Property, Plant and Equipment (Components of Property Plant and Equipment) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Mar. 31, 2016
Property, Plant and Equipment [Line Items]
Less accumulated depreciation	$ (14,503)	$ (9,692)
Property, plant and equipment, net	19,209	20,297
Land [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	260	260
Network Equipment, Site Costs and Related Software [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	21,693	21,500
Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	818	798
Non-network internal use software, office equipment, leased devices and other [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	8,625	6,182
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Construction in progress	$ 2,316	$ 1,249