Segments (Segment Financial Information) (Details) $ in Millions	3 Months Ended								9 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2017 USD ($)	Mar. 31, 2018 USD ($) segments	Mar. 31, 2017 USD ($)	Mar. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments | segments										2
Segment Reporting Information, Total Net Operating Revenues										$ 32,439	$ 33,347	$ 32,180
Inter-segment revenues	$ 8,083	$ 8,239	$ 7,927	$ 8,157	$ 8,539	$ 8,549	$ 8,247	$ 8,012		32,406	33,347	32,180
Total segment operating expenses										(21,370)	(23,413)	(24,034)
Segment earnings										11,069	9,934	8,146
Depreciation, network and other										(3,976)	(3,982)	(4,013)
Depreciation, equipment rentals										(3,792)	(3,116)	(1,781)
Amortization										(812)	(1,052)	(1,294)
Hurricane-related charges										(107)
Other Operating Expenses Net										345	(20)	(748)
Interest expense	(236)	$ (727)	$ (601)	$ (1,163)	(470)	$ (311)	$ (622)	$ (361)		(2,727)	(1,764)	(310)
Interest expense										(2,365)	(2,495)	(2,182)
Other income (expense), net										(59)	(40)	18
Capital expenditures - network and other										3,319	1,950	4,680
Capital expenditures - network and other, inclusive of equipment on lease										10,780	6,926	10,578
Assets	85,459				85,123					85,459	85,123	78,975
Severance, exit costs and asset impairments												(409)
Severance and exit costs										80	66	409
Legal Fees											140	193
Gain (Loss) on Disposition of Property Plant Equipment											65	(166)
Gain (Loss) on Contract Termination										5	(111)	0
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest										303	(771)	(1,854)
Wireless [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues											31,787	30,377
Inter-segment revenues										31,170	31,787	30,377
Total segment operating expenses											(21,973)	(22,326)
Segment earnings											9,814	8,051
Capital expenditures - network and other										10,221	6,568	9,987
Assets	73,834				74,098					73,834	74,098	73,408
Gain (Loss) on Contract Termination									$ (5)		(140)
Wireline [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues										1,251	1,545	1,790
Inter-segment revenues										1,579	2,043	2,382
Total segment operating expenses										(1,697)	(1,924)	(2,290)
Segment earnings										(118)	119	92
Capital expenditures - network and other										166	94	279
Assets	1,117				1,168					1,117	1,168	1,255
Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues										18	15	13
Inter-segment revenues										(310)	(483)	(579)
Total segment operating expenses										292	484	582
Segment earnings										(18)	1	3
Capital expenditures - network and other										393	264	312
Assets	$ 10,508				$ 9,857					10,508	9,857	4,312
Lease and severance [Member]
Segment Reporting Information [Line Items]
Severance and exit costs											66
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues										0	0	0
Intersegment Eliminations [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues											0	0
Intersegment Eliminations [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues										328	498	592
Intersegment Eliminations [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues										(328)	(498)	(592)
Network Equipment, Site Costs and Related Software [Member]
Segment Reporting Information [Line Items]
Gain (Loss) on Disposition of Property Plant Equipment										370
State and Local Jurisdiction [Member]
Segment Reporting Information [Line Items]
Loss Contingency, Loss in Period										114
Other Operating Income (Expense) [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Restructuring reserve, accrual adjustment												20
Other, Net [Member]
Segment Reporting Information [Line Items]
Impairments											(28)
Gain (Loss) on Disposition of Property Plant Equipment										(364)
Gain (Loss) on Disposition of Intangible Assets										479	354
Other, Net [Member] | Lease and severance [Member]
Segment Reporting Information [Line Items]
Severance and exit costs										80
Other, Net [Member] | State and Local Jurisdiction [Member]
Segment Reporting Information [Line Items]
Loss Contingency, Loss in Period										305
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]
Inter-segment revenues										4,524	4,684	3,168
Wireless Equipment Sales [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues										0	0	0
Wireless Equipment Sales [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Inter-segment 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]
Inter-segment revenues										4,048	3,295	1,838
Wireless Equipment Rentals [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues										0	0	0
Wireless Equipment Rentals [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Inter-segment 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]
Inter-segment revenues										1,198	1,053	1,067
Other Products or Services [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues										65	81	87
Other Products or Services [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues										18	12	9
Wireless Service [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues										360	323
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]
Inter-segment revenues										21,400	22,755	24,304
Services [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues										1,514	1,962	2,295
Services [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues										(328)	$ (495)	$ (588)
Sales Revenue, Services, Net [Member] | Other, Net [Member]
Segment Reporting Information [Line Items]
Other Operating Expenses Net										33
Cost of Sales [Member] | Other, Net [Member]
Segment Reporting Information [Line Items]
Other Operating Expenses Net										48
Selling, General and Administrative Expenses [Member] | Other, Net [Member]
Segment Reporting Information [Line Items]
Other Operating Expenses Net										21
Other, Net [Member] | Other, Net [Member]
Segment Reporting Information [Line Items]
Other Operating Expenses Net									$ 5
Sales Revenue, Services, Net [Member] | Other, Net [Member]
Segment Reporting Information [Line Items]
Other Operating Expenses Net										(51)
Wireless Including Hurricane and Other [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues										31,137
Total segment operating expenses										(20,090)
Segment earnings										11,047
Wireless Including Hurricane and Other [Member] | Intersegment Eliminations [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues										0
Wireless Excluding Hurricane and Other [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues										31,170
Total segment operating expenses										(19,965)
Segment earnings										11,205
Wireless Excluding Hurricane and Other [Member] | Intersegment Eliminations [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues										0
Wireless Hurricane and Other [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues										33
Total segment operating expenses										125
Segment earnings										158
Wireless Hurricane and Other [Member] | Intersegment Eliminations [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues										$ 0