Condensed Consolidating Financial Information - Statements of Cash Flows (Details) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017
Condensed Consolidating Financial Information
Net cash provided by operating activities	$ 1,627
INVESTING ACTIVITIES
Capital expenditures	(726)
Purchase of marketable securities	0
Maturity of marketable securities	0
Proceeds from sale of property, plant and equipment and other assets	119
Net cash used in investing activities	(607)
FINANCING ACTIVITIES
Net proceeds from issuance of long-term debt	0
Payments of long-term debt	(5)
Distributions	(1,130)
Increase (decrease) due from/to affiliates, net	0
Net cash used in financing activities	(1,135)
Effect of exchange rates on cash, cash equivalents and restricted cash and securities	0
Net (decrease) increase in cash, cash equivalents and restricted cash and securities	(115)
Cash, cash equivalents and restricted cash and securities at beginning of period	331
Cash, cash equivalents and restricted cash and securities at end of period	216
Eliminations
Condensed Consolidating Financial Information
Net cash provided by operating activities	0
INVESTING ACTIVITIES
Capital expenditures	0
Proceeds from sale of property, plant and equipment and other assets	0
Net cash used in investing activities	0
FINANCING ACTIVITIES
Payments of long-term debt	0
Distributions	0
Increase (decrease) due from/to affiliates, net	0
Net cash used in financing activities	0
Effect of exchange rates on cash, cash equivalents and restricted cash and securities	0
Net (decrease) increase in cash, cash equivalents and restricted cash and securities	0
Cash, cash equivalents and restricted cash and securities at beginning of period	0
Cash, cash equivalents and restricted cash and securities at end of period	0
Level 3 Communications, Inc. \| Reportable Legal Entities
Condensed Consolidating Financial Information
Net cash provided by operating activities	(49)
INVESTING ACTIVITIES
Capital expenditures	0
Proceeds from sale of property, plant and equipment and other assets	68
Net cash used in investing activities	68
FINANCING ACTIVITIES
Payments of long-term debt	0
Distributions	(1,130)
Increase (decrease) due from/to affiliates, net	1,130
Net cash used in financing activities	0
Effect of exchange rates on cash, cash equivalents and restricted cash and securities	0
Net (decrease) increase in cash, cash equivalents and restricted cash and securities	19
Cash, cash equivalents and restricted cash and securities at beginning of period	32
Cash, cash equivalents and restricted cash and securities at end of period	51
Level 3 Financing, Inc. \| Reportable Legal Entities
Condensed Consolidating Financial Information
Net cash provided by operating activities	0
INVESTING ACTIVITIES
Capital expenditures	0
Proceeds from sale of property, plant and equipment and other assets	0
Net cash used in investing activities	0
FINANCING ACTIVITIES
Payments of long-term debt	0
Distributions	0
Increase (decrease) due from/to affiliates, net	0
Net cash used in financing activities	0
Effect of exchange rates on cash, cash equivalents and restricted cash and securities	0
Net (decrease) increase in cash, cash equivalents and restricted cash and securities	0
Cash, cash equivalents and restricted cash and securities at beginning of period	0
Cash, cash equivalents and restricted cash and securities at end of period	0
Level 3 Communications, LLC \| Reportable Legal Entities
Condensed Consolidating Financial Information
Net cash provided by operating activities	1,420
INVESTING ACTIVITIES
Capital expenditures	(392)
Proceeds from sale of property, plant and equipment and other assets	0
Net cash used in investing activities	(392)
FINANCING ACTIVITIES
Payments of long-term debt	0
Distributions	0
Increase (decrease) due from/to affiliates, net	(1,130)
Net cash used in financing activities	(1,130)
Effect of exchange rates on cash, cash equivalents and restricted cash and securities	0
Net (decrease) increase in cash, cash equivalents and restricted cash and securities	(102)
Cash, cash equivalents and restricted cash and securities at beginning of period	186
Cash, cash equivalents and restricted cash and securities at end of period	84
Other Non-Guarantor Subsidiaries \| Reportable Legal Entities
Condensed Consolidating Financial Information
Net cash provided by operating activities	256
INVESTING ACTIVITIES
Capital expenditures	(334)
Proceeds from sale of property, plant and equipment and other assets	51
Net cash used in investing activities	(283)
FINANCING ACTIVITIES
Payments of long-term debt	(5)
Distributions	0
Increase (decrease) due from/to affiliates, net	0
Net cash used in financing activities	(5)
Effect of exchange rates on cash, cash equivalents and restricted cash and securities	0
Net (decrease) increase in cash, cash equivalents and restricted cash and securities	(32)
Cash, cash equivalents and restricted cash and securities at beginning of period	113
Cash, cash equivalents and restricted cash and securities at end of period	$ 81
Predecessor
Condensed Consolidating Financial Information
Net cash provided by operating activities		$ 1,791
INVESTING ACTIVITIES
Capital expenditures		(1,018)
Purchase of marketable securities		(1,127)
Maturity of marketable securities		1,127
Proceeds from sale of property, plant and equipment and other assets		1
