Segments (Segment Financial Information) (Details) $ in Millions	3 Months Ended								12 Months Ended
	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2019 USD ($) segments	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments | segments									2
Segment Reporting Information, Total Net Operating Revenues									$ 33,597	$ 32,439	$ 33,347
Inter-segment revenues	$ 8,441	$ 8,601	$ 8,433	$ 8,125	$ 8,083	$ 8,239	$ 7,927	$ 8,157	33,600	32,406	33,347
Total segment operating expenses									(20,824)	(21,370)	(23,413)
Segment earnings									12,773	11,069	9,934
Depreciation, network and other									(4,245)	(3,976)	(3,982)
Depreciation, equipment rentals									(4,538)	(3,792)	(3,116)
Amortization									(608)	(812)	(1,052)
Hurricane-related charges									(32)	(107)
Merger costs									(346)
Goodwill, Impairment Loss									(2,000)
Other Operating Expenses Net									670	(345)	20
Impairments									(2,000)	0	0
Interest expense	1,674	$ (479)	$ (778)	$ (815)	(236)	$ (727)	$ (601)	$ (1,163)	(398)	(2,727)	(1,764)
Interest expense									(2,563)	(2,365)	(2,495)
Other income (expense), net									187	(59)	(40)
Capital expenditures - network and other									4,963	3,319	1,950
Capital expenditures - network and other, inclusive of equipment on lease									12,404	10,780	6,926
Assets	84,601				85,459				84,601	85,459	85,123
Legal Fees									(74)		(140)
Gain (Loss) on Disposition of Property Plant Equipment										6
Gain (Loss) on Disposition of Assets									(1,135)	(868)	(509)
Gain (Loss) on Contract Termination									0	5	(111)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest									(1,978)	303	(771)
Wireless [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues											31,787
Inter-segment revenues									32,556	31,170	31,787
Total segment operating expenses											21,973
Segment earnings											9,814
Capital expenditures - network and other, inclusive of equipment on lease									11,776	10,221	6,568
Assets	74,929				73,834				74,929	73,834	74,098
Gain (Loss) on Contract Termination									(34)	5	(140)
Wireline [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues									1,024	1,251	1,545
Inter-segment revenues									1,296	1,579	2,043
Total segment operating expenses									1,365	1,697	1,924
Segment earnings									(69)	(118)	119
Capital expenditures - network and other, inclusive of equipment on lease									242	166	94
Assets	1,148				1,117				1,148	1,117	1,168
Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues									17	18	15
Inter-segment revenues									(255)	(310)	(483)
Total segment operating expenses									(261)	(292)	(484)
Segment earnings									6	(18)	1
Capital expenditures - network and other, inclusive of equipment on lease									386	393	264
Assets	$ 8,524				$ 10,508				8,524	10,508	9,857
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
Intersegment Eliminations [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues									272	328	498
Intersegment Eliminations [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues									(272)	(328)	(498)
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, Net [Member]
Segment Reporting Information [Line Items]
Impairments											(28)
Gain (Loss) on Disposition of Property Plant Equipment									(492)	(364)
Gain (Loss) on Disposition of Assets									15
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									85	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									5,606	4,524	4,684
Wireless Equipment Sales [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues									5,606	4,524	4,684
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									5,137	4,048	3,295
Wireless Equipment Rentals [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues									5,137	4,048	3,295
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									1,262	1,281	1,146
Other Products or Services [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues									1,160	1,198	1,053
Other Products or Services [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues									85	65	81
Other Products or Services [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues									17	18	12
Wireless Service [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues											360
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]
Inter-segment revenues									20,653	21,400	22,755
Services [Member] | Wireline [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues									1,211	1,514	1,962
Services [Member] | Corporate, Other And Eliminations [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues									(272)	(328)	$ (495)
Cost of Sales [Member] | Other, Net [Member]
Segment Reporting Information [Line Items]
Other Operating Expenses Net									(6)	48
Selling, General and Administrative Expenses [Member] | Other, Net [Member]
Segment Reporting Information [Line Items]
Other Operating Expenses Net									(1)	21
Other, Net [Member] | Other, Net [Member]
Segment Reporting Information [Line Items]
Other Operating Expenses Net									(22)	5
Revenue from Contract with Customer [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									32,559	31,137
Total segment operating expenses									(19,713)	(20,090)
Segment earnings									12,846	11,047
Wireless Including Hurricane and Other [Member] | Intersegment Eliminations [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues									0	0
Wireless Excluding Hurricane and Other [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues									32,556	31,170
Total segment operating expenses									19,720	19,965
Segment earnings									12,836	11,205
Wireless Excluding Hurricane and Other [Member] | Intersegment Eliminations [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues									0	0
Wireless Hurricane and Other [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information, Total Net Operating Revenues									(3)	33
Total segment operating expenses									7	(125)
Segment earnings									(10)	158
Wireless Hurricane and Other [Member] | Intersegment Eliminations [Member] | Wireless [Member]
Segment Reporting Information [Line Items]
Inter-segment revenues									$ 0	$ 0