Guarantor Financial Information (CONDENSED CONSOLIDATED STATEMENT OF COMPREHENSIVE INCOME (LOSS)) (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Condensed Income Statements, Captions [Line Items]
Net operating revenues	$ 8,142	$ 8,125
Service revenue	8,142	8,125
Revenue, equipment sales, net	1,220	1,173
Revenue, equipment rentals, net	1,359	1,212
Cost of Services (exclusive of depreciation and amortization included below)	1,710	1,677
Cost of equipment sales	1,341	1,270
Cost of equipment rentals	225	124
Net operating expenses
Selling, general and administrative	1,907	1,867
Severance and exit costs	(27)
Depreciation, network and other	1,120	1,023
Depreciation, equipment rentals	1,029	1,136
Amortization	118	171
Other, net	237	42
Total costs and expenses	7,687	7,310
Operating Income	455	815
Other (expense) income
Interest Income	30	42
Interest Expense	(619)	(637)
(Losses) earnings of Subsidiaries	0	0
Other nonoperating income (expense), excluding interest income	(2)	0
Nonoperating Expense	(591)	(595)
Loss (income) before income taxes	(136)	220
Income tax benefit	22	(47)
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	(114)	173
Net Income (Loss) Attributable to Noncontrolling Interest	3	3
Net Income (Loss) Attributable to Parent	(111)	176
Other Comprehensive Income (Loss), Net of Tax	(22)	(4)
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	(136)	169
Consolidation, Eliminations [Member]
Condensed Income Statements, Captions [Line Items]
Net operating revenues	0	0
Revenue, equipment sales, net	0	0
Revenue, equipment rentals, net	0	0
Cost of Services (exclusive of depreciation and amortization included below)	0	0
Cost of equipment sales	0	0
Cost of equipment rentals	0	0
Net operating expenses
Selling, general and administrative	0	0
Depreciation, network and other	0	0
Depreciation, equipment rentals	0	0
Amortization	0	0
Other, net	0	0
Total costs and expenses	0	0
Operating Income	0	0
Other (expense) income
Interest Income	(863)	(733)
Interest Expense	863	733
(Losses) earnings of Subsidiaries	197	(378)
Other nonoperating income (expense), excluding interest income	0	0
Nonoperating Expense	197	(378)
Loss (income) before income taxes	197	(378)
Income tax benefit	0	0
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	197	(378)
Net Income (Loss) Attributable to Noncontrolling Interest	0	0
Net Income (Loss) Attributable to Parent	197	(378)
Other Comprehensive Income (Loss), Net of Tax	22	17
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	219	(361)
Parent Company [Member]
Condensed Income Statements, Captions [Line Items]
Net operating revenues	0	0
Revenue, equipment sales, net	0	0
Revenue, equipment rentals, net	0	0
Cost of Services (exclusive of depreciation and amortization included below)	0	0
Cost of equipment sales	0	0
Cost of equipment rentals	0	0
Net operating expenses
Selling, general and administrative	0	0
Depreciation, network and other	0	0
Depreciation, equipment rentals	0	0
Amortization	0	0
Other, net	0	0
Total costs and expenses	0	0
Operating Income	0	0
Other (expense) income
Interest Income	226	226
Interest Expense	(226)	(226)
(Losses) earnings of Subsidiaries	(111)	176
Other nonoperating income (expense), excluding interest income	0	0
Nonoperating Expense	(111)	176
Loss (income) before income taxes	(111)	176
Income tax benefit	0	0
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	(111)	176
Net Income (Loss) Attributable to Noncontrolling Interest	0	0
Net Income (Loss) Attributable to Parent	(111)	176
Other Comprehensive Income (Loss), Net of Tax	(22)	(4)
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	(133)	172
Guarantor Subsidiaries [Member]
Condensed Income Statements, Captions [Line Items]
Net operating revenues	0	0
Revenue, equipment sales, net	0	0
Revenue, equipment rentals, net	0	0
Cost of Services (exclusive of depreciation and amortization included below)	0	0
Cost of equipment sales	0	0
Cost of equipment rentals	0	0
Net operating expenses
Selling, general and administrative	0	0
Depreciation, network and other	0	0
Depreciation, equipment rentals	0	0
Amortization	0	0
Other, net	0	0
Total costs and expenses	0	0
Operating Income	0	0
Other (expense) income
Interest Income	526	547
Interest Expense	(550)	(571)
(Losses) earnings of Subsidiaries	(86)	202
Other nonoperating income (expense), excluding interest income	(1)	(2)
Nonoperating Expense	(111)	176
Loss (income) before income taxes	(111)	176
Income tax benefit	0	0
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	(111)	176
Net Income (Loss) Attributable to Noncontrolling Interest	0	0
Net Income (Loss) Attributable to Parent	(111)	176
Other Comprehensive Income (Loss), Net of Tax	(22)	(4)
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	(133)	172
Non-Guarantor Subsidiaries [Member]
Condensed Income Statements, Captions [Line Items]
Net operating revenues	8,142	8,125
Revenue, equipment sales, net	1,220	1,173
Revenue, equipment rentals, net	1,359	1,212
Cost of Services (exclusive of depreciation and amortization included below)	1,710	1,677
Cost of equipment sales	1,341	1,270
Cost of equipment rentals	225	124
Net operating expenses
Selling, general and administrative	1,907	1,867
Depreciation, network and other	1,120	1,023
Depreciation, equipment rentals	1,029	1,136
Amortization	118	171
Other, net	237	42
Total costs and expenses	7,687	7,310
Operating Income	455	815
Other (expense) income
Interest Income	141	2
Interest Expense	(706)	(573)
(Losses) earnings of Subsidiaries	0	0
Other nonoperating income (expense), excluding interest income	(1)	2
Nonoperating Expense	(566)	(569)
Loss (income) before income taxes	(111)	246
Income tax benefit	22	(47)
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	(89)	199
Net Income (Loss) Attributable to Noncontrolling Interest	3	3
Net Income (Loss) Attributable to Parent	(86)	202
Other Comprehensive Income (Loss), Net of Tax	0	(13)
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	(89)	186
Services [Member]
Condensed Income Statements, Captions [Line Items]
Service revenue	5,563	5,740
Services [Member] | Consolidation, Eliminations [Member]
Condensed Income Statements, Captions [Line Items]
Service revenue	0	0
Services [Member] | Parent Company [Member]
Condensed Income Statements, Captions [Line Items]
Service revenue	0	0
Services [Member] | Guarantor Subsidiaries [Member]
Condensed Income Statements, Captions [Line Items]
Service revenue	0	0
Services [Member] | Non-Guarantor Subsidiaries [Member]
Condensed Income Statements, Captions [Line Items]
Service revenue	$ 5,563	$ 5,740