Summary of Significant Accounting Policies - Narrative (Details) $ in Thousands	9 Months Ended
Sep. 30, 2020 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss impact	$ 5,541
Cumulative Effect, Period Of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss impact	$ 5,500

Credit Losses, Financial Assets and Other Instruments, Allowance for Credit Loss, Cumulative Effect of Change for Adoption of Credit Loss Guidance

No definition available.

Name:	ofc_CreditLossesFinancialAssetsandOtherInstrumentsAllowanceforCreditLossCumulativeEffectofChangeforAdoptionofCreditLossGuidance
Namespace Prefix:	ofc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration