Loans - Additional Information (Details) - USD ($) $ in Millions		Dec. 31, 2018		Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Long-term Debt	[1],[2]	$ 15,072		$ 9,785
Other Short-term Borrowings		4,857		717
Loans held for investment	[3]	151,839		143,181
Finance Leases Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment		$ 4,100		$ 3,700
Federal National Mortgage Association (FNMA) Insured Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of Loan Portfolio Current		27.00%		28.00%
Loans held for investment		$ 459		$ 560
Government Guarantee Percent		1.00%	[4]	1.00%
Federal Family Education Loan Program (FFELP) Guaranteed Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of Loan Portfolio Current		72.00%		75.00%
Loans held for investment		$ 7,229		$ 6,600
Consumer Indirect [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment		12,419		12,140
Residential Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment		$ 38,900		$ 38,600
Percentage of Loans Held for Investment		26.00%		27.00%
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment		$ 80,940		$ 75,477
Geographic Distribution, Foreign [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment		1,800		1,400
Home Equity Line of Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	[5]	9,468		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		10,300		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		2,700		3,000
Federal Home Loan Bank Advances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other Short-term Borrowings		4,000		0
Federal Home Loan Bank Advances [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Long-term Debt		$ 5,000		$ 4

[1] Includes $289 million and $530 million of long-term debt measured at fair value at December 31, 2018 and 2017, respectively.

[2] Includes debt of consolidated VIEs of $161 million and $189 million at December?31, 2018 and December?31, 2017, respectively.

[3] Includes loans of consolidated VIEs of $153 million and $179 million at December?31, 2018 and December?31, 2017, respectively.

[4] Includes 3,321 loans with a carrying value of $41 million that were modified prior to 2016 and reclassified as TDRs in the fourth quarter of 2016.

[5] Excludes $7.2 billion and $6.6 billion of guaranteed student loans and $459 million and $560 million of guaranteed residential mortgages at December 31, 2018 and 2017, respectively, for which there was nominal risk of principal loss due to the government guarantee.