Segments (Segment Financial Information) (Details) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) segments	Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Document Period End Date			Dec. 31, 2017
Number of Reportable Segments | segments			2
Segment Reporting Information, Total Net Operating Revenues	$ 8,260	$ 8,549	$ 24,356	$ 24,808
Revenues	8,239	8,549	24,323	24,808
Segment Operating Expenses	(5,541)	(6,099)	(16,055)	(17,554)
Segment Reporting Information Operating Income or Loss	2,719	2,450	8,301	7,254
Depreciation	1,977	1,837	5,693	5,227
Amortization	(196)	(255)	(628)	(813)
Other Operating Expenses Net	247	(47)	611	80
Operating Income	727	311	2,491	1,294
Interest Expense	(581)	(619)	(1,789)	(1,864)
Other income, net	(42)	(60)	(50)	(67)
Loss (income) before income taxes	104	(368)	652	(637)
Payments to Acquire Property, Plant and Equipment Inclusive of Equipment on Lease			4,286	2,951
Severance and exit costs	(13)	(19)	(13)	(30)
Legal Fees	(260)		(315)
Loss on disposition of property, plant and equipment			175	28
Gain (Loss) on Contract Termination			(5)	96
Loss on Disposition of Property Plant Equipment			533	368
Wireless [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues		8,172		23,620
Revenues	7,949	8,172	23,380	23,620
Segment Operating Expenses		(5,775)		(16,460)
Segment Reporting Information Operating Income or Loss		2,397		7,160
Payments to Acquire Property, Plant and Equipment Inclusive of Equipment on Lease			3,828	2,654
Gain (Loss) on Contract Termination			5	113
Wireline [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	307	372	963	1,177
Revenues	393	497	1,235	1,563
Segment Operating Expenses	(423)	(449)	(1,305)	(1,473)
Segment Reporting Information Operating Income or Loss	(30)	48	(70)	90
Payments to Acquire Property, Plant and Equipment Inclusive of Equipment on Lease			133	74
Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	4	5	13	11
Revenues	(82)	(120)	(259)	(375)
Segment Operating Expenses	72	125	241	379
Segment Reporting Information Operating Income or Loss	(10)	5	(18)	4
Payments to Acquire Property, Plant and Equipment Inclusive of Equipment on Lease			325	223
Lease and severance [Member]
Segment Reporting Information [Line Items]
Severance and exit costs		(19)	(13)	(30)
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
Intersegment Eliminations [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Revenues	86	125	272	386
Intersegment Eliminations [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	(86)	(125)	(272)	(386)
Other, Net [Member]
Segment Reporting Information [Line Items]
Gain (Loss) on Disposition of Intangible Assets			479	354
Other Operating Expenses Net	66		(100)
Loss on Disposition of Property Plant Equipment	(123)	(137)	(527)	(368)
Write-off leased devices [Member]
Segment Reporting Information [Line Items]
Loss on Disposition of Property Plant Equipment	(123)	$ (109)	(347)	$ (340)
State and Local Jurisdiction [Member]
Segment Reporting Information [Line Items]
Loss Contingency Accrual, Provision			(114)
Selling, General and Administrative Expenses [Member] | Other, Net [Member]
Segment Reporting Information [Line Items]
Other Operating Expenses Net	(15)		(17)
Sales Revenue, Services, Net [Member] | Other, Net [Member]
Segment Reporting Information [Line Items]
Other Operating Expenses Net			51
Wireless Including Hurricane and Other [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	7,928		23,347
Segment Operating Expenses	(5,286)		(15,109)
Segment Reporting Information Operating Income or Loss	2,642		8,238
Wireless Including Hurricane and Other [Member] | Intersegment Eliminations [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	0		0
Wireless Hurricane and Other [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	21		33
Segment Operating Expenses	96		118
Segment Reporting Information Operating Income or Loss	117		151
Wireless Hurricane and Other [Member] | Intersegment Eliminations [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	0		0
Wireless Excluding Hurricane and Other [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	7,949		23,380
Segment Operating Expenses	(5,190)		(14,991)
Segment Reporting Information Operating Income or Loss	2,759		8,389
Wireless Excluding Hurricane and Other [Member] | Intersegment Eliminations [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	$ 0		$ 0