Description of the business and recent developments: Recent Accounting Pronouncements - to be adopted (Details) $ in Millions	Jan. 01, 2019 USD ($)
Description of the business and recent developments:
Asset and lease liability	$ 97.3

Amount of lessee's right to use underlying asset and leases liability under operating lease.

No definition available.

Name:	ccoi_OperatingLeaseRightOfUseAssetAndLeasesLiability
Namespace Prefix:	ccoi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant