COPT Defense Properties (CDP) 10-Q 29 Oct 13 2013 Q3 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	9 Months Ended
	Sep. 30, 2013	Oct. 22, 2013
Entity Information [Line Items]
Entity Registrant Name	CORPORATE OFFICE PROPERTIES TRUST
Entity Central Index Key	860546
Document Type	10-Q
Document Period End Date	30-Sep-13
Amendment Flag	FALSE
Current Fiscal Year End Date	-19
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		87,381,395
Document Fiscal Year Focus	2013
Document Fiscal Period Focus	Q3
Corporate Office Properties, L.P.
Entity Information [Line Items]
Entity Registrant Name	CORPORATE OFFICE PROPERTIES, L.P.
Entity Central Index Key	1577966
Document Type	10-Q
Document Period End Date	30-Sep-13
Amendment Flag	FALSE
Current Fiscal Year End Date	-19
Entity Filer Category	Non-accelerated Filer
Document Fiscal Year Focus	2013
Document Fiscal Period Focus	Q3

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Properties, net:
Operating properties, net	$2,713,579	$2,597,666
Projects in development or held for future development	526,167	565,378
Total properties, net	3,239,746	3,163,044
Assets held for sale, net	133,984	140,229
Cash and cash equivalents	27,318	10,594
Restricted cash and marketable securities	14,698	21,557
Accounts receivable (net of allowance for doubtful accounts of $5,344 and $4,694, respectively)	17,724	19,247
Deferred rent receivable	90,104	85,802
Intangible assets on real estate acquisitions, net	64,372	75,879
Deferred leasing and financing costs, net	63,246	59,952
Mortgage and other investing receivables	40,321	33,396
Prepaid expenses and other assets	64,075	44,059
Total assets	3,755,588	3,653,759
Liabilities:
Debt, net	2,135,031	2,019,168
Accounts payable and accrued expenses	85,291	97,922
Rents received in advance and security deposits	28,539	27,632
Dividends/Distributions payable	29,077	28,698
Deferred revenue associated with operating leases	8,545	11,995
Distributions received in excess of investment in unconsolidated real estate joint venture	6,420	6,420
Interest rate derivatives	3,595	6,185
Other liabilities	8,234	8,942
Total liabilities	2,304,732	2,206,962
Commitments and contingencies (Note 17)
Redeemable noncontrolling interest	16,789	10,298
Corporate Office Properties Trust’s shareholders’ equity:
Preferred Shares of beneficial interest at liquidation preference ($0.01 par value; 25,000,000 shares authorized; shares issued and outstanding of 9,431,667 at September 30, 2013 and 12,821,667 at December 31, 2012)	249,083	333,833
Common Shares of beneficial interest	874	809
Additional paid-in capital	1,812,801	1,653,672
Cumulative distributions in excess of net income	-700,368	-617,455
Accumulated other comprehensive income (loss)	2,925	-5,435
Total Corporate Office Properties Trust’s shareholders’ equity	1,365,315	1,365,424
Noncontrolling interests in subsidiaries:
Common units in COPLP	50,815	52,122
Preferred units in COPLP	8,800	8,800
Other consolidated entities	9,137	10,153
Noncontrolling interests in subsidiaries	68,752	71,075
Total equity	1,434,067	1,436,499
Total liabilities, redeemable noncontrolling interest and equity	3,755,588	3,653,759
Corporate Office Properties, L.P.
Properties, net:
Operating properties, net	2,713,579	2,597,666
Projects in development or held for future development	526,167	565,378
Total properties, net	3,239,746	3,163,044
Assets held for sale, net	133,984	140,229
Cash and cash equivalents	27,318	10,594
Restricted cash and marketable securities	7,517	14,781
Accounts receivable (net of allowance for doubtful accounts of $5,344 and $4,694, respectively)	17,724	19,247
Deferred rent receivable	90,104	85,802
Intangible assets on real estate acquisitions, net	64,372	75,879
Deferred leasing and financing costs, net	63,246	59,952
Mortgage and other investing receivables	40,321	33,396
Prepaid expenses and other assets	64,075	44,059
Total assets	3,748,407	3,646,983
Liabilities:
Debt, net	2,135,031	2,019,168
Accounts payable and accrued expenses	85,291	97,922
Rents received in advance and security deposits	28,539	27,632
Dividends/Distributions payable	29,077	28,698
Deferred revenue associated with operating leases	8,545	11,995
Distributions received in excess of investment in unconsolidated real estate joint venture	6,420	6,420
Interest rate derivatives	3,595	6,185
Other liabilities	1,053	2,166
Total liabilities	2,297,551	2,200,186
Commitments and contingencies (Note 17)
Redeemable noncontrolling interest	16,789	10,298
Corporate Office Properties Trust’s shareholders’ equity:
Common Shares of beneficial interest	1,163,977	1,089,391
Accumulated other comprehensive income (loss)	3,026	-5,708
Total Corporate Office Properties Trust’s shareholders’ equity	1,424,886	1,426,316
Noncontrolling interests in subsidiaries:
Noncontrolling interests in subsidiaries	9,181	10,183
Total equity	1,434,067	1,436,499
Total liabilities, redeemable noncontrolling interest and equity	3,748,407	3,646,983
Corporate Office Properties, L.P. | General Partner
Corporate Office Properties Trust’s shareholders’ equity:
Preferred partners' capital accounts	249,083	333,833
Corporate Office Properties, L.P. | Limited Partner
Corporate Office Properties Trust’s shareholders’ equity:
Preferred partners' capital accounts	$8,800	$8,800

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, except Share data, unless otherwise specified
Statement of Financial Position [Abstract]
Allowance for doubtful accounts	$5,344	$4,694
Preferred Shares of beneficial interest, par value (in dollars per share)	$0.01	$0.01
Preferred Shares of beneficial interest, shares authorized	25,000,000	25,000,000
Preferred Shares of beneficial interest, shares issued	9,431,667	12,821,667
Preferred Shares of beneficial interest, shares outstanding	9,431,667	12,821,667
Common Shares of beneficial interest, par value (in dollars per share)	$0.01	$0.01
Common Shares of beneficial interest, shares authorized	125,000,000	125,000,000
Common Shares of beneficial interest, shares issued	87,381,395	80,952,986
Common Shares of beneficial interest, shares outstanding	87,381,395	80,952,986

Consolidated_Balance_Sheets_Pa1

Consolidated Balance Sheets (Parenthetical 2) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, except Share data, unless otherwise specified
Allowance for doubtful accounts	$5,344	$4,694
Common Shares of beneficial interest, shares outstanding	87,381,395	80,952,986
Corporate Office Properties, L.P.
Allowance for doubtful accounts	$5,344	$4,694
Corporate Office Properties, L.P. | General Partner
Preferred Units, Outstanding	9,431,667	12,821,667
Common Shares of beneficial interest, shares outstanding	87,381,395	80,952,986
Corporate Office Properties, L.P. | Limited Partner
Preferred Units, Outstanding	352,000	352,000
Common Shares of beneficial interest, shares outstanding	3,978,154	4,067,542

Consolidated_Statements_of_Ope

Consolidated Statements of Operations (USD $)	3 Months Ended				9 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Revenues
Rental revenue	$97,442		$92,287		$289,628		$271,634
Tenant recoveries and other real estate operations revenue	21,598		22,075		65,499		63,597
Construction contract and other service revenues	16,991		15,283		52,048		53,812
Total revenues	136,031		129,645		407,175		389,043
Expenses
Property operating expenses	43,482		41,474		129,409		122,102
Depreciation and amortization associated with real estate operations	29,210		28,604		86,239		84,633
Construction contract and other service expenses	16,306		14,410		49,165		51,302
Impairment losses	16,300		46,096		16,300		41,260
General, administrative and leasing expenses	8,027		6,377		22,430		24,797
Business development expenses and land carry costs	1,383		1,632		4,069		4,506
Total operating expenses	114,708		138,593		307,612		328,600
Operating income (loss)	21,323		-8,948		99,563		60,443
Interest expense	-21,242		-23,239		-66,851		-71,909
Interest and other (loss) income	-3		1,095		2,949		3,152
Loss on early extinguishment of debt	-374		-768		-27,028		-937
(Loss) income from continuing operations before equity in income (loss) of unconsolidated entities and income taxes	-296		-31,860		8,633		-9,251
Equity in income (loss) of unconsolidated entities	44		-246		211		-522
Income tax expense	-24		-106		-61		-327
(Loss) income from continuing operations	-276		-32,212		8,783		-10,100
Discontinued operations	-1,724		11,447		-2,594		11,410
(Loss) income before gain on sales of real estate	-2,000		-20,765		6,189		1,310
Gain on sales of real estate	0		0		2,683		21
Net (loss) income	-2,000		-20,765		8,872		1,331
Net loss (income) attributable to noncontrolling interests:
Common units in COPLP	232		1,533		474		738
Preferred units in COPLP	-165		-165		-495		-495
Other consolidated entities	-1,031		235		-2,160		864
Net (loss) income attributable to COPT	-2,964		-19,162		6,691		2,438
Preferred share/unit dividends/distributions	-4,490		-6,546		-15,481		-14,738
Issuance costs associated with redeemed preferred shares/units	0		-1,827		-2,904		-1,827
Net loss attributable to COPT common shareholders	-7,454		-27,535		-11,694		-14,127
Net (loss) income attributable to COPT:
(Loss) income from continuing operations	-1,266		-29,963		9,327		-8,369
Discontinued operations, net	-1,698		10,801		-2,636		10,807
Net (loss) income attributable to COPT	-2,964		-19,162		6,691		2,438
Basic earnings per common share
Loss from continuing operations	($0.07)	[1]	($0.54)	[1]	($0.11)	[1]	($0.35)	[1]
Discontinued operations	($0.02)	[1]	$0.15	[1]	($0.03)	[1]	$0.15	[1]
Net income attributable to COPT common shareholders (in dollars per share/unit)	($0.09)	[1]	($0.39)	[1]	($0.14)	[1]	($0.20)	[1]
Diluted earnings per common share
Loss from continuing operations	($0.07)	[1]	($0.54)	[1]	($0.11)	[1]	($0.35)	[1]
Discontinued operations	($0.02)	[1]	$0.15	[1]	($0.03)	[1]	$0.15	[1]
Net income attributable to COPT common shareholders (in dollars per share/unit)	($0.09)	[1]	($0.39)	[1]	($0.14)	[1]	($0.20)	[1]
Dividends/distributions declared per common share/unit	$0.28		$0.28		$0.83		$0.83
Corporate Office Properties, L.P.
Revenues
Rental revenue	97,442		92,287		289,628		271,634
Tenant recoveries and other real estate operations revenue	21,598		22,075		65,499		63,597
Construction contract and other service revenues	16,991		15,283		52,048		53,812
Total revenues	136,031		129,645		407,175		389,043
Expenses
Property operating expenses	43,482		41,474		129,409		122,102
Depreciation and amortization associated with real estate operations	29,210		28,604		86,239		84,633
Construction contract and other service expenses	16,306		14,410		49,165		51,302
Impairment losses	16,300		46,096		16,300		41,260
General, administrative and leasing expenses	8,027		6,377		22,430		24,797
Business development expenses and land carry costs	1,383		1,632		4,069		4,506
Total operating expenses	114,708		138,593		307,612		328,600
Operating income (loss)	21,323		-8,948		99,563		60,443
Interest expense	-21,242		-23,239		-66,851		-71,909
Interest and other (loss) income	-3		1,095		2,949		3,152
Loss on early extinguishment of debt	-374		-768		-27,028		-937
(Loss) income from continuing operations before equity in income (loss) of unconsolidated entities and income taxes	-296		-31,860		8,633		-9,251
Equity in income (loss) of unconsolidated entities	44		-246		211		-522
Income tax expense	-24		-106		-61		-327
(Loss) income from continuing operations	-276		-32,212		8,783		-10,100
Discontinued operations	-1,724		11,447		-2,594		11,410
(Loss) income before gain on sales of real estate	-2,000		-20,765		6,189		1,310
Gain on sales of real estate	0		0		2,683		21
Net (loss) income	-2,000		-20,765		8,872		1,331
Net loss (income) attributable to noncontrolling interests:
Net (income) loss attributable to noncontrolling interests in consolidated entities	-1,035		-404		-2,172		167
Net (loss) income attributable to COPT	-3,035		-21,169		6,700		1,498
Preferred share/unit dividends/distributions	-4,655		-6,711		-15,976		-15,233
Issuance costs associated with redeemed preferred shares/units	0		-1,827		-2,904		-1,827
Net loss attributable to COPT common shareholders	-7,690		-29,707		-12,180		-15,562
Net (loss) income attributable to COPT:
(Loss) income from continuing operations	-1,262		-31,969		9,441		-9,250
Discontinued operations, net	-1,773		10,800		-2,741		10,748
Net (loss) income attributable to COPT	($3,035)		($21,169)		$6,700		$1,498
Basic earnings per common share
Loss from continuing operations	($0.07)	[2]	($0.54)	[2]	($0.11)	[2]	($0.35)	[2]
Discontinued operations	($0.02)	[2]	$0.15	[2]	($0.03)	[2]	$0.14	[2]
Net income attributable to COPT common shareholders (in dollars per share/unit)	($0.09)	[2]	($0.39)	[2]	($0.14)	[2]	($0.21)	[2]
Diluted earnings per common share
Loss from continuing operations	($0.07)	[2]	($0.54)	[2]	($0.11)	[2]	($0.35)	[2]
Discontinued operations	($0.02)	[2]	$0.15	[2]	($0.03)	[2]	$0.14	[2]
Net income attributable to COPT common shareholders (in dollars per share/unit)	($0.09)	[2]	($0.39)	[2]	($0.14)	[2]	($0.21)	[2]
Dividends/distributions declared per common share/unit	$0.28		$0.28		$0.83		$0.83
[1]	Basic and diluted earnings per common share are calculated based on amounts attributable to common shareholders of Corporate Office Properties Trust.
[2]	Basic and diluted earnings per common unit are calculated based on amounts attributable to common unitholders of Corporate Office Properties, L.P.

