Guarantor Financial Information (CONDENSED CONSOLIDATED STATEMENT OF COMPREHENSIVE INCOME (LOSS)) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Condensed Income Statements, Captions [Line Items]
Service									$ 25,368	$ 27,174	$ 29,542
Equipment									7,979	5,006	4,990
Revenues	$ 8,539	$ 8,549	$ 8,247	$ 8,012	$ 8,071	$ 8,107	$ 7,975	$ 8,027	33,347	32,180	34,532
Cost of services (exclusive of depreciation and amortization below)									7,861	9,439	9,660
Cost of products (exclusive of depreciation and amortization below)									7,077	5,795	9,309
Net operating expenses:
Selling, general and administrative									7,994	8,479	9,563
Depreciation									7,098	5,794	3,797
Amortization									1,052	1,294	1,552
Total costs and expenses									31,583	31,870	36,427
Operating income (loss)	470	311	622	361	8	(197)	(2)	501	1,764	310	(1,895)
Other (expense) income:
Interest and Other Income									60	11	12
Interest Expense									(2,495)	(2,182)	(2,051)
(Losses) earnings of subsidiaries									0	0	0
Other nonoperating income (expense), excluding interest income									(100)	7	15
Nonoperating income (expense)									(2,535)	(2,164)	(2,024)
Income (loss) before income taxes									(771)	(1,854)	(3,919)
Income tax benefit (expense)									(435)	(141)	574
Net (loss) income	$ (283)	$ (479)	$ (142)	$ (302)	$ (554)	$ (836)	$ (585)	$ (20)	(1,206)	(1,995)	(3,345)
Other comprehensive (loss) income									35	(31)	(365)
Comprehensive (loss)									(1,171)	(2,026)	(3,710)
Asset impairments									0	0	2,133	[1]
Severance and exit costs									66	409	304
Other, net									435	660	109
Guarantor Subsidiaries [Member]
Condensed Income Statements, Captions [Line Items]
Service									0	0	0
Equipment									0	0	0
Revenues									0	0	0
Cost of services (exclusive of depreciation and amortization below)									0	0	0
Cost of products (exclusive of depreciation and amortization below)									0	0	0
Net operating expenses:
Selling, general and administrative									0	0	0
Depreciation									0	0	0
Amortization									0	0	0
Total costs and expenses									0	0	1
Operating income (loss)									0	0	(1)
Other (expense) income:
Interest and Other Income									145	165	146
Interest Expense									(1,675)	(1,624)	(1,521)
(Losses) earnings of subsidiaries									402	(538)	(1,970)
Other nonoperating income (expense), excluding interest income									(78)	0	1
Nonoperating income (expense)									(1,206)	(1,997)	(3,344)
Income (loss) before income taxes									(1,206)	(1,997)	(3,345)
Income tax benefit (expense)									0	0	0
Net (loss) income									(1,206)	(1,997)	(3,345)
Other comprehensive (loss) income									35	(31)	(365)
Comprehensive (loss)									(1,171)	(2,028)	(3,710)
Asset impairments											0
Severance and exit costs									0	0	0
Other, net									0	0	1
Non-Guarantor Subsidiaries [Member]
Condensed Income Statements, Captions [Line Items]
Service									25,368	27,174	29,542
Equipment									7,979	5,006	4,990
Revenues									33,347	32,180	34,532
Cost of services (exclusive of depreciation and amortization below)									7,861	9,439	9,660
Cost of products (exclusive of depreciation and amortization below)									7,077	5,795	9,309
Net operating expenses:
Selling, general and administrative									7,994	8,479	9,563
Depreciation									7,098	5,794	3,797
Amortization									1,052	1,294	1,552
Total costs and expenses									31,583	31,870	36,426
Operating income (loss)									1,764	310	(1,894)
Other (expense) income:
Interest and Other Income									21	5	3
Interest Expense									(926)	(717)	(667)
(Losses) earnings of subsidiaries									0	0	0
Other nonoperating income (expense), excluding interest income									(22)	7	14
Nonoperating income (expense)									(927)	(705)	(650)
Income (loss) before income taxes									837	(395)	(2,544)
Income tax benefit (expense)									(435)	(143)	574
Net (loss) income									402	(538)	(1,970)
Other comprehensive (loss) income									42	(21)	(355)
Comprehensive (loss)									444	(559)	(2,325)
Asset impairments											2,133
Severance and exit costs									66	409	304
Other, net									435	660	108
Parent [Member]
Condensed Income Statements, Captions [Line Items]
Service									0	0	0
Equipment									0	0	0
Revenues									0	0	0
Cost of services (exclusive of depreciation and amortization below)									0	0	0
Cost of products (exclusive of depreciation and amortization below)									0	0	0
Net operating expenses:
Selling, general and administrative									0	0	0
Depreciation									0	0	0
Amortization									0	0	0
Total costs and expenses									0	0	0
Operating income (loss)									0	0	0
Other (expense) income:
Interest and Other Income									790	790	687
Interest Expense									(790)	(790)	(687)
(Losses) earnings of subsidiaries									(1,206)	(1,997)	(3,345)
Other nonoperating income (expense), excluding interest income									0	0	0
Nonoperating income (expense)									(1,206)	(1,997)	(3,345)
Income (loss) before income taxes									(1,206)	(1,997)	(3,345)
Income tax benefit (expense)									0	2	0
Net (loss) income									(1,206)	(1,995)	(3,345)
Other comprehensive (loss) income									35	(31)	(365)
Comprehensive (loss)									(1,171)	(2,026)	(3,710)
Asset impairments											0
Severance and exit costs									0	0	0
Other, net									0	0	0
Consolidation, Eliminations [Member]
Condensed Income Statements, Captions [Line Items]
Service									0	0	0
Equipment									0	0	0
Revenues									0	0	0
Cost of services (exclusive of depreciation and amortization below)									0	0	0
Cost of products (exclusive of depreciation and amortization below)									0	0	0
Net operating expenses:
Selling, general and administrative									0	0	0
Depreciation									0	0	0
Amortization									0	0	0
Total costs and expenses									0	0	0
Operating income (loss)									0	0	0
Other (expense) income:
Interest and Other Income									(896)	(949)	(824)
Interest Expense									896	949	824
(Losses) earnings of subsidiaries									804	2,535	5,315
Other nonoperating income (expense), excluding interest income									0	0	0
Nonoperating income (expense)									804	2,535	5,315
Income (loss) before income taxes									804	2,535	5,315
Income tax benefit (expense)									0	0	0
Net (loss) income									804	2,535	5,315
Other comprehensive (loss) income									(77)	52	720
Comprehensive (loss)									727	2,587	6,035
Asset impairments											0
Severance and exit costs									0	0	0
Other, net									$ 0	$ 0	$ 0

[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.