Condensed Consolidating Financial Information - Statements of Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
OPERATING REVENUE
Operating revenues	$ 2,014,000		$ 2,052,000	$ 4,060,000	$ 4,139,000
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)	919,000		980,000	1,886,000	1,978,000
Selling, general and administrative	347,000		388,000	675,000	732,000
Operating expenses - affiliates	87,000		55,000	133,000	108,000
Depreciation and amortization	389,000		433,000	779,000	864,000
Goodwill impairment	0	$ 3,700,000	0	3,708,000	0
Total operating expenses	1,742,000		1,856,000	7,181,000	3,682,000
OPERATING INCOME (LOSS)	272,000		196,000	(3,121,000)	457,000
OTHER (EXPENSE) INCOME
Interest income - affiliate	16,000		16,000	32,000	32,000
Interest expense (income)	(130,000)		(124,000)	(261,000)	(244,000)
Interest income (expense) - intercompany, net	0		0	0	0
Equity in net (losses) earnings of subsidiaries	0		0	0	0
Other (expense) income, net	3,000		(4,000)	15,000	3,000
Total other (expense), net	(111,000)		(112,000)	(214,000)	(209,000)
INCOME (LOSS) BEFORE INCOME TAXES	161,000		84,000	(3,335,000)	248,000
Income tax expense (benefit)	51,000		44,000	140,000	146,000
NET INCOME (LOSS)	110,000		40,000	(3,475,000)	102,000
Other comprehensive loss	(8,000)		(235,000)	(5,000)	(163,000)
COMPREHENSIVE INCOME (LOSS)	102,000		(195,000)	(3,480,000)	(61,000)
Eliminations
OPERATING REVENUE
Operating revenues	(231,000)		(43,000)	(433,000)	(83,000)
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)	0		0	0	0
Selling, general and administrative	(231,000)		(43,000)	(433,000)	(83,000)
Operating expenses - affiliates	0		0	0	0
Depreciation and amortization	0		0	0	0
Goodwill impairment				0
Total operating expenses	(231,000)		(43,000)	(433,000)	(83,000)
OPERATING INCOME (LOSS)	0		0	0	0
OTHER (EXPENSE) INCOME
Interest income - affiliate	0		0	0	0
Interest expense (income)	0		1,000	0	1,000
Interest income (expense) - intercompany, net	1,000		0	1,000	0
Equity in net (losses) earnings of subsidiaries	1,205,000		1,148,000	12,114,000	2,303,000
Other (expense) income, net	0		(1,000)	0	(1,000)
Total other (expense), net	1,206,000		1,148,000	12,115,000	2,303,000
INCOME (LOSS) BEFORE INCOME TAXES	1,206,000		1,148,000	12,115,000	2,303,000
Income tax expense (benefit)	0		0	0	0
NET INCOME (LOSS)	1,206,000		1,148,000	12,115,000	2,303,000
Other comprehensive loss	8,000		235,000	5,000	163,000
COMPREHENSIVE INCOME (LOSS)	1,214,000		1,383,000	12,120,000	2,466,000
Level 3 Communications, Inc. | Reportable Legal Entities
OPERATING REVENUE
Operating revenues	0		0	0	0
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)	0		0	0	0
Selling, general and administrative	0		0	0	0
Operating expenses - affiliates	0		0	0	0
Depreciation and amortization	0		0	0	0
Goodwill impairment				0
Total operating expenses	0		0	0	0
OPERATING INCOME (LOSS)	0		0	0	0
OTHER (EXPENSE) INCOME
Interest income - affiliate	16,000		16,000	32,000	32,000
Interest expense (income)	(8,000)		(8,000)	(16,000)	(16,000)
Interest income (expense) - intercompany, net	948,000		348,000	1,881,000	703,000
Equity in net (losses) earnings of subsidiaries	(848,000)		(316,000)	(5,367,000)	(631,000)
Other (expense) income, net	6,000		0	(2,000)	0
Total other (expense), net	114,000		40,000	(3,472,000)	88,000
INCOME (LOSS) BEFORE INCOME TAXES	114,000		40,000	(3,472,000)	88,000
Income tax expense (benefit)	3,000		0	3,000	(14,000)
NET INCOME (LOSS)	111,000		40,000	(3,475,000)	102,000
Other comprehensive loss	(8,000)		(235,000)	(5,000)	(163,000)
COMPREHENSIVE INCOME (LOSS)	103,000		(195,000)	(3,480,000)	(61,000)
Level 3 Financing, Inc. | Reportable Legal Entities
OPERATING REVENUE
Operating revenues	0		0	0	0
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)	0		0	0	0
Selling, general and administrative	2,000		2,000	3,000	3,000
Operating expenses - affiliates	0		0	0	0
Depreciation and amortization	0		0	0	0
Goodwill impairment				0
Total operating expenses	2,000		2,000	3,000	3,000
OPERATING INCOME (LOSS)	(2,000)		(2,000)	(3,000)	(3,000)
OTHER (EXPENSE) INCOME
Interest income - affiliate	0		0	0	0
Interest expense (income)	(120,000)		(113,000)	(239,000)	(221,000)
Interest income (expense) - intercompany, net	162,000		604,000	326,000	1,212,000
Equity in net (losses) earnings of subsidiaries	(918,000)		(832,000)	(5,511,000)	(1,671,000)
Other (expense) income, net	0		0	0	0
Total other (expense), net	(876,000)		(341,000)	(5,424,000)	(680,000)
