Intangible Assets (Intangible Assets Subject to Amortization) (Details) - USD ($) $ in Millions	9 Months Ended
	Dec. 31, 2016	Mar. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Document Period End Date	Dec. 31, 2016
Gross Carrying Value	$ 8,992	$ 8,996
Accumulated Amortization	(5,410)	(4,527)
Net Carrying Value	3,582	4,469
Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	6,923	6,923
Accumulated Amortization	(4,824)	(4,045)
Net Carrying Value	2,099	2,878
Favorable spectrum leases [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	870	881
Accumulated Amortization	(129)	(110)
Net Carrying Value	741	771
Favorable tower leases [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	589	589
Accumulated Amortization	(367)	(302)
Net Carrying Value	222	287
Trademarks [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	520	520
Accumulated Amortization	(55)	(43)
Net Carrying Value	465	477
Other Finite Lived Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	90	83
Accumulated Amortization	(35)	(27)
Net Carrying Value	55	56
Finite Lived Intangible Assets Excluding Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	2,069	2,073
Accumulated Amortization	(586)	(482)
Net Carrying Value	$ 1,483	$ 1,591
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	3 years
Minimum [Member] | Other Finite Lived Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	4 years
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