Condensed Consolidating Financial Information - Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Oct. 31, 2017
CURRENT ASSETS
Cash and cash equivalents	$ 243	$ 297
Restricted cash and securities - current	4	5
Accounts receivable	712	748
Accounts receivable - affiliates	0	13
Assets held for sale	0	140
Advances to affiliates	0	0
Note receivable - affiliate	1,825	1,825
Other	234	117
Total current assets	3,018	3,145
NET PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment	10,474	9,555
Accumulated depreciation	(1,021)	(143)
Net property, plant and equipment	9,453	9,412
GOODWILL AND OTHER ASSETS
Goodwill	11,119	10,837	$ 10,821
Restricted cash and securities	25	29
Customer relationships, net	7,567	8,845
Other intangible assets, net	410	378
Investment in subsidiaries	0	0
Other, net	699	489
Total goodwill and other assets	19,820	20,578
TOTAL ASSETS	32,291	33,135
CURRENT LIABILITIES
Current maturities of long-term debt	6	8
Accounts payable	726	695
Accounts payable - affiliates	246	41
Accrued expenses and other liabilities
Salaries and benefits	233	136
Income and other taxes	130	100
Interest	95	109
Other	78	59
Advance billings and customer deposits	310	258
Due to affiliates	0	0
Total current liabilities	1,824	1,406
LONG-TERM DEBT	10,838	10,882
DEFERRED REVENUE AND OTHER LIABILITES
Deferred revenue	1,181	1,093
Deferred tax liability	202	212
Other	369	270
Total deferred revenue and other liabilities	1,752	1,575
COMMITMENTS AND CONTINGENCIES
MEMBER'S EQUITY (DEFICIT)	17,877	19,272
TOTAL LIABILITIES AND MEMBER'S EQUITY	32,291	33,135
Eliminations
CURRENT ASSETS
Cash and cash equivalents	0	0
Restricted cash and securities - current	0	0
Accounts receivable	0	0
Accounts receivable - affiliates		(51)
Assets held for sale		0
Advances to affiliates	(51,260)	(42,483)
Note receivable - affiliate	0	0
Other	0	0
Total current assets	(51,260)	(42,534)
NET PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment	0	0
Accumulated depreciation	0	0
Net property, plant and equipment	0	0
GOODWILL AND OTHER ASSETS
Goodwill	0	0
Restricted cash and securities	0	0
Customer relationships, net	0	0
Other intangible assets, net	0	0
Investment in subsidiaries	(37,317)	(38,973)
Other, net	(1,332)	(1,771)
Total goodwill and other assets	(38,649)	(40,744)
TOTAL ASSETS	(89,909)	(83,278)
CURRENT LIABILITIES
Current maturities of long-term debt	0	0
Accounts payable	0	0
Accounts payable - affiliates	0	(51)
Accrued expenses and other liabilities
Salaries and benefits	0	0
Income and other taxes	0	0
Interest	0	0
Other	0	0
Advance billings and customer deposits	0	0
Due to affiliates	(51,260)	(42,483)
Total current liabilities	(51,260)	(42,534)
LONG-TERM DEBT	0	0
DEFERRED REVENUE AND OTHER LIABILITES
Deferred revenue	0	0
Deferred tax liability	(1,332)	(1,771)
Other	0	0
Total deferred revenue and other liabilities	(1,332)	(1,771)
COMMITMENTS AND CONTINGENCIES
MEMBER'S EQUITY (DEFICIT)	(37,317)	(38,973)
TOTAL LIABILITIES AND MEMBER'S EQUITY	(89,909)	(83,278)
Level 3 Parent, LLC | Reportable Legal Entities
CURRENT ASSETS
Cash and cash equivalents	2	13
Restricted cash and securities - current	0	0
Accounts receivable	0	0
Accounts receivable - affiliates		0
Assets held for sale		68
Advances to affiliates	16,852	16,251
Note receivable - affiliate	1,825	1,825
Other	1	0
Total current assets	18,680	18,157
NET PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment	0	0
Accumulated depreciation	0	0
Net property, plant and equipment	0	0
GOODWILL AND OTHER ASSETS
Goodwill	0	0
Restricted cash and securities	15	19
Customer relationships, net	0	0
Other intangible assets, net	0	0
Investment in subsidiaries	15,541	16,954
Other, net	275	280
Total goodwill and other assets	15,831	17,253
TOTAL ASSETS	34,511	35,410
CURRENT LIABILITIES
Current maturities of long-term debt	0	0
Accounts payable	0	0
Accounts payable - affiliates	62	11
Accrued expenses and other liabilities
Salaries and benefits	0	0
Income and other taxes	0	0
Interest	11	11
Other	3	16
Advance billings and customer deposits	0	0
Due to affiliates	0	0
Total current liabilities	76	38
LONG-TERM DEBT	613	616
DEFERRED REVENUE AND OTHER LIABILITES
Deferred revenue	0	0
Deferred tax liability	56	648
Other	0	1
Total deferred revenue and other liabilities	56	649
COMMITMENTS AND CONTINGENCIES
MEMBER'S EQUITY (DEFICIT)	33,766	34,107
