Property, Plant and Equipment (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	$ 428	$ 325	$ 428	$ 325
Depreciation	1,977	1,837	5,693	5,227
Loss on Disposition of Property Plant Equipment			533	368
Gain (Loss) on Disposition of Property Plant Equipment			$ (175)	(28)
Document Period End Date			Dec. 31, 2017
Leased devices [Member]
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	306	166	$ 306	166
Increase (Decrease) in Inventories			3,700	2,300
Depreciation	990	837	2,700	2,200
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	1,000	887	2,900	2,500
Other, Net [Member]
Property, Plant and Equipment [Line Items]
Loss on Disposition of Property Plant Equipment	(123)	(137)	(527)	(368)
Write-off leased devices [Member]
Property, Plant and Equipment [Line Items]
Loss on Disposition of Property Plant Equipment	$ (123)	$ (109)	$ (347)	$ (340)