Consolidated_Statements_of_Com

Consolidated Statements of Comprehensive Income (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Net (loss) income	($2,000)	($20,765)	$8,872	$1,331
Other comprehensive (loss) income
Unrealized (losses) gains on interest rate derivatives	-2,482	-2,760	5,810	-7,386
Losses on interest rate derivatives included in net (loss) income	689	632	2,021	3,034
Equity in other comprehensive income of equity method investee	1,070	0	1,070	0
Other comprehensive (loss) income	-723	-2,128	8,901	-4,352
Comprehensive (loss) income	-2,723	-22,893	17,773	-3,021
Comprehensive (income) loss attributable to noncontrolling interests	-948	1,763	-2,722	1,509
Comprehensive (loss) income attributable to COPT	-3,671	-21,130	15,051	-1,512
Corporate Office Properties, L.P.
Net (loss) income	-2,000	-20,765	8,872	1,331
Other comprehensive (loss) income
Unrealized (losses) gains on interest rate derivatives	-2,482	-2,760	5,810	-7,386
Losses on interest rate derivatives included in net (loss) income	689	632	2,021	3,034
Equity in other comprehensive income of equity method investee	1,070	0	1,070	0
Other comprehensive (loss) income	-723	-2,128	8,901	-4,352
Comprehensive (loss) income	-2,723	-22,893	17,773	-3,021
Comprehensive (income) loss attributable to noncontrolling interests	-1,050	-360	-2,339	336
Comprehensive (loss) income attributable to COPT	($3,773)	($23,253)	$15,434	($2,685)

Consolidated_Statements_of_Equ

Consolidated Statements of Equity (USD $)	Total	Preferred Shares	Common Shares	Additional Paid-in Capital	Cumulative Distributions in Excess of Net Income	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interests	Corporate Office Properties, L.P.	Corporate Office Properties, L.P.	Corporate Office Properties, L.P.	Corporate Office Properties, L.P.	Corporate Office Properties, L.P.	Corporate Office Properties, L.P.	Corporate Office Properties, L.P.	Corporate Office Properties, L.P.
In Thousands, except Share data, unless otherwise specified	USD ($)	USD ($)	USD ($)	USD ($)	USD ($)	USD ($)	USD ($)	USD ($)	Common Shares	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interests	Limited Partner	Limited Partner	General Partner	General Partner
									USD ($)	USD ($)	USD ($)		Preferred Shares		Preferred Shares
													USD ($)		USD ($)
Balance at Dec. 31, 2011	$1,205,899	$216,333	$720	$1,451,078	($534,041)	($1,733)	$73,542	$1,205,899	$972,107	($1,837)	$10,496		$8,800		$216,333
Balance (in units/ shares) at Dec. 31, 2011	72,011,324								76,313,112
Balance (preferred units) at Dec. 31, 2011													352,000		8,121,667
Increase (Decrease) in Shareholders' Equity
Conversion of common units to common shares (94,550 and 310,889 for the nine months ended September 30, 2012 and 2013, respectively)			1	1,187			-1,188
Shares issued to the public (in dollars)	165,662	172,500		-6,838				165,662	-6,838						172,500
Shares issued to the public (in units/ shares)															6,900,000
Stock redeemed or called during period (in units/ shares)															-2,200,000
Stock redeemed or called during period, value	-55,000	-55,000		1,827	-1,827			-55,000							-55,000
Costs associated with common units/shares issued to the public	-5			-5				-5	-5
Exercise of share options (in units/shares)	44,624								44,624
Exercise of share options (in dollars)	666			666				666	666
Share-based compensation (in units/shares)									142,914
Share-based compensation	9,192		1	9,191				9,192	9,192
Restricted common unts/shares redemptions	-135,777								-135,777
Restricted common units/shares redemptions (in dollars)	-3,279			-3,279				-3,279	-3,279
Adjustments to noncontrolling interests resulting from changes in ownership of COPLP				373			-373
Comprehensive income (loss)	-1,218				2,438	-3,955	299	-1,218	-13,735	-4,182	1,466		495		14,738
Dividends/Distributions	-74,203				-74,203			-78,196	-62,963				-495		-14,738
Distributions to owners of common and preferred units in COPLP	-3,993						-3,993
Contributions from noncontrolling interests									-1,602		-1,602
Distributions to noncontrolling interests	-648						-648	-648			-648
Adjustment to arrive at fair value of redeemable noncontrolling interest	-2,827			-2,827				-2,827	-2,827
Increase in tax benefit from share-based compensation	43			43				43	43
Balance at Sep. 30, 2012	1,240,289	333,833	722	1,451,416	-607,633	-5,688	67,639	1,240,289	893,963	-6,019	9,712		8,800		333,833
Balance (in units/ shares) at Sep. 30, 2012	72,157,635								76,364,873
Balance (preferred units) at Sep. 30, 2012													352,000		12,821,667
Balance at Dec. 31, 2012	1,436,499	333,833	809	1,653,672	-617,455	-5,435	71,075	1,436,499	1,089,391	-5,708	10,183		8,800		333,833
Balance (in units/ shares) at Dec. 31, 2012	80,952,986								85,020,528			4,067,542		80,952,986
Balance (preferred units) at Dec. 31, 2012												352,000	352,000	12,821,667	12,821,667
Increase (Decrease) in Shareholders' Equity
Conversion of common units to common shares (94,550 and 310,889 for the nine months ended September 30, 2012 and 2013, respectively)			3	3,988			-3,991
Shares issued to the public (in dollars)	117,961		45	117,916				117,961	117,961
Shares issued to the public (in units/ shares)									4,485,000
Stock redeemed or called during period (in units/ shares)															-3,390,000
Stock redeemed or called during period, value	-84,750	-84,750		2,904	-2,904			-84,750							-84,750
Issuance of common units resulting from common shares issued under at-the-market program (in units)									1,500,000
Issuance of common units resulting from common shares issued under at-the-market program (in dollars)	38,447		15	38,432				38,447	38,447
Acquisition of property and noncontrolling interest in other consolidated entity for COPLP common units	1,369			-1,296			2,665
Acquisition of property and noncontrolling interest in other consolidated entity for COPLP common units (in units)									221,501
Acquisition of property and noncontrolling interest in other consolidated entity for COPLP common units (in dollars)								1,369	3,899		-2,530
Exercise of share options (in units/shares)	32,756								32,756
Exercise of share options (in dollars)	642			642				642	642
Share-based compensation (in units/shares)									171,287
Share-based compensation	5,702		2	5,700				5,702	5,702
Restricted common unts/shares redemptions	-71,523								-71,523
Restricted common units/shares redemptions (in dollars)	-1,848			-1,848				-1,848	-1,848
Adjustments to noncontrolling interests resulting from changes in ownership of COPLP				-773			773
Comprehensive income (loss)	16,885				6,691	8,360	1,834	16,885	-9,276	8,734	1,451		495		15,481
Dividends/Distributions	-86,700				-86,700			-90,381	-74,405				-495		-15,481
Distributions to owners of common and preferred units in COPLP	-3,681						-3,681
Contributions from noncontrolling interests	85						85	85			85
Distributions to noncontrolling interests	-8						-8	-8			-8
Adjustment to arrive at fair value of redeemable noncontrolling interest	-6,414			-6,414				-6,414	-6,414
Increase in tax benefit from share-based compensation	-122			-122				-122	-122
Balance at Sep. 30, 2013	$1,434,067	$249,083	$874	$1,812,801	($700,368)	$2,925	$68,752	$1,434,067	$1,163,977	$3,026	$9,181		$8,800		$249,083
Balance (in units/ shares) at Sep. 30, 2013	87,381,395								91,359,549			3,978,154		87,381,395
Balance (preferred units) at Sep. 30, 2013												352,000	352,000	9,431,667	9,431,667

