Condensed Consolidating Financial Information - Statements of Operations (Details) - USD ($) $ in Millions	2 Months Ended	3 Months Ended								10 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Oct. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
OPERATING REVENUE
Total operating revenue	$ 1,407	$ 2,061	$ 2,064	$ 2,014	$ 2,046	$ 2,071	$ 2,010	$ 2,052	$ 2,087	$ 6,870	$ 8,185	$ 8,220
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)	690									3,493	3,799	3,937
Selling, general and administrative	253									1,208	1,258	1,354
Operating expenses - affiliates	24									0	334	257
Depreciation and amortization	282									1,018	1,613	1,704
Goodwill impairment	0				3,708					0	3,708	0
Total operating expenses	1,249									5,719	10,712	7,252
OPERATING (LOSS) INCOME	158	285	309	272	(3,393)	284	227	196	261	1,151	(2,527)	968
OTHER (EXPENSE) INCOME
Interest income	1									13	9	4
Interest income - affiliate	11									0	61	63
Interest expense	(80)									(441)	(502)	(509)
Loss on modification and extinguishment of debt	0									(44)	5	0
Equity in net (losses) earnings of subsidiaries	0									0	0	0
Other (expense) income, net	3									14	8	11
Total other expense, net	(65)									(458)	(419)	(431)
(LOSS) INCOME BEFORE INCOME TAXES	93									693	(2,946)	537
Income tax expense	(234)									(268)	(255)	(196)
NET (LOSS) INCOME	(141)	$ 160	$ 114	$ 110	$ (3,585)	$ 151	$ 88	$ 40	$ 62	425	(3,201)	341
Other comprehensive loss, net of income taxes	18									80	(8)	(195)
COMPREHENSIVE (LOSS) INCOME	(123)									505	(3,209)	146
Eliminations
OPERATING REVENUE
Total operating revenue	(28)									(129)	(847)	(308)
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)	(28)									(129)	0	0
Selling, general and administrative	0									0	(845)	(308)
Operating expenses - affiliates	0										0	0
Depreciation and amortization	0									0	0	0
Goodwill impairment											0
Total operating expenses	(28)									(129)	(845)	(308)
OPERATING (LOSS) INCOME	0									0	(2)	0
OTHER (EXPENSE) INCOME
Interest income	0									0	0	0
Interest income - affiliate	0									0	1	0
Interest expense	0									0	0	0
Loss on modification and extinguishment of debt										0	0
Equity in net (losses) earnings of subsidiaries	771									2,261	13,891	5,047
Other (expense) income, net	0									0	0	0
Total other expense, net	771									2,261	13,892	5,047
(LOSS) INCOME BEFORE INCOME TAXES	771									2,261	13,890	5,047
Income tax expense	0									0	0	0
NET (LOSS) INCOME	771									2,261	13,890	5,047
Other comprehensive loss, net of income taxes	(18)									0	8	195
COMPREHENSIVE (LOSS) INCOME	753									2,261	13,898	5,242
Level 3 Parent, LLC | Reportable Legal Entities
OPERATING REVENUE
Total operating revenue	0									0	0	0
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)	0									0	0	0
Selling, general and administrative	1									4	(50)	12
Operating expenses - affiliates	0										0	0
Depreciation and amortization	0									0	0	0
Goodwill impairment											0
Total operating expenses	1									4	(50)	12
OPERATING (LOSS) INCOME	(1)									(4)	50	(12)
OTHER (EXPENSE) INCOME
Interest income	0									0	0	0
Interest income - affiliate	262									1,260	3,888	2,430
Interest expense	(5)									(30)	(29)	(33)
Loss on modification and extinguishment of debt										0	10
Equity in net (losses) earnings of subsidiaries	(827)									(815)	(7,109)	(2,044)
Other (expense) income, net	1									3	(8)	(9)
Total other expense, net	(569)									418	(3,248)	344
(LOSS) INCOME BEFORE INCOME TAXES	(570)									414	(3,198)	332
Income tax expense	429									11	(3)	10
NET (LOSS) INCOME	(141)									425	(3,201)	342
Other comprehensive loss, net of income taxes	18									80	(8)	(195)
COMPREHENSIVE (LOSS) INCOME	(123)									505	(3,209)	147
Level 3 Financing, Inc. | Reportable Legal Entities
OPERATING REVENUE
Total operating revenue	0									0	0	0
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)	0									0	0	0
Selling, general and administrative	3									3	5	3
Operating expenses - affiliates	0										0	0
Depreciation and amortization	0									0	0	0
Goodwill impairment											0
Total operating expenses	3									3	5	3
OPERATING (LOSS) INCOME	(3)									(3)	(5)	(3)
