Condensed Consolidating Financial Information - Balance Sheets (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
CURRENT ASSETS
Cash and cash equivalents	$ 259	$ 297
Restricted cash and securities	5	5
Assets held for sale	140	140
Accounts receivable	726	748
Accounts receivable - affiliate	3	13
Intercompany advances	0	0
Note receivable - affiliate	1,825	1,825
Other	183	117
Total current assets	3,141	3,145
Property, plant, and equipment, net	9,542	9,412
Restricted cash and securities	29	29
Goodwill	11,141	10,837
Investment in subsidiaries	0	0
Deferred tax assets	458	426
Other, net	96	63
Total goodwill and other assets	20,291	20,549
TOTAL ASSETS	33,003	33,135
CURRENT LIABILITIES
Current maturities of long-term debt	9	8
Accounts payable	678	695
Accounts payable - affiliate	95	41
Income and other taxes	87	100
Salary and benefits	163	136
Interest	95	109
Current portion of deferred revenue	273	258
Current portion of deferred revenue - affiliate	2	2
Intercompany payables	0	0
Other	67	57
Total current liabilities	1,469	1,406
LONG-TERM DEBT	10,872	10,882
Deferred revenue	1,106	1,093
Deferred revenue - affiliate	6	6
Deferred tax liability	209	212
Other	321	264
Total deferred credits and other liabilities	1,642	1,575
COMMITMENTS AND CONTINGENCIES
Total member's equity	19,020	19,272
Total liabilities and member's equity	33,003	33,135
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	(43,283)	(42,483)
Note receivable - affiliate	0	0
Other	0	0
Total current assets	(43,283)	(42,534)
Property, plant, and equipment, net	0	0
Restricted cash and securities	0	0
Goodwill	0	0
Investment in subsidiaries	(39,352)	(38,973)
Deferred tax assets	(1,771)	(1,771)
Other, net	0	0
Total goodwill and other assets	(41,123)	(40,744)
TOTAL ASSETS	(84,406)	(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
Salary and benefits	0	0
Interest	0	0
Current portion of deferred revenue	0	0
Current portion of deferred revenue - affiliate	0	0
Intercompany payables	(43,283)	(42,483)
Other	0	0
Total current liabilities	(43,283)	(42,534)
LONG-TERM DEBT	0	0
Deferred revenue	0	0
Deferred revenue - affiliate	0	0
Deferred tax liability	(1,771)	(1,771)
Other	0	0
Total deferred credits and other liabilities	(1,771)	(1,771)
COMMITMENTS AND CONTINGENCIES
Total member's equity	(39,352)	(38,973)
Total liabilities and member's equity	(84,406)	(83,278)
Level 3 Communications, Inc. | Reportable Legal Entities
CURRENT ASSETS
Cash and cash equivalents	39	13
Restricted cash and securities	0	0
Assets held for sale	68	68
Accounts receivable	0	0
Accounts receivable - affiliate	0	0
Intercompany advances	16,493	16,251
Note receivable - affiliate	1,825	1,825
Other	0	0
Total current assets	18,425	18,157
Property, plant, and equipment, net	0	0
Restricted cash and securities	19	19
Goodwill	0	0
Investment in subsidiaries	16,955	16,954
Deferred tax assets	279	280
Other, net	0	0
Total goodwill and other assets	17,234	17,234
TOTAL ASSETS	35,678	35,410
CURRENT LIABILITIES
Current maturities of long-term debt	0	0
Accounts payable	0	0
Accounts payable - affiliate	27	11
Income and other taxes	0	0
Salary and benefits	0	0
Interest	3	11
Current portion of deferred revenue	0	0
Current portion of deferred revenue - affiliate	0	0
Intercompany payables	0	0
Other	0	16
Total current liabilities	30	38
LONG-TERM DEBT	615	616
Deferred revenue	0	0
Deferred revenue - affiliate	0	0
Deferred tax liability	648	648
Other	1	1
Total deferred credits and other liabilities	649	649
COMMITMENTS AND CONTINGENCIES
Total member's equity	34,384	34,107
Total liabilities and member's equity	35,678	35,410
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	21,422	21,032
Note receivable - affiliate	0	0
Other	0	0
Total current assets	21,422	21,032
Property, plant, and equipment, net	0	0
Restricted cash and securities	0	0
Goodwill	0	0
Investment in subsidiaries	18,788	18,403
Deferred tax assets	1,826	1,795
Other, net	0	0
Total goodwill and other assets	20,614	20,198
TOTAL ASSETS	42,036	41,230
CURRENT LIABILITIES
Current maturities of long-term debt	0	0
Accounts payable	1	1
Accounts payable - affiliate	0	0
Income and other taxes	1	0
Salary and benefits	0	0
Interest	85	91
Current portion of deferred revenue	0	0
Current portion of deferred revenue - affiliate	0	0
Intercompany payables	0	0
Other	0	0
Total current liabilities	87	92
LONG-TERM DEBT	10,088	10,096
Deferred revenue	0	0
Deferred revenue - affiliate	0	0
Deferred tax liability	0	0
Other	0	1
Total deferred credits and other liabilities	0	1
COMMITMENTS AND CONTINGENCIES
Total member's equity	31,861	31,041
Total liabilities and member's equity	42,036	41,230
Level 3 Communications, LLC | Reportable Legal Entities
CURRENT ASSETS
Cash and cash equivalents	133	175
Restricted cash and securities	1	1
Assets held for sale	5	5
Accounts receivable	10	26
Accounts receivable - affiliate	0	60
Intercompany advances	0	0
Note receivable - affiliate	0	0
Other	95	54
Total current assets	244	321
Property, plant, and equipment, net	3,103	3,237
Restricted cash and securities	10	10
Goodwill	1,658	1,200
Investment in subsidiaries	3,609	3,616
Deferred tax assets	114	0
Other, net	54	32
Total goodwill and other assets	9,804	9,550
TOTAL ASSETS	13,161	13,118
CURRENT LIABILITIES
Current maturities of long-term debt	2	2
Accounts payable	318	323
Accounts payable - affiliate	66	0
Income and other taxes	45	55
Salary and benefits	129	109
Interest	0	0
Current portion of deferred revenue	135	127
Current portion of deferred revenue - affiliate	2	2
Intercompany payables	43,283	42,483
Other	16	23
Total current liabilities	43,996	43,124
LONG-TERM DEBT	13	13
Deferred revenue	843	841
Deferred revenue - affiliate	5	5
Deferred tax liability	849	870
Other	174	98
Total deferred credits and other liabilities	1,871	1,814
COMMITMENTS AND CONTINGENCIES
Total member's equity	(32,719)	(31,833)
Total liabilities and member's equity	13,161	13,118
Other Non-Guarantor Subsidiaries | Reportable Legal Entities
CURRENT ASSETS
Cash and cash equivalents	87	109
Restricted cash and securities	4	4
Assets held for sale	67	67
Accounts receivable	716	722
Accounts receivable - affiliate	3	4
Intercompany advances	5,368	5,200
Note receivable - affiliate	0	0
Other	88	63
Total current assets	6,333	6,169
Property, plant, and equipment, net	6,439	6,175
Restricted cash and securities	0	0
Goodwill	9,483	9,637
Investment in subsidiaries	0	0
Deferred tax assets	10	122
Other, net	42	31
Total goodwill and other assets	13,762	14,311
TOTAL ASSETS	26,534	26,655
CURRENT LIABILITIES
Current maturities of long-term debt	7	6
Accounts payable	359	371
Accounts payable - affiliate	2	81
Income and other taxes	41	45
Salary and benefits	34	27
Interest	7	7
Current portion of deferred revenue	138	131
Current portion of deferred revenue - affiliate	0	0
Intercompany payables	0	0
Other	51	18
Total current liabilities	639	686
LONG-TERM DEBT	156	157
Deferred revenue	263	252
Deferred revenue - affiliate	1	1
Deferred tax liability	483	465
Other	146	164
Total deferred credits and other liabilities	893	882
COMMITMENTS AND CONTINGENCIES
Total member's equity	24,846	24,930
Total liabilities and member's equity	26,534	26,655
Customer Contracts And Relationships
CURRENT ASSETS
Customer relationships, net	8,206	8,845
Customer Contracts And Relationships | Eliminations
CURRENT ASSETS
Customer relationships, net	0	0
Customer Contracts And Relationships | Level 3 Communications, Inc. | Reportable Legal Entities
CURRENT ASSETS
Customer relationships, net	0	0
Customer Contracts And Relationships | Level 3 Financing, Inc. | Reportable Legal Entities
CURRENT ASSETS
Customer relationships, net	0	0
Customer Contracts And Relationships | Level 3 Communications, LLC | Reportable Legal Entities
CURRENT ASSETS
Customer relationships, net	3,979	4,324
Customer Contracts And Relationships | Other Non-Guarantor Subsidiaries | Reportable Legal Entities
CURRENT ASSETS
Customer relationships, net	4,227	4,521
Other Intangible Assets
CURRENT ASSETS
Customer relationships, net	390	378
Other Intangible Assets | Eliminations
CURRENT ASSETS
Customer relationships, net	0	0
Other Intangible Assets | Level 3 Communications, Inc. | Reportable Legal Entities
CURRENT ASSETS
Customer relationships, net	0	0
Other Intangible Assets | Level 3 Financing, Inc. | Reportable Legal Entities
CURRENT ASSETS
Customer relationships, net	0	0
Other Intangible Assets | Level 3 Communications, LLC | Reportable Legal Entities
CURRENT ASSETS
Customer relationships, net	390	378
Other Intangible Assets | Other Non-Guarantor Subsidiaries | Reportable Legal Entities
CURRENT ASSETS
Customer relationships, net	$ 0	$ 0