Condensed Consolidating Financial Information - Statements of Operations (Details) - USD ($) $ in Millions	1 Months Ended	2 Months Ended	3 Months Ended							10 Months Ended	12 Months Ended
	Oct. 31, 2017	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Oct. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
OPERATING REVENUES
Operating revenues		$ 1,391
Operating revenues - affiliate		16
Total operating revenues		1,407									$ 8,277
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)		690
Selling, general and administrative		253
Operating expenses - affiliate		24
Depreciation and amortization		282
Total operating expenses		1,249
OPERATING INCOME		158									1,309
OTHER INCOME (EXPENSE)
Interest income		1
Interest income - affiliate		11
Interest expense		(80)
Interest income (expense) - affiliates, net		0
Loss on modification and extinguishment of debt		0
Equity in net (losses) earnings of subsidiaries		0
Other income, net		3
Total other expense, net		(65)
INCOME BEFORE INCOME TAX (EXPENSE) BENEFIT		93
Income tax (expense) benefit		(234)				$ (16)		$ 40
NET (LOSS) INCOME		(141)									$ 284
Other comprehensive income, net of income taxes		18
COMPREHENSIVE (LOSS) INCOME		(123)
Predecessor
OPERATING REVENUES
Operating revenues										$ 6,870		$ 8,173	$ 8,230
Operating revenues - affiliate										0		0	0
Total operating revenues	$ 701		$ 2,059	$ 2,062	$ 2,048	2,032	$ 2,033	2,057	$ 2,051	6,870		8,173	8,230
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)										3,493		4,162	4,349
Selling, general and administrative										1,208		1,407	1,469
Operating expenses - affiliate										0		0	0
Depreciation and amortization										1,018		1,159	1,082
Total operating expenses										5,719		6,728	6,900
OPERATING INCOME	112		349	353	337	354	354	375	362	1,151		1,445	1,330
OTHER INCOME (EXPENSE)
Interest income										13		4	1
Interest income - affiliate										0		0	0
Interest expense										(441)		(544)	(640)
Loss on modification and extinguishment of debt										(44)		(40)	(218)
Equity in net (losses) earnings of subsidiaries										0		0	0
Other income, net										14		(63)	(408)
Total other expense, net										(458)		(603)	(1,047)
INCOME BEFORE INCOME TAX (EXPENSE) BENEFIT										693		842	283
Income tax (expense) benefit										(268)		(165)	3,150
NET (LOSS) INCOME	$ 19		$ 157	$ 154	$ 95	$ 250	$ 143	$ 156	$ 128	425		677	3,433
Other comprehensive income, net of income taxes										80		(86)	(154)
COMPREHENSIVE (LOSS) INCOME										505		591	3,279
Eliminations
OPERATING REVENUES
Operating revenues		(28)
Operating revenues - affiliate		0
Total operating revenues		(28)
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)		(28)
Selling, general and administrative		0
Operating expenses - affiliate		0
Depreciation and amortization		0
Total operating expenses		(28)
OPERATING INCOME		0
OTHER INCOME (EXPENSE)
Interest income		0
Interest income - affiliate		0
Interest expense		0
Interest income (expense) - affiliates, net		0
Equity in net (losses) earnings of subsidiaries		771
Other income, net		0
Total other expense, net		771
INCOME BEFORE INCOME TAX (EXPENSE) BENEFIT		771
Income tax (expense) benefit		0
NET (LOSS) INCOME		771
Other comprehensive income, net of income taxes		(18)
COMPREHENSIVE (LOSS) INCOME		753
Eliminations | Predecessor
OPERATING REVENUES
Operating revenues										(129)		(132)	(173)
Total operating revenues										(129)		(132)	(173)
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)										(129)		(132)	(173)
Selling, general and administrative										0		0	0
Depreciation and amortization										0		0	0
Total operating expenses										(129)		(132)	(173)
OPERATING INCOME										0		0	0
OTHER INCOME (EXPENSE)
Interest income										0		0	0
Interest income - affiliate										0		0	0
Interest expense										0		0	0
Loss on modification and extinguishment of debt										0
Equity in net (losses) earnings of subsidiaries										2,261		1,945	(646)
Other income, net										0		0	0
Total other expense, net										2,261		1,945	(646)
