| Real Estate Joint Ventures - Narrative (Details) $ in Millions | Aug. 11, 2021  USD ($) | Sep. 30, 2021  USD ($) | Jun. 02, 2021  property | 
|---|---|---|---|
| Stevens Investors, LLC | Construction Loan | |||
| Real Estate [Line Items] | |||
| Related party transaction, amount of transaction | $ 112.0 | ||
| Loans and leases receivable, related parties | $ 90.5 | ||
| B RE COPT DC JV III LLC | |||
| Real Estate [Line Items] | |||
| Nominal Ownership % | 10.00% | 10.00% | |
| Disposed of by sale | Data Center Shells in Northern Virginia | |||
| Real Estate [Line Items] | |||
| Ownership percentage sold | 90.00% | ||
| Number of real estate properties sold | property | 2 | ||
| London Interbank Offered Rate (LIBOR) | Stevens Investors, LLC | Construction Loan | |||
| Real Estate [Line Items] | |||
| Related party transaction, rate | 2.35% | 
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- Definition For an unclassified balance sheet, reflects the carrying amount of unpaid loan amounts due from related parties at the balance sheet date. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef 
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- 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. No definition available. 
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- Definition Amount of transactions with related party during the financial reporting period. Reference 1: http://www.xbrl.org/2003/role/disclosureRef 
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