Basis of Presentation and Principles of Consolidation Basis of Presentation and Principles of Consolidation - Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Cash and cash equivalents	$ 2,687	$ 2,558
Restricted cash, current	111	116
Restricted cash, non-current	50	35
Total cash, cash equivalents, and restricted cash	$ 2,848	$ 2,709	$ 4,626	$ 4,658