Basis of Presentation Deferred Revenue Rollforward in Basis of Presentation (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Deferred Revenue Rollforward [Line Items]
Deferred Revenue	$ 67	$ 72	$ 64	$ 69
Deferred Revenue, Additions	19	12
Deferred Revenue, Revenue Recognized	(16)	(9)
Accounts Payable and Accrued Liabilities [Member]
Deferred Revenue Rollforward [Line Items]
Deferred Revenue	24	19
Other Noncurrent Liabilities [Member]
Deferred Revenue Rollforward [Line Items]
Deferred Revenue	$ 43	53
Prepaid Rentals and Membership Fees [Member]
Deferred Revenue Rollforward [Line Items]
Deferred Revenue		144		$ 125
Deferred Revenue, Additions		1,561
Deferred Revenue, Revenue Recognized		(1,542)
Prepaid Rentals and Membership Fees [Member] | Accounts Payable and Accrued Liabilities [Member]
Deferred Revenue Rollforward [Line Items]
Deferred Revenue		142
Prepaid Rentals and Membership Fees [Member] | Other Noncurrent Liabilities [Member]
Deferred Revenue Rollforward [Line Items]
Deferred Revenue		$ 2