Summary of Significant Accounting Policies - New Accounting Pronouncement (Details) - USD ($) $ in Millions	Sep. 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]
Decrease in retained earnings	$ 25,558		$ 24,431
Loans, allowance for credit losses	$ 134		$ 74	$ 71	$ 72	$ 67
Loans and Other Financial Assets Held at Amortized Cost | Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans, allowance for credit losses		$ 1
Off-Balance Sheet Commitments | Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans, allowance for credit losses		2
Cumulative Effect, Period Of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease in retained earnings		$ (3)