Guarantor Financial Information (CONDENSED CONSOLIDATED STATEMENT OF COMPREHENSIVE INCOME (LOSS)) (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
Condensed Income Statements, Captions [Line Items]
Service									$ 23,834	$ 25,368	$ 27,174
Revenue, equipment sales, net									4,524	4,684	3,168
Revenue, equipment rentals, net									4,048	3,295	1,838
Revenues	$ 8,083	$ 8,239	$ 7,927	$ 8,157	$ 8,539	$ 8,549	$ 8,247	$ 8,012	32,406	33,347	32,180
Net operating expenses:
Cost of services (exclusive of depreciation and amortization below)									6,801	7,861	9,439
Cost of equipment sales									6,109	6,583	5,518
Cost of equipment rentals									493	975	598
Selling, general and administrative									8,087	7,994	8,479
Severance and exit costs									80	66	409
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
Other Operating Income (Expense), Net									(471)	(46)	339
Total costs and expenses									29,679	31,583	31,870
Operating income (loss)	236	727	601	1,163	470	311	622	361	2,727	1,764	310
Other (expense) income:
Interest and Other Income									85	60	11
Interest Expense									(2,365)	(2,495)	(2,182)
(Losses) earnings of subsidiaries									0	0	0
Other nonoperating income (expense), excluding interest income									(144)	(100)	7
Nonoperating income (expense)									(2,424)	(2,535)	(2,164)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest									303	(771)	(1,854)
Income tax benefit (expense)									7,074	(435)	(141)
Net (loss) income									7,377	(1,206)	(1,995)
Net Income (Loss) Attributable to Noncontrolling Interest									12	0	0
Net Income (Loss) Attributable to Parent	$ 0	$ 0	$ 0	$ 0	$ (283)	$ (479)	$ (142)	$ (302)	7,389	(1,206)	(1,995)
Other comprehensive (loss) income									31	35	(31)
Comprehensive (loss)									7,408	(1,171)	(2,026)
Other Operating Expenses Net									(345)	20	748
Parent [Member]
Condensed Income Statements, Captions [Line Items]
Service									0	0	0
Revenue, equipment sales, net									0	0	0
Revenue, equipment rentals, net									0	0	0
Revenues									0	0	0
Net operating expenses:
Cost of services (exclusive of depreciation and amortization below)									0	0	0
Cost of equipment sales									0	0	0
Cost of equipment rentals									0	0	0
Selling, general and administrative									0	0	0
Severance and exit costs									0	0	0
Depreciation, network and other									0	0	0
Depreciation, equipment rentals									0	0	0
Amortization									0	0	0
Other Operating Income (Expense), Net									0	0	0
Total costs and expenses									0	0	0
Operating income (loss)									0	0	0
Other (expense) income:
Interest and Other Income									802	790	790
Interest Expense									(802)	(790)	(790)
(Losses) earnings of subsidiaries									7,389	(1,206)	(1,997)
Other nonoperating income (expense), excluding interest income									0	0	0
Nonoperating income (expense)									7,389	(1,206)	(1,997)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest									7,389	(1,206)	(1,997)
Income tax benefit (expense)									0	0	2
Net (loss) income									7,389
Net Income (Loss) Attributable to Noncontrolling Interest									0
Net Income (Loss) Attributable to Parent									7,389	(1,206)	(1,995)
Other comprehensive (loss) income									31	35	(31)
Comprehensive (loss)									7,420	(1,171)	(2,026)
Guarantor Subsidiaries [Member]
Condensed Income Statements, Captions [Line Items]
Service									0	0	0
Revenue, equipment sales, net									0	0	0
Revenue, equipment rentals, net									0	0	0
Revenues									0	0	0
Net operating expenses:
Cost of services (exclusive of depreciation and amortization below)									0	0	0
Cost of equipment sales									0	0	0
Cost of equipment rentals									0	0	0
Selling, general and administrative									0	0	0
Severance and exit costs									0	0	0
Depreciation, network and other									0	0	0
Depreciation, equipment rentals									0	0	0
