Property, Plant and Equipment (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	$ 1,044	$ 428	$ 1,044	$ 428
Loss on Disposition of Property Plant Equipment			642	533
Leased devices [Member]
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	264	306	264	306
Increase (Decrease) in Inventories			600	600
Network equipment, site costs and related software [Member]
Property, Plant and Equipment [Line Items]
Loss on Disposition of Property Plant Equipment	(117)		(185)	(180)
Other, Net [Member]
Property, Plant and Equipment [Line Items]
Loss on Disposition of Property Plant Equipment	(299)	(123)	(642)	(527)
Write-off leased devices [Member]
Property, Plant and Equipment [Line Items]
Loss on Disposition of Property Plant Equipment	$ (182)	$ (123)	$ (457)	$ (347)