Accounting Policies - Restricted Cash (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Cash and Cash Equivalents	$ 1,539	$ 908	$ 801
Restricted Cash	85	66	72
Total Cash, Cash Equivalents and Restricted Cash	$ 1,624	$ 974	$ 873	$ 1,110