Consolidated_Statements_of_Equ1

Consolidated Statements of Equity (Parenthetical)	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Balance (in units/ shares)	80,952,986	72,011,324
Conversion of common units to common shares	310,889	94,550
Exercise of share options (in units/shares)	32,756	44,624
Restricted common units/shares redemptions	71,523	135,777
Balance (in units/ shares)	87,381,395	72,157,635
Preferred Shares
Shares issued to the public (in units/ shares)		6,900,000
Stock redeemed or called during period (in units/ shares)	3,390,000	2,200,000,000
Common Shares
Shares issued to the public (in units/ shares)	4,485,000
Common shares issued to public under at-the-market program
Shares issued to the public (in units/ shares)	1,500,000

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (USD $)	9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
Cash flows from operating activities
Revenues from real estate operations received	$359,431	$363,877
Construction contract and other service revenues received	48,877	58,637
Property operating expenses paid	-133,799	-133,375
Construction contract and other service expenses paid	-53,191	-50,438
General, administrative, leasing, business development and land carry costs paid	-20,681	-18,526
Interest expense paid	-59,052	-63,811
Previously accreted interest expense paid	-11,116	0
Settlement of interest rate derivatives	0	-29,738
Proceeds from sale of trading marketable securities	298	18,975
Exit costs on property dispositions	-186	-4,066
Payments in connection with early extinguishment of debt	-23,969	-2,637
Interest and other income received	391	786
Income taxes paid	6	-8
Net cash provided by operating activities	107,009	139,676
Cash flows from investing activities
Construction, development and redevelopment	-156,820	-107,621
Tenant improvements on operating properties	-15,868	-20,924
Other capital improvements on operating properties	-17,528	-9,571
Acquisitions of operating properties	0	-48,308
Proceeds from dispositions of properties	12,447	290,607
Mortgage and other loan receivables funded or acquired	-4,111	-11,603
Leasing costs paid	-9,366	-7,289
Other	4,401	-1,527
Net cash (used in) provided by investing activities	-186,845	83,764
Proceeds from debt
Revolving Credit Facility	457,000	262,000
Unsecured senior notes	592,413	0
Other debt proceeds	82,886	399,296
Repayments of debt
Revolving Credit Facility	-457,000	-844,000
Scheduled principal amortization	-7,229	-9,094
Other debt repayments	-542,532	-51,850
Deferred financing costs paid	-9,001	-3,210
Net proceeds from issuance of preferred shares/units	0	165,662
Net proceeds from issuance of common shares/units	157,307	661
Redemption of preferred shares/units	-84,750	-55,000
Common share/unit dividends/distributions paid	-69,451	-69,325
Preferred share/unit dividends/distributions paid	-16,845	-12,345
Distributions paid to noncontrolling interests in COPLP	-3,705	-4,510
Restricted share redemptions	-1,848	-3,279
Other	-685	1,004
Net cash provided by (used in) financing activities	96,560	-223,990
Net increase (decrease) in cash and cash equivalents	16,724	-550
Cash and cash equivalents
Beginning of period	10,594	5,559
End of period	27,318	5,009
Reconciliation of net income to net cash provided by operating activities:
Net income	8,872	1,331
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and other amortization	87,956	95,248
Impairment losses	30,940	60,593
Settlement of previously accreted interest expense	-11,116	0
Amortization of deferred financing costs	4,292	4,696
Increase in deferred rent receivable	-7,995	-7,939
Amortization of net debt discounts	1,207	2,357
Gain on sales of real estate	-2,683	-20,969
Share-based compensation	4,869	8,262
Loss (gain) on early extinguishment of debt	3,059	-3,436
Other	-2,284	-459
Changes in operating assets and liabilities:
Decrease in accounts receivable	1,523	10,063
Decrease in restricted cash and marketable securities	418	15,051
(Increase) decrease in prepaid expenses and other assets	-12,974	1,532
(Decrease) increase in accounts payable, accrued expenses and other liabilities	18	4,124
Increase (decrease) in rents received in advance and security deposits	907	-2,775
Decrease in interest rate derivatives in connection with cash settlement	0	-28,003
Net cash provided by operating activities	107,009	139,676
Supplemental schedule of non-cash investing and financing activities:
Decrease in accrued capital improvements, leasing and other investing activity costs	-13,094	-11,627
Decrease in property in connection with surrender of property in settlement of debt	0	12,812
Decrease in debt in connection with surrender of property in settlement of debt	0	16,304
Increase (decrease) in fair value of derivatives applied to accumulated other comprehensive income (loss) and noncontrolling interests	7,785	-4,398
Equity in other comprehensive income of an equity method investee	1,070	0
Dividends/distributions payable	29,077	26,954
COPLP common units issued to acquire property and noncontrolling interest in other consolidated entity	5,194	0
Decrease in noncontrolling interests and increase in shareholders’ equity in connection with the conversion of common units into common shares	3,991	1,188
Adjustments to noncontrolling interests resulting from changes in COPLP ownership	773	373
Increase in redeemable noncontrolling interest and decrease in shareholders’ equity to carry redeemable noncontrolling interest at fair value	6,414	2,827
Corporate Office Properties, L.P.
Cash flows from operating activities
Revenues from real estate operations received	359,431	363,877
Construction contract and other service revenues received	48,877	58,637
Property operating expenses paid	-133,799	-133,375
Construction contract and other service expenses paid	-53,191	-50,438
General, administrative, leasing, business development and land carry costs paid	-20,681	-18,526
Interest expense paid	-59,052	-63,811
Previously accreted interest expense paid	-11,116	0
Settlement of interest rate derivatives	0	-29,738
Proceeds from sale of trading marketable securities	298	18,975
Exit costs on property dispositions	-186	-4,066
Payments in connection with early extinguishment of debt	-23,969	-2,637
Interest and other income received	391	786
Income taxes paid	6	-8
Net cash provided by operating activities	107,009	139,676
Cash flows from investing activities
Construction, development and redevelopment	-156,820	-107,621
Tenant improvements on operating properties	-15,868	-20,924
Other capital improvements on operating properties	-17,528	-9,571
Acquisitions of operating properties	0	-48,308
Proceeds from dispositions of properties	12,447	290,607
Mortgage and other loan receivables funded or acquired	-4,111	-11,603
Leasing costs paid	-9,366	-7,289
Other	4,401	-1,527
Net cash (used in) provided by investing activities	-186,845	83,764
Proceeds from debt
Revolving Credit Facility	457,000	262,000
Unsecured senior notes	592,413	0
Other debt proceeds	82,886	399,296
Repayments of debt
Revolving Credit Facility	-457,000	-844,000
Scheduled principal amortization	-7,229	-9,094
Other debt repayments	-542,532	-51,850
Deferred financing costs paid	-9,001	-3,210
Net proceeds from issuance of preferred shares/units	0	165,662
Net proceeds from issuance of common shares/units	157,307	661
Redemption of preferred shares/units	-84,750	-55,000
Common share/unit dividends/distributions paid	-72,661	-73,340
Preferred share/unit dividends/distributions paid	-17,340	-12,840
Restricted share redemptions	-1,848	-3,279
Other	-685	1,004
Net cash provided by (used in) financing activities	96,560	-223,990
Net increase (decrease) in cash and cash equivalents	16,724	-550
Cash and cash equivalents
Beginning of period	10,594	5,559
End of period	27,318	5,009
Reconciliation of net income to net cash provided by operating activities:
Net income	8,872	1,331
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and other amortization	87,956	95,248
Impairment losses	30,940	60,593
Settlement of previously accreted interest expense	-11,116	0
Amortization of deferred financing costs	4,292	4,696
Increase in deferred rent receivable	-7,995	-7,939
Amortization of net debt discounts	1,207	2,357
Gain on sales of real estate	-2,683	-20,969
Share-based compensation	4,869	8,262
Loss (gain) on early extinguishment of debt	3,059	-3,436
Other	-2,284	-459
Changes in operating assets and liabilities:
Decrease in accounts receivable	1,523	10,063
Decrease in restricted cash and marketable securities	822	14,399
(Increase) decrease in prepaid expenses and other assets	-12,974	1,532
(Decrease) increase in accounts payable, accrued expenses and other liabilities	-386	4,776
Increase (decrease) in rents received in advance and security deposits	907	-2,775
Decrease in interest rate derivatives in connection with cash settlement	0	-28,003
Net cash provided by operating activities	107,009	139,676
Supplemental schedule of non-cash investing and financing activities:
Decrease in accrued capital improvements, leasing and other investing activity costs	-13,094	-11,627
Decrease in property in connection with surrender of property in settlement of debt	0	12,812
Decrease in debt in connection with surrender of property in settlement of debt	0	16,304
Increase (decrease) in fair value of derivatives applied to accumulated other comprehensive income (loss) and noncontrolling interests	7,785	-4,398
Equity in other comprehensive income of an equity method investee	1,070	0
Dividends/distributions payable	29,077	26,954
COPLP common units issued to acquire property and noncontrolling interest in other consolidated entity	5,194	0
Increase in redeemable noncontrolling interest and decrease in shareholders’ equity to carry redeemable noncontrolling interest at fair value	$6,414	$2,827

Organization

Organization	9 Months Ended
	Sep. 30, 2013
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization	Organization
	Corporate Office Properties Trust (“COPT”) and subsidiaries (collectively, the “Company”) is a fully-integrated and self-managed real estate investment trust (“REIT”). Corporate Office Properties, L.P. (“COPLP”) and subsidiaries (collectively, the “Operating Partnership”) is the entity through which COPT, the sole general partner of COPLP, conducts almost all of its operations and owns almost all of its assets. Unless otherwise expressly stated or the context otherwise requires, “we”, “us” and “our” as used herein refer to each of the Company and the Operating Partnership. We focus primarily on serving the specialized requirements of United States Government agencies and defense contractors, most of whom are engaged in defense information technology and national security related activities. We generally acquire, develop, manage and lease office and data center properties concentrated in large office parks located near knowledge-based government demand drivers and/or in targeted markets or submarkets in the Greater Washington, DC/Baltimore region. As of September 30, 2013, our investments in real estate included the following:
	•	210 operating office properties totaling 19.2 million square feet;
	•	11 office properties under, or contractually committed for, construction or approved for redevelopment that we estimate will total approximately 1.5 million square feet upon completion, including one partially operational property included above;
	•	land held or under pre-construction totaling 1,721 acres (including 56 acres controlled but not owned) that we believe are potentially developable into approximately 20.4 million square feet; and
	•	a partially operational, wholesale data center which upon completion and stabilization is expected to have a critical load of 18 megawatts.
	COPLP owns real estate both directly and through subsidiary partnerships and limited liability companies (“LLCs”).  In addition to owning real estate, COPLP also owns subsidiaries that provide real estate services such as property management and construction and development services primarily for our properties but also for third parties.
	Interests in COPLP are in the form of common and preferred units. As of September 30, 2013, COPT owned 95.6% of the outstanding COPLP common units (“common units”) and 96.4% of the outstanding COPLP preferred units (“preferred units”); the remaining common and preferred units in COPLP were owned by third parties. Three of COPT’s trustees controlled, either directly or through ownership by other entities or family members, 3.4% of COPLP’s common units as of September 30, 2013. Common units in COPLP not owned by COPT carry certain redemption rights. The number of common units in COPLP owned by COPT is equivalent to the number of outstanding common shares of beneficial interest (“common shares”) of COPT, and the entitlement of all COPLP common units to quarterly distributions and payments in liquidation are substantially the same as those of COPT common shareholders. Similarly, in the case of each series of preferred units in COPLP held by COPT, there is a series of preferred shares of beneficial interest (“preferred shares”) that is equivalent in number and carries substantially the same terms as such series of COPLP preferred units. COPT’s common shares are publicly traded on the New York Stock Exchange (“NYSE”) under the ticker symbol “OFC”.
	Because COPLP is managed by COPT, and COPT conducts substantially all of its operations through COPLP, we refer to COPT’s executive officers as COPLP’s executive officers, and although, as a partnership, COPLP does not have a board of trustees, we refer to COPT’s Board of Trustees as COPLP’s Board of Trustees.

