Property, Plant and Equipment (Components of Property Plant and Equipment, specifically Leased Handsets) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017
Property, Plant and Equipment [Line Items]
Less accumulated depreciation	$ (17,423)	$ (14,503)
Leased devices, net	19,925	19,209
Leased devices [Member]
Property, Plant and Equipment [Line Items]
Leased devices	9,592	7,276
Less accumulated depreciation	(3,580)	(3,114)
Leased devices, net	$ 6,012	$ 4,162