| Summary of calculation of numerator and denominator in basic and diluted earning per share | 
        
| 
   
| 
   
 | 
   
 | 
 For the Three Months 
 | 
   
 | 
 For the Six Months 
 | 
   
 | 
 
| 
   
 | 
   
 | 
 Ended June 30, 
 | 
   
 | 
 Ended June 30, 
 | 
   
 | 
 
| 
   
 | 
   
 | 
 2011 
 | 
   
 | 
 2010 
 | 
   
 | 
 2011 
 | 
   
 | 
 2010 
 | 
   
 | 
 
| 
 Numerator: 
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
 
| 
 (Loss) income from continuing operations 
 | 
   
 | 
 $ 
 | 
 (20,556 
 | 
 ) 
 | 
 $ 
 | 
 7,611 
 | 
   
 | 
 $ 
 | 
 (40,992 
 | 
 ) 
 | 
 $ 
 | 
 16,960 
 | 
   
 | 
 
| 
 Gain on sales of real estate, net 
 | 
   
 | 
 16 
 | 
   
 | 
 335 
 | 
   
 | 
 2,717 
 | 
   
 | 
 352 
 | 
   
 | 
 
| 
 Preferred share dividends 
 | 
   
 | 
 (4,026 
 | 
 ) 
 | 
 (4,026 
 | 
 ) 
 | 
 (8,051 
 | 
 ) 
 | 
 (8,051 
 | 
 ) 
 | 
 
| 
 Loss (income) from continuing operations attributable to noncontrolling interests 
 | 
   
 | 
 1,438 
 | 
   
 | 
 (595 
 | 
 ) 
 | 
 2,161 
 | 
   
 | 
 (1,225 
 | 
 ) 
 | 
 
| 
 Income from continuing operations attributable to restricted shares 
 | 
   
 | 
 (237 
 | 
 ) 
 | 
 (250 
 | 
 ) 
 | 
 (519 
 | 
 ) 
 | 
 (540 
 | 
 ) 
 | 
 
| 
 Numerator for basic EPS from continuing operations attributable to COPT common shareholders 
 | 
   
 | 
 (23,365 
 | 
 ) 
 | 
 3,075 
 | 
   
 | 
 (44,684 
 | 
 ) 
 | 
 7,496 
 | 
   
 | 
 
| 
 Dilutive effect of common units in the Operating Partnership on diluted EPS from continuing operations 
 | 
   
 | 
 (1,542 
 | 
 ) 
 | 
  
 | 
   
 | 
 (2,968 
 | 
 ) 
 | 
  
 | 
   
 | 
 
| 
 Numerator for diluted EPS from continuing operations attributable to COPT common shareholders 
 | 
   
 | 
 $ 
 | 
 (24,907 
 | 
 ) 
 | 
 $ 
 | 
 3,075 
 | 
   
 | 
 $ 
 | 
 (47,652 
 | 
 ) 
 | 
 $ 
 | 
 7,496 
 | 
   
 | 
 
| 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
 
| 
 Numerator for basic EPS from continuing operations attributable to COPT common shareholders 
 | 
   
 | 
 $ 
 | 
 (23,365 
 | 
 ) 
 | 
 $ 
 | 
 3,075 
 | 
   
 | 
 $ 
 | 
 (44,684 
 | 
 ) 
 | 
 $ 
 | 
 7,496 
 | 
   
 | 
 
| 
 Discontinued operations 
 | 
   
 | 
 (5,467 
 | 
 ) 
 | 
 1,205 
 | 
   
 | 
 (6,298 
 | 
 ) 
 | 
 2,514 
 | 
   
 | 
 
| 
 Discontinued operations attributable to noncontrolling interests 
 | 
   
 | 
 345 
 | 
   
 | 
 (90 
 | 
 ) 
 | 
 398 
 | 
   
 | 
 (197 
 | 
 ) 
 | 
 
| 
 Numerator for basic EPS on net (loss) income attributable to COPT common shareholders 
 | 
   
