Intangible Assets (Intangible Assets Subject to Amortization) (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Gross carrying value	$ 8,992	$ 8,996
Accumulated amortization	(5,672)	(4,527)
Net carrying value	3,320	4,469
Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying value	6,923	6,923
Accumulated amortization	(5,053)	(4,045)
Net carrying value	1,870	2,878
Favorable spectrum leases [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying value	869	881
Accumulated amortization	(138)	(110)
Net carrying value	731	771
Favorable tower leases [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying value	589	589
Accumulated amortization	(386)	(302)
Net carrying value	203	287
Trademarks [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying value	520	520
Accumulated amortization	(58)	(43)
Net carrying value	462	477
Other Finite Lived Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying value	91	83
Accumulated amortization	(37)	(27)
Net carrying value	54	56
Finite Lived Intangible Assets Excluding Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying value	2,069	2,073
Accumulated amortization	(619)	(482)
Net carrying value	$ 1,450	$ 1,591
Minimum [Member] | Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Useful lives	4 years
Minimum [Member] | Favorable tower leases [Member]
Finite-Lived Intangible Assets [Line Items]
Useful lives
Minimum [Member] | Reacquired Rights [Member]
Finite-Lived Intangible Assets [Line Items]
Useful lives	9 years
Minimum [Member] | Other Finite Lived Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Useful lives
Maximum [Member] | Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Useful lives	8 years
Maximum [Member] | Favorable spectrum leases [Member]
Finite-Lived Intangible Assets [Line Items]
Useful lives	23 years
Maximum [Member] | Favorable tower leases [Member]
Finite-Lived Intangible Assets [Line Items]
Useful lives	7 years
Maximum [Member] | Trademarks [Member]
Finite-Lived Intangible Assets [Line Items]
Useful lives	34 years
Maximum [Member] | Reacquired Rights [Member]
Finite-Lived Intangible Assets [Line Items]
Useful lives	14 years
Maximum [Member] | Other Finite Lived Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Useful lives	10 years