Segments (Statement of Operations Information) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	$ (8,601)	$ (8,260)	$ (25,156)	$ (24,356)
Revenues	8,601	8,239	25,159	24,323
Segment Operating Expenses	5,500	5,541	15,519	16,055
Segment Reporting Information Operating Income or Loss	3,101	2,719	9,637	8,301
Depreciation, network and other	1,088	987	3,132	2,961
Depreciation, equipment rentals	1,137	990	3,454	2,732
Amortization	145	196	475	628
Hurricane-related charges		66		100
Hurricane-related proceeds			(32)
Merger costs	67		216
Other Operating Expenses Net	(185)	247	(320)	611
Selling, general and administrative	2,003	2,108	5,731	6,059
Operating Income	479	727	2,072	2,491
Interest Expense	664	581	1,934	1,789
Other income, net	32	(42)	153	(50)
Loss (income) before income taxes	(153)	104	291	652
Payments to Acquire Property, Plant and Equipment Inclusive of Equipment on Lease			9,553	8,072
Severance and exit costs	(30)		(63)	(13)
Gain (Loss) on Disposition of Property Plant Equipment	(117)		(185)	(175)
Gain (Loss) on Disposition of Assets	12
Loss Contingency Accrual, Period Increase (Decrease)		(260)		(315)
Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	8,351	7,949	24,362	23,380
Payments to Acquire Property, Plant and Equipment Inclusive of Equipment on Lease			9,101	7,612
Gain (Loss) on Contract Termination			34	5
Wireless [Member] | Wireless Including Hurricane and Other [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues		(7,928)	(24,365)	(23,347)
Segment Operating Expenses		5,286	14,650	(15,109)
Segment Reporting Information Operating Income or Loss		2,642	9,715	8,238
Wireless [Member] | Wireless Hurricane and Other [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues		(21)	3	(33)
Segment Operating Expenses		(96)	(7)	118
Segment Reporting Information Operating Income or Loss		117	(10)	151
Wireless [Member] | Wireless Excluding Hurricane and Other [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	(8,351)	(7,949)	(24,362)	(23,380)
Segment Operating Expenses	(5,240)	5,190	(14,657)	14,991
Segment Reporting Information Operating Income or Loss	3,111	2,759	9,705	8,389
Wireline [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	(245)	(307)	(781)	(963)
Revenues	316	393	982	1,235
Segment Operating Expenses	(332)	423	(1,060)	1,305
Segment Reporting Information Operating Income or Loss	(16)	(30)	(78)	(70)
Payments to Acquire Property, Plant and Equipment Inclusive of Equipment on Lease			170	132
Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	(5)	(4)	(13)	(13)
Revenues	(66)	(82)	(188)	(259)
Segment Operating Expenses	72	(72)	198	(241)
Segment Reporting Information Operating Income or Loss	6	(10)	10	(18)
Payments to Acquire Property, Plant and Equipment Inclusive of Equipment on Lease			282	328
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations [Member] | Wireless [Member] | Wireless Including Hurricane and Other [Member]
Segment Reporting Information [Line Items]
Revenues		0	0	0
Intersegment Eliminations [Member] | Wireless [Member] | Wireless Hurricane and Other [Member]
Segment Reporting Information [Line Items]
Revenues		0	0	0
Intersegment Eliminations [Member] | Wireless [Member] | Wireless Excluding Hurricane and Other [Member]
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Revenues	71	(86)	201	272
Intersegment Eliminations [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	(71)	(86)	(201)	(272)
Other, Net [Member]
Segment Reporting Information [Line Items]
Gain (Loss) on Disposition of Intangible Assets				479
Cost of Sales [Member] | Other, Net [Member]
Segment Reporting Information [Line Items]
Other Operating Expenses Net		30	(6)	45
Other, Net [Member] | Other, Net [Member]
Segment Reporting Information [Line Items]
Other Operating Expenses Net			(22)	(5)
Selling, general and administrative			$ 1
Selling, General and Administrative Expenses [Member] | Other, Net [Member]
Segment Reporting Information [Line Items]
Other Operating Expenses Net		$ 15		17
Network equipment, site costs and related software [Member]
Segment Reporting Information [Line Items]
Gain (Loss) on Disposition of Property Plant Equipment				181
Other Property [Member]
Segment Reporting Information [Line Items]
Gain (Loss) on Disposition of Property Plant Equipment				(6)
Sales Revenue, Services, Net [Member] | Other, Net [Member]
Segment Reporting Information [Line Items]
Other Operating Expenses Net				$ 51
New York sales taxes on receipts from sale of wireless services [Member]
Segment Reporting Information [Line Items]
Litigation Settlement, Expense	$ (50)