Basis of Presentation Deferred Revenue Rollforward in Basis of Presentation (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Deferred Revenue Rollforward [Line Items]
Document Period End Date	Mar. 31, 2019
Deferred Revenue	$ 63	$ 72	$ 64	$ 69
Deferred Revenue, Additions	5	7
Deferred Revenue, Revenue Recognized	(6)	(4)
Accounts Payable and Accrued Liabilities [Member]
Deferred Revenue Rollforward [Line Items]
Deferred Revenue	18	18
Other Noncurrent Liabilities [Member]
Deferred Revenue Rollforward [Line Items]
Deferred Revenue	$ 45	54
Prepaid Rentals and Membership Fees [Member]
Deferred Revenue Rollforward [Line Items]
Deferred Revenue		178		$ 125
Deferred Revenue, Additions		412
Deferred Revenue, Revenue Recognized		(359)
Prepaid Rentals and Membership Fees [Member] | Accounts Payable and Accrued Liabilities [Member]
Deferred Revenue Rollforward [Line Items]
Deferred Revenue		176
Prepaid Rentals and Membership Fees [Member] | Other Noncurrent Liabilities [Member]
Deferred Revenue Rollforward [Line Items]
Deferred Revenue		$ 2