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

Percentage of discount on issuance.

+ References

No definition available.

+ Details

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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

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

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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+ Details

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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

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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

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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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

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

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+ Details

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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

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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

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-Publisher FASB
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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of cash inflow (outflow) from long-term debt supported by a written promise to pay an obligation.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 3
-Subparagraph (SX 210.12-04(a))
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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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

- Details

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Balance Type:
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- Details

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Period Type:

- Details

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Long-term debt: 2024 Notes issuances (Details) $ in Thousands, € in Millions		1 Months Ended		3 Months Ended		9 Months Ended
	Jun. 09, 2020 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 EUR (€)	Jun. 30, 2020 EUR (€)	Jun. 03, 2020 EUR (€)	Jun. 30, 2019 EUR (€)
Long-term debt
Realized foreign exchange gain on issuance on 2024 Euro Notes						$ 2,547
Foreign Currency Transaction Gain Loss Unrealized On 2024 Euro Notes				$ (17,315)	$ 6,162	(17,827)	$ 6,339
Net proceeds from issuance of senior unsecured 2024 Euro Notes						240,285	152,128
Redemption and extinguishment of 2021 Notes						(189,225)
Senior secured 2022 notes
Long-term debt
Face amount				$ 445,000		$ 445,000
Interest rate (as a percent)				5.375%		5.375%		5.375%
Senior secured 2022 notes \| Level 2
Long-term debt
Senior notes, fair value				$ 455,000		$ 455,000
Senior unsecured 2024 notes
Long-term debt
Face amount				410,400		410,400		€ 350.0	€ 135.0	€ 215.0	€ 135.0
Interest rate (as a percent)		4.375%							4.375%		4.375%
Discount rate (as a percent)		99.50%
Original amount issued at a discount		$ 240,000							€ 213.9
Foreign currency exchange rate at Euro to USD	1.133	1.112							1.112
Realized foreign exchange gain on issuance on 2024 Euro Notes	$ 2,500
Foreign Currency Transaction Gain Loss Unrealized On 2024 Euro Notes				(17,300)	$ 6,200	(17,800)	$ 6,300
Net proceeds from issuance of senior unsecured 2024 Euro Notes		$ 240,300	$ 152,100
Senior unsecured 2024 notes \| Level 2
Long-term debt
Face amount				410,400		410,400
Senior notes, fair value				$ 416,500		$ 416,500
Senior unsecured 2021 notes
Long-term debt
Face amount		$ 189,200
Interest rate (as a percent)		5.625%							5.625%