Segments (Statement of Operations Information) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Impairments	$ (2)		$ (212)	$ 0
Segment Reporting Information, Total Net Operating Revenues	(7,795)	$ (8,430)	(15,937)	(16,555)
Revenues	7,795	8,433	15,937	16,558
Segment Operating Expenses	5,241	5,174	10,341	10,019
Segment Reporting Information Operating Income or Loss	2,554	3,256	5,596	6,536
Depreciation, network and other	1,065	1,021	2,185	2,044
Depreciation, equipment rentals	1,056	1,181	2,085	2,317
Amortization	106	159	224	330
Hurricane-related charges		(32)		(32)
Merger costs	69	56	152	149
Other Operating Expenses Net	(21)	(93)	(258)	(135)
Selling, general and administrative	1,936	1,861	3,843	3,728
Operating Income	237	778	692	1,593
Interest Expense	594	633	1,213	1,270
Other income, net	14	79	42	121
Loss (income) before income taxes	(343)	224	(479)	444
Payments to Acquire Property, Plant and Equipment Inclusive of Equipment on Lease			5,600	5,922
Severance and exit costs	(19)		(46)
Loss on Disposition of Property Plant Equipment			465	343
Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	7,567	8,166	15,468	16,011
Payments to Acquire Property, Plant and Equipment Inclusive of Equipment on Lease			5,292	5,644
Gain (Loss) on Contract Termination				(34)
Wireless [Member] | Wireless Including Hurricane and Other [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues		(8,169)		(16,014)
Segment Operating Expenses		4,883		(9,410)
Segment Reporting Information Operating Income or Loss		3,286		6,604
Wireless [Member] | Wireless Hurricane and Other [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues		(3)		3
Segment Operating Expenses		(7)		(7)
Segment Reporting Information Operating Income or Loss		(10)		(10)
Wireless [Member] | Wireless Excluding Hurricane and Other [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	(7,567)	(8,166)	(15,468)	(16,011)
Segment Operating Expenses	(5,005)	4,890	(9,869)	9,417
Segment Reporting Information Operating Income or Loss	2,562	3,276	5,599	6,594
Wireline [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	(227)	(260)	(463)	(536)
Revenues	300	328	607	666
Segment Operating Expenses	(305)	348	(612)	728
Segment Reporting Information Operating Income or Loss	(5)	(20)	(5)	(62)
Payments to Acquire Property, Plant and Equipment Inclusive of Equipment on Lease			58	106
Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	(1)	(4)	(6)	(8)
Revenues	(72)	(64)	(138)	(122)
Segment Operating Expenses	69	(64)	140	(126)
Segment Reporting Information Operating Income or Loss	(3)	0	2	4
Payments to Acquire Property, Plant and Equipment Inclusive of Equipment on Lease			250	172
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
Intersegment Eliminations [Member] | Wireless [Member] | Wireless Hurricane and Other [Member]
Segment Reporting Information [Line Items]
Revenues		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	73	(68)	144	130
Intersegment Eliminations [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	(73)	(68)	(144)	(130)
Other, Net [Member]
Segment Reporting Information [Line Items]
Loss on Disposition of Property Plant Equipment	$ (240)	(219)	$ (465)	(343)
Cost of Sales [Member] | Other, Net [Member]
Segment Reporting Information [Line Items]
Other Operating Expenses Net		(6)
Other, Net [Member] | Other, Net [Member]
Segment Reporting Information [Line Items]
Other Operating Expenses Net		(22)
Selling, general and administrative		1
Network equipment, site costs and related software [Member]
Segment Reporting Information [Line Items]
Loss on Disposition of Property Plant Equipment				(68)
Lease and severance [Member]
Segment Reporting Information [Line Items]
Severance and exit costs		$ 25		$ 33