Loans - Additional Information (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other Real Estate	[1]	$ 57	$ 60
Transfer of Portfolio Loans and Leases to Held-for-sale		288	360	$ 1,790
Transfer of Loans Held-for-sale to Portfolio Loans		19	30	$ 741
Loans held for investment sold		705	1,600
Gain (Loss) on Sales of Loans, Net			(6)
Long-term Debt	[2],[3]	9,785	11,748
Other Short-term Borrowings		717	1,015
Letters of Credit Outstanding, Amount		6,700	7,300
Loans and Leases Receivable, Impaired, Commitment to Lend		2	29
Loans held for investment	[4]	143,181	143,298
Finance Leases Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment		$ 3,740	$ 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%	29.00%
Loans held for investment		$ 560	$ 537
Government Guarantee Percent		1.00%	1.00%
Federal Family Education Loan Program (FFELP) Guaranteed Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment purchased		$ 1,729	$ 2,200
Percentage of Loan Portfolio Current		75.00%	75.00%
Loans held for investment		$ 6,633	$ 6,167
Consumer Indirect [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment purchased		233
Loans held for investment		12,140	10,736
Residential Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other Real Estate		51	50
Loans held for investment		$ 38,620	$ 38,990
Percentage of Loans Held for Investment		27.00%	27.00%
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other Real Estate		$ 4	$ 7
Loans held for investment		75,477	78,224
Geographic Distribution, Foreign [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment		1,400	2,200
Home Equity Line of Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	[5]	10,626	11,912
Minimum [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Individually Evaluated For Impairment		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		10,100	10,300
Mortgage Loans on Real Estate [Member] | Credit Concentration Risk [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unused Commitments to Extend Credit		3,000	4,200
Federal Home Loan Bank Advances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Long-term Debt		4	2,754
Federal Reserve Bank Advances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Pledged as Collateral		24,300	22,600
Line of Credit Facility, Remaining Borrowing Capacity		18,200	17,000
Federal Home Loan Bank Advances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Pledged as Collateral		38,000	36,900
Line of Credit Facility, Remaining Borrowing Capacity		$ 30,500	$ 31,900

[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 $45 million and $50 million at December 31, 2017 and 2016, respectively.

[2] Includes $530 million and $963 million of long-term debt measured at fair value at December 31, 2017 and 2016, respectively.

[3] Includes debt of consolidated VIEs of $189 million and $222 million at December 31, 2017 and December 31, 2016, respectively.

[4] Includes loans of consolidated VIEs of $179 million and $211 million at December 31, 2017 and December 31, 2016, respectively.

[5] Excludes $6.6 billion and $6.2 billion of guaranteed student loans and $560 million and $537 million of guaranteed residential mortgages at December 31, 2017 and 2016, respectively, for which there was nominal risk of principal loss due to the government guarantee.