Accounting Policies - Restricted Cash (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounting Policies [Abstract]
Cash and Cash Equivalents	$ 1,043	$ 1,132	$ 1,476
Restricted Cash	67	57	26
Total Cash, Cash Equivalents and Restricted Cash	$ 1,110	$ 1,189	$ 1,502	$ 2,181