Segments (Segment Financial Information) (Details) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) segments	Dec. 31, 2015 USD ($)
Segment Reporting Information [Line Items]
Document Period End Date			Dec. 31, 2016
Number of Reportable Segments | segments			2
Segment Reporting Information, Total Net Operating Revenues	$ 8,549	$ 8,107	$ 24,808	$ 24,109
Revenues	8,549	8,107	24,808	24,109
Segment Operating Expenses	(6,099)	(6,209)	(17,554)	(18,121)
Segment Reporting Information Operating Income or Loss	2,450	1,898	7,254	5,988
Depreciation	1,837	1,549	5,227	4,202
Amortization	(255)	(316)	(813)	(994)
Other Operating Expenses Net	47	230	(80)	490
Operating Income	311	(197)	1,294	302
Interest Expense	(619)	(546)	(1,864)	(1,630)
Other income, net	(60)	4	(67)	13
Loss (income) before income taxes	(368)	(739)	(637)	(1,315)
Payments to Acquire Property, Plant and Equipment Inclusive of Equipment on Lease			2,951	5,682
Severance and exit costs	(19)	(209)	(30)	(247)
Loss on disposition of property, plant and equipment	28			(85)
Gain (Loss) on Contract Termination			96	0
Pension Expense		(59)
Loss Contingency Accrual, Provision		(91)
Loss on Disposition of Property Plant Equipment	137		368	228
Wireless [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	8,172	7,670	23,620	22,726
Revenues	8,172	7,670	23,620	22,726
Segment Operating Expenses	(5,775)	(5,804)	(16,460)	(16,807)
Segment Reporting Information Operating Income or Loss	2,397	1,866	7,160	5,919
Payments to Acquire Property, Plant and Equipment Inclusive of Equipment on Lease			2,654	5,236
Gain (Loss) on Contract Termination			(113)
Wireline [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	372	434	1,177	1,372
Revenues	497	581	1,563	1,820
Segment Operating Expenses	(449)	(548)	(1,473)	(1,749)
Segment Reporting Information Operating Income or Loss	48	33	90	71
Payments to Acquire Property, Plant and Equipment Inclusive of Equipment on Lease			74	205
Impaired Assets to be Disposed of by Method Other than Sale, Amount of Impairment Loss		233
Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	5	3	11	11
Revenues	(120)	(144)	(375)	(437)
Segment Operating Expenses	125	143	379	435
Segment Reporting Information Operating Income or Loss	5	(1)	4	(2)
Payments to Acquire Property, Plant and Equipment Inclusive of Equipment on Lease			223	241
Lease and severance [Member]
Segment Reporting Information [Line Items]
Severance and exit costs	(19)	(209)	(30)	(247)
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	125	147	386	448
Intersegment Eliminations [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	(125)	(147)	(386)	(448)
Trademarks [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Impairment of Intangible Assets, Indefinite-lived (Excluding Goodwill)		1,900
Other, Net [Member]
Segment Reporting Information [Line Items]
Gain (Loss) on Disposition of Intangible Assets			354
Write-off leased devices [Member]
Segment Reporting Information [Line Items]
Loss on Disposition of Property Plant Equipment	$ (109)	78	(340)	163
Other Operating Income (Expense) [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Restructuring Reserve, Accrual Adjustment		41
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				(143)
Loss on sale of devices to MLS under Tranche 1 [Member]
Segment Reporting Information [Line Items]
Loss on disposition of property, plant and equipment				65
New York sales taxes on receipts from sale of wireless services [Member]
Segment Reporting Information [Line Items]
Loss Contingency Accrual, Provision			(180)
Successor [Member]
Segment Reporting Information [Line Items]
Legal Fees		$ 21		178
Successor [Member] | Other Operating Income (Expense) [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Restructuring Reserve, Accrual Adjustment				$ 20
State and Local Jurisdiction [Member]
Segment Reporting Information [Line Items]
Loss Contingency, Loss in Period			$ (103)