Condensed Consolidating Financial Information - Statements of Operations (Details) - USD ($) $ in Millions	1 Months Ended	2 Months Ended	3 Months Ended							10 Months Ended	12 Months Ended
	Oct. 31, 2017	Dec. 31, 2017	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Oct. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
OPERATING REVENUE
Total operating revenue		$ 1,407	$ 2,071	$ 2,010	$ 2,052	$ 2,087				$ 6,870	$ 8,220	$ 8,277	$ 8,173
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)		690									3,937
Selling, general and administrative		253									1,354
Operating expenses - affiliates		24									257
Depreciation and amortization		282									1,704
Total operating expenses		1,249									7,252
OPERATING INCOME		158	284	227	196	261					968	1,309
OTHER (EXPENSE) INCOME
Interest income		1									0
Interest income - affiliate		11									67
Interest expense		(80)									(509)
Loss on modification and extinguishment of debt		0									0
Equity in net (losses) earnings of subsidiaries		0									0
Other (expense) income, net		3									11
Total other expense, net		(65)									(431)
INCOME BEFORE INCOME TAX EXPENSE		93									537
Income tax expense		(234)									(196)
NET INCOME (LOSS)		(141)	$ 151	$ 88	$ 40	$ 62					341	$ 284
Other comprehensive loss, net of income taxes		18									(195)
COMPREHENSIVE INCOME (LOSS)		(123)									146
Predecessor
OPERATING REVENUE
Total operating revenue	$ 701						$ 2,059	$ 2,062	$ 2,048	6,870			8,173
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)										3,493			4,162
Selling, general and administrative										1,208			1,407
Operating expenses - affiliates										0			0
Depreciation and amortization										1,018			1,159
Total operating expenses										5,719			6,728
OPERATING INCOME	112						349	353	337	1,151			1,445
OTHER (EXPENSE) INCOME
Interest income										13			4
Interest income - affiliate										0			0
Interest expense										(441)			(544)
Loss on modification and extinguishment of debt										(44)			(40)
Equity in net (losses) earnings of subsidiaries										0			0
Other (expense) income, net										14			(63)
Total other expense, net										(458)			(603)
INCOME BEFORE INCOME TAX EXPENSE										693			842
Income tax expense										(268)			(165)
NET INCOME (LOSS)	$ 19						$ 157	$ 154	$ 95	425			677
Other comprehensive loss, net of income taxes										80			(86)
COMPREHENSIVE INCOME (LOSS)										505			591
Eliminations
OPERATING REVENUE
Total operating revenue		(28)									(308)
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)		(28)									0
Selling, general and administrative		0									(308)
Operating expenses - affiliates		0									0
Depreciation and amortization		0									0
Total operating expenses		(28)									(308)
OPERATING INCOME		0									0
OTHER (EXPENSE) INCOME
Interest income		0
Interest income - affiliate		0									0
Interest expense		0									0
Equity in net (losses) earnings of subsidiaries		771									5,047
Other (expense) income, net		0									0
Total other expense, net		771									5,047
INCOME BEFORE INCOME TAX EXPENSE		771									5,047
Income tax expense		0									0
NET INCOME (LOSS)		771									5,047
Other comprehensive loss, net of income taxes		(18)									195
COMPREHENSIVE INCOME (LOSS)		753									5,242
Eliminations | Predecessor
OPERATING REVENUE
Total operating revenue										(129)			(132)
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)										(129)			(132)
Selling, general and administrative										0			0
Depreciation and amortization										0			0
Total operating expenses										(129)			(132)
OPERATING INCOME										0			0
OTHER (EXPENSE) INCOME
Interest income										0			0
Interest income - affiliate										0			0
Interest expense										0			0
Loss on modification and extinguishment of debt										0
Equity in net (losses) earnings of subsidiaries										2,261			1,945
Other (expense) income, net										0			0
Total other expense, net										2,261			1,945
INCOME BEFORE INCOME TAX EXPENSE										2,261			1,945
Income tax expense										0			0
NET INCOME (LOSS)										2,261			1,945
Other comprehensive loss, net of income taxes										0			86
COMPREHENSIVE INCOME (LOSS)										2,261			2,031
Level 3 Parent, LLC | Reportable Legal Entities
OPERATING REVENUE
Total operating revenue		0									0
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)		0									0
Selling, general and administrative		1									12
Operating expenses - affiliates		0									0
Depreciation and amortization		0									0