 | 
 (28,487 
 | 
 ) 
 | 
 4,190 
 | 
   
 | 
 (50,584 
 | 
 ) 
 | 
 9,813 
 | 
   
 | 
 
| 
 Dilutive effect of common units in the Operating Partnership 
 | 
   
 | 
 (1,887 
 | 
 ) 
 | 
  
 | 
   
 | 
 (3,366 
 | 
 ) 
 | 
  
 | 
   
 | 
 
| 
 Numerator for diluted EPS on net (loss) income attributable to COPT common shareholders 
 | 
   
 | 
 $ 
 | 
 (30,374 
 | 
 ) 
 | 
 $ 
 | 
 4,190 
 | 
   
 | 
 $ 
 | 
 (53,950 
 | 
 ) 
 | 
 $ 
 | 
 9,813 
 | 
   
 | 
 
| 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
 
| 
 Denominator (all weighted averages): 
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
 
| 
 Denominator for basic EPS (common shares) 
 | 
   
 | 
 68,446 
 | 
   
 | 
 58,489 
 | 
   
 | 
 67,399 
 | 
   
 | 
 58,169 
 | 
   
 | 
 
| 
 Dilutive effect of share-based compensation awards 
 | 
   
 | 
  
 | 
   
 | 
 421 
 | 
   
 | 
  
 | 
   
 | 
 405 
 | 
   
 | 
 
| 
 Dilutive effect of common units 
 | 
   
 | 
 4,382 
 | 
   
 | 
  
 | 
   
 | 
 4,389 
 | 
   
 | 
  
 | 
   
 | 
 
| 
 Denominator for diluted EPS 
 | 
   
 | 
 72,828 
 | 
   
 | 
 58,910 
 | 
   
 | 
 71,788 
 | 
   
 | 
 58,574 
 | 
   
 | 
 
| 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
 
| 
 Basic EPS: 
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
 
| 
 (Loss) income from continuing operations attributable to COPT common shareholders 
 | 
   
 | 
 $ 
 | 
 (0.34 
 | 
 ) 
 | 
 $ 
 | 
 0.05 
 | 
   
 | 
 $ 
 | 
 (0.66 
 | 
 ) 
 | 
 $ 
 | 
 0.13 
 | 
   
 | 
 
| 
 Discontinued operations attributable to COPT common shareholders 
 | 
   
 | 
 (0.08 
 | 
 ) 
 | 
 0.02 
 | 
   
 | 
 (0.09 
 | 
 ) 
 | 
 0.04 
 | 
   
 | 
 
| 
 Net (loss) income attributable to COPT common shareholders 
 | 
   
 | 
 $ 
 | 
 (0.42 
 | 
 ) 
 | 
 $ 
 | 
 0.07 
 | 
   
 | 
 $ 
 | 
 (0.75 
 | 
 ) 
 | 
 $ 
 | 
 0.17 
 | 
   
 | 
 
| 
 Diluted EPS: 
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
 
| 
 (Loss) income from continuing operations attributable to COPT common shareholders 
 | 
   
 | 
 $ 
 | 
 (0.34 
 | 
 ) 
 | 
 $ 
 | 
 0.05 
 | 
   
 | 
 $ 
 | 
 (0.66 
 | 
 ) 
 | 
 $ 
 | 
 0.13 
 | 
   
 | 
 
| 
 Discontinued operations attributable to COPT common shareholders 
 | 
   
 | 
 (0.08 
 | 
 ) 
 | 
 0.02 
 | 
   
 | 
 (0.09 
 | 
 ) 
 | 
 0.04 
 | 
   
 | 
 
| 
 Net (loss) income attributable to COPT common shareholders 
 | 
   
 | 
 $ 
 | 
 (0.42 
 | 
 ) 
 | 
 $ 
 | 
 0.07 
 | 
   
 | 
 $ 
 | 
 (0.75 
 | 
 ) 
 | 
 $ 
 | 
 0.17 
 | 
   
 | 
 
 
 | 
 
 
 | 
 
 
 |