Segments (Statement of Operations Information) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	$ 8,430	$ 7,939	$ 16,555	$ 16,096
Revenues	8,433	7,927	16,558	16,084
Segment Operating Expenses	5,174	5,210	10,019	10,514
Segment Reporting Information Operating Income or Loss	3,256	2,729	6,536	5,582
Depreciation, network and other	1,021	997	2,044	1,974
Depreciation, equipment rentals	1,181	888	2,317	1,742
Amortization	159	209	330	432
Hurricane-related charges		34	(32)	34
Hurricane-related proceeds	32
Merger costs	56		149
Other Operating Expenses Net	93		135	(364)
Selling, general and administrative	1,861	2,013	3,728	3,951
Operating Income	778	601	1,593	1,764
Interest Expense	633	595	1,270	1,208
Other income, net	79	44	121	(8)
Loss (income) before income taxes	224	50	444	548
Payments to Acquire Property, Plant and Equipment Inclusive of Equipment on Lease			5,922	4,908
Severance and exit costs	25	0	33	0
Gain (Loss) on Disposition of Property Plant Equipment	(68)			(175)
Loss Contingency Accrual, Period Increase (Decrease)				(55)
Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	8,166	7,621	16,011	15,431
Payments to Acquire Property, Plant and Equipment Inclusive of Equipment on Lease			5,644	4,579
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	8,169	7,609	16,014	15,419
Segment Operating Expenses	4,883	4,879	9,410	(9,823)
Segment Reporting Information Operating Income or Loss	3,286	2,730	6,604	5,596
Wireless [Member] | Wireless Hurricane and Other [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	3	12	(3)
Segment Operating Expenses	(7)	(22)	(7)	22
Segment Reporting Information Operating Income or Loss	(10)	34	(10)	34
Wireless [Member] | Wireless Excluding Hurricane and Other [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	8,166	7,621	16,011	15,431
Segment Operating Expenses	(4,890)	4,857	(9,417)	9,801
Segment Reporting Information Operating Income or Loss	3,276	2,764	6,594	5,630
Wireline [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	260	314	536	657
Revenues	328	409	666	842
Segment Operating Expenses	(348)	438	(728)	882
Segment Reporting Information Operating Income or Loss	(20)	(29)	(62)	(40)
Payments to Acquire Property, Plant and Equipment Inclusive of Equipment on Lease			106	102
Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	4	4	8	8
Revenues	(64)	(91)	(122)	(177)
Segment Operating Expenses	64	(85)	126	(169)
Segment Reporting Information Operating Income or Loss	0	(6)	4	(8)
Payments to Acquire Property, Plant and Equipment Inclusive of Equipment on Lease			172	227
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	0
Intersegment Eliminations [Member] | Wireless [Member] | Wireless Hurricane and Other [Member]
Segment Reporting Information [Line Items]
Revenues	0	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	68	(95)	130	185
Intersegment Eliminations [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	(68)	(95)	$ (130)	(185)
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	(6)	(15)
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		$ (2)
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)