Condensed Consolidating Financial Information - Statements of Cash Flows (Details) - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017
Condensed Consolidating Financial Information
Net cash provided by operating activities	$ 1,018
INVESTING ACTIVITIES
Capital expenditures	(546)
Purchase of marketable securities	0
Proceeds from sale of property, plant and equipment and other assets	119
Net cash used in investing activities	(427)
Cash Flows from Financing Activities:
Net proceeds from issuance of long-term debt	0
Payments of long-term debt	(3)
Distributions	(605)
Increase (decrease) due from/to affiliates, net	0
Net cash used in financing activities	(608)
Effect of exchange rates on cash, cash equivalents and restricted cash and securities	(2)
Net decrease in cash, cash equivalents and restricted cash and securities	(19)
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	312
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
Cash Flows from 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 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	(85)
INVESTING ACTIVITIES
Capital expenditures	0
Proceeds from sale of property, plant and equipment and other assets	68
Net cash used in investing activities	68
Cash Flows from Financing Activities:
Payments of long-term debt	0
Distributions	(605)
Increase (decrease) due from/to affiliates, net	605
Net cash used in financing activities	0
Effect of exchange rates on cash, cash equivalents and restricted cash and securities	0
Net decrease in cash, cash equivalents and restricted cash and securities	(17)
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	15
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
Cash Flows from 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 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	899
INVESTING ACTIVITIES
Capital expenditures	(289)
Proceeds from sale of property, plant and equipment and other assets	0
Net cash used in investing activities	(289)
Cash Flows from Financing Activities:
Payments of long-term debt	0
Distributions	0
Increase (decrease) due from/to affiliates, net	(605)
Net cash used in financing activities	(605)
Effect of exchange rates on cash, cash equivalents and restricted cash and securities	0
Net decrease in cash, cash equivalents and restricted cash and securities	5
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	191
Other Non-Guarantor Subsidiaries \| Reportable Legal Entities
Condensed Consolidating Financial Information
Net cash provided by operating activities	204
INVESTING ACTIVITIES
Capital expenditures	(257)
Proceeds from sale of property, plant and equipment and other assets	51
Net cash used in investing activities	(206)
Cash Flows from Financing Activities:
Payments of long-term debt	(3)
Distributions	0
Increase (decrease) due from/to affiliates, net	0
Net cash used in financing activities	(3)
Effect of exchange rates on cash, cash equivalents and restricted cash and securities	(2)
Net decrease in cash, cash equivalents and restricted cash and securities	(7)
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	$ 106
Predecessor
Condensed Consolidating Financial Information
Net cash provided by operating activities		$ 1,100
INVESTING ACTIVITIES
Capital expenditures		(696)
Purchase of marketable securities		(1,127)
Proceeds from sale of property, plant and equipment and other assets		0
Net cash used in investing activities		(1,823)
Cash Flows from Financing Activities:
Net proceeds from issuance of long-term debt		4,569
Payments of long-term debt		(4,615)
Distributions		0
Increase (decrease) due from/to affiliates, net		0
Net cash used in financing activities		(46)
Effect of exchange rates on cash, cash equivalents and restricted cash and securities		2
Net decrease in cash, cash equivalents and restricted cash and securities		(767)
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		1,090
Predecessor \| Eliminations
Condensed Consolidating Financial Information
Net cash provided by operating activities		0
INVESTING ACTIVITIES
Capital expenditures		0
Purchase of marketable securities		0
Net cash used in investing activities		0
Cash Flows from 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 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		(14)
INVESTING ACTIVITIES
Capital expenditures		0
Purchase of marketable securities		0
Net cash used in investing activities		0
Cash Flows from Financing Activities:
Net proceeds from issuance of long-term debt		0
Payments of long-term debt		0
Increase (decrease) due from/to affiliates, net		10
Net cash used in financing activities		10
Effect of exchange rates on cash, cash equivalents and restricted cash and securities		0
Net decrease 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		(267)
INVESTING ACTIVITIES
Capital expenditures		0
Purchase of marketable securities		0
Net cash used in investing activities		0
Cash Flows from Financing Activities:
Net proceeds from issuance of long-term debt		4,569
Payments of long-term debt		(4,611)
Increase (decrease) due from/to affiliates, net		309
Net cash used in financing activities		267
Effect of exchange rates on cash, cash equivalents and restricted cash and securities		0
Net decrease 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		352
INVESTING ACTIVITIES
Capital expenditures		(427)
Purchase of marketable securities		(1,127)
Net cash used in investing activities		(1,554)
Cash Flows from Financing Activities:
Net proceeds from issuance of long-term debt		0
Payments of long-term debt		(1)
Increase (decrease) due from/to affiliates, net		442
Net cash used in financing activities		441
Effect of exchange rates on cash, cash equivalents and restricted cash and securities		0
Net decrease in cash, cash equivalents and restricted cash and securities		(761)
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		949
Predecessor \| Other Non-Guarantor Subsidiaries \| Reportable Legal Entities
Condensed Consolidating Financial Information
Net cash provided by operating activities		1,029
INVESTING ACTIVITIES
Capital expenditures		(269)
Purchase of marketable securities		0
Net cash used in investing activities		(269)
Cash Flows from Financing Activities:
Net proceeds from issuance of long-term debt		0
Payments of long-term debt		(3)
Increase (decrease) due from/to affiliates, net		(761)
Net cash used in financing activities		(764)
Effect of exchange rates on cash, cash equivalents and restricted cash and securities		2
Net decrease in cash, cash equivalents and restricted cash and securities		(2)
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		$ 108

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