Quarterly report pursuant to Section 13 or 15(d)

Shareholders' Equity (Details)

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Shareholders' Equity (Details) (USD $)
 1 Months Ended 3 Months Ended 9 Months Ended
May 31, 2011
Sep. 30, 2011
Sep. 30, 2010
Sep. 30, 2011
Sep. 30, 2010
Common Shares          
Common shares issued to the public (in shares) 4,600,000     4,600,000  
Per share value of common shares issued to the public (in dollars per share) $ 33.00        
Net proceeds from issue of shares after underwriter discounts but before offering expenses $ 145,700,000        
Number of operating partnership units converted into common shares (in units)       83,506 620,598
Number of common shares for each converted common unit (in shares)       1  
Dividends declared per common share (in dollars per share)   $ 0.4125 $ 0.4125 $ 1.2375 $ 1.1975
Accumulated other comprehensive loss activity          
Beginning balance       (4,163,000) (1,907,000)
Amount of loss recognized in AOCL (effective portion)   (21,869,000) (1,530,000) (30,463,000) (5,844,000)
Amount of loss reclassified from AOCL to income (effective portion)   1,179,000 887,000 3,446,000 2,684,000
Adjustment to AOCL attributable to noncontrolling interests       2,562,000 206,000
Ending balance   (28,618,000) (4,861,000) (28,618,000) (4,861,000)
Comprehensive income          
Net income (loss)   7,470,000 8,926,000 (37,103,000) 28,752,000
Amount of loss recognized in AOCL (effective portion)   (21,869,000) (1,530,000) (30,463,000) (5,844,000)
Amount of loss reclassified from AOCL to income (effective portion)   1,179,000 887,000 3,446,000 2,684,000
Total comprehensive (loss) income   (13,220,000) 8,283,000 (64,120,000) 25,592,000
Net (income) loss attributable to noncontrolling interests   (904,000) (94,000) 1,655,000 (1,516,000)
Other comprehensive loss attributable to noncontrolling interests   1,206,000 47,000 1,594,000 245,000
Total comprehensive (loss) income attributable to COPT   $ (12,918,000) $ 8,236,000 $ (60,871,000) $ 24,321,000