Represents the premium price of debt instrument as a percentage of par amount.

No definition available.

Name:	ccoi_DebtInstrumentBorrowingsAsPercentageOfPar
Namespace Prefix:	ccoi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the percentage of outstanding principal amount used in calculation of make-whole premium.

+ References

No definition available.

+ Details

Name:	ccoi_DebtInstrumentMakeWholePremiumAsPercentageOfOutstandingPrincipalAmount
Namespace Prefix:	ccoi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the redemption price as a percentage of the principal amount at which the debt instrument is required to be redeemed in the event of a change of control.

+ References

No definition available.

+ Details

Name:	ccoi_DebtInstrumentRedemptionPriceDueToChangeOfControlAsPercentageOfPrincipalAmount
Namespace Prefix:	ccoi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the maximum percentage of the principal amount of the debt instrument that the entity could redeem with net cash proceeds from certain equity offerings.

+ References

No definition available.

+ Details

Name:	ccoi_DebtInstrumentRedemptionWithNetProceedsFromEquityOfferingsAsPercentageOfOriginalPrincipalMaximum
Namespace Prefix:	ccoi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=34725769&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399

+ Details

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage price of original principal amount of debt at which debt can be redeemed by the issuer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph e
-URI http://asc.fasb.org/extlink&oid=6479336&loc=d3e64711-112823

+ Details

Name:	us-gaap_DebtInstrumentRedemptionPricePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of debt premium that was originally recognized at the issuance of the instrument that has yet to be amortized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28541-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=34725769&loc=d3e28878-108400

+ Details

Name:	us-gaap_DebtInstrumentUnamortizedPremium
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The cash outflow paid to third parties in connection with debt origination, which will be amortized over the remaining maturity period of the associated long-term debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3291-108585

+ Details

Name:	us-gaap_PaymentsOfDebtIssuanceCosts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The cash inflow from additional borrowings, net of cash paid to third parties in connection with debt origination.

+ References

No definition available.

+ Details

Name:	us-gaap_ProceedsFromDebtNetOfIssuanceCosts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- References

No definition available.

+ Details

Name:	us-gaap_UnsecuredLongtermDebtCurrentAndNoncurrentAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_DebtInstrumentAxis=ccoi_SeniorSecured2018NotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentRedemptionPeriodAxis=ccoi_DebtInstrumentRedemptionPeriodTwelveMonthPeriodBeginningFebruary2017AndThereafterMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentRedemptionPeriodAxis=ccoi_DebtInstrumentRedemptionPeriodTwelveMonthPeriodBeginningFebruary2016Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentRedemptionPeriodAxis=ccoi_DebtInstrumentRedemptionPeriodPriorToFebruary2015Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentRedemptionPeriodAxis=ccoi_DebtInstrumentRedemptionPeriodTwelveMonthPeriodBeginningFebruary2015Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Long-term debt: Senior secured notes-2018 Notes (Details) - Senior Secured 2018 Notes Member - USD ($) $ in Thousands			12 Months Ended
	Aug. 19, 2013	Jan. 26, 2011	Dec. 31, 2015	Dec. 31, 2014	Mar. 31, 2015
Senior secured notes-2018 Notes
Interest rate (as a percent)	8.375%	8.375%			8.375%
Aggregate principal amount of debt issued	$ 65,000	$ 175,000
Proceeds from issuance of long-term debt, net of issuance costs	69,900	170,500
Debt issuance costs	1,000	$ 4,500	$ 968
Senior secured notes, premium (in dollars)	$ 5,900
Premium price of debt instrument (as a percent)	109.00%
Percentage of principal amount at which notes will be required to be repurchased in the event of a change of control			101.00%
Treasury Rate
Senior secured notes-2018 Notes
Discount rate used to compute make-whole premium, basis points added to reference rate (as a percent)			0.50%
12-month period beginning on February 15, 2017 and thereafter
Senior secured notes-2018 Notes
Percentage of principal amount that the holders of the Convertible Notes may require the Company to repurchase			100.00%
12-month period beginning on February 15, 2016
Senior secured notes-2018 Notes
Percentage of principal amount that the holders of the Convertible Notes may require the Company to repurchase			102.094%
Prior to February 15, 2015
Senior secured notes-2018 Notes
Percentage of principal amount that the holders of the Convertible Notes may require the Company to repurchase			100.00%	108.375%
Percentage of outstanding principal amount used in calculation of make-whole premium			1.00%
Maximum percentage of principal amount of debt instrument which the entity could redeem with proceeds from certain equity offerings				35.00%
12-month period beginning on February 15, 2015
Senior secured notes-2018 Notes
Percentage of principal amount that the holders of the Convertible Notes may require the Company to repurchase			104.188%