Condensed Consolidating Financial Information - Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Current Assets:
Cash and cash equivalents	$ 2,252	$ 1,819	$ 1,569	$ 854
Restricted cash and securities	5	7
Receivables, less allowances for doubtful accounts	750	712
Due from affiliates	0	0
Other	136	115
Total Current Assets	3,143	2,653
Property, Plant and Equipment, net	10,485	10,139
Restricted Cash and Securities	29	31
Goodwill and Other Intangibles Assets, net	8,502	8,644
Investment in Subsidiaries	0	0
Deferred Tax Assets	3,162	3,370
Other Assets, net	52	51
Total Assets	25,373	24,888
Liabilities and Stockholders’ Equity:
Accounts payable	704	706
Current portion of long-term debt	7	7
Accrued payroll and employee benefits	247	195
Accrued interest	95	129
Current portion of deferred revenue	276	266
Due to affiliates	0	0
Other	139	168
Total Current Liabilities	1,468	1,471
Long-Term Debt, less current portion	10,586	10,877
Deferred Revenue, less current portion	1,132	1,001
Other Liabilities	637	622
Commitments and Contingencies
Stockholders' Equity (Deficit)	11,550	10,917
Total Liabilities and Stockholders’ Equity	25,373	24,888
Level 3 Communications, Inc.
Current Assets:
Cash and cash equivalents	14	15	16	12
Restricted cash and securities	0	0
Receivables, less allowances for doubtful accounts	0	0
Due from affiliates	18,214	17,032
Other	0	0
Total Current Assets	18,228	17,047
Property, Plant and Equipment, net	0	0
Restricted Cash and Securities	19	22
Goodwill and Other Intangibles Assets, net	0	0
Investment in Subsidiaries	16,947	16,869
Deferred Tax Assets	58	51
Other Assets, net	0	0
Total Assets	35,252	33,989
Liabilities and Stockholders’ Equity:
Accounts payable	0	0
Current portion of long-term debt	0	0
Accrued payroll and employee benefits	0	0
Accrued interest	3	11
Current portion of deferred revenue	0	0
Due to affiliates	0	0
Other	0	0
Total Current Liabilities	3	11
Long-Term Debt, less current portion	593	592
Deferred Revenue, less current portion	0	0
Other Liabilities	16	16
Commitments and Contingencies
Stockholders' Equity (Deficit)	34,640	33,370
Total Liabilities and Stockholders’ Equity	35,252	33,989
Level 3 Financing, Inc.
Current Assets:
Cash and cash equivalents	0	0	0	6
Restricted cash and securities	0	0
Receivables, less allowances for doubtful accounts	0	0
Due from affiliates	22,749	21,715
Other	0	0
Total Current Assets	22,749	21,715
Property, Plant and Equipment, net	0	0
Restricted Cash and Securities	0	0
Goodwill and Other Intangibles Assets, net	0	0
Investment in Subsidiaries	17,141	17,599
Deferred Tax Assets	2,577	2,687
Other Assets, net	0	0
Total Assets	42,467	42,001
Liabilities and Stockholders’ Equity:
Accounts payable	1	0
Current portion of long-term debt	0	0
Accrued payroll and employee benefits	0	0
Accrued interest	86	110
Current portion of deferred revenue	0	0
Due to affiliates	0	0
Other	(1)	0
Total Current Liabilities	86	110
Long-Term Debt, less current portion	9,820	10,108
Deferred Revenue, less current portion	0	0
Other Liabilities	0	0
Commitments and Contingencies
Stockholders' Equity (Deficit)	32,561	31,783
Total Liabilities and Stockholders’ Equity	42,467	42,001
Level 3 Communications, LLC
Current Assets:
Cash and cash equivalents	2,141	1,700	1,482	727
Restricted cash and securities	1	1
Receivables, less allowances for doubtful accounts	25	26
Due from affiliates	0	0
Other	110	87
Total Current Assets	2,277	1,814
Property, Plant and Equipment, net	4,204	3,869
Restricted Cash and Securities	10	9
Goodwill and Other Intangibles Assets, net	352	353
Investment in Subsidiaries	3,626	3,674
Deferred Tax Assets	0	0
Other Assets, net	18	16
Total Assets	10,487	9,735
Liabilities and Stockholders’ Equity:
Accounts payable	331	307
Current portion of long-term debt	2	2
Accrued payroll and employee benefits	216	160
Accrued interest	0	0
Current portion of deferred revenue	128	116
Due to affiliates	44,216	40,927
Other	92	127
Total Current Liabilities	44,985	41,639
Long-Term Debt, less current portion	13	13
Deferred Revenue, less current portion	843	719
Other Liabilities	185	155
Commitments and Contingencies
Stockholders' Equity (Deficit)	(35,539)	(32,791)
Total Liabilities and Stockholders’ Equity	10,487	9,735
Other Non-Guarantor Subsidiaries
Current Assets:
Cash and cash equivalents	97	104	71	109
Restricted cash and securities	4	6
Receivables, less allowances for doubtful accounts	725	686
Due from affiliates	3,253	2,180
Other	26	28
Total Current Assets	4,105	3,004
Property, Plant and Equipment, net	6,281	6,270
Restricted Cash and Securities	0	0
Goodwill and Other Intangibles Assets, net	8,150	8,291
Investment in Subsidiaries	0	0
Deferred Tax Assets	527	632
Other Assets, net	34	35
Total Assets	19,097	18,232
Liabilities and Stockholders’ Equity:
Accounts payable	372	399
Current portion of long-term debt	5	5
Accrued payroll and employee benefits	31	35
Accrued interest	6	8
Current portion of deferred revenue	148	150
Due to affiliates	0	0
Other	48	41
Total Current Liabilities	610	638
Long-Term Debt, less current portion	160	164
Deferred Revenue, less current portion	289	282
Other Liabilities	436	451
Commitments and Contingencies
Stockholders' Equity (Deficit)	17,602	16,697
Total Liabilities and Stockholders’ Equity	19,097	18,232
Eliminations
Current Assets:
Cash and cash equivalents	0	0	$ 0	$ 0
Restricted cash and securities	0	0
Receivables, less allowances for doubtful accounts	0	0
Due from affiliates	(44,216)	(40,927)
Other	0	0
Total Current Assets	(44,216)	(40,927)
Property, Plant and Equipment, net	0	0
Restricted Cash and Securities	0	0
Goodwill and Other Intangibles Assets, net	0	0
Investment in Subsidiaries	(37,714)	(38,142)
Deferred Tax Assets	0	0
Other Assets, net	0	0
Total Assets	(81,930)	(79,069)
Liabilities and Stockholders’ Equity:
Accounts payable	0	0
Current portion of long-term debt	0	0
Accrued payroll and employee benefits	0	0
Accrued interest	0	0
Current portion of deferred revenue	0	0
Due to affiliates	(44,216)	(40,927)
Other	0	0
Total Current Liabilities	(44,216)	(40,927)
Long-Term Debt, less current portion	0	0
Deferred Revenue, less current portion	0	0
Other Liabilities	0	0
Commitments and Contingencies
Stockholders' Equity (Deficit)	(37,714)	(38,142)
Total Liabilities and Stockholders’ Equity	$ (81,930)	$ (79,069)