INCOME BEFORE INCOME TAX (EXPENSE) BENEFIT										2,261		1,945	(646)
Income tax (expense) benefit										0		0	0
NET (LOSS) INCOME										2,261		1,945	(646)
Other comprehensive income, net of income taxes										0		86	154
COMPREHENSIVE (LOSS) INCOME										2,261		2,031	(492)
Level 3 Parent, LLC | Reportable Legal Entities
OPERATING REVENUES
Operating revenues		0
Operating revenues - affiliate		0
Total operating revenues		0
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)		0
Selling, general and administrative		1
Operating expenses - affiliate		0
Depreciation and amortization		0
Total operating expenses		1
OPERATING INCOME		(1)
OTHER INCOME (EXPENSE)
Interest income		0
Interest income - affiliate		11
Interest expense		(5)
Interest income (expense) - affiliates, net		251
Equity in net (losses) earnings of subsidiaries		(827)
Other income, net		1
Total other expense, net		(569)
INCOME BEFORE INCOME TAX (EXPENSE) BENEFIT		(570)
Income tax (expense) benefit		429
NET (LOSS) INCOME		(141)
Other comprehensive income, net of income taxes		18
COMPREHENSIVE (LOSS) INCOME		(123)
Level 3 Parent, LLC | Reportable Legal Entities | Predecessor
OPERATING REVENUES
Operating revenues										0		0	0
Total operating revenues										0		0	0
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)										0		0	0
Selling, general and administrative										4		16	4
Depreciation and amortization										0		0	0
Total operating expenses										4		16	4
OPERATING INCOME										(4)		(16)	(4)
OTHER INCOME (EXPENSE)
Interest income										0		0	0
Interest income - affiliate										1,260		1,385	1,310
Interest expense										(30)		(36)	(51)
Loss on modification and extinguishment of debt										0
Equity in net (losses) earnings of subsidiaries										(815)		(669)	2,162
Other income, net										3		(1)	(18)
Total other expense, net										418		679	3,403
INCOME BEFORE INCOME TAX (EXPENSE) BENEFIT										414		663	3,399
Income tax (expense) benefit										11		14	34
NET (LOSS) INCOME										425		677	3,433
Other comprehensive income, net of income taxes										80		(86)	(154)
COMPREHENSIVE (LOSS) INCOME										505		591	3,279
Level 3 Financing, Inc. | Reportable Legal Entities
OPERATING REVENUES
Operating revenues		0
Operating revenues - affiliate		0
Total operating revenues		0
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)		0
Selling, general and administrative		3
Operating expenses - affiliate		0
Depreciation and amortization		0
Total operating expenses		3
OPERATING INCOME		(3)
OTHER INCOME (EXPENSE)
Interest income		0
Interest income - affiliate		0
Interest expense		(72)
Interest income (expense) - affiliates, net		368
Equity in net (losses) earnings of subsidiaries		(15)
Other income, net		0
Total other expense, net		281
INCOME BEFORE INCOME TAX (EXPENSE) BENEFIT		278
Income tax (expense) benefit		(1,105)
NET (LOSS) INCOME		(827)
Other comprehensive income, net of income taxes		0
COMPREHENSIVE (LOSS) INCOME		(827)
Level 3 Financing, Inc. | Reportable Legal Entities | Predecessor
OPERATING REVENUES
Operating revenues										0		0	0
Total operating revenues										0		0	0
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)										0		0	0
Selling, general and administrative										3		5	0
Depreciation and amortization										0		0	0
Total operating expenses										3		5	0
OPERATING INCOME										(3)		(5)	0
OTHER INCOME (EXPENSE)
Interest income										0		0	0
Interest income - affiliate										1,890		2,113	1,984
Interest expense										(397)		(505)	(574)
Loss on modification and extinguishment of debt										(44)
Equity in net (losses) earnings of subsidiaries										(2,138)		(2,033)	(1,693)
Other income, net										0		(39)	(200)
Total other expense, net										(689)		(464)	(483)
INCOME BEFORE INCOME TAX (EXPENSE) BENEFIT										(692)		(469)	(483)
Income tax (expense) benefit										(123)		(200)	2,645
NET (LOSS) INCOME										(815)		(669)	2,162
Other comprehensive income, net of income taxes										0		0	0
