Condensed Consolidating Financial Information - Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Oct. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CURRENT ASSETS
Cash and cash equivalents	$ 297
Restricted cash and securities	5
Assets held for sale	140
Accounts receivable	748
Accounts receivable - affiliate	13
Advances to affiliates	0
Note receivable - affiliate	1,825
Other	117
Total current assets	3,145
NET PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment	9,555
Accumulated depreciation	(143)
Net property, plant and equipment	9,412
Restricted cash and securities	29
GOODWILL AND OTHER ASSETS
Goodwill	10,837
Customer relationships, net	8,845
Other intangible assets, net	378
Investment in subsidiaries	0
Deferred tax assets	426
Other, net	63
Total goodwill and other assets	20,549
TOTAL ASSETS	33,135
CURRENT LIABILITIES
Current maturities of long-term debt	8
Accounts payable	695
Accounts payable - affiliate	41
Accrued expenses and other liabilities
Income and other taxes	100
Salaries and benefits	136
Interest	109
Current portion of deferred revenue	258
Current portion of deferred revenue - affiliate	2
Other	57
Due to affiliates	0
Total current liabilities	1,406
LONG-TERM DEBT	10,882
DEFERRED CREDITS AND OTHER LIABILITES
Deferred credits	1,093
Deferred credits - affiliate	6
Deferred tax liability	212
Other	264
Total deferred credits and other liabilities	1,575
COMMITMENTS AND CONTINGENCIES
MEMBER'S/STOCKHOLDERS' EQUITY (DEFICIT)	19,272
TOTAL LIABILITIES AND MEMBER'S/STOCKHOLDERS' EQUITY	33,135
Predecessor
CURRENT ASSETS
Cash and cash equivalents			$ 1,819
Restricted cash and securities			7
Assets held for sale			0
Accounts receivable			712
Accounts receivable - affiliate			0
Advances to affiliates			0
Note receivable - affiliate			0
Other			115
Total current assets			2,653
NET PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment			21,388
Accumulated depreciation			(11,249)
Net property, plant and equipment			10,139
Restricted cash and securities			31
GOODWILL AND OTHER ASSETS
Goodwill			7,729
Customer relationships, net			860
Other intangible assets, net			55
Investment in subsidiaries			0
Deferred tax assets			3,370
Other, net			51
Total goodwill and other assets			12,065
TOTAL ASSETS			24,888
CURRENT LIABILITIES
Current maturities of long-term debt			7
Accounts payable			706
Accounts payable - affiliate			0
Accrued expenses and other liabilities
Income and other taxes			116
Salaries and benefits			195
Interest			129
Current portion of deferred revenue			266
Current portion of deferred revenue - affiliate			0
Other			52
Due to affiliates			0
Total current liabilities			1,471
LONG-TERM DEBT			10,877
DEFERRED CREDITS AND OTHER LIABILITES
Deferred credits			1,001
Deferred credits - affiliate			0
Deferred tax liability			248
Other			374
Total deferred credits and other liabilities			1,623
COMMITMENTS AND CONTINGENCIES
MEMBER'S/STOCKHOLDERS' EQUITY (DEFICIT)		$ 11,554	10,917	$ 10,126
TOTAL LIABILITIES AND MEMBER'S/STOCKHOLDERS' EQUITY			24,888
Eliminations
CURRENT ASSETS
Cash and cash equivalents	0
Restricted cash and securities	0
Assets held for sale	0
Accounts receivable	0
Accounts receivable - affiliate	(51)
Advances to affiliates	(42,483)
Note receivable - affiliate	0
Other	0
Total current assets	(42,534)
NET PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment	0
Accumulated depreciation	0
Net property, plant and equipment	0
Restricted cash and securities	0
GOODWILL AND OTHER ASSETS
Goodwill	0
Customer relationships, net	0
Other intangible assets, net	0
Investment in subsidiaries	(38,973)
Deferred tax assets	(1,771)
Other, net	0
Total goodwill and other assets	(40,744)
TOTAL ASSETS	(83,278)
CURRENT LIABILITIES
Current maturities of long-term debt	0
Accounts payable	0
Accounts payable - affiliate	(51)
Accrued expenses and other liabilities
Income and other taxes	0
Salaries and benefits	0
Interest	0
Current portion of deferred revenue	0
Current portion of deferred revenue - affiliate	0
Other	0
