Property, Plant and Equipment (Components of Property Plant and Equipment) (Details) - USD ($) $ in Millions	9 Months Ended
	Dec. 31, 2016	Mar. 31, 2016
Property, Plant and Equipment [Line Items]
Document Period End Date	Dec. 31, 2016
Less accumulated depreciation	$ (13,341)	$ (9,692)
Property, plant and equipment, net	19,333	20,297
Land [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	260	260
Network equipment, site costs and related software [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	21,696	21,500
Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	808	798
Non-network internal use software, office equipment, leased devices and other [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	8,450	6,182
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Construction in progress	$ 1,460	$ 1,249