Segments (Operating Revenues by Service and Products) (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Segment Reporting Information [Line Items]
Restructuring Charges	$ 0
Net operating revenues	8,157	$ 8,012
Segment Reporting Information, Total Net Operating Revenues	8,157	8,012
Services [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	5,465	5,852
Wireless Equipment [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	2,086	1,496
Voice [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	83	112
Data [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	15	21
Internet [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	225	261
Other Products or Services [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	283	270
Wireless [Member]
Segment Reporting Information [Line Items]
Net operating revenues	7,810	7,597
Segment Reporting Information, Total Net Operating Revenues	7,810	7,597
Wireless [Member] | Services [Member]
Segment Reporting Information [Line Items]
Net operating revenues	5,465	5,852
Prepaid revenue	82	91
Wireless [Member] | Wireless Equipment [Member]
Segment Reporting Information [Line Items]
Net operating revenues	2,086	1,496
Wireless [Member] | Voice [Member]
Segment Reporting Information [Line Items]
Net operating revenues	0	0
Wireless [Member] | Data [Member]
Segment Reporting Information [Line Items]
Net operating revenues	0	0
Wireless [Member] | Internet [Member]
Segment Reporting Information [Line Items]
Net operating revenues	0	0
Wireless [Member] | Other Products or Services [Member]
Segment Reporting Information [Line Items]
Net operating revenues	259	249
Wireline [Member]
Segment Reporting Information [Line Items]
Net operating revenues	433	545
Segment Reporting Information, Total Net Operating Revenues	343	412
Wireline [Member] | Services [Member]
Segment Reporting Information [Line Items]
Net operating revenues	0	0
Wireline [Member] | Wireless Equipment [Member]
Segment Reporting Information [Line Items]
Net operating revenues	0	0
Wireline [Member] | Voice [Member]
Segment Reporting Information [Line Items]
Net operating revenues	124	181
Wireline [Member] | Data [Member]
Segment Reporting Information [Line Items]
Net operating revenues	34	43
Wireline [Member] | Internet [Member]
Segment Reporting Information [Line Items]
Net operating revenues	255	302
Wireline [Member] | Other Products or Services [Member]
Segment Reporting Information [Line Items]
Net operating revenues	20	19
Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Net operating revenues	(86)	(130)
Segment Reporting Information, Total Net Operating Revenues	4	3
Corporate, Other And Eliminations [Member] | Services [Member]
Segment Reporting Information [Line Items]
Net operating revenues	0	0
Corporate, Other And Eliminations [Member] | Wireless Equipment [Member]
Segment Reporting Information [Line Items]
Net operating revenues	0	0
Corporate, Other And Eliminations [Member] | Voice [Member]
Segment Reporting Information [Line Items]
Net operating revenues	(41)	(69)
Corporate, Other And Eliminations [Member] | Data [Member]
Segment Reporting Information [Line Items]
Net operating revenues	(19)	(22)
Corporate, Other And Eliminations [Member] | Internet [Member]
Segment Reporting Information [Line Items]
Net operating revenues	(30)	(41)
Corporate, Other And Eliminations [Member] | Other Products or Services [Member]
Segment Reporting Information [Line Items]
Net operating revenues	4	$ 2
Lease [Member]
Segment Reporting Information [Line Items]
Restructuring Charges	(9)
Lease [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Restructuring Charges	$ (9)