COMPREHENSIVE (LOSS) INCOME										(815)		(669)	2,162
Level 3 Communications, LLC | Reportable Legal Entities
OPERATING REVENUES
Operating revenues		748
Operating revenues - affiliate		16
Total operating revenues		764
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)		418
Selling, general and administrative		179
Operating expenses - affiliate		24
Depreciation and amortization		117
Total operating expenses		738
OPERATING INCOME		26
OTHER INCOME (EXPENSE)
Interest income		1
Interest income - affiliate		0
Interest expense		0
Interest income (expense) - affiliates, net		(578)
Equity in net (losses) earnings of subsidiaries		71
Other income, net		2
Total other expense, net		(504)
INCOME BEFORE INCOME TAX (EXPENSE) BENEFIT		(478)
Income tax (expense) benefit		433
NET (LOSS) INCOME		(45)
Other comprehensive income, net of income taxes		0
COMPREHENSIVE (LOSS) INCOME		(45)
Level 3 Communications, LLC | Reportable Legal Entities | Predecessor
OPERATING REVENUES
Operating revenues										3,108		3,558	3,326
Total operating revenues										3,108		3,558	3,326
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)										1,942		2,249	2,201
Selling, general and administrative										942		1,025	1,064
Depreciation and amortization										356		372	298
Total operating expenses										3,240		3,646	3,563
OPERATING INCOME										(132)		(88)	(237)
OTHER INCOME (EXPENSE)
Interest income										12		3	0
Interest income - affiliate										(2,896)		(3,215)	(3,041)
Interest expense										(2)		(2)	(3)
Loss on modification and extinguishment of debt										0
Equity in net (losses) earnings of subsidiaries										692		757	177
Other income, net										15		2	2
Total other expense, net										(2,179)		(2,455)	(2,865)
INCOME BEFORE INCOME TAX (EXPENSE) BENEFIT										(2,311)		(2,543)	(3,102)
Income tax (expense) benefit										(2)		(2)	(1)
NET (LOSS) INCOME										(2,313)		(2,545)	(3,103)
Other comprehensive income, net of income taxes										0		0	0
COMPREHENSIVE (LOSS) INCOME										(2,313)		(2,545)	(3,103)
Other Non-Guarantor Subsidiaries | Reportable Legal Entities
OPERATING REVENUES
Operating revenues		671
Operating revenues - affiliate		0
Total operating revenues		671
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)		300
Selling, general and administrative		70
Operating expenses - affiliate		0
Depreciation and amortization		165
Total operating expenses		535
OPERATING INCOME		136
OTHER INCOME (EXPENSE)
Interest income		0
Interest income - affiliate		0
Interest expense		(3)
Interest income (expense) - affiliates, net		(41)
Equity in net (losses) earnings of subsidiaries		0
Other income, net		0
Total other expense, net		(44)
INCOME BEFORE INCOME TAX (EXPENSE) BENEFIT		92
Income tax (expense) benefit		9
NET (LOSS) INCOME		101
Other comprehensive income, net of income taxes		18
COMPREHENSIVE (LOSS) INCOME		$ 119
Other Non-Guarantor Subsidiaries | Reportable Legal Entities | Predecessor
OPERATING REVENUES
Operating revenues										3,891		4,747	5,077
Total operating revenues										3,891		4,747	5,077
OPERATING EXPENSES
Cost of services and products (exclusive of depreciation and amortization)										1,680		2,045	2,321
Selling, general and administrative										259		361	401
Depreciation and amortization										662		787	784
Total operating expenses										2,601		3,193	3,506
OPERATING INCOME										1,290		1,554	1,571
OTHER INCOME (EXPENSE)
Interest income										1		1	1
Interest income - affiliate										(254)		(283)	(253)
Interest expense										(12)		(1)	(12)
Loss on modification and extinguishment of debt										0
Equity in net (losses) earnings of subsidiaries										0		0	0
Other income, net										(4)		(25)	(192)
Total other expense, net										(269)		(308)	(456)
INCOME BEFORE INCOME TAX (EXPENSE) BENEFIT										1,021		1,246	1,115
Income tax (expense) benefit										(154)		23	472
NET (LOSS) INCOME										867		1,269	1,587
Other comprehensive income, net of income taxes										0		(86)	(154)
COMPREHENSIVE (LOSS) INCOME										$ 867		$ 1,183	$ 1,433