Basis of Presentation and Principles of Consolidation - Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Cash and cash equivalents	$ 1,563	$ 2,558
Restricted cash, current	123	116
Restricted cash, non-current	40	35
Total cash, cash equivalents, and restricted cash	$ 1,726	$ 2,709	$ 4,449	$ 4,658