Condensed Consolidating Financial Information - Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
CURRENT ASSETS
Cash and cash equivalents	$ 188	$ 297
Restricted cash and securities	3	5
Assets held for sale	15	140
Accounts receivable	715	748
Accounts receivable - affiliate	0	13
Intercompany advances	0	0
Note receivable - affiliate	1,825	1,825
Other	290	117
Total current assets	3,036	3,145
Property, plant, and equipment, net	9,274	9,412
Restricted cash and securities	25	29
GOODWILL AND OTHER ASSETS
Goodwill	11,132	10,837
Investment in subsidiaries	0	0
Deferred tax assets	463	426
Other, net	132	63
Total goodwill and other assets	19,929	20,549
TOTAL ASSETS	32,264	33,135
CURRENT LIABILITIES
Current maturities of long-term debt	6	8
Accounts payable	619	695
Accounts payable - affiliate	88	41
Income and other taxes	114	100
Salaries and benefits	233	136
Interest	96	109
Current portion of deferred revenue	288	260
Intercompany payables	0	0
Other	72	57
Total current liabilities	1,516	1,406
LONG-TERM DEBT	10,848	10,882
DEFERRED REVENUE AND OTHER LIABILITIES
Deferred revenue	1,181	1,099
Deferred income taxes	189	212
Other	358	264
Total deferred revenue and other liabilities	1,728	1,575
COMMITMENTS AND CONTINGENCIES (Note 9)
MEMBER'S EQUITY (DEFICIT)	18,172	19,272
TOTAL LIABILITIES AND MEMBER'S EQUITY	32,264	33,135
Customer Contracts And Relationships
GOODWILL AND OTHER ASSETS
Intangible assets, net	7,801	8,845
Other Intangible Assets
GOODWILL AND OTHER ASSETS
Intangible assets, net	401	378
Eliminations
CURRENT ASSETS
Cash and cash equivalents	0	0
Restricted cash and securities	0	0
Assets held for sale	0	0
Accounts receivable	0	0
Accounts receivable - affiliate	0	(51)
Intercompany advances	(50,834)	(42,483)
Note receivable - affiliate	0	0
Other	0	0
Total current assets	(50,834)	(42,534)
Property, plant, and equipment, net	0	0
Restricted cash and securities	0	0
GOODWILL AND OTHER ASSETS
Goodwill	0	0
Investment in subsidiaries	(38,989)	(38,973)
Deferred tax assets	(1,771)	(1,771)
Other, net	0	0
Total goodwill and other assets	(40,760)	(40,744)
TOTAL ASSETS	(91,594)	(83,278)
CURRENT LIABILITIES
Current maturities of long-term debt	0	0
Accounts payable	0	0
Accounts payable - affiliate	0	(51)
Income and other taxes	0	0
Salaries and benefits	0	0
Interest	0	0
Current portion of deferred revenue	0	0
Current portion of deferred revenue, affiliate	(50,834)
Intercompany payables	0	(42,483)
Other	0	0
Total current liabilities	(50,834)	(42,534)
LONG-TERM DEBT	0	0
DEFERRED REVENUE AND OTHER LIABILITIES
Deferred revenue	0	0
Deferred income taxes	(1,771)	(1,771)
Other	0	0
Total deferred revenue and other liabilities	(1,771)	(1,771)
COMMITMENTS AND CONTINGENCIES (Note 9)
MEMBER'S EQUITY (DEFICIT)	(38,989)	(38,973)
TOTAL LIABILITIES AND MEMBER'S EQUITY	(91,594)	(83,278)
Eliminations | Customer Contracts And Relationships
GOODWILL AND OTHER ASSETS
Intangible assets, net	0	0
Eliminations | Other Intangible Assets
GOODWILL AND OTHER ASSETS
Intangible assets, net	0	0
Level 3 Communications, Inc. | Reportable Legal Entities
CURRENT ASSETS
Cash and cash equivalents	36	13
