Loans - Additional Information (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other Real Estate	[1]	$ 52		$ 52		$ 57
Transfer of Portfolio Loans and Leases to Held-for-sale		122	$ 91	449	$ 218
Transfer of Loans Held-for-sale to Portfolio Loans		5	6	23	16
Financing Receivable, Significant Sales		14	285	187	513
Long-term Debt	[2]	14,289		14,289		9,785
Other Short-term Borrowings		2,856		2,856		717
Letters of Credit Outstanding, Amount		4,300		4,300		6,700
Loans held for investment	[3]	147,215		147,215		143,181
Finance Leases Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment		$ 3,824		$ 3,824		$ 3,693
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%		28.00%
Loans held for investment		$ 452		$ 452		$ 560
Government Guarantee Percent		1.00%		1.00%		1.00%
Federal Family Education Loan Program (FFELP) Guaranteed Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Significant Purchases		$ 433	$ 333	$ 1,440	1,400
Percentage of Loan Portfolio Current		74.00%		74.00%		75.00%
Loans held for investment		$ 7,039		$ 7,039		$ 6,633
Consumer Indirect [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Significant Purchases		213		229	$ 99
Loans held for investment		12,010		12,010		12,140
Residential Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other Real Estate		49		49		51
Loans held for investment		$ 38,505		$ 38,505		$ 38,620
Percentage of Loans Held for Investment		26.00%		26.00%		27.00%
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other Real Estate		$ 2		$ 2		$ 4
Loans held for investment		77,958		77,958		75,477
Geographic Distribution, Foreign [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment		1,400		1,400		1,385
Home Equity Line of Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	[4]	9,669		9,669		10,626
Minimum [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Individually Evaluated For Impairment		3		3		3
Home Equity Line of Credit [Member] | Credit Concentration Risk [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unused Commitments to Extend Credit		10,200		10,200		10,100
Mortgage Loans on Real Estate [Member] | Credit Concentration Risk [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unused Commitments to Extend Credit		3,800		3,800		3,000
Federal Home Loan Bank Advances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Long-term Debt		3,003		3,003		4
Federal Reserve Bank Advances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Pledged as Collateral		26,100		26,100		24,300
Line of Credit Facility, Remaining Borrowing Capacity		19,800		19,800		18,200
Federal Home Loan Bank Advances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Pledged as Collateral		39,400		39,400		38,000
Line of Credit Facility, Remaining Borrowing Capacity		$ 31,500		$ 31,500		$ 30,500

[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 $49 million and $45 million at September 30, 2018 and December 31, 2017, respectively.

[2] Includes debt of consolidated VIEs of $168 million and $189 million at September 30, 2018 and December 31, 2017, respectively.

[3] Includes loans of consolidated VIEs of $159 million and $179 million at September 30, 2018 and December 31, 2017, respectively.

[4] Excludes $7.0 billion and $6.6 billion of guaranteed student loans and $452 million and $560 million of guaranteed residential mortgages at September 30, 2018 and December 31, 2017, respectively, for which there was nominal risk of principal loss due to the government guarantee.