Property, Plant and Equipment (Components of Property Plant and Equipment) (Details) - USD ($) $ in Millions	6 Months Ended
	Sep. 30, 2017	Mar. 31, 2017
Property, Plant and Equipment [Line Items]
Document Period End Date	Sep. 30, 2017
Less accumulated depreciation	$ (16,135)	$ (14,503)
Property, plant and equipment, net	18,901	19,209
Land [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	259	260
Network equipment, site costs and related software [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	21,889	21,693
Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	806	818
Non-network internal use software, office equipment, leased devices and other [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	9,674	8,625
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Construction in progress	$ 2,408	$ 2,316