Basis of Presentation - Components of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Cash and Cash Equivalents	$ 992	$ 675	$ 787	$ 1,180
Restricted Cash	30	38	0	30
Cash, Cash Equivalents, and Restricted Cash	$ 1,022	$ 713	$ 787	$ 1,210