Accounting Policies - Restricted Cash (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Cash and Cash Equivalents	$ 801	$ 1,043	$ 1,132
Restricted Cash	72	67	57
Total Cash, Cash Equivalents and Restricted Cash	$ 873	$ 1,110	$ 1,189	$ 1,502