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Intangible Assets (Intangible Assets Subject to Amortization) (Details) - USD ($)
$ in Millions
6 Months Ended
Sep. 30, 2017
Mar. 31, 2017
Finite-Lived Intangible Assets [Line Items]    
Document Period End Date Sep. 30, 2017  
Gross Carrying Value $ 8,627 $ 8,992
Accumulated Amortization (5,779) (5,672)
Net Carrying Value 2,848 3,320
Customer Relationships [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Value 6,561 6,923
Accumulated Amortization (5,103) (5,053)
Net Carrying Value 1,458 1,870
Favorable spectrum leases [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Value 861 869
Accumulated Amortization (154) (138)
Net Carrying Value 707 731
Favorable tower leases [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Value 589 589
Accumulated Amortization (414) (386)
Net Carrying Value 175 203
Trademarks [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Value 520 520
Accumulated Amortization (66) (58)
Net Carrying Value 454 462
Other Finite Lived Intangible Assets [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Value 96 91
Accumulated Amortization (42) (37)
Net Carrying Value 54 54
Finite Lived Intangible Assets Excluding Customer Relationships [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Value 2,066 2,069
Accumulated Amortization (676) (619)
Net Carrying Value $ 1,390 $ 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