Loans - Additional Information (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Transfer of Portfolio Loans and Leases to Held-for-sale		$ 60	$ 55
Transfer of Loans Held-for-sale to Portfolio Loans		7	5
Loans held for investment sold		$ 100	0
Document Period End Date		Mar. 31, 2017
Long-term Debt	[1]	$ 10,496		$ 11,748
Other Short-term Borrowings		1,955		1,015
Letters of Credit Outstanding, Amount		6,100		7,300
Loans and Leases Receivable, Impaired, Commitment to Lend		17		29
Loans held for investment	[2]	$ 143,529		$ 143,298
Federal National Mortgage Association (FNMA) Insured Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of Loan Portfolio Current		31.00%		29.00%
Loans held for investment		$ 549		$ 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		$ 539	$ 537
Percentage of Loan Portfolio Current		76.00%		75.00%
Loans held for investment		$ 6,396		$ 6,167
Consumer Indirect [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment purchased		99
Loans held for investment	[3]	11,067		10,736
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment		78,253		78,224
Residential Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment		$ 38,550		$ 38,990
Percentage of Loans Held for Investment		27.00%		27.00%
Home Equity Line of Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	[4]	$ 11,511		$ 11,912
Geographic Distribution, Foreign [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment		2,000		2,200
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		10,400		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		4,600		4,200
Federal Home Loan Bank Advances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Long-term Debt		2,300		2,754
Federal Reserve Bank Advances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Pledged as Collateral		22,800		22,600
Line of Credit Facility, Remaining Borrowing Capacity		17,000		17,000
Federal Home Loan Bank Advances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Pledged as Collateral		36,200		36,900
Line of Credit Facility, Remaining Borrowing Capacity		$ 30,800		$ 31,900

[1] Includes debt of consolidated VIEs of $214 million and $222 million at March 31, 2017 and December 31, 2016, respectively.

[2] Includes loans of consolidated VIEs of $201 million and $211 million at March 31, 2017 and December 31, 2016, respectively.

[3] Excludes $6.4 billion and $6.2 billion of guaranteed student loans at March 31, 2017 and December 31, 2016, respectively.

[4] Excludes $549 million and $537 million of guaranteed residential loans at March 31, 2017 and December 31, 2016, respectively.