Summary_of_Significant_Account

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2013
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies
	Basis of Presentation
	The COPT consolidated financial statements include the accounts of COPT, the Operating Partnership, their subsidiaries and other entities in which COPT has a majority voting interest and control.  The COPLP consolidated financial statements include the accounts of COPLP, its subsidiaries and other entities in which COPLP has a majority voting interest and control.  We also consolidate certain entities when control of such entities can be achieved through means other than voting rights (“variable interest entities” or “VIEs”) if they are deemed to be the primary beneficiary of such entities.  We eliminate all significant intercompany balances and transactions in consolidation.
	We use the equity method of accounting when we own an interest in an entity and can exert significant influence over the entity’s operations but cannot control the entity’s operations. We discontinue equity method accounting if our investment in an entity (and net advances) is reduced to zero unless we have guaranteed obligations of the entity or are otherwise committed to provide further financial support for the entity.
	We use the cost method of accounting when we own an interest in an entity and cannot exert significant influence over its operations.
	These interim financial statements should be read together with the consolidated financial statements and notes thereto as of and for the year ended December 31, 2012 for COPT (included in COPT’s 2012 Annual Report on Form 10-K) and COPLP (included in our Current Report on Form 8-K dated July 25, 2013).  The unaudited consolidated financial statements include all adjustments that are necessary, in the opinion of management, to fairly present our financial position and results of operations.  All adjustments are of a normal recurring nature except for the revisions and reclassifications noted below.  The consolidated financial statements have been prepared using the accounting policies described in COPT’s 2012 Annual Report on Form 10-K and our Current Report on Form 8-K dated July 25, 2013.
	Reclassifications
	We reclassified certain amounts from prior periods to conform to the current period presentation of our consolidated financial statements with no effect on previously reported net income or equity.
	Revisions
	As reported in COPT’s 2012 Annual Report on Form 10-K, the following errors were identified in 2012:
	•	the misapplication of accounting guidance related to the recognition of a deferred tax asset resulting from an impairment of assets in the fourth quarter of 2011 that failed to consider a partial reversal of that asset that would result from a cancellation of related inter-company debt in the first quarter of 2012.  The effect of this error was an overstatement of our income tax benefit and an understatement of our net loss for the year ended December 31, 2011 of $4.0 million.  Based on an evaluation against our projected annual net income at that time, this error was previously reported for COPT as an out-of-period adjustment in the three months ended March 31, 2012;
	•	an over-accrual of incentive compensation cost, the effect of which was an overstatement of general and administrative expenses and an overstatement of net loss for the calendar quarter and year ended December 31, 2011 of $711,000. Based on an evaluation against our projected annual net income at that time, this error was previously reported for COPT as an out-of-period adjustment in the three months ended March 31, 2012;
	•	the misapplication of accounting guidance requiring that we recognize loss allocations to a noncontrolling interest holder in a consolidated real estate joint venture associated with decreases in such holder’s claim on the book value of the joint venture’s assets, despite the fact that the real estate held by the joint venture was under development and the joint venture had no underlying losses. The effect of this error was an understatement for COPT of losses attributable to noncontrolling interests in other consolidated entities of $1.8 million for the nine months ended September 30, 2012 and $1.4 million for the year ended December 31, 2011; and
	•	the misapplication of accounting guidance pertaining to our reporting for a noncontrolling interest in a consolidated real estate joint venture formed in March 2010 for which the holder of such interest has the right to require us to acquire the interest at fair value. Accounting guidance requires that this noncontrolling interest be classified outside of permanent equity and reported at fair value as of the end of each reporting period, with changes in such fair value reported as equity transactions with no impact to net income or comprehensive income. This error resulted in an overstatement of equity and offsetting understatement of the line entitled “redeemable noncontrolling interest” in the mezzanine section of COPT’s consolidated balance sheet of $8.9 million as of December 31, 2011. This error had no effect on COPT’s consolidated statements of operations, including reported net income (losses) or earnings per share.
	With respect to the errors in the first two bullets above, we assessed the materiality of these errors on COPT’s consolidated financial statements in connection with previously filed periodic reports, in accordance with ASC 250 (SEC’s Staff Accounting Bulletin No. 99, “Materiality”), and concluded at such time that the errors were not material to any prior annual or interim periods. In assessing the cumulative effect of all such errors, we have considered ASC 250 (SEC Staff Accounting Bulletin No. 108, “Considering the Effects of Prior Year Misstatements when Quantifying Misstatements in Current Year Financial Statements”), and accordingly, the financial statements as of, and for the year ended, December 31, 2011 were revised in COPT’s 2012 Annual Report on Form 10-K. We revised COPT amounts pertaining to the first and second quarter of 2012 in COPT’s Form 10-Qs filed on April 30, 2013 and August 1, 2013, respectively, and amounts pertaining to the third quarter and first nine months of 2012 herein.
	The following are selected line items from COPT’s consolidated financial statements as of, and for the three and nine months ended, September 30, 2012 illustrating the effect of adjustments pertaining to reclassifications and revisions (in thousands):
		Consolidated Balance Sheet as of September 30, 2012
		Per September 30, 2012 10-Q			As Revised			Change			Revisions
	Redeemable noncontrolling interest	$	—		$	9,932		$	9,932		$	9,932
	Additional paid-in capital	$	1,455,558		$	1,451,416		$	(4,142	)	$	(4,142	)
	Cumulative distributions in excess of net income	$	(610,659	)	$	(607,633	)	$	3,026		$	3,026
	Noncontrolling interests in common units in COPLP	$	48,973		$	49,157		$	184		$	184
	Noncontrolling interests in other consolidated entities	$	18,682		$	9,682		$	(9,000	)	$	(9,000	)
	Total equity	$	1,250,221		$	1,240,289		$	(9,932	)	$	(9,932	)
	Total liabilities, redeemable noncontrolling interest and equity	$	3,597,656		$	3,597,656		$	—		$	—
		Consolidated Statements of Operations for the
		Three Months Ended September 30, 2012
		Per September 30, 2012 10-Q			Per September 30, 2013 10-Q			Change			Discontinued Operations			Other Reclassifications			Revisions
	Total revenues	$	130,144		$	129,645		$	(499	)	$	(499	)	$	—		$	—
	Expenses
	Property operating expenses	$	42,799		$	41,474		$	(1,325	)	$	(9	)	$	(1,316	)	$	—
	Depreciation and amortization associated with real estate operations	28,698			28,604			(94		)	(94		)	—			—
	Construction contract and other service expenses	14,410			14,410			—			—			—			—
	Impairment losses	46,096			46,096			—			—			—			—
	General, administrative and leasing expenses	5,061			6,377			1,316			—			1,316			—
	Business development expenses and land carry costs	1,632			1,632			—			—			—			—
	Total operating expenses	$	138,696		$	138,593		$	(103	)	$	(103	)	$	—		$	—
	Operating loss	$	(8,552	)	$	(8,948	)	$	(396	)	$	(396	)	$	—		$	—
	Interest expense	$	(23,239	)	$	(23,239	)	$	—		$	—		$	—		$	—
	Income tax expense	$	(106	)	$	(106	)	$	—		$	—		$	—		$	—
	Loss from continuing operations	$	(31,816	)	$	(32,212	)	$	(396	)	$	(396	)	$	—		$	—
	Discontinued operations	$	11,051		$	11,447		$	396		$	396		$	—		$	—
	Net loss	$	(20,765	)	$	(20,765	)	$	—		$	—		$	—		$	—
	Net loss attributable to noncontrolling interests in common units in COPLP	$	1,569		$	1,533		$	(36	)	$	—		$	—		$	(36	)
	Net loss attributable to noncontrolling interests in other consolidated entities	$	(411	)	$	235		$	646		$	—		$	—		$	646
	Net loss attributable to COPT	$	(19,772	)	$	(19,162	)	$	610		$	—		$	—		$	610
	Basic and diluted earnings per common share:
	Loss from continuing operations	$	(0.54	)	$	(0.54	)	$	—		$	—		$	—		$	—
	Discontinued operations	0.15			0.15			—			—			—			—
	Net loss attributable to COPT common shareholders	$	(0.39	)	$	(0.39	)	$	—		$	—		$	—		$	—
		Consolidated Statements of Operations for the
		Nine Months Ended September 30, 2012
		Per September 30, 2012 10-Q			Per September 30, 2013 10-Q			Change			Discontinued Operations			Other Reclassifications			Revisions
	Total revenues	$	390,499		$	389,043		$	(1,456	)	$	(1,456	)	$	—		$	—
	Expenses
	Property operating expenses	$	126,339		$	122,102		$	(4,237	)	$	32		$	(4,269	)	$	—
	Depreciation and amortization associated with real estate operations	84,920			84,633			(287		)	(287		)	—			—
	Construction contract and other service expenses	51,302			51,302			—			—			—			—
	Impairment losses	41,260			41,260			—			—			—			—
	General, administrative and leasing expenses	19,820			24,797			4,977			(3		)	4,269			711
	Business development expenses and land carry costs	4,506			4,506			—			—			—			—
	Total operating expenses	$	328,147		$	328,600		$	453		$	(258	)	$	—		$	711
	Operating income	$	62,352		$	60,443		$	(1,909	)	$	(1,198	)	$	—		$	(711	)
	Interest expense	$	(71,909	)	$	(71,909	)	$	—		$	—		$	—		$	—
Income tax expense	$	(4,296	)	$	(327	)	$	3,969		$	—		$	—		$	3,969
Loss from continuing operations	$	(12,160	)	$	(10,100	)	$	2,060		$	(1,198	)	$	—		$	3,258
Discontinued operations	$	10,212		$	11,410		$	1,198		$	1,198		$	—		$	—
Net (loss) income	$	(1,927	)	$	1,331		$	3,258		$	—		$	—		$	3,258
Net loss attributable to noncontrolling interests in common units in COPLP	$	1,020		$	738		$	(282	)	$	—		$	—		$	(282	)
Net (income) loss attributable to noncontrolling interests in other consolidated entities	$	(939	)	$	864		$	1,803		$	—		$	—		$	1,803
Net (loss) income attributable to COPT	$	(2,341	)	$	2,438		$	4,779		$	—		$	—		$	4,779
Basic and diluted earnings per common share:
Loss from continuing operations	$	(0.40	)	$	(0.35	)	$	0.05		$	(0.02	)	$	—		$	0.07
Discontinued operations	0.13			0.15			0.02			0.02			—			—
Net loss attributable to COPT common shareholders	$	(0.27	)	$	(0.20	)	$	0.07		$	—		$	—		$	0.07
Recent Accounting Pronouncements
We adopted guidance issued by the Financial Accounting Standards Board (“FASB”) effective January 1, 2013 related to the reporting of the effect of significant reclassifications from accumulated other comprehensive income. This guidance requires an entity to report, either parenthetically on the face of the financial statements or in a single footnote, changes in the components of accumulated other comprehensive income for the period. An entity is required to separately report the amount of such changes attributable to reclassifications (and the statements of operations line affected by such reclassifications) and the amount of such changes attributable to current period other comprehensive income. For amounts that are not required to be reclassified in their entirety to net income in the same reporting period, an entity is required to cross-reference other disclosures that provide additional detail about those amounts. Our adoption of this guidance did not affect our consolidated financial statements or disclosures.

