Segments (Segment Financial Information) (Details) $ in Millions	3 Months Ended
	Jun. 30, 2017 USD ($) segments	Jun. 30, 2016 USD ($)
Segment Reporting Information [Line Items]
Document Period End Date	Jun. 30, 2017
Number of Reportable Segments | segments	2
Segment Reporting Information, Total Net Operating Revenues	$ 8,157	$ 8,012
Revenues	8,157	8,012
Segment Operating Expenses	(5,304)	(5,555)
Segment Reporting Information Operating Income or Loss	2,853	2,457
Depreciation	1,831	1,680
Amortization	(223)	(287)
Other Operating Expenses Net	(364)	129
Operating Income	1,163	361
Interest Expense	(613)	(615)
Other income, net	(52)	8
Loss (income) before income taxes	498	(246)
Payments to Acquire Property, Plant and Equipment Inclusive of Equipment on Lease	1,618	878
Severance and exit costs	0	(16)
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	293	120
Wireless [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	7,810	7,597
Revenues	7,810	7,597
Segment Operating Expenses	(4,944)	(5,157)
Segment Reporting Information Operating Income or Loss	2,866	2,440
Payments to Acquire Property, Plant and Equipment Inclusive of Equipment on Lease	1,435	781
Gain (Loss) on Contract Termination	5	(113)
Wireline [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	343	412
Revenues	433	545
Segment Operating Expenses	(444)	(526)
Segment Reporting Information Operating Income or Loss	(11)	19
Payments to Acquire Property, Plant and Equipment Inclusive of Equipment on Lease	62	20
Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	4	3
Revenues	(86)	(130)
Segment Operating Expenses	84	128
Segment Reporting Information Operating Income or Loss	(2)	(2)
Payments to Acquire Property, Plant and Equipment Inclusive of Equipment on Lease	121	77
Lease and severance [Member]
Segment Reporting Information [Line Items]
Severance and exit costs		(16)
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	0	0
Intersegment Eliminations [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	0	0
Intersegment Eliminations [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Revenues	90	133
Intersegment Eliminations [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	(90)	(133)
Other, Net [Member]
Segment Reporting Information [Line Items]
Gain (Loss) on Disposition of Intangible Assets	479
Loss on Disposition of Property Plant Equipment	287
Write-off leased devices [Member]
Segment Reporting Information [Line Items]
Loss on Disposition of Property Plant Equipment	(112)	120
Write-off leased devices & Loss on sale of devices to MLS under Tranche 1 [Member]
Segment Reporting Information [Line Items]
Loss on Disposition of Property Plant Equipment		$ (120)
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)