| Properties, Net (Details 5) $ / shares in Units, $ in Thousands | 3 Months Ended | 
|---|---|
| Mar. 31, 2015  USD ($)  $ / shares | |
| Real Estate [Abstract] | |
| Pro forma total revenues | $ | $ 169,925 | 
| Pro forma net income attributable to COPT common shareholders | $ | $ 10,847 | 
| Pro Forma EPS, Basic (in dollars per share) | $ / shares | $ 0.11 | 
| Pro Forma EPS, Diluted (in dollars per share) | $ / shares | $ 0.11 | 
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- Definition The pro forma basic net income per share for a period as if the business combination or combinations had been completed at the beginning of a period. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition The pro forma diluted net income per share for a period as if the business combination or combinations had been completed at the beginning of a period. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition The pro forma net Income or Loss for the period as if the business combination or combinations had been completed at the beginning of a period. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition The pro forma revenue for a period as if the business combination or combinations had been completed at the beginning of the period. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- References No definition available. 
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