Face (par) amount of debt instrument after original issue discount.

No definition available.

Name:	ccoi_DebtInstrumentFaceAmountNetOfOriginalIssueDiscount
Namespace Prefix:	ccoi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Minimum percentage of principal amount redeemable under insolvency.

+ References

No definition available.

+ Details

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

- Definition

Percentage of discount on issuance.

+ References

No definition available.

+ Details

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

- Definition

Percentage of additional redemption price in addition to normal redemption price under debt instrument.

+ References

No definition available.

+ Details

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

- Definition

Percentage of minimum redemption price under debt instrument.

+ References

No definition available.

+ Details

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

- Definition

Minimum percentage for redemption under specific arrangements.

+ References

No definition available.

+ Details

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

- Definition

Percentage of redemption price if changes in control taken place under debt instrument.

+ References

No definition available.

+ Details

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

- Definition

Amount before tax of foreign currency transaction unrealized gain (loss) on 2024 Euro notes recognized in the income statement.

+ References

No definition available.

+ Details

Name:	ccoi_ForeignCurrencyTransactionGainLossUnrealizedOn2024EuroNotes
Namespace Prefix:	ccoi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

+ Details

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

- Definition

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=118260190&loc=d3e13433-108611

+ Details

Name:	us-gaap_DebtInstrumentFairValue
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://fasb.org/us-gaap/role/ref/legacyRef
-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

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph e
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

+ Details

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

- Definition

Percentage of principal amount of debt redeemed.

+ References

No definition available.

+ Details

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

- Definition

Foreign exchange rate used to translate amounts denominated in functional currency to reporting currency.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 830
-SubTopic 30
-Section S99
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6897108&loc=SL6897125-166521

+ Details

Name:	us-gaap_ForeignCurrencyExchangeRateTranslation1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount before tax of foreign currency transaction realized gain (loss) recognized in the income statement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 830
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=109240200&loc=d3e30690-110894

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 830
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6450222&loc=d3e30840-110895

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 830
-SubTopic 20
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109240200&loc=d3e30700-110894

+ Details

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

- Definition

The cash inflow from the issuance of long-term debt that is not secured by collateral. Excludes proceeds from tax exempt unsecured debt.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3255-108585

+ Details

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

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SecuredDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=ccoi_SeniorUnsecuredNotesOf4.375Due2024Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Long-term debt: Issuance of 2024 Notes (Details) € in Millions		1 Months Ended			3 Months Ended		6 Months Ended
	Jun. 09, 2020 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2020 EUR (€)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 EUR (€)	Jun. 30, 2019 EUR (€)	Jun. 25, 2019 EUR (€)
Long-term debt
Realized foreign exchange gain on issuance on 2024 Euro Notes					$ 2,547,000		$ 2,547,000
Foreign Currency Transaction Gain Loss Unrealized On 2024 Euro Notes					(3,420,000)	$ 177,000	(512,000)	$ 177,000
Net proceeds from issuance of senior unsecured 2024 Euro Notes							240,285,000	$ 152,128,000
Senior secured 2022 notes
Long-term debt
Senior notes outstanding		$ 445,000,000.0			445,000,000.0		445,000,000.0
Principal amount		$ 393,000,000.0			$ 393,000,000.0		$ 393,000,000.0		€ 350.0
Interest rate (as a percent)		5.375%			5.375%		5.375%		5.375%
Senior unsecured 2024 notes
Long-term debt
Senior notes outstanding		$ 393,000,000.0			$ 393,000,000.0		$ 393,000,000.0		€ 350.0
Principal amount		$ 189,200,000			189,200,000		189,200,000		215.0	€ 135.0	€ 135.0
Discount rate (as a percent)		99.50%	99.50%
Original amount issued at a discount		$ 240,000,000.0			$ 240,000,000.0		$ 240,000,000.0		€ 213.9
Foreign currency exchange rate at Euro to USD	1.133	1.112			1.112		1.112		1.112
Realized foreign exchange gain on issuance on 2024 Euro Notes	$ 2,500,000
Foreign Currency Transaction Gain Loss Unrealized On 2024 Euro Notes					$ (3,400,000)	$ 200,000
Interest rate (as a percent)		4.375%			4.375%		4.375%		4.375%	4.375%
Net proceeds from issuance of senior unsecured 2024 Euro Notes		$ 240,300,000	€ 215.0	$ 152,100,000
Senior notes, fair value		$ 393,000,000.0			$ 393,000,000.0		$ 393,000,000.0
Principal amount plus accrued and unpaid interest (as a percent)		100.00%	100.00%				100.00%
Redemption price ranging, (as a percent)							102.188%
Redemption Price, Additional Percentage							35.00%
Redemption price percentage of principal amount redeemed							104.375%
Redemption Price, Percentage							101.00%
Redemption Price, Minimum Percentage for Redemption							90.00%
Percentage of Principal Amount Redeemable under Insolvency							25.00%