Property, Plant and Equipment (Components of Property Plant and Equipment, specifically Leased Handsets) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Mar. 31, 2016
Property, Plant and Equipment [Line Items]
Less accumulated depreciation	$ (14,503)	$ (9,692)
Leased devices, net	19,209	20,297
Leased devices [Member]
Property, Plant and Equipment [Line Items]
Leased devices	7,276	4,913
Less accumulated depreciation	(3,114)	(1,267)
Leased devices, net	$ 4,162	$ 3,646