Summary of Significant Accounting Policies - New Accounting Pronouncement (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]
Change in retained earnings	$ 26,200		$ 24,431
Allowance for credit loss	$ 122		$ 74	$ 67	$ 54
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Change in retained earnings		$ (3)
Accounting Standards Update 2016-13 [Member] | Loans and Other Financial Assets Held at Amortized Cost
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss		1
Accounting Standards Update 2016-13 [Member] | Off Balance Sheet Commitments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss		$ 2