Basis of Presentation and Principles of Consolidation - Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($) reportable_segment	Dec. 31, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of Reportable Segments | reportable_segment	3
Cash and cash equivalents	$ 1,395	$ 1,491
Restricted cash, current	119	115
Restricted cash, non-current	52	51
Total cash, cash equivalents, and restricted cash	$ 1,566	$ 1,657	$ 2,642	$ 2,709