Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Millions	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Accounting Policies [Line Items]
Retained earnings	$ 509		$ 681
Available-for-sale-debt securities	0
U.S. Treasury Securities
Schedule Of Accounting Policies [Line Items]
Portfolio based securities loss expectation	0
U.S. Government Agency Securities
Schedule Of Accounting Policies [Line Items]
Portfolio based securities loss expectation	$ 0
Cumulative Effect, Period of Adoption, Adjustment | Revision of Prior Period, Adjustment
Schedule Of Accounting Policies [Line Items]
Retained earnings		$ (3)	$ (3)	$ 3