Loans - Additional Information (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Transfer of Portfolio Loans and Leases to Held-for-sale		$ 91	$ 153	$ 218	$ 315
Transfer of Loans Held-for-sale to Portfolio Loans		6	13	16	23
Loans held for investment sold		285	1,200	513	1,500
Gain (Loss) on Sales of Loans, Net		1	(8)		(6)
Long-term Debt	[1]	11,280		11,280		$ 11,748
Other Short-term Borrowings		909		909		1,015
Letters of Credit Outstanding, Amount		5,000		5,000		7,300
Loans and Leases Receivable, Impaired, Commitment to Lend		1		1		29
Loans held for investment	[2]	$ 144,264		$ 144,264		$ 143,298
Federal National Mortgage Association (FNMA) Insured Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of Loan Portfolio Current		32.00%		32.00%		29.00%
Loans held for investment		$ 497		$ 497		$ 537
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		$ 333	$ 506	$ 1,365	$ 1,579
Percentage of Loan Portfolio Current		76.00%		76.00%		75.00%
Loans held for investment		$ 6,559		$ 6,559		$ 6,167
Consumer Indirect [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment purchased				99
Loans held for investment	[3]	11,952		11,952		10,736
Residential Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment		$ 38,730		$ 38,730		$ 38,990
Percentage of Loans Held for Investment		27.00%		27.00%		27.00%
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment		$ 76,960		$ 76,960		$ 78,224
Geographic Distribution, Foreign [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment		1,500		1,500		2,200
Home Equity Line of Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	[4]	10,865		10,865		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		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		10,300
Mortgage Loans on Real Estate [Member] | Credit Concentration Risk [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unused Commitments to Extend Credit		4,100		4,100		4,200
Federal Home Loan Bank Advances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Long-term Debt		1,254		1,254		2,754
Federal Reserve Bank Advances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Pledged as Collateral		23,900		23,900		22,600
Line of Credit Facility, Remaining Borrowing Capacity		17,800		17,800		17,000
Federal Home Loan Bank Advances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Pledged as Collateral		38,200		38,200		36,900
Line of Credit Facility, Remaining Borrowing Capacity		$ 30,800		$ 30,800		$ 31,900

[1] Includes debt of consolidated VIEs of $195 million and $222 million at September 30, 2017 and December 31, 2016, respectively.

[2] Includes loans of consolidated VIEs of $186 million and $211 million at September 30, 2017 and December 31, 2016, respectively.

[3] Excludes $6.6 billion and $6.2 billion of guaranteed student loans at September 30, 2017 and December 31, 2016, respectively.

[4] Excludes $497 million and $537 million of guaranteed residential loans at September 30, 2017 and December 31, 2016, respectively.