Condensed Consolidating Financial Information - Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Oct. 31, 2017
CURRENT ASSETS
Cash and cash equivalents	$ 316	$ 243	$ 297	$ 2,274
Restricted cash and securities - current	3	4	5	5
Accounts receivable	667	712
Advances to affiliates	0	0
Note receivable - affiliate	1,590	1,825
Other	266	234
Total current assets	2,842	3,018
Net property, plant and equipment	9,936	9,453
GOODWILL AND OTHER ASSETS
Goodwill	7,415	11,119	10,837
Operating lease assets	1,060
Restricted cash	19	25	29	29
Customer relationships, net		7,567
Other intangible assets, net	469	410
Investment in subsidiaries	0	0
Other, net	492	699
Total goodwill and other assets	16,320	19,820
TOTAL ASSETS	29,098	32,291
CURRENT LIABILITIES
Current maturities of long-term debt	11	6
Accounts payable	654	726
Accounts payable - affiliates	669	246
Accrued expenses and other liabilities
Salaries and benefits	240	233
Income and other taxes	152	130
Current operating lease liabilities	249
Interest	85	95
Other	77	78
Current portion of deferred revenue	309	310
Due to affiliates	0	0
Total current liabilities	2,446	1,824
LONG-TERM DEBT	10,356	10,838
DEFERRED REVENUE AND OTHER LIABILITES
Deferred revenue	1,343	1,181
Deferred tax liability	241	202
Noncurrent operating lease liabilities	854
Other	313	369
Total deferred revenue and other liabilities	2,751	1,752
COMMITMENTS AND CONTINGENCIES
MEMBER'S EQUITY (DEFICIT)	13,545	17,877	$ 19,272	$ 11,554
TOTAL LIABILITIES AND MEMBER'S EQUITY (DEFICIT)	29,098	32,291
Eliminations
CURRENT ASSETS
Cash and cash equivalents	0	0
Restricted cash and securities - current	0	0
Accounts receivable	0	0
Advances to affiliates	(54,059)	(51,260)
Note receivable - affiliate	0	0
Other	0	0
Total current assets	(54,059)	(51,260)
Net property, plant and equipment	0	0
GOODWILL AND OTHER ASSETS
Goodwill	0	0
Operating lease assets	(474)
Restricted cash	0	0
Customer relationships, net		0
Other intangible assets, net	0	0
Investment in subsidiaries	(23,427)	(37,317)
Other, net	(1,554)	(1,332)
Total goodwill and other assets	(25,455)	(38,649)
TOTAL ASSETS	(79,514)	(89,909)
CURRENT LIABILITIES
Current maturities of long-term debt	0	0
Accounts payable	0	0
Accounts payable - affiliates	0	0
Accrued expenses and other liabilities
Salaries and benefits	0	0
Income and other taxes	0	0
Current operating lease liabilities	(94)
Interest	0	0
Other	0	0
Current portion of deferred revenue	0	0
Due to affiliates	(54,059)	(51,260)
Total current liabilities	(54,153)	(51,260)
LONG-TERM DEBT	0	0
DEFERRED REVENUE AND OTHER LIABILITES
Deferred revenue	0	0
Deferred tax liability	(1,554)	(1,332)
Noncurrent operating lease liabilities	(380)
Other	0	0
Total deferred revenue and other liabilities	(1,934)	(1,332)
COMMITMENTS AND CONTINGENCIES
MEMBER'S EQUITY (DEFICIT)	(23,427)	(37,317)
TOTAL LIABILITIES AND MEMBER'S EQUITY (DEFICIT)	(79,514)	(89,909)
Level 3 Parent, LLC | Reportable Legal Entities
CURRENT ASSETS
Cash and cash equivalents	3	2
Restricted cash and securities - current	0	0
Accounts receivable	0	0
Advances to affiliates	19,302	16,852
Note receivable - affiliate	1,590	1,825
Other	1	1
Total current assets	20,896	18,680
Net property, plant and equipment	0	0
GOODWILL AND OTHER ASSETS
Goodwill	0	0
Operating lease assets	0
Restricted cash	12	15
Customer relationships, net		0
Other intangible assets, net	0	0
Investment in subsidiaries	8,432	15,541
Other, net	272	275
Total goodwill and other assets	8,716	15,831
TOTAL ASSETS	29,612	34,511
CURRENT LIABILITIES
Current maturities of long-term debt	0	0
Accounts payable	0	0
Accounts payable - affiliates	80	62
Accrued expenses and other liabilities
Salaries and benefits	0	0
Income and other taxes	0	0
Current operating lease liabilities	0
Interest	0	11
Other	1	3
Current portion of deferred revenue	0	0
Due to affiliates	0	0
Total current liabilities	81	76
LONG-TERM DEBT	0	613
DEFERRED REVENUE AND OTHER LIABILITES
Deferred revenue	0	0
Deferred tax liability	56	56
Noncurrent operating lease liabilities	0
Other	1	0
Total deferred revenue and other liabilities	57	56
COMMITMENTS AND CONTINGENCIES
MEMBER'S EQUITY (DEFICIT)	29,474	33,766
TOTAL LIABILITIES AND MEMBER'S EQUITY (DEFICIT)	29,612	34,511
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
Advances to affiliates	24,293	23,957
Note receivable - affiliate	0	0
Other	0	3
Total current assets	24,293	23,960
Net property, plant and equipment	0	0
GOODWILL AND OTHER ASSETS
Goodwill	0	0
Operating lease assets	0
