Intangible Assets (Narrative) (Details) - USD ($) $ in Millions		12 Months Ended
		Mar. 31, 2017	Mar. 31, 2015	Dec. 31, 2014
FCC Licenses [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Indefinite-lived Intangible Assets Acquired		$ 85
Trademarks [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Estimated fair value of Sprint trade name				$ 3,300
Impairment due to Sprint trade name	[1]	0
Other, Net [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Gain (Loss) on Disposition of Intangible Assets		$ 354
Wireless [Member] | Trademarks [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Impairment due to Sprint trade name			$ 1,900

[1] .