Fair_Value_Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2013
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements
	For a description on how we estimate fair value, see Note 3 to the consolidated financial statements in COPT’s 2012 Annual Report on Form 10-K and Note 3 to the COPLP consolidated financial statements in our Current Report on Form 8-K dated July 25, 2013.
	Recurring Fair Value Measurements
	Our partner in a real estate joint venture has the right to require us to acquire its interest at fair value beginning in March 2020; accordingly, we classify the fair value of our partner’s interest as a redeemable noncontrolling interest in the mezzanine section of our consolidated balance sheet. In determining the fair value of our partner’s interest as of September 30, 2013, we used discount rates ranging from 10.0% to 20.0% (15.3% weighted average), which factored in risk appropriate to the level of future property development expected to be undertaken by the joint venture; a significant increase (decrease) in the discount rate used in determining the fair value would result in a significantly (lower) higher fair value. Given our reliance on the unobservable inputs, the valuations are classified in Level 3 of the fair value hierarchy.
	The carrying values of cash and cash equivalents, restricted cash, accounts receivable, other assets (excluding investing receivables) and accounts payable and accrued expenses are reasonable estimates of their fair values because of the short maturities of these instruments.  As discussed in Note 6, we estimated the fair values of our mortgage and other investing receivables based on the discounted estimated future cash flows of the loans (categorized within Level 3 of the fair value hierarchy); the discount rates used approximate current market rates for loans with similar maturities and credit quality, and the estimated cash payments include scheduled principal and interest payments.  For our disclosure of debt fair values in Note 8 to the consolidated financial statements, we estimated the fair value of our unsecured senior notes and exchangeable senior notes based on quoted market rates for publicly-traded debt (categorized within Level 2 of the fair value hierarchy) and estimated the fair value of our other debt based on the discounted estimated future cash payments to be made on such debt (categorized within Level 3 of the fair value hierarchy); the discount rates used approximate current market rates for loans, or groups of loans, with similar maturities and credit quality, and the estimated future payments include scheduled principal and interest payments.  Fair value estimates are made at a specific point in time, are subjective in nature and involve uncertainties and matters of significant judgment.  Settlement at such fair value amounts may not be possible and may not be a prudent management decision.
	For additional fair value information, please refer to Note 6 for mortgage loans receivable, Note 8 for debt and Note 9 for interest rate derivatives.
	COPT and Subsidiaries
	The table below sets forth financial assets and liabilities of COPT and its subsidiaries that are accounted for at fair value on a recurring basis as of September 30, 2013 and the hierarchy level of inputs used in measuring their respective fair values under applicable accounting standards (in thousands):
	Description		Quoted Prices in		Significant Other			Significant		Total
			Active Markets for		Observable Inputs(Level 2)			Unobservable Inputs(Level 3)
			Identical Assets(Level 1)
	Assets:
	Marketable securities in deferred compensation plan (1)
	Mutual funds		$	6,753	$	—		$	—	$	6,753
	Common stocks		217		—			—		217
	Other		211		—			—		211
	Common stock (1)		520		—			—		520
	Interest rate derivatives (2)		—		5,195			—		5,195
	Warrants to purchase common stock (2)		—		301			—		301
	Total Assets		$	7,701	$	5,496		$	—	$	13,197
	Liabilities:
	Deferred compensation plan liability (3)		$	7,181	$	—		$	—	$	7,181
	Interest rate derivatives		—		3,595			—		3,595
	Total Liabilities		$	7,181	$	3,595		$	—	$	10,776
	Redeemable noncontrolling interest		$	—	$	—		$	16,789	$	16,789
	(1) Included in the line entitled “restricted cash and marketable securities” on COPT’s consolidated balance sheet.
	(2) Included in the line entitled “prepaid expenses and other assets” on COPT’s consolidated balance sheet.
	(3) Included in the line entitled “other liabilities” on COPT’s consolidated balance sheet.
	COPLP and Subsidiaries
	The table below sets forth financial assets and liabilities of COPLP and its subsidiaries that are accounted for at fair value on a recurring basis as of September 30, 2013 and the hierarchy level of inputs used in measuring their respective fair values under applicable accounting standards (in thousands):
	Description		Quoted Prices in		Significant Other			Significant		Total
			Active Markets for		Observable Inputs(Level 2)			Unobservable Inputs(Level 3)
			Identical Assets(Level 1)
	Assets:
	Common stock (1)		$	520	$	—		$	—	$	520
	Interest rate derivatives (2)		—		5,195			—		5,195
	Warrants to purchase common stock (2)		—		301			—		301
	Total Assets		$	520	$	5,496		$	—	$	6,016
	Liabilities:
	Interest rate derivatives		$	—	$	3,595		$	—	$	3,595
	Redeemable noncontrolling interest		$	—	$	—		$	16,789	$	16,789
	(1) Included in the line entitled “restricted cash and marketable securities” on COPLP’s consolidated balance sheet.
	(2) Included in the line entitled “prepaid expenses and other assets” on COPLP’s consolidated balance sheet.
	Nonrecurring Fair Value Measurements
	Nine Months Ended September 30, 2013
	During the nine months ended September 30, 2013, we recognized the following impairment losses:
	•	for certain of our operating properties serving as collateral for a non recourse loan, we expect that the cash flows that will be generated by the properties will be insufficient to fund debt service requirements on the loan. While we sought to negotiate various alternatives with the lender, during the three months ended September 30, 2013, we determined that the probable outcome will be the conveyance of the properties to the lender to extinguish the loan. We expect that the conveyance will occur in a series of transactions in the fourth quarter of 2013 and the first quarter of 2014. We determined that the carrying amount of certain of these properties located in Colorado Springs, Colorado exceed their fair value, and will not likely be recovered from the cash flows from the operations of the properties over the likely remaining holding period. Accordingly, we recognized non-cash impairment losses of $10.4 million on these properties during the three months ended September 30, 2013;
	•	$14.8 million (all classified as discontinued operations and including $186,000 in exit costs) in connection with properties and land no longer aligned with our strategy that we sold or have classified as held for sale, most of which was attributable to our continuing negotiations to sell certain properties in Colorado Springs; and
	•	$5.9 million on two properties in the Greater Baltimore region during the three months ended September 30, 2013. After shortening our expected holding period for these properties during the period, we determined that the carrying amount of the properties will not likely be recovered from the cash flows from the operations and sales of the properties over the shortened period.
	The table below sets forth the fair value hierarchy of the valuation technique used by us in determining the fair value of the properties (dollars in thousands):
			Quoted Prices in					Significant				Impairment Losses Recognized (1)
			Active Markets for		Significant Other			Unobservable				Three Months		Nine Months
			Identical Assets		Observable Inputs			Inputs				Ended		Ended
	Description		(Level 1)		(Level 2)			(Level 3)		Total		30-Sep-13		30-Sep-13
	Assets (2):
	Properties, net		$	—	$	—		$	245,535	$	245,535	$	21,888	$	30,940
	(1) Represents impairment losses, excluding exit costs incurred of $186,000 for the three months and nine months ended September 30, 2013.
	(2) Reflects balance sheet classifications of assets at time of fair value measurement, excluding the effect of held for sale classifications.
	The table below sets forth quantitative information about significant unobservable inputs used for the Level 3 fair value measurements reported above (dollars in thousands):
	Description		Fair Value on		Valuation Technique		Unobservable Input	Range (Weighted Average)
			Measurement Date
	Properties on which impairment losses were recognized		$	245,535	Bids for property indicative of value		Indicative bids (1)	-1
					Contract of sale		Contract price (1)	-1
					Discounted cash flow		Discount rate	10.0% to 11.0% (10.9%)
						Terminal capitalization rate	9.5% to 10.0% (9.7%)
						Market rent growth rate	3.0% (2)
						Expense growth rate	3.0% (2)
(1) These fair value measurements were developed as a result of negotiations between us and potential, or actual, purchasers of properties.
(2) Only one value applied for this unobservable input.
Nine Months Ended September 30, 2012
During the nine months ended September 30, 2012, we recognized non-cash impairment losses of $60.6 million for the amount by which the carrying values of certain properties exceeded their estimated fair values. The table below sets forth the fair value hierarchy of the valuation techniques used by us in determining such fair values (dollars in thousands):
		Quoted Prices in					Significant				Impairment Losses Recognized (1)
		Active Markets for		Significant Other			Unobservable				Three Months		Nine Months
		Identical Assets		Observable Inputs			Inputs				Ended		Ended
Description		(Level 1)		(Level 2)			(Level 3)		Total		30-Sep-12		30-Sep-12
Assets (2):
Properties, net		$	—	$	—		$	369,312	$	369,312	$	52,900	60,593
(1) Represents impairment losses, excluding exit costs incurred of $2.9 million for the three months ended September 30, 2012 and $4.2 million for the nine months ended September 30, 2012.
(2) Reflects balance sheet classifications of assets at time of fair value measurement, excluding the effect of held for sale classifications.
The table below sets forth quantitative information about significant unobservable inputs used for the Level 3 fair value measurements reported above (dollars in thousands):
Description		Fair Value on		Valuation Technique		Unobservable Input	Range (Weighted Average)
		Measurement Date
Properties on which impairment losses were recognized		$	369,312	Bid for properties indicative of value		Indicative bid (1)	-1
				Contract of sale		Contract price (1)	-1
				Discounted cash flow		Discount rate	10.1% to 11.0% (10.4%)
						Terminal capitalization rate	8.7% to 10.0% (8.9%)
						Market rent growth rate	3.0% (2)
						Expense growth rate	3.0% (2)
				Yield Analysis		Yield	12% (2)
						Market rent rate	$8.50 per square foot(2)
						Leasing costs	$20.00 per square foot (2)
(1) These fair value measurements were developed as a result of negotiations between us and potential, or actual, purchasers of properties.
(2) Only one value applied for this unobservable input.

