Segments (Operating Revenues by Service and Products) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Revenues	$ 8,441	$ 8,601	$ 8,433	$ 8,125	$ 8,083	$ 8,239	$ 7,927	$ 8,157	$ 33,600	$ 32,406	$ 33,347
Segment Reporting Information, Total Net Operating Revenues									33,597	32,439	33,347
Other Operating Expenses Net									(670)	345	(20)
Wireless [Member]
Segment Reporting Information [Line Items]
Revenues									32,556	31,170	31,787
Segment Reporting Information, Total Net Operating Revenues											31,787
Wireline [Member]
Segment Reporting Information [Line Items]
Revenues									1,296	1,579	2,043
Segment Reporting Information, Total Net Operating Revenues									1,024	1,251	1,545
Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues									(255)	(310)	(483)
Segment Reporting Information, Total Net Operating Revenues									17	18	15
Services [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues									21,592	22,586	24,222
Services [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues									20,653	21,400	22,755
Services [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Revenues									1,211	1,514	1,962
Services [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues									(272)	(328)	(495)
Wireless Equipment Sales [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues									5,606	4,524	4,684
Wireless Equipment Sales [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues									5,606	4,524	4,684
Wireless Equipment Sales [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Revenues									0	0	0
Wireless Equipment Sales [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues									0	0	0
Wireless Equipment Rentals [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues									5,137	4,048	3,295
Wireless Equipment Rentals [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues									5,137	4,048	3,295
Wireless Equipment Rentals [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Revenues									0	0	0
Wireless Equipment Rentals [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues									0	0	0
Other Products or Services [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues									1,262	1,281	1,146
Other Products or Services [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues									1,160	1,198	1,053
Other Products or Services [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Revenues									85	65	81
Other Products or Services [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues									17	18	12
Wireless Service [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues											$ 360
Revenue from Contract with Customer [Member] | Other, Net [Member]
Segment Reporting Information [Line Items]
Other Operating Expenses Net									$ 3	$ (33)