Condensed Consolidating Financial Information - Statements of Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Condensed Consolidating Financial Information
Operating revenues	$ 2,052		$ 4,139
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)	980		1,978
Selling, general and administrative	388		732
Operating expenses - affiliates	55		108
Depreciation and amortization	433		864
Total operating expenses	1,856		3,682
OPERATING INCOME	196		457
OTHER INCOME (EXPENSE)
Interest income	0		1
Interest income - affiliate	16		32
Interest expense	(124)		(244)
Interest income (expense) - intercompany, net	0		0
Equity in net earnings (losses) of subsidiaries	0		0
Loss on modification and extinguishment of debt	0		0
Other (expense) income, net	(4)		2
Total other expense, net	(112)		(209)
INCOME (LOSS) BEFORE INCOME TAXES	84		248
Income tax expense	(44)		(146)
NET INCOME	40		102
Other comprehensive (loss) income	(235)		(163)
COMPREHENSIVE INCOME (LOSS)	(195)		(61)
Eliminations
Condensed Consolidating Financial Information
Operating revenues	(43)		(83)
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)	(43)		(83)
Selling, general and administrative	0		0
Operating expenses - affiliates	0		0
Depreciation and amortization	0		0
Total operating expenses	(43)		(83)
OPERATING INCOME	0		0
OTHER INCOME (EXPENSE)
Interest income	(1)		(1)
Interest income - affiliate	0		0
Interest expense	1		1
Interest income (expense) - intercompany, net	0		0
Equity in net earnings (losses) of subsidiaries	1,148		2,303
Other (expense) income, net	0		0
Total other expense, net	1,148		2,303
INCOME (LOSS) BEFORE INCOME TAXES	1,148		2,303
Income tax expense	0		0
NET INCOME	1,148		2,303
Other comprehensive (loss) income	235		163
COMPREHENSIVE INCOME (LOSS)	1,383		2,466
Level 3 Communications, Inc. | Reportable Legal Entities
Condensed Consolidating Financial Information
Operating revenues	0		0
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)	0		0
Selling, general and administrative	0		0
Operating expenses - affiliates	0		0
Depreciation and amortization	0		0
Total operating expenses	0		0
OPERATING INCOME	0		0
OTHER INCOME (EXPENSE)
Interest income	0		0
Interest income - affiliate	16		32
Interest expense	(8)		(16)
Interest income (expense) - intercompany, net	348		703
Equity in net earnings (losses) of subsidiaries	(316)		(631)
Other (expense) income, net	0		0
Total other expense, net	40		88
INCOME (LOSS) BEFORE INCOME TAXES	40		88
Income tax expense	0		14
NET INCOME	40		102
Other comprehensive (loss) income	(235)		(163)
COMPREHENSIVE INCOME (LOSS)	(195)		(61)
Level 3 Financing, Inc. | Reportable Legal Entities
Condensed Consolidating Financial Information
Operating revenues	0		0
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)	0		0
Selling, general and administrative	2		0
Operating expenses - affiliates	0		3
Depreciation and amortization	0		0
Total operating expenses	2		3
OPERATING INCOME	(2)		(3)
OTHER INCOME (EXPENSE)
Interest income	0		0
Interest income - affiliate	0		0
Interest expense	(113)		(221)
Interest income (expense) - intercompany, net	604		1,212
Equity in net earnings (losses) of subsidiaries	(832)		(1,671)
Other (expense) income, net	0		0
Total other expense, net	(341)		(680)
INCOME (LOSS) BEFORE INCOME TAXES	(343)		(683)
Income tax expense	27		52
NET INCOME	(316)		(631)
Other comprehensive (loss) income	0		0
COMPREHENSIVE INCOME (LOSS)	(316)		(631)
Level 3 Communications, LLC | Reportable Legal Entities
Condensed Consolidating Financial Information
Operating revenues	983		1,964
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)	600		1,189
Selling, general and administrative	286		344
Operating expenses - affiliates	37		545
Depreciation and amortization	174		90
Total operating expenses	1,097		2,168
OPERATING INCOME	(114)		(204)
OTHER INCOME (EXPENSE)
Interest income	1		1
Interest income - affiliate	0		0
Interest expense	0		(1)
Interest income (expense) - intercompany, net	(878)		(1,759)
Equity in net earnings (losses) of subsidiaries	0		(1)
Other (expense) income, net	2		3
Total other expense, net	(875)		(1,757)
INCOME (LOSS) BEFORE INCOME TAXES	(989)		(1,961)
Income tax expense	13		(34)
NET INCOME	(976)		(1,995)
Other comprehensive (loss) income	0		0
COMPREHENSIVE INCOME (LOSS)	(976)		(1,995)
Other Non-Guarantor Subsidiaries | Reportable Legal Entities
Condensed Consolidating Financial Information
Operating revenues	1,112		2,258
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)	423		872
Selling, general and administrative	100		520
Operating expenses - affiliates	18		184
Depreciation and amortization	259		18
Total operating expenses	800		1,594
OPERATING INCOME	312		664