Properties_net

Properties, net	9 Months Ended
	Sep. 30, 2013
Real Estate [Abstract]
Properties, net	Properties, net
	Operating properties, net consisted of the following (in thousands):
		September 30,			December 31,
		2013			2012
	Land	$	436,565		$	427,766
	Buildings and improvements	2,889,383			2,725,875
	Less: accumulated depreciation	(612,369		)	(555,975		)
	Operating properties, net	$	2,713,579		$	2,597,666
	Projects we had in development or held for future development consisted of the following (in thousands):
		September 30,			December 31,
		2013			2012
	Land	$	253,810		$	236,324
	Construction in progress, excluding land	272,357			329,054
	Projects in development or held for future development	$	526,167		$	565,378
	During the nine months ended September 30, 2013, we sold 920 Elkridge Landing Road, a 103,000 square foot office property in Linthicum, Maryland (in the Baltimore/Washington Corridor), for $6.9 million.
	2013 Construction Activities
	During the nine months ended September 30, 2013, we placed into service an aggregate of 392,000 square feet in four newly constructed office properties located in the Baltimore/Washington Corridor, Northern Virginia and Huntsville, Alabama. As of September 30, 2013, we had nine office properties under construction, or for which we were contractually committed to construct, that we estimate will total 1.3 million square feet upon completion, including four in Northern Virginia, three in Huntsville, Alabama and two in the Baltimore/Washington Corridor. We also had redevelopment underway, or otherwise approved, for two office properties that we estimate will total 235,000 square feet upon completion, including one in Greater Philadelphia and one in the Baltimore/Washington Corridor.

Real_Estate_Joint_Ventures

Real Estate Joint Ventures	9 Months Ended
	Sep. 30, 2013
Equity Method Investments and Joint Ventures [Abstract]
Real Estate Joint Ventures	Real Estate Joint Ventures
	During the nine months ended September 30, 2013, we had an investment in one unconsolidated real estate joint venture accounted for using the equity method of accounting.  Information pertaining to this investment is set forth below (dollars in thousands):
	Investment Balance at (1)								Date				Nature of		Maximum Exposure
	September 30, 2013				31-Dec-12				Acquired		Ownership		Activity		to Loss (2)
	$	(6,420	)		$	(6,420	)		9/29/05		20%		Operates 16 Buildings		$	—
	(1)   The carrying amount of our investment in this joint venture was lower than our share of the equity in the joint venture by $3.0 million at September 30, 2013 and $4.5 million at December 31, 2012 due to our deferral of gain on the contribution by us of real estate into the joint venture upon its formation and our discontinuance of loss recognition under the equity method effective October 2012, as discussed below.  A difference will continue to exist to the extent the nature of our continuing involvement in the joint venture remains the same and we continue to no longer recognize income or losses under the equity method.
	(2)   Derived from the sum of our investment balance and maximum additional unilateral capital contributions or loans required from us.  Not reported above are additional amounts that we and our partner are required to fund when needed by this joint venture; these funding requirements are proportional to our respective ownership percentages.  Also not reported above are additional unilateral contributions or loans from us, the amounts of which are uncertain, that we would be required to make if certain contingent events occur (see Note 17).
	The following table sets forth condensed balance sheets for this unconsolidated real estate joint venture (in thousands):
		September 30,				December 31,
		2013				2012
	Properties, net	$	57,222			$	58,460
	Other assets	6,248				4,376
	Total assets	$	63,470			$	62,836
	Liabilities (primarily debt)	$	80,424			$	72,693
	Owners’ equity	(16,954		)		(9,857		)
	Total liabilities and owners’ equity	$	63,470			$	62,836
	The following table sets forth condensed statements of operations for this unconsolidated real estate joint venture (in thousands):
		For the Three Months Ended September 30,								For the Nine Months Ended September 30,
		2013				2012				2013				2012
	Revenues	$	1,739			$	1,778			$	5,342			$	5,497
	Property operating expenses	(755		)		(758		)		(2,250		)		(2,219		)
	Interest expense	(2,966		)		(1,138		)		(8,521		)		(3,291		)
	Depreciation and amortization expense	(575		)		(568		)		(1,668		)		(1,732		)
	Net loss	$	(2,557	)		$	(686	)		$	(7,097	)		$	(1,745	)
	As discussed further in our 2012 Annual Report on Form 10-K and our Current Report on Form 8-K dated July 25, 2013, in 2012, the holder of mortgage debt encumbering all of the joint venture’s properties notified us of the debt’s default, initiated foreclosure proceedings and terminated our property management responsibilities; accordingly, we discontinued recognition of losses on this investment under the equity method effective in October 2012 due to our having neither the obligation nor intent to support the joint venture.
	The table below sets forth information pertaining to our investments in consolidated real estate joint ventures at September 30, 2013 (dollars in thousands):
					Ownership				September 30, 2013								-1
			Date		% at				Total				Pledged				Total
			Acquired		9/30/13		Nature of Activity		Assets				Assets				Liabilities
	LW Redstone Company, LLC		3/23/10		85%		Developing business park (2)		$	114,141			$	45,209			$	22,141
	M Square Associates, LLC		6/26/07		50%		Operating two buildings and developing others (3)		60,879				47,854				41,463
	COPT-FD Indian Head, LLC		10/23/06		75%		Holding land parcel (4)		6,447				—				—
	MOR Forbes 2 LLC		12/24/02		50%		Operating one building (5)		4,132				—				90
									$	185,599			$	93,063			$	63,694
	(1) Excludes amounts eliminated in consolidation.
	(2) This joint venture’s property is in Huntsville, Alabama.
	(3) This joint venture’s properties are in College Park, Maryland (in the Suburban Maryland region).
	(4) This joint venture’s property is in Charles County, Maryland. In 2012, the joint venture exercised its option under a development agreement to require Charles County to repurchase the land parcel at its original acquisition cost. Under the terms of the agreement with Charles County, the repurchase is expected to occur by August 2014.
	(5) This joint venture’s property is in Lanham, Maryland (in the Suburban Maryland region).
	These ventures include only ones in which parties other than COPLP and COPT own interests. During the periods included herein, we also owned a controlling interest in Arundel Preserve #5, LLC, a consolidated real estate joint venture owning property in Hanover Maryland (in the Baltimore/Washington Corridor), until September 17, 2013, at which time we acquired our partner’s noncontrolling interest, along with incremental additional land value in the venture, in exchange for 221,501 common units in COPLP valued at $5.2 million.
	Our commitments and contingencies pertaining to our real estate joint ventures are disclosed in Note 17.

Mortgage_and_Other_Investing_R

Mortgage and Other Investing Receivables	9 Months Ended
	Sep. 30, 2013
Receivables [Abstract]
Mortgage and Other Investing Receivables	Mortgage and Other Investing Receivables
	Mortgage and other investing receivables consisted of the following (in thousands):
		September 30,		December 31,
		2013		2012
	Notes receivable from City of Huntsville	$	40,321	$	33,252
	Mortgage loan receivable	—		144
		$	40,321	$	33,396
	Our notes receivable from the City of Huntsville funded infrastructure costs in connection with our LW Redstone Company, LLC joint venture (see Note 5).  We did not have an allowance for credit losses in connection with our notes receivable at September 30, 2013 or December 31, 2012.  The fair value of these receivables approximated their carrying amounts at September 30, 2013 and December 31, 2012.

Prepaid_Expenses_and_Other_Ass

Prepaid Expenses and Other Assets	9 Months Ended
	Sep. 30, 2013
Prepaid Expense and Other Assets [Abstract]
Prepaid Expenses and Other Assets	Prepaid Expenses and Other Assets
	Prepaid expenses and other assets consisted of the following (in thousands):
		September 30,		December 31,
		2013		2012
	Prepaid expenses	$	23,552	$	19,270
	Lease incentives	8,751		5,578
	Furniture, fixtures and equipment, net	6,705		7,991
	Deferred tax asset	6,222		6,612
	Construction contract costs incurred in excess of billings	5,465		—
	Interest rate derivatives	5,195		—
	Other equity method investments	4,928		2,425
	Other assets	3,257		2,183
	Prepaid expenses and other assets	$	64,075	$	44,059
	Other assets, as reported above, include operating notes receivable due from tenants with terms exceeding one year totaling $186,000 at September 30, 2013 and $271,000 at December 31, 2012.  We carried allowances for estimated losses for most of these balances.