Total operating expenses		1									12
OPERATING INCOME		(1)									(12)
OTHER (EXPENSE) INCOME
Interest income		0
Interest income - affiliate		262									2,430
Interest expense		(5)									(33)
Equity in net (losses) earnings of subsidiaries		(827)									(2,044)
Other (expense) income, net		1									(9)
Total other expense, net		(569)									344
INCOME BEFORE INCOME TAX EXPENSE		(570)									332
Income tax expense		429									10
NET INCOME (LOSS)		(141)									342
Other comprehensive loss, net of income taxes		18									(195)
COMPREHENSIVE INCOME (LOSS)		(123)									147
Level 3 Parent, LLC | Reportable Legal Entities | Predecessor
OPERATING REVENUE
Total operating revenue										0			0
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)										0			0
Selling, general and administrative										4			16
Depreciation and amortization										0			0
Total operating expenses										4			16
OPERATING INCOME										(4)			(16)
OTHER (EXPENSE) INCOME
Interest income										0			0
Interest income - affiliate										1,260			1,385
Interest expense										(30)			(36)
Loss on modification and extinguishment of debt										0
Equity in net (losses) earnings of subsidiaries										(815)			(669)
Other (expense) income, net										3			(1)
Total other expense, net										418			679
INCOME BEFORE INCOME TAX EXPENSE										414			663
Income tax expense										11			14
NET INCOME (LOSS)										425			677
Other comprehensive loss, net of income taxes										80			(86)
COMPREHENSIVE INCOME (LOSS)										505			591
Level 3 Financing, Inc. | Reportable Legal Entities
OPERATING REVENUE
Total operating revenue		0									0
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)		0									0
Selling, general and administrative		3									3
Operating expenses - affiliates		0									0
Depreciation and amortization		0									0
Total operating expenses		3									3
OPERATING INCOME		(3)									(3)
OTHER (EXPENSE) INCOME
Interest income		0
Interest income - affiliate		368									1,562
Interest expense		(72)									(457)
Equity in net (losses) earnings of subsidiaries		(15)									(3,257)
Other (expense) income, net		0									0
Total other expense, net		281									(2,152)
INCOME BEFORE INCOME TAX EXPENSE		278									(2,155)
Income tax expense		(1,105)									111
NET INCOME (LOSS)		(827)									(2,044)
Other comprehensive loss, net of income taxes		0									0
COMPREHENSIVE INCOME (LOSS)		(827)									(2,044)
Level 3 Financing, Inc. | Reportable Legal Entities | Predecessor
OPERATING REVENUE
Total operating revenue										0			0
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)										0			0
Selling, general and administrative										3			5
Depreciation and amortization										0			0
Total operating expenses										3			5
OPERATING INCOME										(3)			(5)
OTHER (EXPENSE) INCOME
Interest income										0			0
Interest income - affiliate										1,890			2,113
Interest expense										(397)			(505)
Loss on modification and extinguishment of debt										(44)
Equity in net (losses) earnings of subsidiaries										(2,138)			(2,033)
Other (expense) income, net										0			(39)
Total other expense, net										(689)			(464)
INCOME BEFORE INCOME TAX EXPENSE										(692)			(469)
Income tax expense										(123)			(200)
NET INCOME (LOSS)										(815)			(669)
Other comprehensive loss, net of income taxes										0			0
COMPREHENSIVE INCOME (LOSS)										(815)			(669)
Level 3 Communications, LLC | Reportable Legal Entities
OPERATING REVENUE
Total operating revenue		764									4,014
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)		418									2,209
Selling, general and administrative		179									392
Operating expenses - affiliates		24									176
Depreciation and amortization		117									688
Total operating expenses		738									3,465
OPERATING INCOME		26									549
OTHER (EXPENSE) INCOME
Interest income		1
Interest income - affiliate		(578)									(3,800)
Interest expense		0									(3)
Equity in net (losses) earnings of subsidiaries		71									254
Other (expense) income, net		2									1
Total other expense, net		(504)									(3,548)
INCOME BEFORE INCOME TAX EXPENSE		(478)									(2,999)
Income tax expense		433									(232)
NET INCOME (LOSS)		(45)									(3,231)
Other comprehensive loss, net of income taxes		0									0
COMPREHENSIVE INCOME (LOSS)		(45)									(3,231)
Level 3 Communications, LLC | Reportable Legal Entities | Predecessor
OPERATING REVENUE
Total operating revenue										3,108			3,558
