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

No definition available.

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- 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

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Namespace Prefix:	ccoi_
Data Type:	num:percentItemType
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- Definition

Represents the percentage of redemption price of principal amount accrued interest.

+ References

No definition available.

+ Details

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Data Type:	num:percentItemType
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- Definition

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

+ References

No definition available.

+ Details

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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

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Namespace Prefix:	ccoi_
Data Type:	num:percentItemType
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- Definition

Refers to the accrued interest expense related to the repurchase of debt.

+ References

No definition available.

+ Details

Name:	ccoi_InterestExpenseDebtRelatedToRepurchaseOfDebt
Namespace Prefix:	ccoi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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

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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

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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

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Face (par) amount of the original debt instrument that was repurchased.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of gain (loss) from the difference between the repurchase price of a debt instrument initially issued by the entity and the net carrying amount of the debt at the time of its repurchase.

+ References

No definition available.

+ Details

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

- Definition

Amount of the cost of borrowed funds accounted for as interest expense for debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.8)
-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

+ Details

Name:	us-gaap_InterestExpenseDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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

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

- Details

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- Details

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Long-term debt: Senior unsecured notes (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
	Apr. 09, 2014	Jun. 30, 2016	Dec. 31, 2016
2021 Notes \| Prior to April 15, 2017
Senior unsecured notes- $189.2 million 2021 Notes
Percentage of principal amount that the holders of the Convertible Notes may require the Company to repurchase			100.00%
Percentage of redemption price of principal amount accrued interest			104.219%
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%
2021 Notes \| 12-month period beginning on April 15, 2017
Senior unsecured notes- $189.2 million 2021 Notes
Percentage of principal amount that the holders of the Convertible Notes may require the Company to repurchase			104.219%
2021 Notes \| 12-month period beginning on April 15, 2018
Senior unsecured notes- $189.2 million 2021 Notes
Percentage of principal amount that the holders of the Convertible Notes may require the Company to repurchase			102.813%
2021 Notes \| 12-month period beginning on April 15, 2019
Senior unsecured notes- $189.2 million 2021 Notes
Percentage of principal amount that the holders of the Convertible Notes may require the Company to repurchase			101.406%
2021 Notes \| 12-month period beginning on April 15, 2020
Senior unsecured notes- $189.2 million 2021 Notes
Percentage of principal amount that the holders of the Convertible Notes may require the Company to repurchase			100.00%
2021 Notes \| Cogent Finance
Senior unsecured notes- $189.2 million 2021 Notes
Aggregate principal amount of debt issued	$ 200.0
Interest rate (as a percent)	5.625%
Interest expense on original debt		$ 10.9
Interest expense on repurchased debt		10.8
Repurchased amount		189.2
Loss on debt purchases		$ 0.2
Proceeds from issuance of long-term debt, net of issuance costs	$ 195.8
Senior Unsecured Notes \| Prior to April 15, 2017
Senior unsecured notes- $189.2 million 2021 Notes
Percentage of redemption price which may redeem with net cash proceeds from certain equity offerings			105.625%
Senior Unsecured Notes \| Cogent Finance
Senior unsecured notes- $189.2 million 2021 Notes
Aggregate principal amount of debt issued			$ 189.2
Percentage of principal amount at which notes will be required to be repurchased in the event of a change of control			101.00%
Senior Secured Notes
Senior unsecured notes- $189.2 million 2021 Notes
Aggregate principal amount of debt issued			$ 375.0
Senior Secured Notes \| Cogent Finance
Senior unsecured notes- $189.2 million 2021 Notes
Aggregate principal amount of debt issued			$ 240.0
Treasury Rate \| 2021 Notes
Senior unsecured notes- $189.2 million 2021 Notes
Discount rate used to compute make-whole premium, basis points added to reference rate (as a percent)			0.50%