Debt

Debt	9 Months Ended
	Sep. 30, 2013
Debt Disclosure [Abstract]
Debt	Debt
	Our debt consisted of the following (dollars in thousands):
		Maximum
		Availability at			Carrying Value at						Scheduled Maturity
		September 30,			September 30,		December 31,		Stated Interest Rates at		Dates at
		2013			2013		2012		September 30, 2013		September 30, 2013
	Mortgage and Other Secured Loans:
	Fixed rate mortgage loans (1)	N/A			$	882,311	$	948,414	3.96% - 7.87% (2)		2014-2034
	Variable rate secured loan	N/A			37,894		38,475		LIBOR + 2.25% (3)		11/2/15
	Other construction loan facilities	$	26,150		—		29,557		N/A		N/A
	Total mortgage and other secured loans				920,205		1,016,446
	Revolving Credit Facility (4)	800,000			—		—		LIBOR + 0.975% to 1.75% (5)		7/1/17
	Term Loan Facilities (6)	(6		)	620,000		770,000		LIBOR + 1.10% to 2.60% (7)		2015-2019
	Unsecured Senior Notes (8)
	3.600% Senior Notes	N/A			347,183		—		3.60%		5/15/23
	5.250% Senior Notes	N/A			245,360		—		5.25%		2/15/24
	Unsecured notes payable	N/A			1,723		1,788		0% (9)		5/1/26
	4.25% Exchangeable Senior Notes (10)	N/A			560		230,934		4.25%		4/15/30
	Total debt, net				$	2,135,031	$	2,019,168
	(1)	Several of the fixed rate mortgages carry interest rates that were above or below market rates upon assumption and therefore were recorded at their fair value based on applicable effective interest rates.  The carrying values of these loans reflect net unamortized premiums totaling $465,000 at September 30, 2013 and $1.3 million at December 31, 2012.
	-2	The weighted average interest rate on these loans was 5.98% at September 30, 2013.
	(3)	The interest rate on the loan outstanding was 2.43% at September 30, 2013.
	-4	Refer to the paragraph below for disclosure pertaining to the Revolving Credit Facility.
	-5	The weighted average interest rate on the Revolving Credit Facility was 1.45% at September 30, 2013.
	(6)	We have the ability to borrow an aggregate of an additional $180.0 million under these term loan facilities, provided that there is no default under the facilities and subject to the approval of the lenders.
	(7)	The weighted average interest rate on these loans was 1.80% at September 30, 2013.
	-8	Refer to the paragraphs below for disclosure pertaining to these notes.
	(9)	These notes carry interest rates that were below market rates upon assumption and therefore were recorded at their fair value based on applicable effective interest rates.  The carrying value of these notes reflects an unamortized discount totaling $788,000 at September 30, 2013 and $873,000 at December 31, 2012.
	(10)	As described further in our 2012 Annual Report on Form 10-K, these notes have an exchange settlement feature that provides that the notes may, under certain circumstances, be exchangeable for cash and, at COPLP’s discretion, COPT common shares at an exchange rate (subject to adjustment) of 20.8513 shares per one thousand dollar principal amount of the notes (exchange rate is as of September 30, 2013 and is equivalent to an exchange price of $47.96 per common share).  During the nine months ended September 30, 2013, we repaid $239.4 million principal amount of these notes and recognized a $25.9 million loss on early extinguishment of debt. The carrying value of these notes included a principal amount of $575,000 and an unamortized discount totaling $15,000 at September 30, 2013 and a principal amount of $240.0 million and an unamortized discount totaling $9.1 million at December 31, 2012.  The effective interest rate under the notes, including amortization of the issuance costs, was 6.05%.  Because the closing price of our common shares at September 30, 2013 and December 31, 2012 was less than the exchange price per common share applicable to these notes, the if-converted value of the notes did not exceed the principal amount.  The table below sets forth interest expense recognized on these notes before deductions for amounts capitalized (in thousands):
		For the Three Months Ended September 30,					For the Nine Months Ended September 30,
		2013			2012		2013		2012
	Interest expense at stated interest rate	$	6		$	2,550	$	4,201	$	7,650
	Interest expense associated with amortization of discount	2			919		1,613		2,717
	Total	$	8		$	3,469	$	5,814	$	10,367
	All debt is owed by the Operating Partnership. While COPT is not directly obligated by any debt, it has guaranteed our Revolving Credit Facility, Term Loan Facilities, Unsecured Senior Notes and 4.25% Exchangeable Senior Notes.
	On July 16, 2013, we entered into an amendment to our credit agreement providing for an unsecured revolving credit facility (the “Revolving Credit Facility”). The amendment extended the maturity date of the facility from September 2014 to July 2017, with the ability for us to extend such maturity by one year at our option, provided that there is no default under the facility and we pay an extension fee of 0.15% of the total availability of the facility. The amendment also changed the interest rate on the facility to be based on LIBOR (customarily the 30-day rate) plus 0.975% to 1.750%, as determined by the credit ratings assigned to COPLP by Standard & Poor’s Rating Services, Moody’s Investor Services, Inc. or Fitch Ratings Ltd. (collectively, the “Ratings Agencies”). Also on July 16, 2013, we entered into amendments on $550.0 million in existing unsecured term loan agreements with the same group of lenders as the Revolving Credit Facility that added one year in extension options and changed the interest rate applicable to the facilities to be based on LIBOR (customarily the 30-day rate) plus 1.10% to 2.00%, as determined by the credit ratings assigned to COPLP by the Ratings Agencies.
	During the nine months ended September 30, 2013, we issued the following senior notes:
	•	a $350.0 million aggregate principal amount of 3.600% Senior Notes at an initial offering price of 99.816% of their face value on May 6, 2013, resulting in proceeds, after deducting discounts of the initial purchasers of the notes, but before other offering expenses, of $347.1 million. The notes mature on May 15, 2023. The carrying value of these notes reflects an unamortized discount totaling $2.8 million at September 30, 2013. The effective interest rate under the notes, including amortization of the issuance costs, was 3.70%; and
	•	a $250.0 million aggregate principal amount of 5.250% Senior Notes at an initial offering price of 98.783% of their face value on September 16, 2013, resulting in proceeds, after deducting discounts of the initial purchasers of the notes, but before other offering expenses, of $245.3 million. The notes mature on February 15, 2024. The carrying value of these notes reflects an unamortized discount totaling $4.6 million at September 30, 2013. The effective interest rate under the notes, including amortization of the issuance costs, was 5.49%.
	We may redeem these notes, in whole at any time or in part from time to time, at our option, at a redemption price equal to the greater of (1) the aggregate principal amount of the notes being redeemed and (2) the sum of the present values of the remaining scheduled payments of principal and interest thereon (not including any portion of such payments of interest accrued as of the date of redemption) discounted to its present value, on a semi-annual basis at an adjusted treasury rate plus a spread (30 basis points for the 3.600% Senior Notes and 40 basis points for the 5.250% Senior Notes), plus, in each case, accrued and unpaid interest thereon to the date of redemption. However, in each case, if this redemption occurs on or after three months prior to the maturity date, the redemption price will be equal to 100% of the principal amount of the notes being redeemed, plus accrued and unpaid interest thereon to, but not including, the applicable redemption date. These notes are unconditionally guaranteed by COPT.
	We capitalized interest costs of $2.2 million in the three months ended September 30, 2013, $3.4 million in the three months ended September 30, 2012, $6.7 million in the nine months ended September 30, 2013 and $10.8 million in the nine months ended September 30, 2012.
	The following table sets forth information pertaining to the fair value of our debt (in thousands):
		September 30, 2013					December 31, 2012
		Carrying			Estimated		Carrying		Estimated
		Amount			Fair Value		Amount		Fair Value
	Fixed-rate debt
	Unsecured Senior Notes	$	592,543		$	579,654	$	—	$	—
	4.25% Exchangeable Senior Notes	560			575		230,934		240,282
	Other fixed-rate debt	884,034			812,246		950,202		968,180
	Variable-rate debt	657,894			661,593		838,032		845,558
		$	2,135,031		$	2,054,068	$	2,019,168	$	2,054,020

Interest_Rate_Derivatives

Interest Rate Derivatives	9 Months Ended
	Sep. 30, 2013
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Interest Rate Derivatives	Interest Rate Derivatives
	The following table sets forth the key terms and fair values of our interest rate swap derivatives (dollars in thousands):
													Fair Value at
	Notional Amount				Fixed Rate		Floating Rate Index		Effective Date		Expiration Date		September 30,				December 31,
													2013				2012
	$	100,000			0.61%		One-Month LIBOR		1/3/12		9/1/14		$	(365	)		$	(594	)
	100,000				0.61%		One-Month LIBOR		1/3/12		9/1/14		(362		)		(591		)
	100,000				0.83%		One-Month LIBOR		1/3/12		9/1/15		(963		)		(1,313		)
	100,000				0.83%		One-Month LIBOR		1/3/12		9/1/15		(963		)		(1,313		)
	37,894			-1	3.83%		One-Month LIBOR + 2.25%		11/2/10		11/2/15		(933		)		(1,268		)
	100,000				0.81%		One-Month LIBOR		9/2/14		9/1/16		1				(263		)
	100,000				0.81%		One-Month LIBOR		9/2/14		9/1/16		(9		)		(272		)
	100,000				1.67%		One-Month LIBOR		9/1/15		8/1/19		2,685				(154		)
	100,000				1.73%		One-Month LIBOR		9/1/15		8/1/19		2,509				(417		)
													$	1,600			$	(6,185	)
	-1	The notional amount of this instrument is scheduled to amortize to $36.2 million.
	Each of the one-month LIBOR interest rate swaps set forth in the table above was designated as cash flow hedges of interest rate risk.
	The table below sets forth the fair value of our interest rate derivatives as well as their classification on our consolidated balance sheet (in thousands):
			September 30, 2013						31-Dec-12
	Derivatives		Balance Sheet Location		Fair Value				Balance Sheet Location		Fair Value
	Interest rate swaps designated as cash flow hedges		Prepaid expenses and other assets		$	5,195			Prepaid expenses and other assets		$	—
	Interest rate swaps designated as cash flow hedges		Interest rate derivatives		(3,595		)		Interest rate derivatives		(6,185		)
	The table below presents the effect of our interest rate derivatives on our consolidated statements of operations and comprehensive income (in thousands):
		For the Three Months Ended September 30,								For the Nine Months Ended September 30,
		2013				2012				2013				2012
	Amount of (loss) gain recognized in accumulated other comprehensive income (loss) (“AOCI”) (effective portion)	$	(2,482	)		$	(2,760	)		$	5,810			$	(7,386	)
	Amount of loss reclassified from AOCI into interest expense (effective portion)	689				632				2,021				3,034
	Over the next 12 months, we estimate that approximately $2.6 million of losses will be reclassified from AOCI to interest expense.
	We have agreements with each of our interest rate derivative counterparties that contain provisions under which, if we default or are capable of being declared in default on any of our indebtedness, we could also be declared in default on our derivative obligations.  These agreements also incorporate the loan covenant provisions of our indebtedness with a lender affiliate of the derivative counterparties.  Failure to comply with the loan covenant provisions could result in our being declared in default on any derivative instrument obligations covered by the agreements.  As of September 30, 2013, the fair value of interest rate derivatives in a liability position related to these agreements was $3.6 million, excluding the effects of accrued interest. As of September 30, 2013, we had not posted any collateral related to these agreements.  We are not in default with any of these provisions.  If we breached any of these provisions, we could be required to settle our obligations under the agreements at their termination value of $3.8 million.

Redeemable_Noncontrolling_Inte

Redeemable Noncontrolling Interest	9 Months Ended
	Sep. 30, 2013
Noncontrolling Interest [Abstract]
Redeemable Noncontrolling Interest	Redeemable Noncontrolling Interest
	The table below sets forth activity in our redeemable noncontrolling interest (in thousands):
		Nine Months Ended September 30,
		2013			2012
	Beginning balance	$	10,298		$	8,908
	Distribution to noncontrolling interest	(811		)	—
	Net income (loss) attributable to noncontrolling interest	888			(1,803		)
	Adjustment to arrive at fair value of interest	6,414			2,827
	Ending balance	$	16,789		$	9,932

Equity

Equity	9 Months Ended
	Sep. 30, 2013
Equity [Abstract]
Equity	Equity
	On April 22, 2013, COPT redeemed all of its outstanding 7.625% Series J Preferred Shares of beneficial interest (the “Series J Preferred Shares”) at a price of $25 per share, or $84.8 million in the aggregate, plus accrued and unpaid dividends thereon through the date of redemption. Concurrently, COPLP redeemed the Series J Preferred Units previously owned by COPT that carried terms substantially the same as the Series J Preferred Shares. At the time of the redemption, we recognized a $2.9 million decrease to net income available to common shareholders/unitholders pertaining to the original issuance costs incurred on these securities.
	On March 19, 2013, COPT completed a public offering of 4,485,000 common shares at a price of $26.34 per share for net proceeds of $118.1 million (after underwriter discounts but before offering expenses) that were contributed to COPLP in exchange for 4,485,000 common units.
	On July 15, 2013, COPT issued 1.5 million common shares at a weighted average price of $26.05 per share under its at-the-market (“ATM”) stock offering program established in October 2012, representing its first issuance under the ATM program. Net proceeds from the shares issued totaled $38.5 million, after payment of $586,000 in commissions to sales agents. The proceeds from the shares issued were contributed to COPLP in exchange for 1.5 million common units. COPT’s remaining capacity under the ATM Plan is an aggregate gross sales price of $110.9 million in stock sales.
	During the nine months ended September 30, 2013, certain COPLP limited partners redeemed 310,889 common units in COPLP for common shares in COPT on the basis of one common share for each common unit.
	See Note 13 for disclosure of COPT common share and COPLP common unit activity pertaining to our share-based compensation plans.

