Property, Plant and Equipment (Components of Property Plant and Equipment, specifically Leased Handsets) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Mar. 31, 2018
Property, Plant and Equipment [Line Items]
Less accumulated depreciation	$ (20,539)	$ (17,423)
Leased devices, net	21,201	19,925
Leased devices [Member]
Property, Plant and Equipment [Line Items]
Leased devices	10,972	9,592
Less accumulated depreciation	(4,360)	(3,580)
Leased devices, net	$ 6,612	$ 6,012