Condensed Consolidating Financial Information - Balance Sheets (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
CURRENT ASSETS
Cash and cash equivalents	$ 228	$ 243	$ 282
Restricted cash	3	4	5
Accounts receivable	771	712
Intercompany advances	0	0
Note receivable - affiliate	1,825	1,825
Other	311	234
Total current assets	3,138	3,018
Property, plant, and equipment, net	9,654	9,453
GOODWILL AND OTHER ASSETS
Goodwill	7,408	11,119
Operating lease assets	1,183
Restricted cash	22	25	25
Investment in subsidiaries	0	0
Other, net	627	699
Total goodwill and other assets	16,891	19,820
TOTAL ASSETS	29,683	32,291
CURRENT LIABILITIES
Current maturities of long-term debt	7	6
Accounts payable	777	726
Accounts payable - affiliates	458	246
Salaries and benefits	197	233
Income and other taxes	117	130
Current operating lease liabilities	269
Interest	91	95
Intercompany payables	0	0
Other	67	78
Current portion of deferred revenue	297	310
Total current liabilities	2,280	1,824
LONG-TERM DEBT	10,820	10,838
DEFERRED REVENUE AND OTHER LIABILITIES
Deferred revenue	1,236	1,181
Deferred income taxes, net	257	202
Noncurrent operating lease liabilities	962
Other	296	369
Total deferred revenue and other liabilities	2,751	1,752
MEMBER'S EQUITY (DEFICIT)	13,832	17,877	$ 18,610
TOTAL LIABILITIES AND MEMBER'S EQUITY	29,683	32,291
Customer Contracts And Relationships
GOODWILL AND OTHER ASSETS
Intangible assets, net	7,219	7,567
Other Intangible Assets
GOODWILL AND OTHER ASSETS
Intangible assets, net	432	410
Eliminations
CURRENT ASSETS
Cash and cash equivalents	0	0
Restricted cash	0	0
Accounts receivable	0	0
Intercompany advances	(52,981)	(51,260)
Note receivable - affiliate	0	0
Other	0	0
Total current assets	(52,981)	(51,260)
Property, plant, and equipment, net	0	0
GOODWILL AND OTHER ASSETS
Goodwill	0	0
Operating lease assets	(533)
Restricted cash	0	0
Investment in subsidiaries	(25,204)	(37,317)
Other, net	(1,390)	(1,332)
Total goodwill and other assets	(27,127)	(38,649)
TOTAL ASSETS	(80,108)	(89,909)
CURRENT LIABILITIES
Current maturities of long-term debt	0	0
Accounts payable	0	0
Accounts payable - affiliates	0	0
Salaries and benefits	0	0
Income and other taxes	0	0
Current operating lease liabilities	(93)
Interest	0	0
Intercompany payables	(52,980)	(51,260)
Other	0	0
Current portion of deferred revenue	0	0
Total current liabilities	(53,073)	(51,260)
LONG-TERM DEBT	0	0
DEFERRED REVENUE AND OTHER LIABILITIES
Deferred revenue	0	0
Deferred income taxes, net	(1,390)	(1,332)
Noncurrent operating lease liabilities	(440)
Other	0	0
Total deferred revenue and other liabilities	(1,830)	(1,332)
MEMBER'S EQUITY (DEFICIT)	(25,205)	(37,317)
TOTAL LIABILITIES AND MEMBER'S EQUITY	(80,108)	(89,909)
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	3	2
Restricted cash	0	0
Accounts receivable	0	0
Intercompany advances	18,195	16,852
Note receivable - affiliate	1,825	1,825
Other	0	1
Total current assets	20,023	18,680
Property, plant, and equipment, net	0	0
GOODWILL AND OTHER ASSETS
Goodwill	0	0
Operating lease assets	0
Restricted cash	12	15
Investment in subsidiaries	10,175	15,541
Other, net	271	275
Total goodwill and other assets	10,458	15,831
TOTAL ASSETS	30,481	34,511
CURRENT LIABILITIES
Current maturities of long-term debt	0	0
Accounts payable	0	0
Accounts payable - affiliates	80	62
Salaries and benefits	0	0
Income and other taxes	0	0
Current operating lease liabilities	0
Interest	12	11
Intercompany payables	0	0
Other	0	3
Current portion of deferred revenue	0	0
Total current liabilities	92	76
LONG-TERM DEBT	612	613
DEFERRED REVENUE AND OTHER LIABILITIES
Deferred revenue	0	0
Deferred income taxes, net	56	56
Noncurrent operating lease liabilities	0
Other	0	0
Total deferred revenue and other liabilities	56	56
MEMBER'S EQUITY (DEFICIT)	29,721	33,766
TOTAL LIABILITIES AND MEMBER'S EQUITY	30,481	34,511
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	0	0
Accounts receivable	0	0
Intercompany advances	24,063	23,957
Note receivable - affiliate	0	0
Other	0	3
Total current assets	24,063	23,960
Property, plant, and equipment, net	0	0
GOODWILL AND OTHER ASSETS
Goodwill	0	0
Operating lease assets	0
Restricted cash	0	0
Investment in subsidiaries	12,404	17,915
Other, net	1,482	1,421
Total goodwill and other assets	13,886	19,336
TOTAL ASSETS	37,949	43,296
CURRENT LIABILITIES
Current maturities of long-term debt	0	0
Accounts payable	31	0
Accounts payable - affiliates	16	11
Salaries and benefits	0	0
Income and other taxes	6	4
Current operating lease liabilities	0
Interest	75	78
Intercompany payables	0	0
Other	0	1
Current portion of deferred revenue	0	0
Total current liabilities	128	94
LONG-TERM DEBT	10,054	10,068
DEFERRED REVENUE AND OTHER LIABILITIES
Deferred revenue	0	0
Deferred income taxes, net	0	0
Noncurrent operating lease liabilities	0
Other	0	0
Total deferred revenue and other liabilities	0	0
MEMBER'S EQUITY (DEFICIT)	27,767	33,134
TOTAL LIABILITIES AND MEMBER'S EQUITY	37,949	43,296
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	157	164
Restricted cash	0	0
Accounts receivable	103	70
Intercompany advances	7,783	7,744
Note receivable - affiliate	0	0
Other	137	97
Total current assets	8,180	8,075
Property, plant, and equipment, net	3,384	3,136
GOODWILL AND OTHER ASSETS
Goodwill	423	1,665
Operating lease assets	1,302
Restricted cash	8	9
Investment in subsidiaries	2,625	3,861
Other, net	118	110
Total goodwill and other assets	8,451	9,877
TOTAL ASSETS	20,015	21,088
CURRENT LIABILITIES
Current maturities of long-term debt	0	1
Accounts payable	387	380
Accounts payable - affiliates	371	162
Salaries and benefits	160	189
Income and other taxes	39	72
Current operating lease liabilities	272
Interest	1	1
Intercompany payables	49,046	45,347
Other	4	8
Current portion of deferred revenue	140	168
Total current liabilities	50,420	46,328
LONG-TERM DEBT	6	7
DEFERRED REVENUE AND OTHER LIABILITIES
Deferred revenue	1,044	971
Deferred income taxes, net	817	841
Noncurrent operating lease liabilities	1,077
Other	152	197
Total deferred revenue and other liabilities	3,090	2,009
MEMBER'S EQUITY (DEFICIT)	(33,501)	(27,256)
TOTAL LIABILITIES AND MEMBER'S EQUITY	20,015	21,088
Level 3 Communications, LLC | Reportable Legal Entities | Customer Contracts And Relationships
GOODWILL AND OTHER ASSETS
Intangible assets, net	3,546	3,823
Level 3 Communications, LLC | Reportable Legal Entities | Other Intangible Assets
GOODWILL AND OTHER ASSETS
Intangible assets, net	429	409
Other Non-Guarantor Subsidiaries | Reportable Legal Entities
CURRENT ASSETS
Cash and cash equivalents	68	77
Restricted cash	3	4
Accounts receivable	668	642
Intercompany advances	2,940	2,707
Note receivable - affiliate	0	0
Other	174	133
Total current assets	3,853	3,563
Property, plant, and equipment, net	6,270	6,317
GOODWILL AND OTHER ASSETS
Goodwill	6,985	9,454
Operating lease assets	414
Restricted cash	2	1
Investment in subsidiaries	0	0
Other, net	146	225
Total goodwill and other assets	11,223	13,425
TOTAL ASSETS	21,346	23,305
CURRENT LIABILITIES
Current maturities of long-term debt	7	5
Accounts payable	359	346
Accounts payable - affiliates	(9)	11
Salaries and benefits	37	44
Income and other taxes	72	54
Current operating lease liabilities	90
Interest	3	5
Intercompany payables	3,934	5,913
Other	63	66
Current portion of deferred revenue	157	142
Total current liabilities	4,713	6,586
LONG-TERM DEBT	148	150
DEFERRED REVENUE AND OTHER LIABILITIES
Deferred revenue	192	210
Deferred income taxes, net	774	637
Noncurrent operating lease liabilities	325
Other	144	172
Total deferred revenue and other liabilities	1,435	1,019
MEMBER'S EQUITY (DEFICIT)	15,050	15,550
TOTAL LIABILITIES AND MEMBER'S EQUITY	21,346	23,305
Other Non-Guarantor Subsidiaries | Reportable Legal Entities | Customer Contracts And Relationships
GOODWILL AND OTHER ASSETS
Intangible assets, net	3,673	3,744
Other Non-Guarantor Subsidiaries | Reportable Legal Entities | Other Intangible Assets
GOODWILL AND OTHER ASSETS
Intangible assets, net	$ 3	$ 1