Revenues Deferred Revenue (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Deferred Revenue Rollforward [Line Items]
Document Period End Date	Sep. 30, 2018
Deferred Revenue	$ 216	$ 194
Deferred Revenue, Additions	1,573
Deferred Revenue, Revenue Recognized	1,551
Prepaid Rentals [Member]
Deferred Revenue Rollforward [Line Items]
Deferred Revenue	123	101
Deferred Revenue, Additions	1,406
Deferred Revenue, Revenue Recognized	1,384
Other Deferred Revenue [Member]
Deferred Revenue Rollforward [Line Items]
Deferred Revenue	93	$ 93
Deferred Revenue, Additions	167
Deferred Revenue, Revenue Recognized	167
Accounts Payable and Accrued Liabilities [Member] | Other Deferred Revenue [Member]
Deferred Revenue Rollforward [Line Items]
Deferred Revenue	38
Other Noncurrent Liabilities [Member] | Other Deferred Revenue [Member]
Deferred Revenue Rollforward [Line Items]
Deferred Revenue	$ 55