Property, Plant and Equipment (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	$ 1,151	$ 360	$ 1,151	$ 360
Loss on Disposition of Property Plant Equipment			343	410
Leased devices [Member]
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	207	210	207	210
Increase (Decrease) in Inventories			400	300
Network equipment, site costs and related software [Member]
Property, Plant and Equipment [Line Items]
Loss on Disposition of Property Plant Equipment			(68)	(180)
Other, Net [Member]
Property, Plant and Equipment [Line Items]
Loss on Disposition of Property Plant Equipment	(219)	(117)	(343)	(404)
Write-off leased devices [Member]
Property, Plant and Equipment [Line Items]
Loss on Disposition of Property Plant Equipment	$ (151)	$ (112)	$ (275)	$ (224)