Earnings Per Share (EPS) (Details) (USD $) In Thousands, except Per Share data	3 Months Ended				9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010		Sep. 30, 2011		Sep. 30, 2010
Numerator:
Income (loss) from continuing operations	$ 4,893		$ 4,696		$ (27,700)		$ 21,647
Gain on sales of real estate, net			2,477		2,717		2,829
Preferred share dividends	(4,025)		(4,025)		(12,076)		(12,076)
(Income) loss from continuing operations attributable to noncontrolling interests	(755)		33		877		(1,192)
Income from continuing operations attributable to restricted shares	(262)		(267)		(781)		(807)
Numerator for basic EPS from continuing operations attributable to COPT common shareholders	(149)		2,914		(36,963)		10,401
Dilutive effect of common units in the Operating Partnership on diluted EPS from continuing operations					(2,410)
Numerator for diluted EPS from continuing operations attributable to COPT common shareholders	(149)		2,914		(39,373)		10,401
Discontinued operations	2,577		1,753		(12,120)		4,276
Discontinued operations attributable to noncontrolling interests	(149)		(127)		778		(324)
Numerator for basic EPS on net income (loss) attributable to COPT common shareholders	2,279		4,540		(48,305)		14,353
Dilutive effect of common units in the Operating Partnership					(3,188)
Numerator for diluted EPS on net income (loss) income attributable to COPT common shareholders	$ 2,279		$ 4,540		$ (51,493)		$ 14,353
Denominator (all weighted averages):
Denominator for basic EPS (common shares) (in shares)	71,312		58,656		68,718		58,333
Dilutive effect of share-based compensation awards (in shares)			296				367
Dilutive effect of common units (in shares)					4,371
Denominator for diluted EPS (in shares)	71,312		58,952		73,089		58,700
Basic EPS:
Income (loss) from continuing operations attributable to COPT common shareholders (in dollars per share)			$ 0.05	[1]	$ (0.54)	[1]	$ 0.18	[1]
Discontinued operations attributable to COPT common shareholders (in dollars per share)	$ 0.03	[1]	$ 0.03	[1]	$ (0.16)	[1]	$ 0.07	[1]
Net income (loss) attributable to COPT common shareholders (in dollars per share)	$ 0.03	[1]	$ 0.08	[1]	$ (0.70)	[1]	$ 0.25	[1]
Diluted EPS:
Income (loss) from continuing operations attributable to COPT common shareholders (in dollars per share)			$ 0.05	[1]	$ (0.54)	[1]	$ 0.17	[1]
Discontinued operations attributable to COPT common shareholders (in dollars per share)	$ 0.03	[1]	$ 0.03	[1]	$ (0.16)	[1]	$ 0.07	[1]
Net income (loss) attributable to COPT common shareholders (in dollars per share)	$ 0.03	[1]	$ 0.08	[1]	$ (0.70)	[1]	$ 0.24	[1]

[1] Basic and diluted earnings per common share are calculated based on amounts attributable to common shareholders of Corporate Office Properties Trust.