Condensed Consolidating Financial Information - Statements of Comprehensive Income (Loss) (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
Revenue	$ 2,059	$ 2,033	$ 6,168	$ 6,140
Costs and Expenses:
Network access costs	678	675	2,044	2,045
Network related expenses	345	337	1,018	1,014
Depreciation and amortization	333	319	983	930
Selling, general and administrative expenses	348	348	1,078	1,061
Total Costs and Expenses	1,704	1,679	5,123	5,050
Operating Income (Loss)	355	354	1,045	1,090
Other Income (Expense):
Interest income	6	1	11	3
Interest expense	(134)	(139)	(400)	(414)
Interest income (expense) affiliates, net	0	0	0	0
Equity in net earnings (losses) of subsidiaries	0	0	0	0
Other, net	6	1	(35)	(54)
Total Other Expense	(122)	(137)	(424)	(465)
Income (Loss) before Income Taxes	233	217	621	625
Income Tax Expense	(76)	(74)	(215)	(198)
Net Income (Loss)	157	143	406	427
Other Comprehensive Income (Loss), Net of Income Taxes	44	(15)	106	8
Comprehensive Income (Loss)	201	128	512	435
Level 3 Communications, Inc.
Condensed Consolidating Financial Information
Revenue	0	0	0	0
Costs and Expenses:
Network access costs	0	0	0	0
Network related expenses	0	0	0	0
Depreciation and amortization	0	0	0	0
Selling, general and administrative expenses	2	1	4	3
Total Costs and Expenses	2	1	4	3
Operating Income (Loss)	(2)	(1)	(4)	(3)
Other Income (Expense):
Interest income	0	0	0	0
Interest expense	(9)	(9)	(27)	(27)
Interest income (expense) affiliates, net	377	342	1,132	1,027
Equity in net earnings (losses) of subsidiaries	(212)	(193)	(703)	(581)
Other, net	0	0	0	0
Total Other Expense	156	140	402	419
Income (Loss) before Income Taxes	154	139	398	416
Income Tax Expense	3	4	8	11
Net Income (Loss)	157	143	406	427
Other Comprehensive Income (Loss), Net of Income Taxes	44	(15)	106	8
Comprehensive Income (Loss)	201	128	512	435
Level 3 Financing, Inc.
Condensed Consolidating Financial Information
Revenue	0	0	0	0
Costs and Expenses:
Network access costs	0	0	0	0
Network related expenses	0	0	0	0
Depreciation and amortization	0	0	0	0
Selling, general and administrative expenses	1	1	3	4
Total Costs and Expenses	1	1	3	4
Operating Income (Loss)	(1)	(1)	(3)	(4)
Other Income (Expense):
Interest income	0	0	0	0
Interest expense	(121)	(124)	(358)	(380)
Interest income (expense) affiliates, net	562	526	1,703	1,585
Equity in net earnings (losses) of subsidiaries	(614)	(539)	(1,892)	(1,568)
Other, net	0	0	(44)	(39)
Total Other Expense	(173)	(137)	(591)	(402)
Income (Loss) before Income Taxes	(174)	(138)	(594)	(406)
Income Tax Expense	(38)	(55)	(109)	(175)
Net Income (Loss)	(212)	(193)	(703)	(581)
Other Comprehensive Income (Loss), Net of Income Taxes	0	0	0	0
Comprehensive Income (Loss)	(212)	(193)	(703)	(581)
Level 3 Communications, LLC
Condensed Consolidating Financial Information
Revenue	932	907	2,785	2,652
Costs and Expenses:
Network access costs	329	330	991	965
Network related expenses	254	242	743	716
Depreciation and amortization	114	100	327	280
Selling, general and administrative expenses	272	262	843	775
Total Costs and Expenses	969	934	2,904	2,736
Operating Income (Loss)	(37)	(27)	(119)	(84)
Other Income (Expense):
Interest income	6	1	11	2
Interest expense	0	(1)	(2)	(2)
Interest income (expense) affiliates, net	(868)	(804)	(2,605)	(2,407)
Equity in net earnings (losses) of subsidiaries	215	169	618	569
Other, net	12	(1)	16	2
Total Other Expense	(635)	(636)	(1,962)	(1,836)
Income (Loss) before Income Taxes	(672)	(663)	(2,081)	(1,920)
Income Tax Expense	(1)	0	(2)	(2)
Net Income (Loss)	(673)	(663)	(2,083)	(1,922)
Other Comprehensive Income (Loss), Net of Income Taxes	0	0	0	0
Comprehensive Income (Loss)	(673)	(663)	(2,083)	(1,922)
Other Non-Guarantor Subsidiaries
Condensed Consolidating Financial Information
Revenue	1,168	1,160	3,499	3,590
Costs and Expenses:
Network access costs	390	379	1,169	1,182
Network related expenses	91	95	275	298
Depreciation and amortization	219	219	656	650
Selling, general and administrative expenses	73	84	228	279
Total Costs and Expenses	773	777	2,328	2,409
Operating Income (Loss)	395	383	1,171	1,181
Other Income (Expense):
Interest income	0	0	0	1
Interest expense	(4)	(5)	(13)	(5)
Interest income (expense) affiliates, net	(71)	(64)	(230)	(205)
Equity in net earnings (losses) of subsidiaries	0	0	0	0
Other, net	(6)	2	(7)	(17)
Total Other Expense	(81)	(67)	(250)	(226)
Income (Loss) before Income Taxes	314	316	921	955
Income Tax Expense	(40)	(23)	(112)	(32)
Net Income (Loss)	274	293	809	923
Other Comprehensive Income (Loss), Net of Income Taxes	44	(15)	106	8
Comprehensive Income (Loss)	318	278	915	931
Eliminations
Condensed Consolidating Financial Information
Revenue	(41)	(34)	(116)	(102)
Costs and Expenses:
Network access costs	(41)	(34)	(116)	(102)
Network related expenses	0	0	0	0
Depreciation and amortization	0	0	0	0
Selling, general and administrative expenses	0	0	0	0
Total Costs and Expenses	(41)	(34)	(116)	(102)
Operating Income (Loss)	0	0	0	0
Other Income (Expense):
Interest income	0	0	0	0
Interest expense	0	0	0	0
Interest income (expense) affiliates, net	0	0	0	0
Equity in net earnings (losses) of subsidiaries	611	563	1,977	1,580
Other, net	0	0	0	0
Total Other Expense	611	563	1,977	1,580
Income (Loss) before Income Taxes	611	563	1,977	1,580
Income Tax Expense	0	0	0	0
Net Income (Loss)	611	563	1,977	1,580
Other Comprehensive Income (Loss), Net of Income Taxes	(44)	15	(106)	(8)
Comprehensive Income (Loss)	$ 567	$ 578	$ 1,871	$ 1,572