Guarantor Financial Information (CONDENSED CONSOLIDATED STATEMENT OF COMPREHENSIVE INCOME (LOSS)) (Details) - USD ($)	3 Months Ended								12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Condensed Income Statements, Captions [Line Items]
Service									$ 22,857,000,000	$ 23,834,000,000	$ 25,368,000,000
Revenue, equipment sales, net									5,606,000,000	4,524,000,000	4,684,000,000
Revenue, equipment rentals, net									5,137,000,000	4,048,000,000	3,295,000,000
Revenues	$ 8,441,000,000	$ 8,601,000,000	$ 8,433,000,000	$ 8,125,000,000	$ 8,083,000,000	$ 8,239,000,000	$ 7,927,000,000	$ 8,157,000,000	33,600,000,000	32,406,000,000	33,347,000,000
Net operating expenses:
Cost of services (exclusive of depreciation and amortization below)									6,664,000,000	6,801,000,000	7,861,000,000
Cost of equipment sales									6,082,000,000	6,109,000,000	6,583,000,000
Cost of equipment rentals									643,000,000	493,000,000	975,000,000
Selling, general and administrative									7,774,000,000	8,087,000,000	7,994,000,000
Depreciation, network and other									4,245,000,000	3,976,000,000	3,982,000,000
Depreciation, equipment rentals									4,538,000,000	3,792,000,000	3,116,000,000
Amortization									608,000,000	812,000,000	1,052,000,000
Goodwill, Impairment Loss									2,000,000,000
Other Operating Income (Expense), Net									648,000,000	391,000,000	(20,000,000)
Total costs and expenses									33,202,000,000	29,679,000,000	31,583,000,000
Operating income (loss)	(1,674,000,000)	479,000,000	778,000,000	815,000,000	236,000,000	727,000,000	601,000,000	1,163,000,000	398,000,000	2,727,000,000	1,764,000,000
Other (expense) income:
Interest and Other Income									169,000,000	85,000,000	60,000,000
Interest Expense									(2,563,000,000)	(2,365,000,000)	(2,495,000,000)
(Losses) earnings of subsidiaries									0	0	0
Other nonoperating income (expense), excluding interest income									18,000,000	(144,000,000)	(100,000,000)
Nonoperating income (expense)									(2,376,000,000)	(2,424,000,000)	(2,535,000,000)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest									(1,978,000,000)	303,000,000	(771,000,000)
Income tax benefit (expense)									35,000,000	7,074,000,000	(435,000,000)
Net (loss) income									(1,943,000,000)	7,377,000,000	(1,206,000,000)
Net Income (Loss) Attributable to Noncontrolling Interest									0	12,000,000	0
Net Income (Loss) Attributable to Parent	$ (2,174)	$ (141)	$ 196	$ 176	$ 0	$ 0	$ 0	$ 0	(1,943,000,000)	7,389,000,000	(1,206,000,000)
Other comprehensive (loss) income									(71,000,000)	31,000,000	35,000,000
Comprehensive (loss)									(2,014,000,000)	7,408,000,000	(1,171,000,000)
Impairments									2,000,000,000	0	0
Other Operating Expenses Net									(670,000,000)	345,000,000	(20,000,000)
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
Depreciation, network and other									0	0	0
Depreciation, equipment rentals									0	0	0
Amortization									0	0	0
Goodwill, Impairment Loss									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									905,000,000	802,000,000	790,000,000
Interest Expense									(905,000,000)	(802,000,000)	(790,000,000)
(Losses) earnings of subsidiaries									(1,943,000,000)	7,389,000,000	(1,206,000,000)
Other nonoperating income (expense), excluding interest income									0	0	0
Nonoperating income (expense)									(1,943,000,000)	7,389,000,000	(1,206,000,000)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest									(1,943,000,000)	7,389,000,000	(1,206,000,000)
Income tax benefit (expense)									0	0	0
Net (loss) income									(1,943,000,000)	7,389,000,000
Net Income (Loss) Attributable to Noncontrolling Interest									0	0
Net Income (Loss) Attributable to Parent									(1,943,000,000)	7,389,000,000	(1,206,000,000)
Other comprehensive (loss) income									(71,000,000)	31,000,000	35,000,000
Comprehensive (loss)									(2,014,000,000)	7,420,000,000	(1,171,000,000)
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
Depreciation, network and other									0	0	0
Depreciation, equipment rentals									0	0	0
Amortization									0	0	0
Goodwill, Impairment Loss									0
Other Operating Income (Expense), Net									0	55,000,000	0
Total costs and expenses									0	(55,000,000)	0
Operating income (loss)									0	55,000,000	0
Other (expense) income:
