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Property, Plant and Equipment (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Property, Plant and Equipment      
Land $ 36 $ 35  
Buildings 297 297  
Leasehold and land improvements 1,178 1,153  
Cell site equipment 3,411 3,383  
Switching equipment 988 976  
Office furniture and equipment 389 420  
Other operating assets and equipment 57 53  
System development 1,060 1,217  
Work in process 212 178  
Property, plant and equipment, gross 7,628 7,712  
Accumulated depreciation and amortization (5,308) (5,242)  
Property, plant and equipment, net 2,320 2,470  
Depreciation and amortization expense 604 607 $ 596
(Gain) loss on asset disposals, net $ 17 $ 22 $ 16
Buildings      
Property, Plant and Equipment      
Useful life 20 years    
Leasehold and land improvements | Minimum      
Property, Plant and Equipment      
Useful life 1 year    
Leasehold and land improvements | Maximum      
Property, Plant and Equipment      
Useful life 30 years    
Cell site equipment | Minimum      
Property, Plant and Equipment      
Useful life 7 years    
Cell site equipment | Maximum      
Property, Plant and Equipment      
Useful life 25 years    
Switching equipment | Minimum      
Property, Plant and Equipment      
Useful life 5 years    
Switching equipment | Maximum      
Property, Plant and Equipment      
Useful life 8 years    
Office furniture and equipment | Minimum      
Property, Plant and Equipment      
Useful life 3 years    
Office furniture and equipment | Maximum      
Property, Plant and Equipment      
Useful life 5 years    
Other operating assets and equipment | Minimum      
Property, Plant and Equipment      
Useful life 3 years    
Other operating assets and equipment | Maximum      
Property, Plant and Equipment      
Useful life 5 years    
System development | Minimum      
Property, Plant and Equipment      
Useful life 1 year    
System development | Maximum      
Property, Plant and Equipment      
Useful life 7 years