Accounting Policies - Restricted Cash (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Cash and Cash Equivalents	$ 908	$ 801	$ 1,043
Restricted Cash	66	72	67
Total Cash, Cash Equivalents and Restricted Cash	$ 974	$ 873	$ 1,110	$ 1,189