Summary of Significant Accounting Policies - New Accounting Pronouncement (Details) - USD ($) $ in Millions	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans, allowance for losses	$ 97		$ 74	$ 70	$ 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 losses		$ 1
Off-Balance Sheet Commitments | Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans, allowance for losses		2
Cumulative Effect, Period Of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of ASC 326		$ 3