Segments (Operating Revenues by Service and Products) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Revenues	$ 7,795	$ 8,433	$ 15,937	$ 16,558
Segment Reporting Information, Total Net Operating Revenues	7,795	8,430	15,937	16,555
Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	7,567	8,166	15,468	16,011
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	300	328	607	666
Segment Reporting Information, Total Net Operating Revenues	227	260	463	536
Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	(72)	(64)	(138)	(122)
Segment Reporting Information, Total Net Operating Revenues	1	4	6	8
Voice [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues			2,689	2,465
Voice [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues			2,689	2,465
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	5,219	5,441	10,479	10,863
Services [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	5,010	5,206	10,052	10,376
Services [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Revenues	282	303	571	617
Services [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	(73)	(68)	(144)	(130)
Wireless Equipment Sales [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	1,192	1,418
Wireless Equipment Sales [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	1,192	1,418
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	1,330	1,253
Wireless Equipment Rentals [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	1,330	1,253
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	54	318	357	636
Other Products or Services [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	35	289	315	579
Other Products or Services [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Revenues	18	25	36	49
Other Products or Services [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	$ 1	$ 4	6	8
Wireless Equipment [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues			2,412	2,591
Wireless Equipment [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues			2,412	2,591
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