Segments (Segment Financial Information) (Details) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) segments	Sep. 30, 2016 USD ($)
Segment Reporting Information [Line Items]
Document Period End Date			Sep. 30, 2017
Number of Reportable Segments | segments			2
Segment Reporting Information, Total Net Operating Revenues	$ 7,939	$ 8,247	$ 16,096	$ 16,259
Revenues	7,927	8,247	16,084	16,259
Segment Operating Expenses	(5,210)	(5,900)	(10,514)	(11,455)
Segment Reporting Information Operating Income or Loss	2,729	2,347	5,582	4,804
Depreciation	1,885	1,710	3,716	3,390
Amortization	(209)	(271)	(432)	(558)
Other Operating Expenses Net		256	364	127
Operating Income	601	622	1,764	983
Interest Expense	(595)	(630)	(1,208)	(1,245)
Other income, net	44	(15)	(8)	(7)
Loss (income) before income taxes	50	(23)	548	(269)
Payments to Acquire Property, Plant and Equipment Inclusive of Equipment on Lease			2,908	1,706
Severance and exit costs	0	5	0	(11)
Legal Fees			(55)
Loss on disposition of property, plant and equipment			175
Gain (Loss) on Contract Termination			(5)	96
Loss on Disposition of Property Plant Equipment			410	231
Wireless [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	7,621	7,851	15,431	15,448
Revenues	7,621	7,851	15,431	15,448
Segment Operating Expenses	(4,857)	(5,528)	(9,801)	(10,685)
Segment Reporting Information Operating Income or Loss	2,764	2,323	5,630	4,763
Payments to Acquire Property, Plant and Equipment Inclusive of Equipment on Lease			2,581	1,497
Gain (Loss) on Contract Termination			5	113
Wireline [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	314	392	657	804
Revenues	409	521	842	1,066
Segment Operating Expenses	(438)	(498)	(882)	(1,024)
Segment Reporting Information Operating Income or Loss	(29)	23	(40)	42
Payments to Acquire Property, Plant and Equipment Inclusive of Equipment on Lease			102	51
Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	4	4	8	7
Revenues	(91)	(125)	(177)	(255)
Segment Operating Expenses	85	126	169	254
Segment Reporting Information Operating Income or Loss	(6)	1	(8)	(1)
Payments to Acquire Property, Plant and Equipment Inclusive of Equipment on Lease			225	158
Lease and severance [Member]
Segment Reporting Information [Line Items]
Severance and exit costs		5		(11)
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Revenues	95	129	185	262
Intersegment Eliminations [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	(95)	(129)	(185)	(262)
Other, Net [Member]
Segment Reporting Information [Line Items]
Gain (Loss) on Disposition of Intangible Assets			479	354
Other Operating Expenses Net	(34)		(34)
Loss on Disposition of Property Plant Equipment	(117)	(111)	(404)	(231)
Write-off leased devices [Member]
Segment Reporting Information [Line Items]
Loss on Disposition of Property Plant Equipment	$ (112)	$ (111)	(224)	$ (231)
New York sales taxes on receipts from sale of wireless services [Member]
Segment Reporting Information [Line Items]
Loss Contingency Accrual, Provision			(200)
State and Local Jurisdiction [Member]
Segment Reporting Information [Line Items]
Loss Contingency Accrual, Provision			$ (114)