Restricted cash and securities	0	0
Assets held for sale	0	68
Accounts receivable	0	0
Accounts receivable - affiliate	0	0
Intercompany advances	16,935	16,251
Note receivable - affiliate	1,825	1,825
Other	0	0
Total current assets	18,796	18,157
Property, plant, and equipment, net	0	0
Restricted cash and securities	15	19
GOODWILL AND OTHER ASSETS
Goodwill	0	0
Investment in subsidiaries	16,265	16,954
Deferred tax assets	283	280
Other, net	0	0
Total goodwill and other assets	16,548	17,234
TOTAL ASSETS	35,359	35,410
CURRENT LIABILITIES
Current maturities of long-term debt	0	0
Accounts payable	17	0
Accounts payable - affiliate	(4)	11
Income and other taxes	17	0
Salaries and benefits	0	0
Interest	3	11
Current portion of deferred revenue	0	0
Current portion of deferred revenue, affiliate	0
Intercompany payables	0	0
Other	0	16
Total current liabilities	33	38
LONG-TERM DEBT	614	616
DEFERRED REVENUE AND OTHER LIABILITIES
Deferred revenue	0	0
Deferred income taxes	651	648
Other	0	1
Total deferred revenue and other liabilities	651	649
COMMITMENTS AND CONTINGENCIES (Note 9)
MEMBER'S EQUITY (DEFICIT)	34,061	34,107
TOTAL LIABILITIES AND MEMBER'S EQUITY	35,359	35,410
Level 3 Communications, Inc. | Reportable Legal Entities | Customer Contracts And Relationships
GOODWILL AND OTHER ASSETS
Intangible assets, net	0	0
Level 3 Communications, Inc. | Reportable Legal Entities | Other Intangible Assets
GOODWILL AND OTHER ASSETS
Intangible assets, net	0	0
Level 3 Financing, Inc. | Reportable Legal Entities
CURRENT ASSETS
Cash and cash equivalents	0	0
Restricted cash and securities	0	0
Assets held for sale	0	0
Accounts receivable	0	0
Accounts receivable - affiliate	0	0
Intercompany advances	23,527	21,032
Note receivable - affiliate	0	0
Other	0	0
Total current assets	23,527	21,032
Property, plant, and equipment, net	0	0
Restricted cash and securities	0	0
GOODWILL AND OTHER ASSETS
Goodwill	0	0
Investment in subsidiaries	18,667	18,403
Deferred tax assets	1,902	1,795
Other, net	0	0
Total goodwill and other assets	20,569	20,198
TOTAL ASSETS	44,096	41,230
CURRENT LIABILITIES
Current maturities of long-term debt	0	0
Accounts payable	57	1
Accounts payable - affiliate	(3)	0
Income and other taxes	3	0
Salaries and benefits	0	0
Interest	87	91
Current portion of deferred revenue	0	0
Current portion of deferred revenue, affiliate	0
Intercompany payables	0	0
Other	1	0
Total current liabilities	145	92
LONG-TERM DEBT	10,075	10,096
DEFERRED REVENUE AND OTHER LIABILITIES
Deferred revenue	0	0
Deferred income taxes	19	0
Other	0	1
Total deferred revenue and other liabilities	19	1
COMMITMENTS AND CONTINGENCIES (Note 9)
MEMBER'S EQUITY (DEFICIT)	33,857	31,041
TOTAL LIABILITIES AND MEMBER'S EQUITY	44,096	41,230
Level 3 Financing, Inc. | Reportable Legal Entities | Customer Contracts And Relationships
GOODWILL AND OTHER ASSETS
Intangible assets, net	0	0
Level 3 Financing, Inc. | Reportable Legal Entities | Other Intangible Assets
GOODWILL AND OTHER ASSETS
Intangible assets, net	0	0
Level 3 Communications, LLC | Reportable Legal Entities
CURRENT ASSETS
Cash and cash equivalents	73	175
