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Property and Equipment - Additional Information (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Property, Plant and Equipment [Abstract]      
Depreciation $ 313us-gaap_DepreciationNonproduction $ 318us-gaap_DepreciationNonproduction $ 290us-gaap_DepreciationNonproduction
Net capital lease assets included in property and equipment 149us-gaap_CapitalLeasesBalanceSheetAssetsByMajorClassNet 130us-gaap_CapitalLeasesBalanceSheetAssetsByMajorClassNet  
Accumulated depreciation and amortization of capital lease assets included in property and equipment $ 64us-gaap_CapitalLeasesLesseeBalanceSheetAssetsByMajorClassAccumulatedDeprecation $ 59us-gaap_CapitalLeasesLesseeBalanceSheetAssetsByMajorClassAccumulatedDeprecation