Intangible Assets (Intangible Assets Subject to Amortization) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
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	8,988	$ 8,976
Accumulated amortization	(3,095)	(1,418)
Net carrying value	5,893	7,558
Successor [Member] | Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying value	6,923	6,923
Accumulated amortization	(2,791)	(1,289)
Net carrying value	4,132	5,634
Successor [Member] | Favorable spectrum leases [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying value	884	884
Accumulated amortization	(71)	(30)
Net carrying value	813	854
Successor [Member] | Favorable tower leases [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying value	589	589
Accumulated amortization	(189)	(80)
Net carrying value	400	509
Successor [Member] | Trademarks [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying value	520	520
Accumulated amortization	(27)	(12)
Net carrying value	493	508
Successor [Member] | Other Finite Lived Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying value	72	60
Accumulated amortization	(17)	(7)
Net carrying value	55	53
Successor [Member] | Finite Lived Intangible Assets Excluding Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying value	2,065	2,053
Accumulated amortization	(304)	(129)
Net carrying value	1,761	$ 1,924