Due to affiliates	(42,483)
Total current liabilities	(42,534)
LONG-TERM DEBT	0
DEFERRED CREDITS AND OTHER LIABILITES
Deferred credits	0
Deferred credits - affiliate	0
Deferred tax liability	(1,771)
Other	0
Total deferred credits and other liabilities	(1,771)
MEMBER'S/STOCKHOLDERS' EQUITY (DEFICIT)	(38,973)
TOTAL LIABILITIES AND MEMBER'S/STOCKHOLDERS' EQUITY	(83,278)
Eliminations | Predecessor
CURRENT ASSETS
Cash and cash equivalents			0
Restricted cash and securities			0
Accounts receivable			0
Advances to affiliates			(40,927)
Other			0
Total current assets			(40,927)
NET PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment			0
Accumulated depreciation			0
Net property, plant and equipment			0
Restricted cash and securities			0
GOODWILL AND OTHER ASSETS
Goodwill			0
Customer relationships, net			0
Investment in subsidiaries			(38,142)
Deferred tax assets			0
Other, net			0
Total goodwill and other assets			(38,142)
TOTAL ASSETS			(79,069)
CURRENT LIABILITIES
Current maturities of long-term debt			0
Accounts payable			0
Accrued expenses and other liabilities
Income and other taxes			0
Salaries and benefits			0
Interest			0
Current portion of deferred revenue			0
Other			0
Due to affiliates			(40,927)
Total current liabilities			(40,927)
LONG-TERM DEBT			0
DEFERRED CREDITS AND OTHER LIABILITES
Deferred credits			0
Deferred tax liability			0
Other			0
Total deferred credits and other liabilities			0
MEMBER'S/STOCKHOLDERS' EQUITY (DEFICIT)			(38,142)
TOTAL LIABILITIES AND MEMBER'S/STOCKHOLDERS' EQUITY			(79,069)
Level 3 Parent, LLC | Reportable Legal Entities
CURRENT ASSETS
Cash and cash equivalents	13
Restricted cash and securities	0
Assets held for sale	68
Accounts receivable	0
Accounts receivable - affiliate	0
Advances to affiliates	16,251
Note receivable - affiliate	1,825
Other	0
Total current assets	18,157
NET PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment	0
Accumulated depreciation	0
Net property, plant and equipment	0
Restricted cash and securities	19
GOODWILL AND OTHER ASSETS
Goodwill	0
Customer relationships, net	0
Other intangible assets, net	0
Investment in subsidiaries	16,954
Deferred tax assets	280
Other, net	0
Total goodwill and other assets	17,234
TOTAL ASSETS	35,410
CURRENT LIABILITIES
Current maturities of long-term debt	0
Accounts payable	0
Accounts payable - affiliate	11
Accrued expenses and other liabilities
Income and other taxes	0
Salaries and benefits	0
Interest	11
Current portion of deferred revenue	0
Current portion of deferred revenue - affiliate	0
Other	16
Due to affiliates	0
Total current liabilities	38
LONG-TERM DEBT	616
DEFERRED CREDITS AND OTHER LIABILITES
Deferred credits	0
Deferred credits - affiliate	0
Deferred tax liability	648
Other	1
Total deferred credits and other liabilities	649
MEMBER'S/STOCKHOLDERS' EQUITY (DEFICIT)	34,107
TOTAL LIABILITIES AND MEMBER'S/STOCKHOLDERS' EQUITY	35,410
Level 3 Parent, LLC | Reportable Legal Entities | Predecessor
CURRENT ASSETS
Cash and cash equivalents			15
Restricted cash and securities			0
Accounts receivable			0
Advances to affiliates			17,032
Other			0
Total current assets			17,047
NET PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment			0
Accumulated depreciation			0
Net property, plant and equipment			0
Restricted cash and securities			22
GOODWILL AND OTHER ASSETS
Goodwill			0
Customer relationships, net			0
Other intangible assets, net			0
Investment in subsidiaries			16,869
Deferred tax assets			51
Other, net			0
Total goodwill and other assets			16,920
TOTAL ASSETS			33,989
CURRENT LIABILITIES
Current maturities of long-term debt			0
Accounts payable			0
Accrued expenses and other liabilities
Income and other taxes			0
Salaries and benefits			0
Interest			11
Current portion of deferred revenue			0
Other			0
Due to affiliates			0
Total current liabilities			11
LONG-TERM DEBT			592
DEFERRED CREDITS AND OTHER LIABILITES
Deferred credits			0
Deferred tax liability			0
Other			16
Total deferred credits and other liabilities			16
