Property, Plant and Equipment (Components of Property Plant and Equipment) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017
Property, Plant and Equipment [Line Items]
Less accumulated depreciation	$ (17,423)	$ (14,503)
Property, plant and equipment, net	19,925	19,209
Land [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	254	260
Network Equipment, Site Costs and Related Software [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	22,930	21,693
Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	813	818
Non-network internal use software, office equipment, leased devices and other [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	11,149	8,625
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Construction in progress	$ 2,202	$ 2,316