OTHER INCOME (EXPENSE)
Interest income	0		1
Interest income - affiliate	0		0
Interest expense	(4)		(7)
Interest income (expense) - intercompany, net	(74)		(156)
Equity in net earnings (losses) of subsidiaries	0		0
Other (expense) income, net	(6)		(1)
Total other expense, net	(84)		(163)
INCOME (LOSS) BEFORE INCOME TAXES	228		501
Income tax expense	(84)		(178)
NET INCOME	144		323
Other comprehensive (loss) income	(235)		(163)
COMPREHENSIVE INCOME (LOSS)	(91)		160
Predecessor
Condensed Consolidating Financial Information
Operating revenues		$ 2,062		$ 4,110
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)		1,035		2,086
Selling, general and administrative		367		731
Operating expenses - affiliates		0		0
Depreciation and amortization		307		603
Total operating expenses		1,709		3,420
OPERATING INCOME		353		690
OTHER INCOME (EXPENSE)
Interest income		3		5
Interest income - affiliate		0		0
Interest expense		(131)		(265)
Interest income (expense) - intercompany, net		0		0
Equity in net earnings (losses) of subsidiaries		0		0
Loss on modification and extinguishment of debt		0		(44)
Other (expense) income, net		(2)		2
Total other expense, net		(130)		(302)
INCOME (LOSS) BEFORE INCOME TAXES		223		388
Income tax expense		(69)		(139)
NET INCOME		154		249
Other comprehensive (loss) income		41		62
COMPREHENSIVE INCOME (LOSS)		195		311
Predecessor | Eliminations
Condensed Consolidating Financial Information
Operating revenues		(46)		(75)
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)		(46)		(75)
Selling, general and administrative		0		0
Operating expenses - affiliates		0		0
Depreciation and amortization		0		0
Total operating expenses		(46)		(75)
OPERATING INCOME		0		0
OTHER INCOME (EXPENSE)
Interest income		0		0
Interest income - affiliate		0		0
Interest expense		0		0
Interest income (expense) - intercompany, net		0		0
Equity in net earnings (losses) of subsidiaries		648		1,366
Loss on modification and extinguishment of debt				0
Other (expense) income, net		0		0
Total other expense, net		648		1,366
INCOME (LOSS) BEFORE INCOME TAXES		648		1,366
Income tax expense		0		0
NET INCOME		648		1,366
Other comprehensive (loss) income		(41)		(62)
COMPREHENSIVE INCOME (LOSS)		607		1,304
Predecessor | Level 3 Communications, Inc. | Reportable Legal Entities
Condensed Consolidating Financial Information
Operating revenues		0		0
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)		0		0
Selling, general and administrative		1		2
Operating expenses - affiliates		0		0
Depreciation and amortization		0		0
Total operating expenses		1		2
OPERATING INCOME		(1)		(2)
OTHER INCOME (EXPENSE)
Interest income		0		0
Interest income - affiliate		0		0
Interest expense		(9)		(18)
Interest income (expense) - intercompany, net		378		755
Equity in net earnings (losses) of subsidiaries		(216)		(491)
Loss on modification and extinguishment of debt				0
Other (expense) income, net		0		0
Total other expense, net		153		246
INCOME (LOSS) BEFORE INCOME TAXES		152		244
Income tax expense		2		5
NET INCOME		154		249
Other comprehensive (loss) income		41		62
COMPREHENSIVE INCOME (LOSS)		195		311
Predecessor | Level 3 Financing, Inc. | Reportable Legal Entities
Condensed Consolidating Financial Information
Operating revenues		0		0
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)		0		0
Selling, general and administrative		1		2
Operating expenses - affiliates		0		0
Depreciation and amortization		0		0
Total operating expenses		1		2
OPERATING INCOME		(1)		(2)
OTHER INCOME (EXPENSE)
Interest income		0		0
Interest income - affiliate		0		0
Interest expense		(117)		(237)
Interest income (expense) - intercompany, net		567		1,141
Equity in net earnings (losses) of subsidiaries		(632)		(1,278)
Loss on modification and extinguishment of debt				(44)
Other (expense) income, net		0		0
Total other expense, net		(182)		(418)
INCOME (LOSS) BEFORE INCOME TAXES		(183)		(420)
Income tax expense		(33)		(71)
NET INCOME		(216)		(491)
Other comprehensive (loss) income		0		0
COMPREHENSIVE INCOME (LOSS)		(216)		(491)
Predecessor | Level 3 Communications, LLC | Reportable Legal Entities
Condensed Consolidating Financial Information
Operating revenues		934		1,854
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)		602		1,185
Selling, general and administrative		294		571
Operating expenses - affiliates		0		0
Depreciation and amortization		92		179
Total operating expenses		988		1,935
OPERATING INCOME		(54)		(81)
OTHER INCOME (EXPENSE)
Interest income		3		5
Interest income - affiliate		0		0
Interest expense		(1)		(2)
Interest income (expense) - intercompany, net		(868)		(1,737)
Equity in net earnings (losses) of subsidiaries		200		403
