Summary of Significant Accounting Policies Restricted Cash Adjustment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease Increase In Program Cash		$ 0	$ 0
Cash and cash equivalents	$ 611	490
Payments for (Proceeds from) Other Investing Activities	5	1	3
Net Cash Provided by (Used in) Investing Activities	(2,204)	(2,182)	(2,685)
Effect of changes in exchange rates on cash and cash equivalents, program and restricted cash	45	(6)	(50)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect	181	3	(36)
Program cash	283	225
Restricted Cash and Cash Equivalents	7	5
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 901	720	717	$ 753
Scenario, Previously Reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease Increase In Program Cash		31	(148)
Payments for (Proceeds from) Other Investing Activities		3	6
Net Cash Provided by (Used in) Investing Activities		(2,149)	(2,830)
Effect of changes in exchange rates on cash and cash equivalents, program and restricted cash		(4)	(41)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect		38	(172)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents		490	452	624
Restatement Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease Increase In Program Cash		(31)	148
Payments for (Proceeds from) Other Investing Activities		(2)	(3)
Net Cash Provided by (Used in) Investing Activities		(33)	145
Effect of changes in exchange rates on cash and cash equivalents, program and restricted cash		(2)	(9)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect		(35)	136
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents		$ 230	$ 265	$ 129