New Accounting Pronouncements (Details) - USD ($) $ in Millions	Sep. 30, 2020	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	$ 1,084	$ 1,192	$ 467	$ 637	$ 676	$ 613	$ 541
Liability for off-balance sheet credit exposures	225	$ 253	158	158	$ 89	$ 90	$ 96
Retained earnings	$ 491			1,534
Accounting Standards Update 2016-13 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			(170)
Liability for off-balance sheet credit exposures			54
Commercial Loans [Member] | Accounting Standards Update 2016-13 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			(126)
Consumer Loans [Member] | Accounting Standards Update 2016-13 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			(44)
Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses				$ 467
Cumulative Effect, Period of Adoption, Adjusted Balance [Member] | Accounting Standards Update 2016-13 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings			82
Cumulative Effect, Period of Adoption, Adjusted Balance [Member] | Student Loan [Member] | Accounting Standards Update 2016-13 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings			$ 2