Accounting Policies - Restricted Cash (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Cash and Cash Equivalents	$ 1,223	$ 1,539	$ 971
Restricted Cash	49		65
Total Cash, Cash Equivalents and Restricted Cash	$ 1,272	$ 1,624	$ 1,036	$ 974