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Intangible Assets (Intangible Assets Subject to Amortization) (Details) - USD ($)
$ in Millions
3 Months Ended
Jun. 30, 2018
Mar. 31, 2018
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Value $ 8,404 $ 8,402
Accumulated Amortization (6,159) (5,937)
Net Carrying Value 2,245 2,465
Customer Relationships [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Value 6,562 6,562
Accumulated Amortization (5,623) (5,462)
Net Carrying Value 939 1,100
Favorable spectrum leases [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Value 856 856
Accumulated Amortization (216) (172)
Net Carrying Value 640 684
Favorable tower leases [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Value 335 335
Accumulated Amortization (188) (179)
Net Carrying Value 147 156
Trademarks [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Value 520 520
Accumulated Amortization (78) (74)
Net Carrying Value 442 446
Other Finite Lived Intangible Assets [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Value 131 129
Accumulated Amortization (54) (50)
Net Carrying Value 77 79
Finite Lived Intangible Assets Excluding Customer Relationships [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Value 1,842 1,840
Accumulated Amortization (536) (475)
Net Carrying Value $ 1,306 $ 1,365
Minimum [Member] | Customer Relationships [Member]    
Finite-Lived Intangible Assets [Line Items]    
Finite-Lived Intangible Asset, Useful Life 4 years  
Minimum [Member] | Favorable tower leases [Member]    
Finite-Lived Intangible Assets [Line Items]    
Finite-Lived Intangible Asset, Useful Life  
Minimum [Member] | Other Finite Lived Intangible Assets [Member]    
Finite-Lived Intangible Assets [Line Items]    
Finite-Lived Intangible Asset, Useful Life  
Maximum [Member] | Customer Relationships [Member]    
Finite-Lived Intangible Assets [Line Items]    
Finite-Lived Intangible Asset, Useful Life 8 years  
Maximum [Member] | Favorable spectrum leases [Member]    
Finite-Lived Intangible Assets [Line Items]    
Finite-Lived Intangible Asset, Useful Life 23 years  
Maximum [Member] | Favorable tower leases [Member]    
Finite-Lived Intangible Assets [Line Items]    
Finite-Lived Intangible Asset, Useful Life 7 years  
Maximum [Member] | Trademarks [Member]    
Finite-Lived Intangible Assets [Line Items]    
Finite-Lived Intangible Asset, Useful Life 34 years  
Maximum [Member] | Other Finite Lived Intangible Assets [Member]    
Finite-Lived Intangible Assets [Line Items]    
Finite-Lived Intangible Asset, Useful Life 10 years