TOTAL LIABILITIES AND MEMBER'S EQUITY	34,511	35,410
Level 3 Financing, Inc. | Reportable Legal Entities
CURRENT ASSETS
Cash and cash equivalents	0	0
Restricted cash and securities - current	0	0
Accounts receivable	0	0
Accounts receivable - affiliates		0
Assets held for sale		0
Advances to affiliates	23,957	21,032
Note receivable - affiliate	0	0
Other	3	0
Total current assets	23,960	21,032
NET PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment	0	0
Accumulated depreciation	0	0
Net property, plant and equipment	0	0
GOODWILL AND OTHER ASSETS
Goodwill	0	0
Restricted cash and securities	0	0
Customer relationships, net	0	0
Other intangible assets, net	0	0
Investment in subsidiaries	17,915	18,403
Other, net	1,421	1,795
Total goodwill and other assets	19,336	20,198
TOTAL ASSETS	43,296	41,230
CURRENT LIABILITIES
Current maturities of long-term debt	0	0
Accounts payable	0	1
Accounts payable - affiliates	11	0
Accrued expenses and other liabilities
Salaries and benefits	0	0
Income and other taxes	4	0
Interest	78	91
Other	1	0
Advance billings and customer deposits	0	0
Due to affiliates	0	0
Total current liabilities	94	92
LONG-TERM DEBT	10,068	10,096
DEFERRED REVENUE AND OTHER LIABILITES
Deferred revenue	0	0
Deferred tax liability	0	0
Other	0	1
Total deferred revenue and other liabilities	0	1
COMMITMENTS AND CONTINGENCIES
MEMBER'S EQUITY (DEFICIT)	33,134	31,041
TOTAL LIABILITIES AND MEMBER'S EQUITY	43,296	41,230
Level 3 Communications, LLC | Reportable Legal Entities
CURRENT ASSETS
Cash and cash equivalents	164	175
Restricted cash and securities - current	0	1
Accounts receivable	70	26
Accounts receivable - affiliates		60
Assets held for sale		5
Advances to affiliates	7,744	0
Note receivable - affiliate	0	0
Other	97	54
Total current assets	8,075	321
NET PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment	3,456	3,285
Accumulated depreciation	(320)	(48)
Net property, plant and equipment	3,136	3,237
GOODWILL AND OTHER ASSETS
Goodwill	1,665	1,200
Restricted cash and securities	9	10
Customer relationships, net	3,823	4,324
Other intangible assets, net	409	378
Investment in subsidiaries	3,861	3,616
Other, net	110	32
Total goodwill and other assets	9,877	9,560
TOTAL ASSETS	21,088	13,118
CURRENT LIABILITIES
Current maturities of long-term debt	1	2
Accounts payable	380	323
Accounts payable - affiliates	162	0
Accrued expenses and other liabilities
Salaries and benefits	189	109
Income and other taxes	72	55
Interest	1	0
Other	8	25
Advance billings and customer deposits	168	127
Due to affiliates	45,347	42,483
Total current liabilities	46,328	43,124
LONG-TERM DEBT	7	13
DEFERRED REVENUE AND OTHER LIABILITES
Deferred revenue	971	841
Deferred tax liability	841	870
Other	197	103
Total deferred revenue and other liabilities	2,009	1,814
COMMITMENTS AND CONTINGENCIES
MEMBER'S EQUITY (DEFICIT)	(27,256)	(31,833)
TOTAL LIABILITIES AND MEMBER'S EQUITY	21,088	13,118
Other Non-Guarantor Subsidiaries | Reportable Legal Entities
CURRENT ASSETS
Cash and cash equivalents	77	109
Restricted cash and securities - current	4	4
Accounts receivable	642	722
Accounts receivable - affiliates		4
Assets held for sale		67
Advances to affiliates	2,707	5,200
Note receivable - affiliate	0	0
Other	133	63
Total current assets	3,563	6,169
NET PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment	7,018	6,270
Accumulated depreciation	(701)	(95)
Net property, plant and equipment	6,317	6,175
GOODWILL AND OTHER ASSETS
Goodwill	9,454	9,637
Restricted cash and securities	1	0
Customer relationships, net	3,744	4,521
Other intangible assets, net	1	0
Investment in subsidiaries	0	0
Other, net	225	153
Total goodwill and other assets	13,425	14,311
TOTAL ASSETS	23,305	26,655
CURRENT LIABILITIES
Current maturities of long-term debt	5	6
Accounts payable	346	371
Accounts payable - affiliates	11	81
Accrued expenses and other liabilities
Salaries and benefits	44	27
Income and other taxes	54	45
Interest	5	7
Other	66	18
Advance billings and customer deposits	142	131
Due to affiliates	5,913	0
Total current liabilities	6,586	686
LONG-TERM DEBT	150	157
DEFERRED REVENUE AND OTHER LIABILITES
Deferred revenue	210	252
Deferred tax liability	637	465
Other	172	165
Total deferred revenue and other liabilities	1,019	882
COMMITMENTS AND CONTINGENCIES
MEMBER'S EQUITY (DEFICIT)	15,550	24,930
TOTAL LIABILITIES AND MEMBER'S EQUITY	$ 23,305	$ 26,655