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Summary of Significant Accounting Policies Deferred Revenue (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]    
Deferred Revenue $ 187 $ 194
Deferred Revenue, Additions 1,982  
Deferred Revenue, Revenue Recognized 1,989  
Prepaid Rentals [Member]    
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]    
Deferred Revenue 104 101
Deferred Revenue, Additions 1,764  
Deferred Revenue, Revenue Recognized 1,761  
Other Deferred Revenue [Member]    
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]    
Deferred Revenue 83 $ 93
Deferred Revenue, Additions 218  
Deferred Revenue, Revenue Recognized 228  
Accounts Payable and Accrued Liabilities [Member] | Other Deferred Revenue [Member]    
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]    
Deferred Revenue 36  
Other Noncurrent Liabilities [Member] | Other Deferred Revenue [Member]    
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]    
Deferred Revenue $ 47