Segments (Operating Revenues by Service and Products) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Other Operating Expenses Net	$ (247)	$ 47	$ (611)	$ (80)
Gain (Loss) on Contract Termination			5	(96)
Gain (Loss) on Disposition of Property Plant Equipment			(175)	(28)
Restructuring Charges			13
Net operating revenues	8,239	8,549	24,323	24,808
Segment Reporting Information, Total Net Operating Revenues	8,260	8,549	24,356	24,808
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property			(533)	(368)
Severance and exit costs	13	19	13	30
Wireless Equipment [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	2,309	2,226	6,355	5,556
Voice [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	62	92	213	310
Data [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	9	18	37	60
Internet [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	218	243	665	752
Other Products or Services [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	351	299	945	850
Services [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	5,311	5,671	16,141	17,280
Wireless [Member]
Segment Reporting Information [Line Items]
Gain (Loss) on Contract Termination			(5)	(113)
Net operating revenues	7,949	8,172	23,380	23,620
Segment Reporting Information, Total Net Operating Revenues		8,172		23,620
Wireless [Member] | Wireless Service [Member]
Segment Reporting Information [Line Items]
Net operating revenues		92		275
Wireless [Member] | Wireless Equipment [Member]
Segment Reporting Information [Line Items]
Net operating revenues	2,309	2,226	6,355	5,556
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	329	275	884	784
Wireless [Member] | Services [Member]
Segment Reporting Information [Line Items]
Net operating revenues	5,311	5,671	16,141	17,280
Wireline [Member]
Segment Reporting Information [Line Items]
Net operating revenues	393	497	1,235	1,563
Segment Reporting Information, Total Net Operating Revenues	307	372	963	1,177
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	94	153	327	506
Wireline [Member] | Data [Member]
Segment Reporting Information [Line Items]
Net operating revenues	29	41	96	127
Wireline [Member] | Internet [Member]
Segment Reporting Information [Line Items]
Net operating revenues	254	281	765	871
Wireline [Member] | Other Products or Services [Member]
Segment Reporting Information [Line Items]
Net operating revenues	16	22	47	59
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	(82)	(120)	(259)	(375)
Segment Reporting Information, Total Net Operating Revenues	4	5	13	11
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	(32)	(61)	(114)	(196)
Corporate, Other And Eliminations [Member] | Data [Member]
Segment Reporting Information [Line Items]
Net operating revenues	(20)	(23)	(59)	(67)
Corporate, Other And Eliminations [Member] | Internet [Member]
Segment Reporting Information [Line Items]
Net operating revenues	(36)	(38)	(100)	(119)
Corporate, Other And Eliminations [Member] | Other Products or Services [Member]
Segment Reporting Information [Line Items]
Net operating revenues	6	2	14	7
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		19	13	30
Lease [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Restructuring Charges	(3)		(12)
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	123	109	347	340
Other, Net [Member]
Segment Reporting Information [Line Items]
Other Operating Expenses Net	(66)		100
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property	123	$ 137	527	368
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	21		33
Cost of Sales [Member] | Other, Net [Member]
Segment Reporting Information [Line Items]
Other Operating Expenses Net	30		45
Selling, General and Administrative Expenses [Member] | Other, Net [Member]
Segment Reporting Information [Line Items]
Other Operating Expenses Net	$ 15		17
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)