Restricted cash	0	0
Customer relationships, net		0
Other intangible assets, net	0	0
Investment in subsidiaries	10,564	17,915
Other, net	1,485	1,421
Total goodwill and other assets	12,049	19,336
TOTAL ASSETS	36,342	43,296
CURRENT LIABILITIES
Current maturities of long-term debt	0	0
Accounts payable	15	0
Accounts payable - affiliates	17	11
Accrued expenses and other liabilities
Salaries and benefits	0	0
Income and other taxes	7	4
Current operating lease liabilities	0
Interest	81	78
Other	1	1
Current portion of deferred revenue	0	0
Due to affiliates	0	0
Total current liabilities	121	94
LONG-TERM DEBT	10,196	10,068
DEFERRED REVENUE AND OTHER LIABILITES
Deferred revenue	0	0
Deferred tax liability	0	0
Noncurrent operating lease liabilities	0
Other	0	0
Total deferred revenue and other liabilities	0	0
COMMITMENTS AND CONTINGENCIES
MEMBER'S EQUITY (DEFICIT)	26,025	33,134
TOTAL LIABILITIES AND MEMBER'S EQUITY (DEFICIT)	36,342	43,296
Level 3 Communications, LLC | Reportable Legal Entities
CURRENT ASSETS
Cash and cash equivalents	231	164
Restricted cash and securities - current	0	0
Accounts receivable	44	70
Advances to affiliates	7,634	7,744
Note receivable - affiliate	0	0
Other	105	97
Total current assets	8,014	8,075
Net property, plant and equipment	3,688	3,136
GOODWILL AND OTHER ASSETS
Goodwill	252	1,665
Operating lease assets	1,210
Restricted cash	5	9
Customer relationships, net		3,823
Other intangible assets, net	447	409
Investment in subsidiaries	4,431	3,861
Other, net	88	110
Total goodwill and other assets	9,816	9,877
TOTAL ASSETS	21,518	21,088
CURRENT LIABILITIES
Current maturities of long-term debt	3	1
Accounts payable	350	380
Accounts payable - affiliates	569	162
Accrued expenses and other liabilities
Salaries and benefits	192	189
Income and other taxes	95	72
Current operating lease liabilities	254
Interest	1	1
Other	22	8
Current portion of deferred revenue	169	168
Due to affiliates	50,865	45,347
Total current liabilities	52,520	46,328
LONG-TERM DEBT	15	7
DEFERRED REVENUE AND OTHER LIABILITES
Deferred revenue	1,137	971
Deferred tax liability	739	841
Noncurrent operating lease liabilities	986
Other	154	197
Total deferred revenue and other liabilities	3,016	2,009
COMMITMENTS AND CONTINGENCIES
MEMBER'S EQUITY (DEFICIT)	(34,033)	(27,256)
TOTAL LIABILITIES AND MEMBER'S EQUITY (DEFICIT)	21,518	21,088
Other Non-Guarantor Subsidiaries | Reportable Legal Entities
CURRENT ASSETS
Cash and cash equivalents	82	77
Restricted cash and securities - current	3	4
Accounts receivable	623	642
Advances to affiliates	2,830	2,707
Note receivable - affiliate	0	0
Other	160	133
Total current assets	3,698	3,563
Net property, plant and equipment	6,248	6,317
GOODWILL AND OTHER ASSETS
Goodwill	7,163	9,454
Operating lease assets	324
Restricted cash	2	1
Customer relationships, net		3,744
Other intangible assets, net	22	1
Investment in subsidiaries	0	0
Other, net	201	225
Total goodwill and other assets	11,194	13,425
TOTAL ASSETS	21,140	23,305
CURRENT LIABILITIES
Current maturities of long-term debt	8	5
Accounts payable	289	346
Accounts payable - affiliates	3	11
Accrued expenses and other liabilities
Salaries and benefits	48	44
Income and other taxes	50	54
Current operating lease liabilities	89
Interest	3	5
Other	53	66
Current portion of deferred revenue	140	142
Due to affiliates	3,194	5,913
Total current liabilities	3,877	6,586
LONG-TERM DEBT	145	150
DEFERRED REVENUE AND OTHER LIABILITES
Deferred revenue	206	210
Deferred tax liability	1,000	637
Noncurrent operating lease liabilities	248
Other	158	172
Total deferred revenue and other liabilities	1,612	1,019
COMMITMENTS AND CONTINGENCIES
MEMBER'S EQUITY (DEFICIT)	15,506	15,550
TOTAL LIABILITIES AND MEMBER'S EQUITY (DEFICIT)	21,140	23,305
Customer relationships
GOODWILL AND OTHER ASSETS
Customer relationships, net	6,865	$ 7,567
Customer relationships | Eliminations
GOODWILL AND OTHER ASSETS
Customer relationships, net	0
Customer relationships | Level 3 Parent, LLC | Reportable Legal Entities
GOODWILL AND OTHER ASSETS
Customer relationships, net	0
Customer relationships | Level 3 Financing, Inc. | Reportable Legal Entities
GOODWILL AND OTHER ASSETS
Customer relationships, net	0
Customer relationships | Level 3 Communications, LLC | Reportable Legal Entities
GOODWILL AND OTHER ASSETS
Customer relationships, net	3,383
Customer relationships | Other Non-Guarantor Subsidiaries | Reportable Legal Entities
GOODWILL AND OTHER ASSETS
Customer relationships, net	$ 3,482