Loans - Additional Information (Details) - USD ($) $ in Millions		Mar. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Long-term Debt	[1]	$ 17,395	$ 15,072
Other Short-term Borrowings		7,259	4,857
Loans held for investment	[2]	155,233	151,839
Finance Leases Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment		$ 4,100	$ 4,059
Federal National Mortgage Association (FNMA) Insured Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of Loan Portfolio Current		30.00%	27.00%
Loans held for investment		$ 467	$ 459
Government Guarantee Percent		1.00%	1.00%
Federal Family Education Loan Program (FFELP) Guaranteed Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of Loan Portfolio Current		73.00%	72.00%
Loans held for investment		$ 7,300	$ 7,200
Consumer Indirect [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment		13,268	12,419
Residential Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment		$ 38,300	$ 38,900
Percentage of Loans Held for Investment		25.00%	26.00%
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment		$ 83,729	$ 80,940
Geographic Distribution, Foreign [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment		2,000	1,800
Home Equity Line of Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	[3]	9,167	9,468
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		10,500	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		3,100	2,700
Federal Home Loan Bank Advances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Long-term Debt		$ 8,300	$ 5,000

[1] Includes debt of consolidated VIEs of $156 million and $161 million at March 31, 2019 and December 31, 2018, respectively.

[2] Includes loans of consolidated VIEs of $147 million and $153 million at March 31, 2019 and December 31, 2018, respectively.

[3] Excludes $7.3 billion and $7.2 billion of guaranteed student loans and $467 million and $459 million of guaranteed residential mortgages at March 31, 2019 and December 31, 2018, respectively, for which there was nominal risk of principal loss due to the government guarantee.