Amortization									0	0	0
Other Operating Income (Expense), Net									(55)	0	0
Total costs and expenses									(55)	0	0
Operating income (loss)									55	0	0
Other (expense) income:
Interest and Other Income									1,289	145	165
Interest Expense									(1,643)	(1,675)	(1,624)
(Losses) earnings of subsidiaries									7,784	402	(538)
Other nonoperating income (expense), excluding interest income									(96)	(78)	0
Nonoperating income (expense)									7,334	(1,206)	(1,997)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest									7,389	(1,206)	(1,997)
Income tax benefit (expense)									0	0	0
Net (loss) income									7,389
Net Income (Loss) Attributable to Noncontrolling Interest									0
Net Income (Loss) Attributable to Parent									7,389	(1,206)	(1,997)
Other comprehensive (loss) income									31	35	(31)
Comprehensive (loss)									7,420	(1,171)	(2,028)
Non-Guarantor Subsidiaries [Member]
Condensed Income Statements, Captions [Line Items]
Service									23,834	25,368	27,174
Revenue, equipment sales, net									4,524	4,684	3,168
Revenue, equipment rentals, net									4,048	3,295	1,838
Revenues									32,406	33,347	32,180
Net operating expenses:
Cost of services (exclusive of depreciation and amortization below)									6,801	7,861	9,439
Cost of equipment sales									6,109	6,583	5,518
Cost of equipment rentals									493	975	598
Selling, general and administrative									8,087	7,994	8,479
Severance and exit costs									80	66	409
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
Other Operating Income (Expense), Net									(416)	(46)	339
Total costs and expenses									29,734	31,583	31,870
Operating income (loss)									2,672	1,764	310
Other (expense) income:
Interest and Other Income									11	21	5
Interest Expense									(1,937)	(926)	(717)
(Losses) earnings of subsidiaries									0	0	0
Other nonoperating income (expense), excluding interest income									(48)	(22)	7
Nonoperating income (expense)									(1,974)	(927)	(705)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest									698	837	(395)
Income tax benefit (expense)									7,074	(435)	(143)
Net (loss) income									7,772
Net Income (Loss) Attributable to Noncontrolling Interest									12
Net Income (Loss) Attributable to Parent									7,784	402	(538)
Other comprehensive (loss) income									48	42	(21)
Comprehensive (loss)									7,820	444	(559)
Consolidation, Eliminations [Member]
Condensed Income Statements, Captions [Line Items]
Service									0	0	0
Revenue, equipment sales, net									0	0	0
Revenue, equipment rentals, net									0	0	0
Revenues									0	0	0
Net operating expenses:
Cost of services (exclusive of depreciation and amortization below)									0	0	0
Cost of equipment sales									0	0	0
Cost of equipment rentals									0	0	0
Selling, general and administrative									0	0	0
Severance and exit costs									0	0	0
Depreciation, network and other									0	0	0
Depreciation, equipment rentals									0	0	0
Amortization									0	0	0
Other Operating Income (Expense), Net									0	0	0
Total costs and expenses									0	0	0
Operating income (loss)									0	0	0
Other (expense) income:
Interest and Other Income									(2,017)	(896)	(949)
Interest Expense									2,017	896	949
(Losses) earnings of subsidiaries									(15,173)	804	2,535
Other nonoperating income (expense), excluding interest income									0	0	0
Nonoperating income (expense)									(15,173)	804	2,535
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest									(15,173)	804	2,535
Income tax benefit (expense)									0	0	0
Net (loss) income									(15,173)
Net Income (Loss) Attributable to Noncontrolling Interest									0
Net Income (Loss) Attributable to Parent									(15,173)	804	2,535
Other comprehensive (loss) income									(79)	(77)	52
Comprehensive (loss)									$ (15,252)	$ 727	$ 2,587