Cash and Cash Equivalents and Restricted Cash and Cash Equivalents Supplemental Cash Flow Disclosure (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Condensed Cash Flow Statements, Captions [Line Items]
Cash and cash equivalents	$ 1,016	$ 935
Restricted cash and cash equivalents	358	327
Restricted Cash and Cash Equivalents, Noncurrent	28	21
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 1,402	$ 1,283	$ 1,459	$ 1,003