Segments (Operating Revenues by Service and Products) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Revenues	$ 8,433	$ 7,927	$ 16,558	$ 16,084
Segment Reporting Information, Total Net Operating Revenues	8,430	7,939	16,555	16,096
Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	8,166	7,621	16,011	15,431
Wireless [Member] | Wireless Hurricane and Other [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	3	12	(3)
Wireline [Member]
Segment Reporting Information [Line Items]
Revenues	328	409	666	842
Segment Reporting Information, Total Net Operating Revenues	260	314	536	657
Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	(64)	(91)	(122)	(177)
Segment Reporting Information, Total Net Operating Revenues	4	4	8	8
Voice [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues			2,465	1,865
Voice [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues			2,465	1,865
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	5,441	5,668	10,863	11,456
Services [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	5,206	5,365	10,376	10,830
Services [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Revenues	303	398	617	811
Services [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	(68)	(95)	(130)	(185)
Wireless Equipment Sales [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	1,418	994
Wireless Equipment Sales [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	1,418	994
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	1,253	966
Wireless Equipment Rentals [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	1,253	966
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	318	311	636	594
Other Products or Services [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	289	296	579	555
Other Products or Services [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Revenues	25	11	49	31
Other Products or Services [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	$ 4	$ 4	8	8
Wireless Equipment [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues			2,591	2,181
Wireless Equipment [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues			2,591	2,181
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