Loans - Additional Information (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other Real Estate		$ 53		$ 53		$ 57
Transfer of Portfolio Loans and Leases to Held-for-sale		123	$ 67	327	$ 127
Transfer of Loans Held-for-sale to Portfolio Loans		12	3	18	10
Loans held for investment sold		137	110	172	228
Loans held for investment purchased		532	$ 493
Long-term Debt	[1]	11,995		11,995		9,785
Other Short-term Borrowings		2,470		2,470		717
Letters of Credit Outstanding, Amount		4,300		4,300		6,700
Loans held for investment	[2]	144,935		144,935		143,181
Finance Leases Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment		$ 3,755		$ 3,755		$ 3,693
Federal National Mortgage Association (FNMA) Insured Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of Loan Portfolio Current		30.00%		30.00%		28.00%
Loans held for investment		$ 525		$ 525		$ 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]
Loans held for investment purchased				$ 1,007	1,000
Percentage of Loan Portfolio Current		77.00%		77.00%		75.00%
Loans held for investment		$ 6,892		$ 6,892		$ 6,633
Consumer Indirect [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment purchased				16	$ 99
Loans held for investment		11,712		11,712		12,140
Residential Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other Real Estate		49		49		51
Loans held for investment		$ 38,216		$ 38,216		$ 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		77,101		77,101		75,477
Geographic Distribution, Foreign [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment		1,300		1,300		1,400
Home Equity Line of Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	[3]	9,918		9,918		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		10,100		10,100		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		1,750		1,750		4
Federal Reserve Bank Advances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Pledged as Collateral		25,000		25,000		24,300
Line of Credit Facility, Remaining Borrowing Capacity		18,700		18,700		18,200
Federal Home Loan Bank Advances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Pledged as Collateral		39,300		39,300		38,000
Line of Credit Facility, Remaining Borrowing Capacity		$ 31,300		$ 31,300		$ 30,500

[1] Includes debt of consolidated VIEs of $174 million and $189 million at June 30, 2018 and December 31, 2017, respectively.

[2] Includes loans of consolidated VIEs of $165 million and $179 million at June 30, 2018 and December 31, 2017, respectively.

[3] Excludes $6.9 billion and $6.6 billion of guaranteed student loans and $525 million and $560 million of guaranteed residential mortgages at June 30, 2018 and December 31, 2017, respectively, for which there was nominal risk of principal loss due to the government guarantee.