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Intangible Assets (Intangible Assets Subject to Amortization) (Details) - USD ($)
$ in Millions
9 Months Ended
Dec. 31, 2017
Mar. 31, 2017
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Value $ 8,556 $ 8,992
Accumulated Amortization (5,889) (5,672)
Net Carrying Value 2,667 3,320
Customer Relationships [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Value 6,561 6,923
Accumulated Amortization (5,289) (5,053)
Net Carrying Value 1,272 1,870
Favorable spectrum leases [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Value 861 869
Accumulated Amortization (163) (138)
Net Carrying Value 698 731
Favorable tower leases [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Value 487 589
Accumulated Amortization (322) (386)
Net Carrying Value 165 203
Trademarks [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Value 520 520
Accumulated Amortization (70) (58)
Net Carrying Value 450 462
Other Finite Lived Intangible Assets [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Value 127 91
Accumulated Amortization (45) (37)
Net Carrying Value 82 54
Finite Lived Intangible Assets Excluding Customer Relationships [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Value 1,995 2,069
Accumulated Amortization (600) (619)
Net Carrying Value $ 1,395 $ 1,450
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