Amount of contractual obligation, including but not limited to, long-term debt, capital lease obligations, operating lease obligations, purchase obligations, and other commitments that has been fulfilled..

No definition available.

Name:	cor_ContractualObligationMet
Namespace Prefix:	cor_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Commitments and Contingencies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018
Telecommunications Capacity
Commitments and Contingencies
Open commitments		$ 76.9
Telecommunications Capacity \| Forecast
Commitments and Contingencies
Open commitments met	$ 1.3
Construction Contracts
Commitments and Contingencies
Open commitments		$ 189.6