Property, Plant and Equipment (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	$ 387	$ 309
Depreciation	1,831	1,680
Loss on Disposition of Property Plant Equipment	293	120
Gain (Loss) on Disposition of Property Plant Equipment	$ (175)
Document Period End Date	Jun. 30, 2017
Leased devices [Member]
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	$ 210	142
Increase (Decrease) in Inventories	800	500
Depreciation	854	644
Network equipment, site costs and related software [Member]
Property, Plant and Equipment [Line Items]
Loss on Disposition of Property Plant Equipment	175
Gain (Loss) on Disposition of Property Plant Equipment	181
Other Property [Member]
Property, Plant and Equipment [Line Items]
Gain (Loss) on Disposition of Property Plant Equipment	(6)
Other, Net [Member]
Property, Plant and Equipment [Line Items]
Loss on Disposition of Property Plant Equipment	287
Write-off leased devices [Member]
Property, Plant and Equipment [Line Items]
Loss on Disposition of Property Plant Equipment	$ (112)	$ 120