Segments (Operating Revenues by Service and Products) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenues	$ 8,080	$ 8,601	$ 24,017	$ 25,159
Segment Reporting Information, Total Net Operating Revenues	8,080	8,601	24,017	25,156
Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	7,859	8,351	23,327	24,362
Wireless [Member] | Wireless Hurricane and Other [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues		3		(3)
Wireline [Member]
Segment Reporting Information [Line Items]
Revenues	296	316	903	982
Segment Reporting Information, Total Net Operating Revenues	222	245	685	781
Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	(75)	(66)	(213)	(188)
Segment Reporting Information, Total Net Operating Revenues	(1)	5	5	13
Voice [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues			3,981	3,778
Voice [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues			3,981	3,778
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	5,174	5,386	15,653	16,249
Services [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	4,969	5,160	15,021	15,536
Services [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Revenues	279	297	850	914
Services [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	(74)	(71)	(218)	(201)
Wireless Equipment Sales [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	1,372	1,589
Wireless Equipment Sales [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	1,372	1,589
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	1,292	1,313
Wireless Equipment Rentals [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	1,292	1,313
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	242	313	599	949
Other Products or Services [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	226	289	541	868
Other Products or Services [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Revenues	17	19	53	68
Other Products or Services [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	$ (1)	$ 5	5	13
Wireless Equipment [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues			3,784	4,180
Wireless Equipment [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues			3,784	4,180
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