MEMBER'S/STOCKHOLDERS' EQUITY (DEFICIT)			33,370
TOTAL LIABILITIES AND MEMBER'S/STOCKHOLDERS' EQUITY			33,989
Level 3 Financing, Inc. | Reportable Legal Entities
CURRENT ASSETS
Cash and cash equivalents	0
Restricted cash and securities	0
Assets held for sale	0
Accounts receivable	0
Accounts receivable - affiliate	0
Advances to affiliates	21,032
Note receivable - affiliate	0
Other	0
Total current assets	21,032
NET PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment	0
Accumulated depreciation	0
Net property, plant and equipment	0
Restricted cash and securities	0
GOODWILL AND OTHER ASSETS
Goodwill	0
Customer relationships, net	0
Other intangible assets, net	0
Investment in subsidiaries	18,403
Deferred tax assets	1,795
Other, net	0
Total goodwill and other assets	20,198
TOTAL ASSETS	41,230
CURRENT LIABILITIES
Current maturities of long-term debt	0
Accounts payable	1
Accounts payable - affiliate	0
Accrued expenses and other liabilities
Income and other taxes	0
Salaries and benefits	0
Interest	91
Current portion of deferred revenue	0
Current portion of deferred revenue - affiliate	0
Other	0
Due to affiliates	0
Total current liabilities	92
LONG-TERM DEBT	10,096
DEFERRED CREDITS AND OTHER LIABILITES
Deferred credits	0
Deferred credits - affiliate	0
Deferred tax liability	0
Other	1
Total deferred credits and other liabilities	1
MEMBER'S/STOCKHOLDERS' EQUITY (DEFICIT)	31,041
TOTAL LIABILITIES AND MEMBER'S/STOCKHOLDERS' EQUITY	41,230
Level 3 Financing, Inc. | Reportable Legal Entities | Predecessor
CURRENT ASSETS
Cash and cash equivalents			0
Restricted cash and securities			0
Accounts receivable			0
Advances to affiliates			21,715
Other			0
Total current assets			21,715
NET PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment			0
Accumulated depreciation			0
Net property, plant and equipment			0
Restricted cash and securities			0
GOODWILL AND OTHER ASSETS
Goodwill			0
Customer relationships, net			0
Other intangible assets, net			0
Investment in subsidiaries			17,599
Deferred tax assets			2,687
Other, net			0
Total goodwill and other assets			20,286
TOTAL ASSETS			42,001
CURRENT LIABILITIES
Current maturities of long-term debt			0
Accounts payable			0
Accrued expenses and other liabilities
Income and other taxes			0
Salaries and benefits			0
Interest			110
Current portion of deferred revenue			0
Other			0
Due to affiliates			0
Total current liabilities			110
LONG-TERM DEBT			10,108
DEFERRED CREDITS AND OTHER LIABILITES
Deferred credits			0
Deferred tax liability			0
Other			0
Total deferred credits and other liabilities			0
MEMBER'S/STOCKHOLDERS' EQUITY (DEFICIT)			31,783
TOTAL LIABILITIES AND MEMBER'S/STOCKHOLDERS' EQUITY			42,001
Level 3 Communications, LLC | Reportable Legal Entities
CURRENT ASSETS
Cash and cash equivalents	175
Restricted cash and securities	1
Assets held for sale	5
Accounts receivable	26
Accounts receivable - affiliate	60
Advances to affiliates	0
Note receivable - affiliate	0
Other	54
Total current assets	321
NET PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment	3,285
Accumulated depreciation	(48)
Net property, plant and equipment	3,237
Restricted cash and securities	10
GOODWILL AND OTHER ASSETS
Goodwill	1,200
Customer relationships, net	4,324
Other intangible assets, net	378
Investment in subsidiaries	3,616
Deferred tax assets	0
Other, net	32
Total goodwill and other assets	9,550
TOTAL ASSETS	13,118
CURRENT LIABILITIES
Current maturities of long-term debt	2
Accounts payable	323
Accounts payable - affiliate	0
Accrued expenses and other liabilities
Income and other taxes	55
Salaries and benefits	109
Interest	0
Current portion of deferred revenue	127
Current portion of deferred revenue - affiliate	2
Other	23
Due to affiliates	42,483
Total current liabilities	43,124
LONG-TERM DEBT	13
DEFERRED CREDITS AND OTHER LIABILITES
Deferred credits	841
Deferred credits - affiliate	5
Deferred tax liability	870
Other	98
Total deferred credits and other liabilities	1,814
MEMBER'S/STOCKHOLDERS' EQUITY (DEFICIT)	(31,833)
