Property, Plant and Equipment (Components of Property Plant and Equipment) (Details) - USD ($) $ in Millions	9 Months Ended
	Dec. 31, 2017	Mar. 31, 2017
Property, Plant and Equipment [Line Items]
Document Period End Date	Dec. 31, 2017
Less accumulated depreciation	$ (16,727)	$ (14,503)
Property, plant and equipment, net	19,712	19,209
Land [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	254	260
Network equipment, site costs and related software [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	22,649	21,693
Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	809	818
Non-network internal use software, office equipment, leased devices and other [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	10,626	8,625
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Construction in progress	$ 2,101	$ 2,316