Condensed Consolidating Financial Information - Statements of Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2017	Jun. 30, 2016
Condensed Consolidating Financial Information
Revenue	$ 2,061	$ 2,061	$ 2,056	$ 2,056	$ 4,109	$ 4,107
Costs and Expenses:
Network access costs	675	675	676	676	1,366	1,370
Network related expenses	337	337	339	339	673	677
Depreciation and amortization	330	330	310	310	650	611
Selling, general and administrative Expenses	366	366	357	357	730	713
Total Costs and Expenses	1,708	1,708	1,682	1,682	3,419	3,371
Operating Income (Loss)	353	353	374	374	690	736
Other Income (Expense):
Interest income	3	3	1	1	5	2
Interest expense	(132)	(132)	(140)	(140)	(266)	(275)
Interest income (expense) affiliates, net		0		0	0	0
Equity in net earnings (losses) of subsidiaries		0		0	0	0
Other, net		(1)		(45)	(41)	(55)
Total Other Expense	(130)	(130)	(184)	(184)	(302)	(328)
Income (Loss) before Income Taxes	223	223	190	190	388	408
Income Tax Expense	(69)	(69)	(34)	(34)	(139)	(124)
Net Income (Loss)	154	154	156	156	249	284
Other Comprehensive Income (Loss), Net of Income Taxes		41		(22)	62	23
Comprehensive Income (Loss)	$ 195	195	$ 134	134	311	307
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		1		1	2	2
Total Costs and Expenses		1		1	2	2
Operating Income (Loss)		(1)		(1)	(2)	(2)
Other Income (Expense):
Interest income		0		0	0	0
Interest expense		(9)		(9)	(18)	(18)
Interest income (expense) affiliates, net		378		343	755	685
Equity in net earnings (losses) of subsidiaries		(216)		(187)	(491)	(399)
Other, net		0		0	0	0
Total Other Expense		153		147	246	268
Income (Loss) before Income Taxes		152		146	244	266
Income Tax Expense		2		3	5	7
Net Income (Loss)		154		149	249	273
Other Comprehensive Income (Loss), Net of Income Taxes		41		(22)	62	23
Comprehensive Income (Loss)		195		127	311	296
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		2	2	3
Total Costs and Expenses		1		2	2	3
Operating Income (Loss)		(1)		(2)	(2)	(3)
Other Income (Expense):
Interest income		0		0	0	0
Interest expense		(117)		(128)	(237)	(256)
Interest income (expense) affiliates, net		567		528	1,141	1,059
Equity in net earnings (losses) of subsidiaries		(632)		(489)	(1,278)	(1,030)
Other, net		0		(39)	(44)	(39)
Total Other Expense		(182)		(128)	(418)	(266)
Income (Loss) before Income Taxes		(183)		(130)	(420)	(269)
Income Tax Expense		(33)		(50)	(71)	(119)
Net Income (Loss)		(216)		(180)	(491)	(388)
Other Comprehensive Income (Loss), Net of Income Taxes		0		0	0	0
Comprehensive Income (Loss)		(216)		(180)	(491)	(388)
Level 3 Communications, LLC
Condensed Consolidating Financial Information
Revenue		933		879	1,853	1,745
Costs and Expenses:
Network access costs		339		315	662	635
Network related expenses		246		237	489	474
Depreciation and amortization		109		92	213	180
Selling, general and administrative Expenses		294		263	571	513
Total Costs and Expenses		988		907	1,935	1,802
Operating Income (Loss)		(55)		(28)	(82)	(57)
Other Income (Expense):
Interest income		3		1	5	1
Interest expense		(1)		0	(2)	(1)
Interest income (expense) affiliates, net		(868)		(802)	(1,737)	(1,603)
Equity in net earnings (losses) of subsidiaries		200		201	403	400
Other, net		(1)		1	4	3
Total Other Expense		(667)		(599)	(1,327)	(1,200)
Income (Loss) before Income Taxes		(722)		(627)	(1,409)	(1,257)
Income Tax Expense		(1)		(1)	(1)	(2)
Net Income (Loss)		(723)		(628)	(1,410)	(1,259)
Other Comprehensive Income (Loss), Net of Income Taxes		0		0	0	0
Comprehensive Income (Loss)		(723)		(628)	(1,410)	(1,259)
Other Non-Guarantor Subsidiaries
Condensed Consolidating Financial Information
Revenue		1,174		1,211	2,331	2,430
Costs and Expenses:
Network access costs		382		395	779	803
Network related expenses		91		102	184	203
Depreciation and amortization		221		218	437	431
Selling, general and administrative Expenses		70		91	155	195
Total Costs and Expenses		764		806	1,555	1,632
Operating Income (Loss)		410		405	776	798
Other Income (Expense):
Interest income		0		0	0	1
Interest expense		(5)		(3)	(9)	0
Interest income (expense) affiliates, net		(77)		(69)	(159)	(141)
Equity in net earnings (losses) of subsidiaries		0		0	0	0
Other, net		0		(7)	(1)	(19)
Total Other Expense		(82)		(79)	(169)	(159)
Income (Loss) before Income Taxes		328		326	607	639
Income Tax Expense		(37)		14	(72)	(10)
Net Income (Loss)		291		340	535	629
Other Comprehensive Income (Loss), Net of Income Taxes		41		(22)	62	23
Comprehensive Income (Loss)		332		318	597	652
Eliminations
Condensed Consolidating Financial Information
Revenue		(46)		(34)	(75)	(68)
Costs and Expenses:
Network access costs		(46)		(34)	(75)	(68)
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		(46)		(34)	(75)	(68)
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		648		475	1,366	1,029
Other, net		0		0	0	0
Total Other Expense		648		475	1,366	1,029
Income (Loss) before Income Taxes		648		475	1,366	1,029
Income Tax Expense		0		0	0	0
Net Income (Loss)		648		475	1,366	1,029
Other Comprehensive Income (Loss), Net of Income Taxes		(41)		22	(62)	(23)
Comprehensive Income (Loss)		$ 607		$ 497	$ 1,304	$ 1,006