OTHER (EXPENSE) INCOME
Interest income	0									0	0	0
Interest income - affiliate	368									1,890	658	1,562
Interest expense	(72)									(397)	(468)	(457)
Loss on modification and extinguishment of debt										(44)	(5)
Equity in net (losses) earnings of subsidiaries	(15)									(2,138)	(7,352)	(3,257)
Other (expense) income, net	0									0	0	0
Total other expense, net	281									(689)	(7,167)	(2,152)
(LOSS) INCOME BEFORE INCOME TAXES	278									(692)	(7,172)	(2,155)
Income tax expense	(1,105)									(123)	63	111
NET (LOSS) INCOME	(827)									(815)	(7,109)	(2,044)
Other comprehensive loss, net of income taxes	0									0	0	0
COMPREHENSIVE (LOSS) INCOME	(827)									(815)	(7,109)	(2,044)
Level 3 Communications, LLC | Reportable Legal Entities
OPERATING REVENUE
Total operating revenue	764									3,108	4,138	4,014
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)	418									1,942	1,987	2,209
Selling, general and administrative	179									942	1,438	392
Operating expenses - affiliates	24										241	176
Depreciation and amortization	117									356	652	688
Goodwill impairment											1,369
Total operating expenses	738									3,240	5,687	3,465
OPERATING (LOSS) INCOME	26									(132)	(1,549)	549
OTHER (EXPENSE) INCOME
Interest income	1									12	7	3
Interest income - affiliate	(578)									(2,896)	(5,829)	(3,803)
Interest expense	0									(2)	13	(3)
Loss on modification and extinguishment of debt										0	0
Equity in net (losses) earnings of subsidiaries	71									692	570	254
Other (expense) income, net	2									15	8	1
Total other expense, net	(504)									(2,179)	(5,231)	(3,548)
(LOSS) INCOME BEFORE INCOME TAXES	(478)									(2,311)	(6,780)	(2,999)
Income tax expense	433									(2)	30	(232)
NET (LOSS) INCOME	(45)									(2,313)	(6,750)	(3,231)
Other comprehensive loss, net of income taxes	0									0	0	0
COMPREHENSIVE (LOSS) INCOME	(45)									(2,313)	(6,750)	(3,231)
Other Non-Guarantor Subsidiaries | Reportable Legal Entities
OPERATING REVENUE
Total operating revenue	671									3,891	4,894	4,514
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)	300									1,680	1,812	1,728
Selling, general and administrative	70									259	710	1,255
Operating expenses - affiliates	0										93	81
Depreciation and amortization	165									662	961	1,016
Goodwill impairment											2,339
Total operating expenses	535									2,601	5,915	4,080
OPERATING (LOSS) INCOME	136									1,290	(1,021)	434
OTHER (EXPENSE) INCOME
Interest income	0									1	2	1
Interest income - affiliate	(41)									(254)	1,343	(126)
Interest expense	(3)									(12)	(18)	(16)
Loss on modification and extinguishment of debt										0	0
Equity in net (losses) earnings of subsidiaries	0									0	0	0
Other (expense) income, net	0									(4)	8	19
Total other expense, net	(44)									(269)	1,335	(122)
(LOSS) INCOME BEFORE INCOME TAXES	92									1,021	314	312
Income tax expense	9									(154)	(345)	(85)
NET (LOSS) INCOME	101									867	(31)	227
Other comprehensive loss, net of income taxes	18									0	(8)	(195)
COMPREHENSIVE (LOSS) INCOME	119									867	(39)	32
Non-Affiliate Services
OPERATING REVENUE
Total operating revenue	1,391									6,870	8,005	8,113
Non-Affiliate Services | Eliminations
OPERATING REVENUE
Total operating revenue	(28)									(129)	0	0
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	3,912	3,884
Non-Affiliate Services | Other Non-Guarantor Subsidiaries | Reportable Legal Entities
OPERATING REVENUE
Total operating revenue	671									3,891	4,093	4,229
Affiliate revenue
OPERATING REVENUE
Total operating revenue	16									$ 0	180	107
Affiliate revenue | Eliminations
OPERATING REVENUE
Total operating revenue	0										(847)	(308)
Affiliate revenue | Level 3 Parent, LLC | Reportable Legal Entities
OPERATING REVENUE
Total operating revenue	0										0	0
Affiliate revenue | Level 3 Financing, Inc. | Reportable Legal Entities
OPERATING REVENUE
Total operating revenue	0										0	0
Affiliate revenue | Level 3 Communications, LLC | Reportable Legal Entities
OPERATING REVENUE
Total operating revenue	16										226	130
Affiliate revenue | Other Non-Guarantor Subsidiaries | Reportable Legal Entities
OPERATING REVENUE
Total operating revenue	$ 0										$ 801	$ 285