Condensed Consolidating Financial Information - Statements of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Condensed Consolidating Financial Information
Net cash provided by operating activities	$ 483	$ 571
INVESTING ACTIVITIES
Capital expenditures	(285)	(252)
Proceeds from sale of property, plant and equipment and other assets	0	1
Deposits received on assets held for sale	0	34
Net cash used in investing activities	(285)	(217)
FINANCING ACTIVITIES
Distributions	(225)	(390)
Other	(1)	(2)
Increase (decrease) due from/to affiliates, net	0	0
Net cash used in financing activities	(226)	(392)
Net decrease in cash, cash equivalents and restricted cash	(28)	(38)
Cash, cash equivalents and restricted cash at beginning of period	272	331
Cash, cash equivalents and restricted cash at end of period	244	293
Eliminations
Condensed Consolidating Financial Information
Net cash provided by operating activities	0	0
INVESTING ACTIVITIES
Capital expenditures	0	0
Proceeds from sale of property, plant and equipment and other assets		0
Deposits received on assets held for sale		0
Net cash used in investing activities	0	0
FINANCING ACTIVITIES
Distributions	0	0
Other	0	0
Increase (decrease) due from/to affiliates, net	0	0
Net cash used in financing activities	0	0
Net decrease in cash, cash equivalents and restricted cash	0	0
Cash, cash equivalents and restricted cash at beginning of period	0	0
Cash, cash equivalents and restricted cash at end of period	0	0
Level 3 Communications, Inc. \| Reportable Legal Entities
Condensed Consolidating Financial Information
Net cash provided by operating activities	17	(8)
INVESTING ACTIVITIES
Capital expenditures	0	0
Proceeds from sale of property, plant and equipment and other assets		0
Deposits received on assets held for sale		34
Net cash used in investing activities	0	34
FINANCING ACTIVITIES
Distributions	(225)	(390)
Other	0	0
Increase (decrease) due from/to affiliates, net	225	390
Net cash used in financing activities	0	0
Net decrease in cash, cash equivalents and restricted cash	17	26
Cash, cash equivalents and restricted cash at beginning of period	17	32
Cash, cash equivalents and restricted cash at end of period	34	58
Level 3 Financing, Inc. \| Reportable Legal Entities
Condensed Consolidating Financial Information
Net cash provided by operating activities	0	0
INVESTING ACTIVITIES
Capital expenditures	0	0
Proceeds from sale of property, plant and equipment and other assets		0
Deposits received on assets held for sale		0
Net cash used in investing activities	0	0
FINANCING ACTIVITIES
Distributions	0	0
Other	0	0
Increase (decrease) due from/to affiliates, net	0	0
Net cash used in financing activities	0	0
Net decrease in cash, cash equivalents and restricted cash	0	0
Cash, cash equivalents and restricted cash at beginning of period	0	0
Cash, cash equivalents and restricted cash at end of period	0	0
Level 3 Communications, LLC \| Reportable Legal Entities
Condensed Consolidating Financial Information
Net cash provided by operating activities	389	490
INVESTING ACTIVITIES
Capital expenditures	(189)	(142)
Proceeds from sale of property, plant and equipment and other assets		0
Deposits received on assets held for sale		0
Net cash used in investing activities	(189)	(142)
FINANCING ACTIVITIES
Distributions	0	0
Other	0	0
Increase (decrease) due from/to affiliates, net	(225)	(390)
Net cash used in financing activities	(225)	(390)
Net decrease in cash, cash equivalents and restricted cash	(25)	(42)
Cash, cash equivalents and restricted cash at beginning of period	173	186
Cash, cash equivalents and restricted cash at end of period	148	144
Other Non-Guarantor Subsidiaries \| Reportable Legal Entities
Condensed Consolidating Financial Information
Net cash provided by operating activities	77	89
INVESTING ACTIVITIES
Capital expenditures	(96)	(110)
Proceeds from sale of property, plant and equipment and other assets		1
Deposits received on assets held for sale		0
Net cash used in investing activities	(96)	(109)
FINANCING ACTIVITIES
Distributions	0	0
Other	(1)	(2)
Increase (decrease) due from/to affiliates, net	0	0
Net cash used in financing activities	(1)	(2)
Net decrease in cash, cash equivalents and restricted cash	(20)	(22)
Cash, cash equivalents and restricted cash at beginning of period	82	113
Cash, cash equivalents and restricted cash at end of period	$ 62	$ 91

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