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Property and Equipment (Narrative) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Property, Plant and Equipment [Line Items]      
Capital lease obligations incurred $ 43 $ 9 $ 26
Depreciation and amortization 1,174 669 626
Property Plant and Equipment and Assets Held under Capital Leases [Member]      
Property, Plant and Equipment [Line Items]      
Depreciation and amortization $ 185 $ 139 $ 130