INCOME (LOSS) BEFORE INCOME TAXES	(878,000)		(343,000)	(5,427,000)	(683,000)
Income tax expense (benefit)	(30,000)		(27,000)	(60,000)	(52,000)
NET INCOME (LOSS)	(848,000)		(316,000)	(5,367,000)	(631,000)
Other comprehensive loss	0		0	0	0
COMPREHENSIVE INCOME (LOSS)	(848,000)		(316,000)	(5,367,000)	(631,000)
Level 3 Communications, LLC | Reportable Legal Entities
OPERATING REVENUE
Operating revenues	1,007,000		983,000	2,017,000	1,964,000
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)	464,000		600,000	968,000	1,189,000
Selling, general and administrative	393,000		286,000	762,000	545,000
Operating expenses - affiliates	65,000		37,000	87,000	90,000
Depreciation and amortization	163,000		174,000	308,000	344,000
Goodwill impairment	0			1,369,000
Total operating expenses	1,085,000		1,097,000	3,494,000	2,168,000
OPERATING INCOME (LOSS)	(78,000)		(114,000)	(1,477,000)	(204,000)
OTHER (EXPENSE) INCOME
Interest income - affiliate	0		0	0	0
Interest expense (income)	3,000		0	3,000	(1,000)
Interest income (expense) - intercompany, net	(1,786,000)		(878,000)	(3,546,000)	(1,759,000)
Equity in net (losses) earnings of subsidiaries	561,000		0	(1,236,000)	(1,000)
Other (expense) income, net	(5,000)		3,000	8,000	4,000
Total other (expense), net	(1,227,000)		(875,000)	(4,771,000)	(1,757,000)
INCOME (LOSS) BEFORE INCOME TAXES	(1,305,000)		(989,000)	(6,248,000)	(1,961,000)
Income tax expense (benefit)	(18,000)		(13,000)	0	34,000
NET INCOME (LOSS)	(1,287,000)		(976,000)	(6,248,000)	(1,995,000)
Other comprehensive loss	0		0	0	0
COMPREHENSIVE INCOME (LOSS)	(1,287,000)		(976,000)	(6,248,000)	(1,995,000)
Other Non-Guarantor Subsidiaries | Reportable Legal Entities
OPERATING REVENUE
Operating revenues	1,238,000		1,112,000	2,476,000	2,258,000
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)	455,000		380,000	918,000	789,000
Selling, general and administrative	183,000		143,000	343,000	267,000
Operating expenses - affiliates	22,000		18,000	46,000	18,000
Depreciation and amortization	226,000		259,000	471,000	520,000
Goodwill impairment	0			2,339,000
Total operating expenses	886,000		800,000	4,117,000	1,594,000
OPERATING INCOME (LOSS)	352,000		312,000	(1,641,000)	664,000
OTHER (EXPENSE) INCOME
Interest income - affiliate	0		0	0	0
Interest expense (income)	(5,000)		(4,000)	(9,000)	(7,000)
Interest income (expense) - intercompany, net	675,000		(74,000)	1,338,000	(156,000)
Equity in net (losses) earnings of subsidiaries	0		0	0	0
Other (expense) income, net	2,000		(6,000)	9,000	0
Total other (expense), net	672,000		(84,000)	1,338,000	(163,000)
INCOME (LOSS) BEFORE INCOME TAXES	1,024,000		228,000	(303,000)	501,000
Income tax expense (benefit)	96,000		84,000	197,000	178,000
NET INCOME (LOSS)	928,000		144,000	(500,000)	323,000
Other comprehensive loss	(8,000)		(235,000)	(5,000)	(163,000)
COMPREHENSIVE INCOME (LOSS)	920,000		(91,000)	(505,000)	160,000
Non-Affiliate Revenue
OPERATING REVENUE
Operating revenues	1,976,000		2,025,000	3,967,000	4,087,000
Non-Affiliate Revenue | Eliminations
OPERATING REVENUE
Operating revenues	0		0	0	0
Non-Affiliate Revenue | Level 3 Communications, Inc. | Reportable Legal Entities
OPERATING REVENUE
Operating revenues	0		0	0	0
Non-Affiliate Revenue | Level 3 Financing, Inc. | Reportable Legal Entities
OPERATING REVENUE
Operating revenues	0		0	0	0
Non-Affiliate Revenue | Level 3 Communications, LLC | Reportable Legal Entities
OPERATING REVENUE
Operating revenues	953,000		977,000	1,908,000	1,933,000
Non-Affiliate Revenue | Other Non-Guarantor Subsidiaries | Reportable Legal Entities
OPERATING REVENUE
Operating revenues	1,023,000		1,048,000	2,059,000	2,154,000
Affiliate Revenues
OPERATING REVENUE
Operating revenues	38,000		27,000	93,000	52,000
Affiliate Revenues | Eliminations
OPERATING REVENUE
Operating revenues	(231,000)		(43,000)	(433,000)	(83,000)
Affiliate Revenues | Level 3 Communications, Inc. | Reportable Legal Entities
OPERATING REVENUE
Operating revenues	0		0	0	0
Affiliate Revenues | Level 3 Financing, Inc. | Reportable Legal Entities
OPERATING REVENUE
Operating revenues	0		0	0	0
Affiliate Revenues | Level 3 Communications, LLC | Reportable Legal Entities
OPERATING REVENUE
Operating revenues	54,000		6,000	109,000	31,000
Affiliate Revenues | Other Non-Guarantor Subsidiaries | Reportable Legal Entities
OPERATING REVENUE
Operating revenues	$ 215,000		$ 64,000	$ 417,000	$ 104,000