Condensed Consolidating Financial Information - Statements of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Condensed Consolidating Financial Information
Net Cash Provided by (Used in) Operating Activities			$ 1,100	$ 1,141
Cash Flows from Investing Activities:
Capital expenditures	$ (328)	$ (367)	(696)	(664)
(Increase) decrease in restricted cash and securities, net			4	11
Other				1
Purchases of marketable securities			(1,127)	0
Proceeds from sale of property, plant, and equipment and other assets			0	1
Net Cash Used in Investing Activities			(1,819)	(652)
Cash Flows from Financing Activities:
Long-term debt borrowings, net of issuance costs			4,569	764
Payments on and repurchases of long-term debt and capital leases			(4,615)	(816)
Increase (decrease) due from-to affiliates, net			0	0
Net Cash (Used in) Provided by Financing Activities			(46)	(52)
Effect of Exchange Rates on Cash and Cash Equivalents			2	0
Net Change in Cash and Cash Equivalents			(763)	437
Cash and Cash Equivalents at Beginning of Period			1,819	854
Cash and Cash Equivalents at End of Period	1,056	1,291	1,056	1,291
Level 3 Communications, Inc.
Condensed Consolidating Financial Information
Net Cash Provided by (Used in) Operating Activities			(14)	(18)
Cash Flows from Investing Activities:
Capital expenditures			0	0
(Increase) decrease in restricted cash and securities, net			3	5
Other				0
Purchases of marketable securities			0
Proceeds from sale of property, plant, and equipment and other assets			0
Net Cash Used in Investing Activities			3	5
Cash Flows from Financing Activities:
Long-term debt borrowings, net of issuance costs			0	0
Payments on and repurchases of long-term debt and capital leases			0	0
Increase (decrease) due from-to affiliates, net			10	12
Net Cash (Used in) Provided by Financing Activities			10	12
Effect of Exchange Rates on Cash and Cash Equivalents			0	0
Net Change in Cash and Cash Equivalents			(1)	(1)
Cash and Cash Equivalents at Beginning of Period			15	12
Cash and Cash Equivalents at End of Period	14	11	14	11
Level 3 Financing, Inc.
Condensed Consolidating Financial Information
Net Cash Provided by (Used in) Operating Activities			(267)	(225)
Cash Flows from Investing Activities:
Capital expenditures			0	0
(Increase) decrease in restricted cash and securities, net			0	0
Other				0
Purchases of marketable securities			0
Proceeds from sale of property, plant, and equipment and other assets			0
Net Cash Used in Investing Activities			0	0
Cash Flows from Financing Activities:
Long-term debt borrowings, net of issuance costs			4,569	764
Payments on and repurchases of long-term debt and capital leases			(4,611)	(806)
Increase (decrease) due from-to affiliates, net			309	267
Net Cash (Used in) Provided by Financing Activities			267	225
Effect of Exchange Rates on Cash and Cash Equivalents			0	0
Net Change in Cash and Cash Equivalents			0	0
Cash and Cash Equivalents at Beginning of Period			0	6
Cash and Cash Equivalents at End of Period	0	6	0	6
Level 3 Communications, LLC
Condensed Consolidating Financial Information
Net Cash Provided by (Used in) Operating Activities			352	274
Cash Flows from Investing Activities:
Capital expenditures			(427)	(333)
(Increase) decrease in restricted cash and securities, net			(1)	6
Other				0
Purchases of marketable securities			(1,127)
Proceeds from sale of property, plant, and equipment and other assets			0
Net Cash Used in Investing Activities			(1,555)	(327)
Cash Flows from Financing Activities:
Long-term debt borrowings, net of issuance costs			0	0
Payments on and repurchases of long-term debt and capital leases			(1)	0
Increase (decrease) due from-to affiliates, net			442	528
Net Cash (Used in) Provided by Financing Activities			441	528
Effect of Exchange Rates on Cash and Cash Equivalents			0	0
Net Change in Cash and Cash Equivalents			(762)	475
Cash and Cash Equivalents at Beginning of Period			1,700	727
Cash and Cash Equivalents at End of Period	938	1,202	938	1,202
Other Non-Guarantor Subsidiaries
Condensed Consolidating Financial Information
Net Cash Provided by (Used in) Operating Activities			1,029	1,110
Cash Flows from Investing Activities:
Capital expenditures			(269)	(331)
(Increase) decrease in restricted cash and securities, net			2	0
Other				1
Purchases of marketable securities			0
Proceeds from sale of property, plant, and equipment and other assets			0
Net Cash Used in Investing Activities			(267)	(330)
Cash Flows from Financing Activities:
Long-term debt borrowings, net of issuance costs			0	0
Payments on and repurchases of long-term debt and capital leases			(3)	(10)
Increase (decrease) due from-to affiliates, net			(761)	(807)
Net Cash (Used in) Provided by Financing Activities			(764)	(817)
Effect of Exchange Rates on Cash and Cash Equivalents			2	0
Net Change in Cash and Cash Equivalents			0	(37)
Cash and Cash Equivalents at Beginning of Period			104	109
Cash and Cash Equivalents at End of Period	104	72	104	72
Eliminations
Condensed Consolidating Financial Information
Net Cash Provided by (Used in) Operating Activities			0	0
Cash Flows from Investing Activities:
Capital expenditures			0	0
(Increase) decrease in restricted cash and securities, net			0	0
Other				0
Purchases of marketable securities			0
Proceeds from sale of property, plant, and equipment and other assets			0
Net Cash Used in Investing Activities			0	0
Cash Flows from Financing Activities:
Long-term debt borrowings, net of issuance costs			0	0
Payments on and repurchases of long-term debt and capital leases			0	0
Increase (decrease) due from-to affiliates, net			0	0
Net Cash (Used in) Provided by Financing Activities			0	0
Effect of Exchange Rates on Cash and Cash Equivalents			0	0
Net Change in Cash and Cash Equivalents			0	0
Cash and Cash Equivalents at Beginning of Period			0	0
Cash and Cash Equivalents at End of Period	$ 0	$ 0	$ 0	$ 0