Accounting Policies - Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Cash and Cash Equivalents	$ 837	$ 1,043	$ 961
Restricted Cash	51		90
Total Cash, Cash Equivalents and Restricted Cash	$ 888	$ 1,110	$ 1,051	$ 1,189