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	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Cash and cash equivalents	$ 2,485	$ 2,558
Restricted Cash and Cash Equivalents, Current	115	116
Restricted Cash and Cash Equivalents, Noncurrent	42	35
Total cash, cash equivalents, and restricted cash	$ 2,642	$ 2,709	$ 4,632	$ 4,658