Interest and Other Income									2,166,000,000	1,289,000,000	145,000,000
Interest Expense									(2,315,000,000)	(1,643,000,000)	(1,675,000,000)
(Losses) earnings of subsidiaries									(1,811,000,000)	7,784,000,000	402,000,000
Other nonoperating income (expense), excluding interest income									17,000,000	(96,000,000)	(78,000,000)
Nonoperating income (expense)									(1,943,000,000)	7,334,000,000	(1,206,000,000)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest									(1,943,000,000)	7,389,000,000	(1,206,000,000)
Income tax benefit (expense)									0	0	0
Net (loss) income									(1,943,000,000)	7,389,000,000
Net Income (Loss) Attributable to Noncontrolling Interest									0	0
Net Income (Loss) Attributable to Parent									(1,943,000,000)	7,389,000,000	(1,206,000,000)
Other comprehensive (loss) income									(71,000,000)	31,000,000	35,000,000
Comprehensive (loss)									(2,014,000,000)	7,420,000,000	(1,171,000,000)
Non-Guarantor Subsidiaries [Member]
Condensed Income Statements, Captions [Line Items]
Service									22,857,000,000	23,834,000,000	25,368,000,000
Revenue, equipment sales, net									5,606,000,000	4,524,000,000	4,684,000,000
Revenue, equipment rentals, net									5,137,000,000	4,048,000,000	3,295,000,000
Revenues									33,600,000,000	32,406,000,000	33,347,000,000
Net operating expenses:
Cost of services (exclusive of depreciation and amortization below)									6,664,000,000	6,801,000,000	7,861,000,000
Cost of equipment sales									6,082,000,000	6,109,000,000	6,583,000,000
Cost of equipment rentals									643,000,000	493,000,000	975,000,000
Selling, general and administrative									7,774,000,000	8,087,000,000	7,994,000,000
Depreciation, network and other									4,245,000,000	3,976,000,000	3,982,000,000
Depreciation, equipment rentals									4,538,000,000	3,792,000,000	3,116,000,000
Amortization									608,000,000	812,000,000	1,052,000,000
Goodwill, Impairment Loss									2,000,000,000
Other Operating Income (Expense), Net									(648,000,000)	336,000,000	(20,000,000)
Total costs and expenses									33,202,000,000	29,734,000,000	31,583,000,000
Operating income (loss)									398,000,000	2,672,000,000	1,764,000,000
Other (expense) income:
Interest and Other Income									682,000,000	11,000,000	21,000,000
Interest Expense									(2,927,000,000)	(1,937,000,000)	(926,000,000)
(Losses) earnings of subsidiaries									0	0	0
Other nonoperating income (expense), excluding interest income									1,000,000	(48,000,000)	(22,000,000)
Nonoperating income (expense)									(2,244,000,000)	(1,974,000,000)	(927,000,000)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest									(1,846,000,000)	698,000,000	837,000,000
Income tax benefit (expense)									35,000,000	7,074,000,000	(435,000,000)
Net (loss) income									(1,811,000,000)	7,772,000,000
Net Income (Loss) Attributable to Noncontrolling Interest									0	12,000,000
Net Income (Loss) Attributable to Parent									(1,811,000,000)	7,784,000,000	402,000,000
Other comprehensive (loss) income									(49,000,000)	48,000,000	42,000,000
Comprehensive (loss)									(1,860,000,000)	7,820,000,000	444,000,000
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
Depreciation, network and other									0	0	0
Depreciation, equipment rentals									0	0	0
Amortization									0	0	0
Goodwill, Impairment Loss									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									(3,584,000,000)	(2,017,000,000)	(896,000,000)
Interest Expense									3,584,000,000	2,017,000,000	896,000,000
(Losses) earnings of subsidiaries									3,754,000,000	(15,173,000,000)	804,000,000
Other nonoperating income (expense), excluding interest income									0	0	0
Nonoperating income (expense)									3,754,000,000	(15,173,000,000)	804,000,000
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest									3,754,000,000	(15,173,000,000)	804,000,000
Income tax benefit (expense)									0	0	0
Net (loss) income									3,754,000,000	(15,173,000,000)
Net Income (Loss) Attributable to Noncontrolling Interest									0	0
Net Income (Loss) Attributable to Parent									3,754,000,000	(15,173,000,000)	804,000,000
Other comprehensive (loss) income									120,000,000	(79,000,000)	(77,000,000)
Comprehensive (loss)									$ 3,874,000,000	$ (15,252,000,000)	$ 727,000,000