Restricted cash and securities	1	1
Assets held for sale	1	5
Accounts receivable	24	26
Accounts receivable - affiliate	0	60
Intercompany advances	(106)	0
Note receivable - affiliate	0	0
Other	122	54
Total current assets	115	321
Property, plant, and equipment, net	3,137	3,237
Restricted cash and securities	10	10
GOODWILL AND OTHER ASSETS
Goodwill	9,576	1,200
Investment in subsidiaries	4,057	3,616
Deferred tax assets	203	0
Other, net	75	32
Total goodwill and other assets	18,099	9,550
TOTAL ASSETS	21,361	13,118
CURRENT LIABILITIES
Current maturities of long-term debt	1	2
Accounts payable	215	323
Accounts payable - affiliate	93	0
Income and other taxes	59	55
Salaries and benefits	196	109
Interest	1	0
Current portion of deferred revenue	147	129
Current portion of deferred revenue, affiliate	45,320
Intercompany payables	0	42,483
Other	1	23
Total current liabilities	46,033	43,124
LONG-TERM DEBT	7	13
DEFERRED REVENUE AND OTHER LIABILITIES
Deferred revenue	962	846
Deferred income taxes	837	870
Other	170	98
Total deferred revenue and other liabilities	1,969	1,814
COMMITMENTS AND CONTINGENCIES (Note 9)
MEMBER'S EQUITY (DEFICIT)	(26,648)	(31,833)
TOTAL LIABILITIES AND MEMBER'S EQUITY	21,361	13,118
Level 3 Communications, LLC | Reportable Legal Entities | Customer Contracts And Relationships
GOODWILL AND OTHER ASSETS
Intangible assets, net	3,815	4,324
Level 3 Communications, LLC | Reportable Legal Entities | Other Intangible Assets
GOODWILL AND OTHER ASSETS
Intangible assets, net	373	378
Other Non-Guarantor Subsidiaries | Reportable Legal Entities
CURRENT ASSETS
Cash and cash equivalents	79	109
Restricted cash and securities	2	4
Assets held for sale	14	67
Accounts receivable	691	722
Accounts receivable - affiliate	0	4
Intercompany advances	10,478	5,200
Note receivable - affiliate	0	0
Other	168	63
Total current assets	11,432	6,169
Property, plant, and equipment, net	6,137	6,175
Restricted cash and securities	0	0
GOODWILL AND OTHER ASSETS
Goodwill	1,556	9,637
Investment in subsidiaries	0	0
Deferred tax assets	(154)	122
Other, net	57	31
Total goodwill and other assets	5,473	14,311
TOTAL ASSETS	23,042	26,655
CURRENT LIABILITIES
Current maturities of long-term debt	5	6
Accounts payable	330	371
Accounts payable - affiliate	2	81
Income and other taxes	35	45
Salaries and benefits	37	27
Interest	5	7
Current portion of deferred revenue	141	131
Current portion of deferred revenue, affiliate	5,514
Intercompany payables	0	0
Other	70	18
Total current liabilities	6,139	686
LONG-TERM DEBT	152	157
DEFERRED REVENUE AND OTHER LIABILITIES
Deferred revenue	219	253
Deferred income taxes	453	465
Other	188	164
Total deferred revenue and other liabilities	860	882
COMMITMENTS AND CONTINGENCIES (Note 9)
MEMBER'S EQUITY (DEFICIT)	15,891	24,930
TOTAL LIABILITIES AND MEMBER'S EQUITY	23,042	26,655
Other Non-Guarantor Subsidiaries | Reportable Legal Entities | Customer Contracts And Relationships
GOODWILL AND OTHER ASSETS
Intangible assets, net	3,986	4,521
Other Non-Guarantor Subsidiaries | Reportable Legal Entities | Other Intangible Assets
GOODWILL AND OTHER ASSETS
Intangible assets, net	$ 28	$ 0