Amount of contractual obligation, including but not limited to, long-term debt, capital lease obligations, operating lease obligations, purchase obligations, and other commitments that will be fulfilled in the next fiscal year.

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, 2019	Dec. 31, 2018
Telecommunications Capacity
Commitments and Contingencies
Open commitments		$ 75.9
Telecommunications Capacity \| Forecast
Commitments and Contingencies
Open commitments met	$ 5.6
Construction Contracts
Commitments and Contingencies
Open commitments		$ 179.1