Condensed Consolidating Financial Information - Statements of Cash Flows (Details) - USD ($) $ in Millions	2 Months Ended		10 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Oct. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Condensed Consolidating Financial Information
Net cash (used in) provided by operating activities	$ 308	$ 308	$ 1,914	$ 2,683	$ 2,397
INVESTING ACTIVITIES
Capital expenditures	(207)	(207)	(1,119)	(1,341)	(1,038)
Purchase of marketable securities	0		(1,127)	0	0
Maturity of marketable securities	0		1,127	0	0
Proceeds from sale of property, plant and equipment and other assets	0		1	28	134
Note receivable - affiliate	(1,825)	(1,825)	0		0
Note receivable - affiliate				235
Net cash used in investing activities	(2,032)	(2,032)	(1,118)	(1,078)	(904)
FINANCING ACTIVITIES
Net proceeds from issuance of long-term debt	0		4,569	2,479	0
Payments of long-term debt	(1)	(1)	(4,917)	(2,906)	(7)
Distributions	(250)	(250)	0	(1,084)	(1,545)
Increase (decrease) due from/to affiliates, net		0	0	0	0
Other	(2)	(2)	3	(28)	0
Net cash used in financing activities	(253)	(253)	(345)	(1,539)	(1,552)
Net increase (decrease) in cash, cash equivalents, restricted cash and securities	(1,977)	(1,977)	451	66	(59)
Cash, cash equivalents, restricted cash and securities at beginning of period	2,308	2,308	1,857	272	331
Cash, cash equivalents, restricted cash and securities at end of period	331	331	2,308	338	272
Eliminations
Condensed Consolidating Financial Information
Net cash (used in) provided by operating activities		0	0	0	0
INVESTING ACTIVITIES
Capital expenditures		0	0	0	0
Purchase of marketable securities			0
Maturity of marketable securities			0
Proceeds from sale of property, plant and equipment and other assets			0	0	0
Note receivable - affiliate		0
Note receivable - affiliate				0
Net cash used in investing activities		0	0	0	0
FINANCING ACTIVITIES
Net proceeds from issuance of long-term debt			0	0
Payments of long-term debt		0	0	0	0
Distributions		0		0	0
Increase (decrease) due from/to affiliates, net		0	0	0	0
Other		0	0	0
Net cash used in financing activities		0	0	0	0
Net increase (decrease) in cash, cash equivalents, restricted cash and securities		0	0	0	0
Cash, cash equivalents, restricted cash and securities at beginning of period	0	0	0	0	0
Cash, cash equivalents, restricted cash and securities at end of period	0	0	0	0	0
Level 3 Parent, LLC | Reportable Legal Entities
Condensed Consolidating Financial Information
Net cash (used in) provided by operating activities		(1)	(61)	313	(98)
INVESTING ACTIVITIES
Capital expenditures		0	0	0	0
Purchase of marketable securities			0
Maturity of marketable securities			0
Proceeds from sale of property, plant and equipment and other assets			0	50	83
Note receivable - affiliate		0
Note receivable - affiliate				235
Net cash used in investing activities		0	0	285	83
FINANCING ACTIVITIES
Net proceeds from issuance of long-term debt			0	0
Payments of long-term debt		0	0	(600)	0
Distributions		(250)		(1,084)	(1,545)
Increase (decrease) due from/to affiliates, net		250	57	1,084	1,545
Other		0	0	0
Net cash used in financing activities		0	57	(600)	0
Net increase (decrease) in cash, cash equivalents, restricted cash and securities		(1)	(4)	(2)	(15)
Cash, cash equivalents, restricted cash and securities at beginning of period	33	33	37	17	32
Cash, cash equivalents, restricted cash and securities at end of period	32	32	33	15	17
Level 3 Financing, Inc. | Reportable Legal Entities
Condensed Consolidating Financial Information
Net cash (used in) provided by operating activities		0	(401)	(151)	0
INVESTING ACTIVITIES
Capital expenditures		0	0	0	0
Purchase of marketable securities			0
Maturity of marketable securities			0
Proceeds from sale of property, plant and equipment and other assets			0	0	0
Note receivable - affiliate		0
Note receivable - affiliate				0
Net cash used in investing activities		0	0	0	0
FINANCING ACTIVITIES
Net proceeds from issuance of long-term debt			4,569	2,479
Payments of long-term debt		0	(4,911)	(2,300)	0
Distributions		0		0	0
Increase (decrease) due from/to affiliates, net		0	743	0	0
Other		0	0	(28)
Net cash used in financing activities		0	401	151	0
Net increase (decrease) in cash, cash equivalents, restricted cash and securities		0	0	0	0
Cash, cash equivalents, restricted cash and securities at beginning of period	0	0	0	0	0
Cash, cash equivalents, restricted cash and securities at end of period	0	0	0	0	0
Level 3 Communications, LLC | Reportable Legal Entities
Condensed Consolidating Financial Information
Net cash (used in) provided by operating activities		172	1,615	1,952	2,059
INVESTING ACTIVITIES
Capital expenditures		(110)	(667)	(782)	(527)
Purchase of marketable securities			(1,127)
Maturity of marketable securities			1,127
Proceeds from sale of property, plant and equipment and other assets			1	(23)	0
Note receivable - affiliate		(1,825)
Note receivable - affiliate				0
Net cash used in investing activities		(1,935)	(666)	(805)	(527)
FINANCING ACTIVITIES
Net proceeds from issuance of long-term debt			0	0
Payments of long-term debt		0	1	0	0
Distributions		0		0	0
Increase (decrease) due from/to affiliates, net		(250)	(460)	(1,084)	(1,545)
Other		0	0	0
Net cash used in financing activities		(250)	(459)	(1,084)	(1,545)
Net increase (decrease) in cash, cash equivalents, restricted cash and securities		(2,013)	490	63	(13)
Cash, cash equivalents, restricted cash and securities at beginning of period	2,200	2,199	1,710	173	186
Cash, cash equivalents, restricted cash and securities at end of period	186	186	2,200	236	173
Other Non-Guarantor Subsidiaries | Reportable Legal Entities
Condensed Consolidating Financial Information
Net cash (used in) provided by operating activities		137	761	569	436
INVESTING ACTIVITIES
Capital expenditures		(97)	(452)	(559)	(511)
Purchase of marketable securities			0
Maturity of marketable securities			0
Proceeds from sale of property, plant and equipment and other assets			0	1	51
Note receivable - affiliate		0
Note receivable - affiliate				0
Net cash used in investing activities		(97)	(452)	(558)	(460)
FINANCING ACTIVITIES
Net proceeds from issuance of long-term debt			0	0
Payments of long-term debt		(1)	(7)	(6)	(7)
Distributions		0		0	0
Increase (decrease) due from/to affiliates, net		0	(340)	0	0
Other		(2)	3	0
Net cash used in financing activities		(3)	(344)	(6)	(7)
Net increase (decrease) in cash, cash equivalents, restricted cash and securities		37	(35)	5	(31)
Cash, cash equivalents, restricted cash and securities at beginning of period	75	76	110	82	113
Cash, cash equivalents, restricted cash and securities at end of period	$ 113	$ 113	$ 75	$ 87	$ 82