Condensed Consolidating Financial Information - Statements of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Condensed Consolidating Financial Information
Net Cash Provided by (Used in) Operating Activities			$ 1,791	$ 1,786
Cash Flows from Investing Activities:
Capital expenditures	$ (322)	$ (364)	(1,018)	(1,028)
(Increase) decrease in restricted cash and securities, net			4	11
Purchases of marketable securities			(1,127)	0
Maturity of marketable securities			1,127	0
Proceeds from sale of property, plant, and equipment and other assets			1	1
Net Cash Used in Investing Activities			(1,013)	(1,016)
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			(4,917)	(818)
Increase (decrease) due from-to affiliates, net			0	0
Net Cash Used in Financing Activities			(348)	(54)
Effect of Exchange Rates on Cash and Cash Equivalents			3	(1)
Net Change in Cash and Cash Equivalents			433	715
Cash and Cash Equivalents at Beginning of Period			1,819	854
Cash and Cash Equivalents at End of Period	2,252	1,569	2,252	1,569
Level 3 Communications, Inc.
Condensed Consolidating Financial Information
Net Cash Provided by (Used in) Operating Activities			(32)	(35)
Cash Flows from Investing Activities:
Capital expenditures			0	0
(Increase) decrease in restricted cash and securities, net			3	5
Purchases of marketable securities			0
Maturity of marketable securities			0
Proceeds from sale of property, plant, and equipment and other assets			0	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			28	34
Net Cash Used in Financing Activities			28	34
Effect of Exchange Rates on Cash and Cash Equivalents			0	0
Net Change in Cash and Cash Equivalents			(1)	4
Cash and Cash Equivalents at Beginning of Period			15	12
Cash and Cash Equivalents at End of Period	14	16	14	16
Level 3 Financing, Inc.
Condensed Consolidating Financial Information
Net Cash Provided by (Used in) Operating Activities			(378)	(332)
Cash Flows from Investing Activities:
Capital expenditures			0	0
(Increase) decrease in restricted cash and securities, net			0	0
Purchases of marketable securities			0
Maturity of marketable securities			0
Proceeds from sale of property, plant, and equipment and other assets			0	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			(4,911)	(806)
Increase (decrease) due from-to affiliates, net			720	368
Net Cash Used in Financing Activities			378	326
Effect of Exchange Rates on Cash and Cash Equivalents			0	0
Net Change in Cash and Cash Equivalents			0	(6)
Cash and Cash Equivalents at Beginning of Period			0	6
Cash and Cash Equivalents at End of Period	0	0	0	0
Level 3 Communications, LLC
Condensed Consolidating Financial Information
Net Cash Provided by (Used in) Operating Activities			698	489
Cash Flows from Investing Activities:
Capital expenditures			(614)	(537)
(Increase) decrease in restricted cash and securities, net			(1)	6
Purchases of marketable securities			(1,127)
Maturity of marketable securities			1,127
Proceeds from sale of property, plant, and equipment and other assets			1	0
Net Cash Used in Investing Activities			(614)	(531)
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	(1)
Increase (decrease) due from-to affiliates, net			356	798
Net Cash Used in Financing Activities			357	797
Effect of Exchange Rates on Cash and Cash Equivalents			0	0
Net Change in Cash and Cash Equivalents			441	755
Cash and Cash Equivalents at Beginning of Period			1,700	727
Cash and Cash Equivalents at End of Period	2,141	1,482	2,141	1,482
Other Non-Guarantor Subsidiaries
Condensed Consolidating Financial Information
Net Cash Provided by (Used in) Operating Activities			1,503	1,664
Cash Flows from Investing Activities:
Capital expenditures			(404)	(491)
(Increase) decrease in restricted cash and securities, net			2	0
Purchases of marketable securities			0
Maturity of marketable securities			0
Proceeds from sale of property, plant, and equipment and other assets			0	1
Net Cash Used in Investing Activities			(402)	(490)
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			(7)	(11)
Increase (decrease) due from-to affiliates, net			(1,104)	(1,200)
Net Cash Used in Financing Activities			(1,111)	(1,211)
Effect of Exchange Rates on Cash and Cash Equivalents			3	(1)
Net Change in Cash and Cash Equivalents			(7)	(38)
Cash and Cash Equivalents at Beginning of Period			104	109
Cash and Cash Equivalents at End of Period	97	71	97	71
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
Purchases of marketable securities			0
Maturity of marketable securities			0
Proceeds from sale of property, plant, and equipment and other assets			0	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 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