Segments (Segment Financial Information) (Details) $ in Millions		3 Months Ended									12 Months Ended
		Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)	Mar. 31, 2017 USD ($) segments		Mar. 31, 2016 USD ($)		Mar. 31, 2015 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segments											2
Net operating revenues											$ 33,347		$ 32,180		$ 34,532
Inter-segment revenues		$ 8,539	$ 8,549	$ 8,247	$ 8,012	$ 8,071	$ 8,107	$ 7,975	$ 8,027		33,347		32,180		34,532
Total segment operating expenses											(23,413)		(24,034)		(28,532)
Segment earnings											9,934		8,146		6,000
Depreciation											(7,098)		(5,794)		(3,797)
Amortization											(1,052)		(1,294)		(1,552)
Impairments											0		0		(2,133)	[1]
Other, net											(20)		(748)	[2]	(413)	[2]
Operating (loss) income		470	$ 311	$ 622	$ 361	8	$ (197)	$ (2)	$ 501		1,764		310		(1,895)
Interest expense											(2,495)		(2,182)		(2,051)
Other income (expense), net											(40)		18		27
Income (loss) before income taxes											(771)		(1,854)		(3,919)
Capital expenditures - network and other											1,950		4,680		5,422
Capital expenditures - network and other, inclusive of equipment on lease											3,875		6,972		6,004
Assets		85,123				78,975					85,123		78,975		82,841
Severance, exit costs and asset impairments															(304)
Legal fees											(140)		(193)		(91)
Pension expense															59
Severance and exit costs											66		409		304
Restructuring charges											(68)		(391)
Other, net											(435)		(660)		(109)
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property											509		487
Gain (Loss) on Disposition of Property Plant Equipment													65
Gain (Loss) on Contract Termination											(111)		0		0
Wireless [Member]
Segment Reporting Information [Line Items]
Net operating revenues											31,787		30,377		32,327
Inter-segment revenues											31,787		30,377		32,327
Total segment operating expenses											(21,973)		(22,326)		(26,433)
Segment earnings											9,814		8,051		5,894
Capital expenditures - network and other											3,517		6,381		5,442
Assets		74,098				73,408					74,098		73,408		75,533
Gain (Loss) on Contract Termination											140
Wireline [Member]
Segment Reporting Information [Line Items]
Net operating revenues											1,545		1,790		2,191
Inter-segment revenues											2,043		2,382		2,814
Total segment operating expenses											(1,924)		(2,290)		(2,701)
Segment earnings											119		92		113
Capital expenditures - network and other											94		279		275
Assets		1,168				1,255					1,168		1,255		1,262
Impairment loss										$ 233
Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Net operating revenues											15		13		14
Inter-segment revenues	[3]										(483)		(579)		(609)
Total segment operating expenses											484		582		602
Segment earnings											1		3		(7)
Capital expenditures - network and other											264		312		287
Assets		$ 9,857				$ 4,312					9,857		4,312		6,046
Lease and severance [Member]
Segment Reporting Information [Line Items]
Severance and exit costs													409
Lease [Member]
Segment Reporting Information [Line Items]
Restructuring charges											(17)	[4]	(156)	[5]
Lease [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Restructuring charges	[4]										(14)
Lease [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Restructuring charges	[4]										(3)
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues	[6]										0		0		0
Intersegment Eliminations [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues	[6]										0		0		0
Intersegment Eliminations [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues	[6]										498		592		623
Intersegment Eliminations [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues	[6]										(498)		(592)		(623)
Trademarks [Member]
Segment Reporting Information [Line Items]
Impairment of intangible assets, indefinite-lived (excluding goodwill)	[7]										0
Trademarks [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Impairment of intangible assets, indefinite-lived (excluding goodwill)															1,900
Leased devices [Member]
Segment Reporting Information [Line Items]
Depreciation											(3,100)		(1,800)		$ (206)
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property											481		256
Gain (Loss) on Disposition of Property Plant Equipment													65
State and Local Jurisdiction [Member]
Segment Reporting Information [Line Items]
Loss Contingency, Loss in Period											103
Other, Net [Member]
Segment Reporting Information [Line Items]
Impairments													(166)
Gain (Loss) on Disposition of Property Plant Equipment											(28)
Gain (Loss) on Disposition of Intangible Assets											354
Other, Net [Member] | Lease and severance [Member]
Segment Reporting Information [Line Items]
Severance and exit costs											66
Other, Net [Member] | State and Local Jurisdiction [Member]
Segment Reporting Information [Line Items]
Loss Contingency, Loss in Period											140
Write-off leased devices [Member]
Segment Reporting Information [Line Items]
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property											$ 481		$ 321

[1] (2)Impairments for the year ended March 31, 2015 consist of a $1.9 billion trade name impairment related to the Wireless segment and a $233 million impairment related to Wireline long-lived assets.

[2] (3)Other, net for the year ended March 31, 2017 consists of $66 million of severance and exit costs, a $140 million charge for a state tax matter combined with legal reserves related to other pending legal suits and proceedings, and a $28 million loss on disposal of property, plant and equipment related to cell site construction costs that are no longer recoverable as a result of changes in our network plans. In addition, the year ended March 31, 2017 included a $354 million non-cash gain related to spectrum license exchanges with other carriers and $140 million of contract termination costs, primarily related to the termination of our pre-existing wholesale arrangement with nTelos, as a result of the Shentel transaction combined with the costs related to the termination of our relationship with General Wireless Operations Inc. (Radio Shack). Losses totaling $481 million related to the write-off of leased devices associated with lease cancellations were excluded from Other, net and included within Wireless segment earnings. Other, net for the year ended March 31, 2016 consists of $409 million of severance and exit costs, combined with $193 million for legal reserves related to various pending legal suits and proceedings and a $166 million loss on disposal of property, plant and equipment related to cell site construction costs and other network costs that are no longer recoverable as a result of changes in the Company's network plans, partially offset by $20 million of income resulting from a revision to our estimate of a previously recorded reserve. Losses totaling $321 million relating to the write-off of leased devices associated with lease cancellations of $256 million and the loss on sale of devices to MLS under the Handset Sale-Leaseback Tranche 1 transaction for $65 million was excluded from Other, net and included within Wireless segment earnings. Other, net for the year ended March 31, 2015 consists of $304 million of severance and exit costs, combined with $91 million for legal reserves related to various pending legal suits and proceedings and $59 million for a partial pension settlement, partially offset by $41 million of income resulting from a revision to

[3] (1)Revenues eliminated in consolidation consist primarily of wireline services provided to the Wireless segment for resale to or use by wireless subscribers.

[4] (1)For the year ended March 31, 2017, we recognized costs of $17 million ($14 million Wireless, $3 million Wireline).

[5] (4)We recognized costs of $176 million (Wireless only) for the year ended March 31, 2016, which were offset by $20 million of income (Wireless only) resulting from a revision to our estimate of a previously recorded reserve.

[6] (1)Inter-segment revenues consist primarily of wireline services provided to the Wireless segment for resale to or use by wireless subscribers.

[7] .