Loss on modification and extinguishment of debt				0
Other (expense) income, net		(2)		3
Total other expense, net		(668)		(1,328)
INCOME (LOSS) BEFORE INCOME TAXES		(722)		(1,409)
Income tax expense		(1)		(2)
NET INCOME		(723)		(1,411)
Other comprehensive (loss) income		0		0
COMPREHENSIVE INCOME (LOSS)		(723)		(1,411)
Predecessor | Other Non-Guarantor Subsidiaries | Reportable Legal Entities
Condensed Consolidating Financial Information
Operating revenues		1,174		2,331
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)		479		976
Selling, general and administrative		71		156
Operating expenses - affiliates		0		0
Depreciation and amortization		215		424
Total operating expenses		765		1,556
OPERATING INCOME		409		775
OTHER INCOME (EXPENSE)
Interest income		0		0
Interest income - affiliate		0		0
Interest expense		(4)		(8)
Interest income (expense) - intercompany, net		(77)		(159)
Equity in net earnings (losses) of subsidiaries		0		0
Loss on modification and extinguishment of debt				0
Other (expense) income, net		0		(1)
Total other expense, net		(81)		(168)
INCOME (LOSS) BEFORE INCOME TAXES		328		607
Income tax expense		(37)		(71)
NET INCOME		291		536
Other comprehensive (loss) income		41		62
COMPREHENSIVE INCOME (LOSS)		332		598
Non-Affiliate Revenue
Condensed Consolidating Financial Information
Operating revenues	2,025		4,087
Non-Affiliate Revenue | Eliminations
Condensed Consolidating Financial Information
Operating revenues	(43)		(83)
Non-Affiliate Revenue | Level 3 Communications, Inc. | Reportable Legal Entities
Condensed Consolidating Financial Information
Operating revenues	0		0
Non-Affiliate Revenue | Level 3 Financing, Inc. | Reportable Legal Entities
Condensed Consolidating Financial Information
Operating revenues	0		0
Non-Affiliate Revenue | Level 3 Communications, LLC | Reportable Legal Entities
Condensed Consolidating Financial Information
Operating revenues	977		1,933
Non-Affiliate Revenue | Other Non-Guarantor Subsidiaries | Reportable Legal Entities
Condensed Consolidating Financial Information
Operating revenues	1,091		2,237
Non-Affiliate Revenue | Predecessor
Condensed Consolidating Financial Information
Operating revenues		2,062		4,110
Non-Affiliate Revenue | Predecessor | Eliminations
Condensed Consolidating Financial Information
Operating revenues		(46)		(75)
Non-Affiliate Revenue | Predecessor | Level 3 Communications, Inc. | Reportable Legal Entities
Condensed Consolidating Financial Information
Operating revenues		0		0
Non-Affiliate Revenue | Predecessor | Level 3 Financing, Inc. | Reportable Legal Entities
Condensed Consolidating Financial Information
Operating revenues		0		0
Non-Affiliate Revenue | Predecessor | Level 3 Communications, LLC | Reportable Legal Entities
Condensed Consolidating Financial Information
Operating revenues		934		1,854
Non-Affiliate Revenue | Predecessor | Other Non-Guarantor Subsidiaries | Reportable Legal Entities
Condensed Consolidating Financial Information
Operating revenues		1,174		2,331
Affiliate Revenues
Condensed Consolidating Financial Information
Operating revenues	27		52
Affiliate Revenues | Eliminations
Condensed Consolidating Financial Information
Operating revenues	0		0
Affiliate Revenues | Level 3 Communications, Inc. | Reportable Legal Entities
Condensed Consolidating Financial Information
Operating revenues	0		0
Affiliate Revenues | Level 3 Financing, Inc. | Reportable Legal Entities
Condensed Consolidating Financial Information
Operating revenues	0		0
Affiliate Revenues | Level 3 Communications, LLC | Reportable Legal Entities
Condensed Consolidating Financial Information
Operating revenues	6		31
Affiliate Revenues | Other Non-Guarantor Subsidiaries | Reportable Legal Entities
Condensed Consolidating Financial Information
Operating revenues	$ 21		$ 21
Affiliate Revenues | Predecessor
Condensed Consolidating Financial Information
Operating revenues		0		0
Affiliate Revenues | Predecessor | Eliminations
Condensed Consolidating Financial Information
Operating revenues		0		0
Affiliate Revenues | Predecessor | Level 3 Communications, Inc. | Reportable Legal Entities
Condensed Consolidating Financial Information
Operating revenues		0		0
Affiliate Revenues | Predecessor | Level 3 Financing, Inc. | Reportable Legal Entities
Condensed Consolidating Financial Information
Operating revenues		0		0
Affiliate Revenues | Predecessor | Level 3 Communications, LLC | Reportable Legal Entities
Condensed Consolidating Financial Information
Operating revenues		0		0
Affiliate Revenues | Predecessor | Other Non-Guarantor Subsidiaries | Reportable Legal Entities
Condensed Consolidating Financial Information
Operating revenues		$ 0		$ 0