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)										1,942			2,249
Selling, general and administrative										942			1,025
Depreciation and amortization										356			372
Total operating expenses										3,240			3,646
OPERATING INCOME										(132)			(88)
OTHER (EXPENSE) INCOME
Interest income										12			3
Interest income - affiliate										(2,896)			(3,215)
Interest expense										(2)			(2)
Loss on modification and extinguishment of debt										0
Equity in net (losses) earnings of subsidiaries										692			757
Other (expense) income, net										15			2
Total other expense, net										(2,179)			(2,455)
INCOME BEFORE INCOME TAX EXPENSE										(2,311)			(2,543)
Income tax expense										(2)			(2)
NET INCOME (LOSS)										(2,313)			(2,545)
Other comprehensive loss, net of income taxes										0			0
COMPREHENSIVE INCOME (LOSS)										(2,313)			(2,545)
Other Non-Guarantor Subsidiaries | Reportable Legal Entities
OPERATING REVENUE
Total operating revenue		671									4,514
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)		300									1,728
Selling, general and administrative		70									1,255
Operating expenses - affiliates		0									81
Depreciation and amortization		165									1,016
Total operating expenses		535									4,080
OPERATING INCOME		136									434
OTHER (EXPENSE) INCOME
Interest income		0
Interest income - affiliate		(41)									(125)
Interest expense		(3)									(16)
Equity in net (losses) earnings of subsidiaries		0									0
Other (expense) income, net		0									19
Total other expense, net		(44)									(122)
INCOME BEFORE INCOME TAX EXPENSE		92									312
Income tax expense		9									(85)
NET INCOME (LOSS)		101									227
Other comprehensive loss, net of income taxes		18									(195)
COMPREHENSIVE INCOME (LOSS)		119									32
Other Non-Guarantor Subsidiaries | Reportable Legal Entities | Predecessor
OPERATING REVENUE
Total operating revenue										3,891			4,747
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)										1,680			2,045
Selling, general and administrative										259			361
Depreciation and amortization										662			787
Total operating expenses										2,601			3,193
OPERATING INCOME										1,290			1,554
OTHER (EXPENSE) INCOME
Interest income										1			1
Interest income - affiliate										(254)			(283)
Interest expense										(12)			(1)
Loss on modification and extinguishment of debt										0
Equity in net (losses) earnings of subsidiaries										0			0
Other (expense) income, net										(4)			(25)
Total other expense, net										(269)			(308)
INCOME BEFORE INCOME TAX EXPENSE										1,021			1,246
Income tax expense										(154)			23
NET INCOME (LOSS)										867			1,269
Other comprehensive loss, net of income taxes										0			(86)
COMPREHENSIVE INCOME (LOSS)										867			1,183
Non-Affiliate Services
OPERATING REVENUE
Total operating revenue		1,391								6,870	8,113		8,173
Non-Affiliate Services | Predecessor
OPERATING REVENUE
Total operating revenue										6,870			8,173
Non-Affiliate Services | Eliminations
OPERATING REVENUE
Total operating revenue		(28)								(129)	0		(132)
Non-Affiliate Services | Level 3 Parent, LLC | Reportable Legal Entities
OPERATING REVENUE
Total operating revenue		0								0	0		0
Non-Affiliate Services | Level 3 Financing, Inc. | Reportable Legal Entities
OPERATING REVENUE
Total operating revenue		0								0	0		0
Non-Affiliate Services | Level 3 Communications, LLC | Reportable Legal Entities
OPERATING REVENUE
Total operating revenue		748								3,108	3,884		3,558
Non-Affiliate Services | Other Non-Guarantor Subsidiaries | Reportable Legal Entities
OPERATING REVENUE
Total operating revenue		671								3,891	4,229		4,747
Affiliate revenue
OPERATING REVENUE
Total operating revenue		16								0	107		0
Affiliate revenue | Predecessor
OPERATING REVENUE
Total operating revenue										$ 0			$ 0
Affiliate revenue | Eliminations
OPERATING REVENUE
Total operating revenue		0									(308)
Affiliate revenue | Level 3 Parent, LLC | Reportable Legal Entities
OPERATING REVENUE
Total operating revenue		0									0
Affiliate revenue | Level 3 Financing, Inc. | Reportable Legal Entities
OPERATING REVENUE
Total operating revenue		0									0
Affiliate revenue | Level 3 Communications, LLC | Reportable Legal Entities
OPERATING REVENUE
Total operating revenue		16									130
Affiliate revenue | Other Non-Guarantor Subsidiaries | Reportable Legal Entities
OPERATING REVENUE
Total operating revenue		$ 0									$ 285