Property and equipment: Installment payment agreements (Details) - System infrastructure - Note obligations $ in Millions	1 Months Ended
Mar. 31, 2015 payment installment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Installment payment agreement
Term of debt	24 months
Number of payments during the first six months \| payment	0
Number of equal payments \| installment	18
Remaining term	18 months
Outstanding obligation		$ 5.5	$ 21.2
Unamortized discount		$ 0.3	$ 0.8

Represents the number of equal payments on note obligation.

No definition available.

Name:	ccoi_NoteObligationNumberOfEqualPayments
Namespace Prefix:	ccoi_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the remaining term for the note obligation, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	ccoi_NoteObligationRemainingTerm
Namespace Prefix:	ccoi_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the number of payments during the first six months of the note obligation.

+ References

No definition available.

+ Details

Name:	ccoi_NumberOfPaymentsDuringFirstSixMonthsOfNoteObligation
Namespace Prefix:	ccoi_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of the recorded unconditional purchase obligation outstanding. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

No definition available.

+ Details

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

- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of debt discount 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_DebtInstrumentUnamortizedDiscount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

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

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_TechnologyEquipmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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