Summary of Significant Accounting Policies - New Accounting Standards (Details) - USD ($) $ in Millions	Dec. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	$ 1,015	$ 467	$ 637	$ 541	$ 681
Liability for off-balance sheet credit exposures	237	158	158	$ 96	$ 106
Retained earnings	$ 601		1,534
Cumulative Effect, Period of Adoption, Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			(170)
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	$ 54
Accounting Standards Update 2016-13 [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings		82
Student Loan [Member] | Accounting Standards Update 2016-13 [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings		2
Commercial Loan [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)