Basis of Presentation Deferred Revenue Rollforward in Basis of Presentation (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Deferred Revenue Rollforward [Line Items]
Deferred Revenue	$ 66	$ 72	$ 64	$ 69
Deferred Revenue, Additions	12	10
Deferred Revenue, Revenue Recognized	(10)	(7)
Accounts Payable and Accrued Liabilities [Member]
Deferred Revenue Rollforward [Line Items]
Deferred Revenue	22	18
Other Noncurrent Liabilities [Member]
Deferred Revenue Rollforward [Line Items]
Deferred Revenue	$ 44	54
Prepaid Rentals and Membership Fees [Member]
Deferred Revenue Rollforward [Line Items]
Deferred Revenue		231		$ 125
Deferred Revenue, Additions		989
Deferred Revenue, Revenue Recognized		(883)
Prepaid Rentals and Membership Fees [Member] | Accounts Payable and Accrued Liabilities [Member]
Deferred Revenue Rollforward [Line Items]
Deferred Revenue		229
Prepaid Rentals and Membership Fees [Member] | Other Noncurrent Liabilities [Member]
Deferred Revenue Rollforward [Line Items]
Deferred Revenue		$ 2