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Commitments and contingencies: (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
employee
case
Former Sales Employees Fair Labor Standards Act  
Commitments and contingencies  
Number of employees who opted to file an action 70ccoi_LossContingencyNumberOfFormerEmployeesOptedToFileAnAction
/ us-gaap_LitigationCaseAxis
= ccoi_FormerSalesEmployeesFairLaborStandardsActMember
Number of pending claims 4us-gaap_LossContingencyPendingClaimsNumber
/ us-gaap_LitigationCaseAxis
= ccoi_FormerSalesEmployeesFairLaborStandardsActMember
Former Sales Employees Fair Labor Standards Act | Settled Litigation  
Commitments and contingencies  
Number of pending claims 2us-gaap_LossContingencyPendingClaimsNumber
/ us-gaap_LitigationCaseAxis
= ccoi_FormerSalesEmployeesFairLaborStandardsActMember
/ us-gaap_LitigationStatusAxis
= us-gaap_SettledLitigationMember
Former Sales Employees Fair Labor Standards Act | Pending Litigation  
Commitments and contingencies  
Number of pending claims 2us-gaap_LossContingencyPendingClaimsNumber
/ us-gaap_LitigationCaseAxis
= ccoi_FormerSalesEmployeesFairLaborStandardsActMember
/ us-gaap_LitigationStatusAxis
= us-gaap_PendingLitigationMember
Leased circuit and dark fiber costs | Maximum  
Commitments and contingencies  
Estimate of possible loss in excess of the amount accrued 2.2us-gaap_LossContingencyEstimateOfPossibleLoss
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= ccoi_LeasedCircuitAndDarkFiberMember