Segments (Operating Revenues by Service and Products) (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Revenues	$ 8,142	$ 8,125
Segment Reporting Information, Total Net Operating Revenues	8,142	8,125
Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	7,901	7,845
Wireline [Member]
Segment Reporting Information [Line Items]
Revenues	307	338
Segment Reporting Information, Total Net Operating Revenues	236	276
Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	(66)	(58)
Segment Reporting Information, Total Net Operating Revenues	5	4
Services [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	5,260	5,422
Services [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	5,042	5,170
Services [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Revenues	289	314
Services [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	(71)	(62)
Wireless Equipment Sales [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues	1,220	1,173
Wireless Equipment Sales [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	1,220	1,173
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	1,359	1,212
Wireless Equipment Rentals [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	1,359	1,212
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	303	318
Other Products or Services [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues	280	290
Other Products or Services [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Revenues	18	24
Other Products or Services [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	$ 5	$ 4