Cash and Cash Equivalents and Restricted Cash and Cash Equivalents Supplemental Cash Flow Disclosure (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Condensed Cash Flow Statements, Captions [Line Items]
Cash and cash equivalents	$ 1,224	$ 935
Restricted cash and cash equivalents	297	327
Restricted Cash and Cash Equivalents, Noncurrent	23	21
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 1,544	$ 1,283	$ 1,065	$ 1,003