Basis of Presentation and Principles of Consolidation - Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Details) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2019 USD ($) reportable_segment	Jun. 30, 2019 USD ($) reportable_segment	Dec. 31, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of Reportable Segments | reportable_segment	3	3
Cash and cash equivalents	$ 1,520	$ 1,520	$ 1,491
Restricted cash, current	120	120	115
Restricted cash, non-current	61	61	51
Total cash, cash equivalents, and restricted cash	$ 1,701	$ 1,701	$ 1,657	$ 2,848	$ 2,709