Segments (Operating Revenues by Service and Products) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Other Operating Expenses Net		$ (256)	$ (364)	$ (127)
Gain (Loss) on Contract Termination			5	(96)
Gain (Loss) on Disposition of Property Plant Equipment			(175)
Restructuring Charges			0
Net operating revenues	$ 7,927	8,247	16,084	16,259
Segment Reporting Information, Total Net Operating Revenues	7,939	8,247	16,096	16,259
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property			(410)	(231)
Severance and exit costs	0	(5)	0	11
Wireless Equipment [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	1,960	1,834	4,046	3,330
Voice [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	68	106	151	218
Data [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	13	21	28	42
Internet [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	222	248	447	509
Other Products or Services [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	311	281	594	551
Services [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	5,365	5,757	10,830	11,609
Wireless [Member]
Segment Reporting Information [Line Items]
Gain (Loss) on Contract Termination			(5)	(113)
Net operating revenues	7,621	7,851	15,431	15,448
Segment Reporting Information, Total Net Operating Revenues	7,621	7,851	15,431	15,448
Wireless [Member] | Wireless Service [Member]
Segment Reporting Information [Line Items]
Net operating revenues		92		183
Wireless [Member] | Wireless Equipment [Member]
Segment Reporting Information [Line Items]
Net operating revenues	1,960	1,834	4,046	3,330
Wireless [Member] | Voice [Member]
Segment Reporting Information [Line Items]
Net operating revenues	0	0	0	0
Wireless [Member] | Data [Member]
Segment Reporting Information [Line Items]
Net operating revenues	0	0	0	0
Wireless [Member] | Internet [Member]
Segment Reporting Information [Line Items]
Net operating revenues	0	0	0	0
Wireless [Member] | Other Products or Services [Member]
Segment Reporting Information [Line Items]
Net operating revenues	296	260	555	509
Wireless [Member] | Services [Member]
Segment Reporting Information [Line Items]
Net operating revenues	5,365	5,757	10,830	11,609
Wireline [Member]
Segment Reporting Information [Line Items]
Net operating revenues	409	521	842	1,066
Segment Reporting Information, Total Net Operating Revenues	314	392	657	804
Wireline [Member] | Wireless Equipment [Member]
Segment Reporting Information [Line Items]
Net operating revenues	0	0	0	0
Wireline [Member] | Voice [Member]
Segment Reporting Information [Line Items]
Net operating revenues	109	172	233	353
Wireline [Member] | Data [Member]
Segment Reporting Information [Line Items]
Net operating revenues	33	43	67	86
Wireline [Member] | Internet [Member]
Segment Reporting Information [Line Items]
Net operating revenues	256	288	511	590
Wireline [Member] | Other Products or Services [Member]
Segment Reporting Information [Line Items]
Net operating revenues	11	18	31	37
Wireline [Member] | Services [Member]
Segment Reporting Information [Line Items]
Net operating revenues	0	0	0	0
Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Net operating revenues	(91)	(125)	(177)	(255)
Segment Reporting Information, Total Net Operating Revenues	4	4	8	7
Corporate, Other And Eliminations [Member] | Wireless Equipment [Member]
Segment Reporting Information [Line Items]
Net operating revenues	0	0	0	0
Corporate, Other And Eliminations [Member] | Voice [Member]
Segment Reporting Information [Line Items]
Net operating revenues	(41)	(66)	(82)	(135)
Corporate, Other And Eliminations [Member] | Data [Member]
Segment Reporting Information [Line Items]
Net operating revenues	(20)	(22)	(39)	(44)
Corporate, Other And Eliminations [Member] | Internet [Member]
Segment Reporting Information [Line Items]
Net operating revenues	(34)	(40)	(64)	(81)
Corporate, Other And Eliminations [Member] | Other Products or Services [Member]
Segment Reporting Information [Line Items]
Net operating revenues	4	3	8	5
Corporate, Other And Eliminations [Member] | Services [Member]
Segment Reporting Information [Line Items]
Net operating revenues	0	0	0	0
Lease and severance [Member]
Segment Reporting Information [Line Items]
Severance and exit costs		(5)		11
Lease [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Restructuring Charges			(9)
Network equipment, site costs and related software [Member]
Segment Reporting Information [Line Items]
Gain (Loss) on Disposition of Property Plant Equipment			181
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property			180
Other Property [Member]
Segment Reporting Information [Line Items]
Gain (Loss) on Disposition of Property Plant Equipment			(6)
Write-off leased devices [Member]
Segment Reporting Information [Line Items]
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property	112	111	224	231
Other, Net [Member]
Segment Reporting Information [Line Items]
Other Operating Expenses Net	34		34
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property	$ 117	$ 111	404	231
Gain (Loss) on Disposition of Intangible Assets			479	354
Sales Revenue, Services, Net [Member] | Other, Net [Member]
Segment Reporting Information [Line Items]
Other Operating Expenses Net			12
Cost of Sales [Member] | Other, Net [Member]
Segment Reporting Information [Line Items]
Other Operating Expenses Net			15
Selling, General and Administrative Expenses [Member] | Other, Net [Member]
Segment Reporting Information [Line Items]
Other Operating Expenses Net			2
Other, Net [Member] | Other, Net [Member]
Segment Reporting Information [Line Items]
Other Operating Expenses Net			$ 5
State and Local Jurisdiction [Member]
Segment Reporting Information [Line Items]
Loss Contingency, Loss in Period				$ (103)