Intangible Assets (Intangible Assets Subject to Amortization) (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Finite-Lived Intangible Assets [Line Items]
Net carrying value	$ 4,469	$ 5,893
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	3 years
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	4 years
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
Successor [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying value	$ 8,996	8,988
Accumulated amortization	(4,527)	(3,095)
Net carrying value	4,469	5,893
Successor [Member] | Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying value	6,923	6,923
Accumulated amortization	(4,045)	(2,791)
Net carrying value	2,878	4,132
Successor [Member] | Favorable spectrum leases [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying value	881	884
Accumulated amortization	(110)	(71)
Net carrying value	771	813
Successor [Member] | Favorable tower leases [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying value	589	589
Accumulated amortization	(302)	(189)
Net carrying value	287	400
Successor [Member] | Trademarks [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying value	520	520
Accumulated amortization	(43)	(27)
Net carrying value	477	493
Successor [Member] | Other Finite Lived Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying value	83	72
Accumulated amortization	(27)	(17)
Net carrying value	56	55
Successor [Member] | Finite Lived Intangible Assets Excluding Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying value	2,073	2,065
Accumulated amortization	(482)	(304)
Net carrying value	$ 1,591	$ 1,761