TOTAL LIABILITIES AND MEMBER'S/STOCKHOLDERS' EQUITY	13,118
Level 3 Communications, LLC | Reportable Legal Entities | Predecessor
CURRENT ASSETS
Cash and cash equivalents			1,700
Restricted cash and securities			1
Accounts receivable			26
Advances to affiliates			0
Other			87
Total current assets			1,814
NET PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment			10,424
Accumulated depreciation			(6,555)
Net property, plant and equipment			3,869
Restricted cash and securities			9
GOODWILL AND OTHER ASSETS
Goodwill			352
Customer relationships, net			0
Other intangible assets, net			1
Investment in subsidiaries			3,674
Deferred tax assets			0
Other, net			16
Total goodwill and other assets			4,043
TOTAL ASSETS			9,735
CURRENT LIABILITIES
Current maturities of long-term debt			2
Accounts payable			307
Accrued expenses and other liabilities
Income and other taxes			103
Salaries and benefits			160
Interest			0
Current portion of deferred revenue			116
Other			24
Due to affiliates			40,927
Total current liabilities			41,639
LONG-TERM DEBT			13
DEFERRED CREDITS AND OTHER LIABILITES
Deferred credits			719
Deferred tax liability			0
Other			155
Total deferred credits and other liabilities			874
MEMBER'S/STOCKHOLDERS' EQUITY (DEFICIT)			(32,791)
TOTAL LIABILITIES AND MEMBER'S/STOCKHOLDERS' EQUITY			9,735
Other Non-Guarantor Subsidiaries | Reportable Legal Entities
CURRENT ASSETS
Cash and cash equivalents	109
Restricted cash and securities	4
Assets held for sale	67
Accounts receivable	722
Accounts receivable - affiliate	4
Advances to affiliates	5,200
Note receivable - affiliate	0
Other	63
Total current assets	6,169
NET PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment	6,270
Accumulated depreciation	(95)
Net property, plant and equipment	6,175
Restricted cash and securities	0
GOODWILL AND OTHER ASSETS
Goodwill	9,637
Customer relationships, net	4,521
Other intangible assets, net	0
Investment in subsidiaries	0
Deferred tax assets	122
Other, net	31
Total goodwill and other assets	14,311
TOTAL ASSETS	26,655
CURRENT LIABILITIES
Current maturities of long-term debt	6
Accounts payable	371
Accounts payable - affiliate	81
Accrued expenses and other liabilities
Income and other taxes	45
Salaries and benefits	27
Interest	7
Current portion of deferred revenue	131
Current portion of deferred revenue - affiliate	0
Other	18
Due to affiliates	0
Total current liabilities	686
LONG-TERM DEBT	157
DEFERRED CREDITS AND OTHER LIABILITES
Deferred credits	252
Deferred credits - affiliate	1
Deferred tax liability	465
Other	164
Total deferred credits and other liabilities	882
MEMBER'S/STOCKHOLDERS' EQUITY (DEFICIT)	24,930
TOTAL LIABILITIES AND MEMBER'S/STOCKHOLDERS' EQUITY	$ 26,655
Other Non-Guarantor Subsidiaries | Reportable Legal Entities | Predecessor
CURRENT ASSETS
Cash and cash equivalents			104
Restricted cash and securities			6
Accounts receivable			686
Advances to affiliates			2,180
Other			28
Total current assets			3,004
NET PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment			10,964
Accumulated depreciation			(4,694)
Net property, plant and equipment			6,270
Restricted cash and securities			0
GOODWILL AND OTHER ASSETS
Goodwill			7,377
Customer relationships, net			860
Other intangible assets, net			54
Investment in subsidiaries			0
Deferred tax assets			632
Other, net			35
Total goodwill and other assets			8,958
TOTAL ASSETS			18,232
CURRENT LIABILITIES
Current maturities of long-term debt			5
Accounts payable			399
Accrued expenses and other liabilities
Income and other taxes			13
Salaries and benefits			35
Interest			8
Current portion of deferred revenue			150
Other			28
Due to affiliates			0
Total current liabilities			638
LONG-TERM DEBT			164
DEFERRED CREDITS AND OTHER LIABILITES
Deferred credits			282
Deferred tax liability			248
Other			203
Total deferred credits and other liabilities			733
MEMBER'S/STOCKHOLDERS' EQUITY (DEFICIT)			16,697
TOTAL LIABILITIES AND MEMBER'S/STOCKHOLDERS' EQUITY			$ 18,232