Segments (Operating Revenues by Service and Products) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Hurricane-related proceeds			$ (32)
Revenues	$ 8,601	$ 8,239	25,159	$ 24,323
Segment Reporting Information, Total Net Operating Revenues	8,601	8,260	25,156	24,356
Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	8,351	7,949	24,362	23,380
Wireless [Member] | Wireless Hurricane and Other [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues		21	(3)	33
Wireline [Member]
Segment Reporting Information [Line Items]
Revenues	316	393	982	1,235
Segment Reporting Information, Total Net Operating Revenues	245	307	781	963
Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	(66)	(82)	(188)	(259)
Segment Reporting Information, Total Net Operating Revenues	5	4	13	13
Voice [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues			3,778	2,912
Voice [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues			3,778	2,912
Voice [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Revenues			0	0
Voice [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues			0	0
Services [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	5,386	5,600	16,249	17,056
Services [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	5,160	5,311	15,536	16,141
Services [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Revenues	297	377	914	1,188
Services [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	(71)	(88)	(201)	(273)
Wireless Equipment Sales [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	1,589	1,262
Wireless Equipment Sales [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	1,589	1,262
Wireless Equipment Sales [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Revenues	0	0
Wireless Equipment Sales [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	0	0
Wireless Equipment Rentals [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	1,313	1,047
Wireless Equipment Rentals [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	1,313	1,047
Wireless Equipment Rentals [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Revenues	0	0
Wireless Equipment Rentals [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	0	0
Other Products or Services [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	313	351	949	945
Other Products or Services [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	289	329	868	884
Other Products or Services [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Revenues	19	16	68	47
Other Products or Services [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	$ 5	$ 6	13	14
Wireless Equipment [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues			4,180	3,443
Wireless Equipment [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues			4,180	3,443
Wireless Equipment [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Revenues			0	0
Wireless Equipment [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues			0	$ 0
Total service revenue
Segment Reporting Information [Line Items]
Hurricane-related proceeds			$ 3