Segments (Operating Revenues by Service and Products) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Revenues	$ 8,083	$ 8,239	$ 7,927	$ 8,157	$ 8,539	$ 8,549	$ 8,247	$ 8,012	$ 32,406	$ 33,347	$ 32,180
Segment Reporting Information, Total Net Operating Revenues									32,439	33,347	32,180
Other Operating Expenses Net									(345)	20	748
Wireless [Member]
Segment Reporting Information [Line Items]
Revenues									31,170	31,787	30,377
Segment Reporting Information, Total Net Operating Revenues										31,787	30,377
Wireline [Member]
Segment Reporting Information [Line Items]
Revenues									1,579	2,043	2,382
Segment Reporting Information, Total Net Operating Revenues									1,251	1,545	1,790
Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues									(310)	(483)	(579)
Segment Reporting Information, Total Net Operating Revenues									18	15	13
Services [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues									22,586	24,222	26,011
Services [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues									21,400	22,755	24,304
Services [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Revenues									1,514	1,962	2,295
Services [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues									(328)	(495)	(588)
Wireless Equipment Sales [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues									4,524	4,684	3,168
Wireless Equipment Sales [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues									4,524	4,684	3,168
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									4,048	3,295	1,838
Wireless Equipment Rentals [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues									4,048	3,295	1,838
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									1,281	1,146	1,163
Other Products or Services [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues									1,198	1,053	1,067
Other Products or Services [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Revenues									65	81	87
Other Products or Services [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues									18	12	$ 9
Wireless Service [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Revenues									360	$ 323
Sales Revenue, Services, Net [Member] | Other, Net [Member]
Segment Reporting Information [Line Items]
Other Operating Expenses Net									$ (33)