Net cash used in investing activities		(1,017)
FINANCING ACTIVITIES
Net proceeds from issuance of long-term debt		4,569
Payments of long-term debt		(4,917)
Distributions		0
Increase (decrease) due from/to affiliates, net		0
Net cash used in financing activities		(348)
Effect of exchange rates on cash, cash equivalents and restricted cash and securities		3
Net (decrease) increase in cash, cash equivalents and restricted cash and securities		429
Cash, cash equivalents and restricted cash and securities at beginning of period		1,857
Cash, cash equivalents and restricted cash and securities at end of period		2,286
Predecessor \| Eliminations
Condensed Consolidating Financial Information
Net cash provided by operating activities		0
INVESTING ACTIVITIES
Capital expenditures		0
Purchase of marketable securities		0
Maturity of marketable securities		0
Proceeds from sale of property, plant and equipment and other assets		0
Net cash used in investing activities		0
FINANCING ACTIVITIES
Net proceeds from issuance of long-term debt		0
Payments of long-term debt		0
Increase (decrease) due from/to affiliates, net		0
Net cash used in financing activities		0
Effect of exchange rates on cash, cash equivalents and restricted cash and securities		0
Net (decrease) increase in cash, cash equivalents and restricted cash and securities		0
Cash, cash equivalents and restricted cash and securities at beginning of period		0
Cash, cash equivalents and restricted cash and securities at end of period		0
Predecessor \| Level 3 Communications, Inc. \| Reportable Legal Entities
Condensed Consolidating Financial Information
Net cash provided by operating activities		(32)
INVESTING ACTIVITIES
Capital expenditures		0
Purchase of marketable securities		0
Maturity of marketable securities		0
Proceeds from sale of property, plant and equipment and other assets		0
Net cash used in investing activities		0
FINANCING ACTIVITIES
Net proceeds from issuance of long-term debt		0
Payments of long-term debt		0
Increase (decrease) due from/to affiliates, net		28
Net cash used in financing activities		28
Effect of exchange rates on cash, cash equivalents and restricted cash and securities		0
Net (decrease) increase in cash, cash equivalents and restricted cash and securities		(4)
Cash, cash equivalents and restricted cash and securities at beginning of period		37
Cash, cash equivalents and restricted cash and securities at end of period		33
Predecessor \| Level 3 Financing, Inc. \| Reportable Legal Entities
Condensed Consolidating Financial Information
Net cash provided by operating activities		(378)
INVESTING ACTIVITIES
Capital expenditures		0
Purchase of marketable securities		0
Maturity of marketable securities		0
Proceeds from sale of property, plant and equipment and other assets		0
Net cash used in investing activities		0
FINANCING ACTIVITIES
Net proceeds from issuance of long-term debt		4,569
Payments of long-term debt		(4,911)
Increase (decrease) due from/to affiliates, net		720
Net cash used in financing activities		378
Effect of exchange rates on cash, cash equivalents and restricted cash and securities		0
Net (decrease) increase in cash, cash equivalents and restricted cash and securities		0
Cash, cash equivalents and restricted cash and securities at beginning of period		0
Cash, cash equivalents and restricted cash and securities at end of period		0
Predecessor \| Level 3 Communications, LLC \| Reportable Legal Entities
Condensed Consolidating Financial Information
Net cash provided by operating activities		698
INVESTING ACTIVITIES
Capital expenditures		(614)
Purchase of marketable securities		(1,127)
Maturity of marketable securities		1,127
Proceeds from sale of property, plant and equipment and other assets		1
Net cash used in investing activities		(613)
FINANCING ACTIVITIES
Net proceeds from issuance of long-term debt		0
Payments of long-term debt		1
Increase (decrease) due from/to affiliates, net		356
Net cash used in financing activities		357
Effect of exchange rates on cash, cash equivalents and restricted cash and securities		0
Net (decrease) increase in cash, cash equivalents and restricted cash and securities		442
Cash, cash equivalents and restricted cash and securities at beginning of period		1,710
Cash, cash equivalents and restricted cash and securities at end of period		2,152
Predecessor \| Other Non-Guarantor Subsidiaries \| Reportable Legal Entities
Condensed Consolidating Financial Information
Net cash provided by operating activities		1,503
INVESTING ACTIVITIES
Capital expenditures		(404)
Purchase of marketable securities		0
Maturity of marketable securities		0
Proceeds from sale of property, plant and equipment and other assets		0
Net cash used in investing activities		(404)
FINANCING ACTIVITIES
Net proceeds from issuance of long-term debt		0
Payments of long-term debt		(7)
Increase (decrease) due from/to affiliates, net		(1,104)
Net cash used in financing activities		(1,111)
Effect of exchange rates on cash, cash equivalents and restricted cash and securities		3
Net (decrease) increase in cash, cash equivalents and restricted cash and securities		(9)
Cash, cash equivalents and restricted cash and securities at beginning of period		110
Cash, cash equivalents and restricted cash and securities at end of period		$ 101

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