Property, Plant and Equipment (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	$ 360	$ 310	$ 360	$ 310
Depreciation	1,885	1,710	3,716	3,390
Loss on Disposition of Property Plant Equipment			410	231
Gain (Loss) on Disposition of Property Plant Equipment			$ (175)
Document Period End Date			Sep. 30, 2017
Leased devices [Member]
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	210	96	$ 210	96
Increase (Decrease) in Inventories			1,900	1,200
Depreciation	888	724	1,700	1,400
Network equipment, site costs and related software [Member]
Property, Plant and Equipment [Line Items]
Loss on Disposition of Property Plant Equipment			(180)
Gain (Loss) on Disposition of Property Plant Equipment			181
Hurricane [Member]
Property, Plant and Equipment [Line Items]
Loss on Disposition of Property Plant Equipment	(5)
Other Property [Member]
Property, Plant and Equipment [Line Items]
Gain (Loss) on Disposition of Property Plant Equipment			(6)
Equipment revenue [Member]
Property, Plant and Equipment [Line Items]
Operating Leases, Income Statement, Lease Revenue	966	811	1,900	1,600
Other, Net [Member]
Property, Plant and Equipment [Line Items]
Loss on Disposition of Property Plant Equipment	(117)	(111)	(404)	(231)
Write-off leased devices [Member]
Property, Plant and Equipment [Line Items]
Loss on Disposition of Property Plant Equipment	$ (112)	$ (111)	$ (224)	$ (231)