Property, Plant and Equipment (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	$ 987			$ 387
Depreciation - network and other			$ 5,693
Loss on Disposition of Property Plant Equipment	124	$ 293
Gain (Loss) on Disposition of Property Plant Equipment	$ (175)
Document Period End Date	Jun. 30, 2018
Leased devices [Member]
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	$ 221			$ 210
Increase (Decrease) in Inventories	200	100
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)	(6)
Other, Net [Member]
Property, Plant and Equipment [Line Items]
Loss on Disposition of Property Plant Equipment	(124)	(287)
Write-off leased devices [Member]
Property, Plant and Equipment [Line Items]
Loss on Disposition of Property Plant Equipment	$ (124)	$ (112)