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Commitments and Contingencies Future Minimum Lease Payments (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]      
Minimum capital lease payments - 2018 $ 75    
Minimum capital lease payments - 2019 71    
Minimum capital lease payments - 2020 69    
Minimum capital lease payments - 2021 200    
Minimum capital lease payments - 2022 35    
Minimum capital lease payments - Thereafter 129    
Total minimum capital leases payments 579    
Capital lease payments - amount representing interest (113)    
Present value of minimum capital lease payments 466    
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]      
Minimum operating lease payments - 2018 [1] 458    
Minimum operating lease payments - 2019 [1] 452    
Minimum operating lease payments - 2020 [1] 453    
Minimum operating lease payments - 2021 [1] 378    
Minimum operating lease payments - 2022 [1] 294    
Minimum operating lease payments - Thereafter [1] 984    
Total minimum operating lease payments [1] 3,019    
Rent Expense for Operating Leases, excluding per diem leases $ 587 $ 600 $ 627
[1] Excludes leases having non-cancelable lease terms of less than one year and per diem leases.