Represents the notice period required for cancellation of lease.

No definition available.

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

- Definition

Amount of the fixed annual rent per year under lease agreement.

+ References

No definition available.

+ Details

Name:	ccoi_OperatingLeaseFixedAnnualRent
Namespace Prefix:	ccoi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the number of owners of LLC.

+ References

No definition available.

+ Details

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

- Definition

Represents the percentage of interest held by the related parties in the LLC.

+ References

No definition available.

+ Details

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

- Definition

Term of the lessee's leasing arrangement, 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_LesseeLeasingArrangementsOperatingLeasesTermOfContract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Expenses recognized resulting from transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 850

-SubTopic 10

-Section 50

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

+ Details

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

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

- Details

Name:	dei_LegalEntityAxis=ccoi_NiobiumLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=us-gaap_LeaseAgreementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=ccoi_SodiumLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=us-gaap_ChiefExecutiveOfficerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=us-gaap_ImmediateFamilyMemberOfManagementOrPrincipalOwnerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Related party transactions: (Details) $ in Millions	1 Months Ended	3 Months Ended		4 Months Ended	6 Months Ended
Related party transactions: (Details) $ in Millions	Apr. 30, 2015 USD ($) item	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	May. 10, 2015 item	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)
Office lease
Payment for rent and related costs (in dollars)		$ 0.3	$ 0.1		$ 0.4	$ 0.3
Niobium LLC
Office lease
Number of owners of the LLC \| item				2
Niobium LLC \| Lease
Office lease
Notice period for cancellation of lease				60 days
Sodium LLC
Office lease
Number of owners of the LLC \| item	2
Sodium LLC \| Lease
Office lease
Notice period for cancellation of lease	60 days
Fixed annual rent	$ 1.0
Lease term (in years)	5 years
Chief Executive Officer \| Niobium LLC
Office lease
Ownership interest of related parties held in the partnership (as a percent)				51.00%
Chief Executive Officer \| Sodium LLC
Office lease
Ownership interest of related parties held in the partnership (as a percent)	51.00%
Chief Executive Officer's wife \| Niobium LLC
Office lease
Ownership interest of related parties held in the partnership (as a percent)				49.00%
Chief Executive Officer's wife \| Sodium LLC
Office lease
Ownership interest of related parties held in the partnership (as a percent)	49.00%