 million at September 30, 2019 and December 31, 2018, respectively.

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

[4] The net gain on LHFI sales was $47 million for the nine months ended September 30, 2019, and was immaterial for the three months ended September 30, 2019 as well as the three and nine months ended September 30, 2018.

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

[6] Represents UPB of loans purchased.

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