Loans - Additional Information (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other Real Estate		$ 59		$ 57
Transfer of Portfolio Loans and Leases to Held-for-sale		204	$ 60
Transfer of Loans Held-for-sale to Portfolio Loans		6	7
Loans held for investment sold		36	118
Long-term Debt	[1]	10,692		9,785
Other Short-term Borrowings		706		717
Letters of Credit Outstanding, Amount		4,800		6,700
Loans and Leases Receivable, Impaired, Commitment to Lend		2		2
Loans held for investment	[2]	142,618		143,181
Finance Leases Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment		$ 3,577		$ 3,693
Federal National Mortgage Association (FNMA) Insured Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of Loan Portfolio Current		29.00%		28.00%
Loans held for investment		$ 611		$ 560
Government Guarantee Percent		2.00%		1.00%
Federal Family Education Loan Program (FFELP) Guaranteed Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment purchased		$ 475	539
Percentage of Loan Portfolio Current		77.00%		75.00%
Loans held for investment		$ 6,693		$ 6,633
Consumer Indirect [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment purchased			$ 99
Loans held for investment		11,869		12,140
Residential Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other Real Estate		54		51
Loans held for investment		$ 38,273		$ 38,620
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		$ 3		$ 4
Loans held for investment		75,324		75,477
Geographic Distribution, Foreign [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment		1,400		1,400
Home Equity Line of Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	[3]	10,241		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
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		10,100
Mortgage Loans on Real Estate [Member] | Credit Concentration Risk [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unused Commitments to Extend Credit		3,400		3,000
Federal Home Loan Bank Advances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Long-term Debt		4		4
Federal Reserve Bank Advances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Pledged as Collateral		23,500		24,300
Line of Credit Facility, Remaining Borrowing Capacity		17,600		18,200
Federal Home Loan Bank Advances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Pledged as Collateral		38,200		38,000
Line of Credit Facility, Remaining Borrowing Capacity		$ 30,300		$ 30,500

[1] Includes debt of consolidated VIEs of $182 million and $189 million at March 31, 2018 and December 31, 2017, respectively.

[2] Includes loans of consolidated VIEs of $171 million and $179 million at March 31, 2018 and December 31, 2017, respectively.

[3] Excludes $6.7 billion and $6.6 billion of guaranteed student loans and $611 million and $560 million of guaranteed residential mortgages at March 31, 2018 and December 31, 2017, respectively, for which there was nominal risk of principal loss due to the government guarantee.