Information_by_Business_Segmen

Information by Business Segment	9 Months Ended
	Sep. 30, 2013
Segment Reporting [Abstract]
Information by Business Segment	Information by Business Segment
	We have ten reportable operating office property segments (comprised of: the Baltimore/Washington Corridor; Northern Virginia; San Antonio; Washington, DC — Capitol Riverfront; St. Mary’s and King George Counties; Greater Baltimore; Suburban Maryland; Colorado Springs; Greater Philadelphia; and other). We also have an operating wholesale data center segment. The table below reports segment financial information for our reportable segments (in thousands).  We measure the performance of our segments through the measure we define as NOI from real estate operations, which is derived by subtracting property operating expenses from revenues from real estate operations.
		Operating Office Property Segments
		Baltimore/			Northern			San			Washington,			St. Mary’s &		Greater		Suburban			Colorado		Greater		Other			Operating			Total
		Washington			Virginia			Antonio			DC - Capitol			King George		Baltimore		Maryland			Springs		Philadelphia					Wholesale
		Corridor									Riverfront			Counties														Data Center
	Three Months Ended September 30, 2013
	Revenues from real estate operations	$	57,226		$	23,062		$	7,898		$	4,295		$	4,270	$	10,703	$	2,332		$	6,622	$	3,258	$	3,819		$	1,076		$	124,561
	Property operating expenses	18,780			7,999			4,061			1,946			1,299		4,143		862			2,371		924		654			1,846			44,885
	NOI from real estate operations	$	38,446		$	15,063		$	3,837		$	2,349		$	2,971	$	6,560	$	1,470		$	4,251	$	2,334	$	3,165		$	(770	)	$	79,676
	Additions to long-lived assets	$	7,204		$	2,459		$	251		$	86		$	630	$	1,707	$	88		$	1,413	$	102	$	(84	)	$	86		$	13,942
	Transfers from non-operating properties	$	8,706		$	26,355		$	—		$	—		$	—	$	9	$	(183	)	$	1,860	$	3,414	$	(33	)	$	2,714		$	42,842
	Three Months Ended September 30, 2012
	Revenues from real estate operations	$	55,799		$	20,363		$	8,125		$	4,389		$	4,085	$	11,918	$	2,371		$	6,278	$	2,541	$	3,589		$	1,806		$	121,264
	Property operating expenses	18,534			7,115			4,272			1,924			1,241		4,539		1,041			2,432		663		686			1,252			43,699
	NOI from real estate operations	$	37,265		$	13,248		$	3,853		$	2,465		$	2,844	$	7,379	$	1,330		$	3,846	$	1,878	$	2,903		$	554		$	77,565
	Additions to long-lived assets	$	4,454		$	54,781		$	—		$	414		$	355	$	1,055	$	36		$	548	$	195	$	(35	)	$	—		$	61,803
	Transfers from non-operating properties	$	12,431		$	12,979		$	—		$	—		$	24	$	99	$	(88	)	$	315	$	1,843	$	548		$	(5	)	$	28,146
	Nine Months Ended September 30, 2013
	Revenues from real estate operations	$	171,049		$	68,992		$	24,019		$	12,716		$	12,355	$	32,246	$	6,809		$	19,874	$	8,529	$	10,878		$	4,446		$	371,913
	Property operating expenses	57,030			24,020			12,427			5,769			3,715		12,408		2,393			7,014		2,482		1,667			4,680			133,605
	NOI from real estate operations	$	114,019		$	44,972		$	11,592		$	6,947		$	8,640	$	19,838	$	4,416		$	12,860	$	6,047	$	9,211		$	(234	)	$	238,308
	Additions to long-lived assets	$	14,029		$	6,699		$	268		$	484		$	1,634	$	1,761	$	143		$	2,323	$	387	$	3,588		$	262		$	31,578
	Transfers from non-operating properties	$	37,270		$	37,209		$	—		$	—		$	12	$	113	$	337		$	4,540	$	28,031	$	20,925		$	69,163		$	197,600
	Segment assets at September 30, 2013	$	1,223,246		$	593,335		$	118,701		$	99,852		$	96,679	$	305,947	$	52,726		$	159,578	$	105,112	$	130,598		$	168,039		$	3,053,813
	Nine Months Ended September 30, 2012
	Revenues from real estate operations	$	167,726		$	57,974		$	23,563		$	12,515		$	12,436	$	41,954	$	12,680		$	18,880	$	7,171	$	10,977		$	4,660		$	370,536
	Property operating expenses	56,677			21,270			11,998			5,485			3,524		15,911		5,357			6,754		1,851		2,088			3,378			134,293
	NOI from real estate operations	$	111,049		$	36,704		$	11,565		$	7,030		$	8,912	$	26,043	$	7,323		$	12,126	$	5,320	$	8,889		$	1,282		$	236,243
	Additions to long-lived assets	$	9,393		$	57,161		$	259		$	116		$	815	$	5,390	$	1,219		$	1,334	$	285	$	124		$	11		$	76,107
	Transfers from non-operating properties	$	42,488		$	12,979		$	464		$	—		$	218	$	493	$	793		$	2,611	$	11,233	$	394		$	57,675		$	129,348
	Segment assets at September 30, 2012	$	1,187,700		$	535,521		$	119,752		$	106,090		$	97,977	$	283,996	$	53,395		$	173,918	$	66,301	$	111,426		$	100,471		$	2,836,547
	The following table reconciles our segment revenues to total revenues as reported on our consolidated statements of operations (in thousands):
		For the Three Months Ended September 30,						For the Nine Months Ended September 30,
		2013			2012			2013			2012
	Segment revenues from real estate operations	$	124,561		$	121,264		$	371,913		$	370,536
	Construction contract and other service revenues	16,991			15,283			52,048			53,812
	Less: Revenues from discontinued operations (Note 15)	(5,521		)	(6,902		)	(16,786		)	(35,305		)
	Total revenues	$	136,031		$	129,645		$	407,175		$	389,043
	The following table reconciles our segment property operating expenses to property operating expenses as reported on our consolidated statements of operations (in thousands):
		For the Three Months Ended September 30,						For the Nine Months Ended September 30,
		2013			2012			2013			2012
	Segment property operating expenses	$	44,885		$	43,699		$	133,605		$	134,293
	Less: Property operating expenses from discontinued operations (Note 15)	(1,403		)	(2,225		)	(4,196		)	(12,191		)
	Total property operating expenses	$	43,482		$	41,474		$	129,409		$	122,102
	As previously discussed, we provide real estate services such as property management and construction and development services primarily for our properties but also for third parties.  The primary manner in which we evaluate the operating performance of our service activities is through a measure we define as net operating income from service operations (“NOI from service operations”), which is based on the net of revenues and expenses from these activities.  Construction contract and other service revenues and expenses consist primarily of subcontracted costs that are reimbursed to us by the customer along with a management fee. The operating margins from these activities are small relative to the revenue.  We believe NOI from service operations is a useful measure in assessing both our level of activity and our profitability in conducting such operations. The table below sets forth the computation of our NOI from service operations (in thousands):
		For the Three Months Ended September 30,						For the Nine Months Ended September 30,
		2013			2012			2013			2012
	Construction contract and other service revenues	$	16,991		$	15,283		$	52,048		$	53,812
	Construction contract and other service expenses	(16,306		)	(14,410		)	(49,165		)	(51,302		)
	NOI from service operations	$	685		$	873		$	2,883		$	2,510
	The following table reconciles our NOI from real estate operations for reportable segments and NOI from service operations to (loss) income from continuing operations as reported on our consolidated statements of operations (in thousands):
		For the Three Months Ended September 30,						For the Nine Months Ended September 30,
		2013			2012			2013			2012
	NOI from real estate operations	$	79,676		$	77,565		$	238,308		$	236,243
	NOI from service operations	685			873			2,883			2,510
	Interest and other (loss) income	(3		)	1,095			2,949			3,152
	Equity in income (loss) of unconsolidated entities	44			(246		)	211			(522		)
	Income tax expense	(24		)	(106		)	(61		)	(327		)
	Other adjustments:	—						—
	Depreciation and other amortization associated with real estate operations	(29,210		)	(28,604		)	(86,239		)	(84,633		)
	Impairment losses	(16,300		)	(46,096		)	(16,300		)	(41,260		)
	General, administrative and leasing expenses	(8,027		)	(6,377		)	(22,430		)	(24,797		)
	Business development expenses and land carry costs	(1,383		)	(1,632		)	(4,069		)	(4,506		)
Interest expense on continuing operations	(21,242		)	(23,239		)	(66,851		)	(71,909		)
NOI from discontinued operations	(4,118		)	(4,677		)	(12,590		)	(23,114		)
Loss on early extinguishment of debt	(374		)	(768		)	(27,028		)	(937		)
(Loss) income from continuing operations	$	(276	)	$	(32,212	)	$	8,783		$	(10,100	)
The following table reconciles our segment assets to COPT’s consolidated total assets (in thousands):
	September 30,			September 30,
	2013			2012
Segment assets	$	3,053,813		$	2,836,547
Non-operating property assets	530,389			616,526
Other assets	171,386			144,583
Total COPT consolidated assets	$	3,755,588		$	3,597,656
The accounting policies of the segments are the same as those used to prepare our consolidated financial statements, except that discontinued operations are not presented separately for segment purposes.  In the segment reporting presented above, we did not allocate interest expense, depreciation and amortization and impairment losses to our real estate segments since they are not included in the measure of segment profit reviewed by management.  We also did not allocate general and administrative expenses, business development expenses and land carry costs, interest and other income, equity in loss of unconsolidated entities, income taxes and noncontrolling interests because these items represent general corporate or non-operating property items not attributable to segments.

ShareBased_Compensation

Share-Based Compensation	9 Months Ended
	Sep. 30, 2013
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-Based Compensation	Share-Based Compensation
	Performance Share Units (“PSUs”)
	On March 1, 2013, our Board of Trustees granted 69,579 PSUs with an aggregate grant date fair value of $1.9 million to executives.  The PSUs have a performance period beginning on January 1, 2013 and concluding on the earlier of December 31, 2015 or the date of: (1) termination by us without cause, death or disability of the executive or constructive discharge of the executive (collectively, “qualified termination”); or (2) a sale event.  The number of PSUs earned (“earned PSUs”) at the end of the performance period will be determined based on the percentile rank of COPT’s total shareholder return relative to a peer group of companies, as set forth in the following schedule:
	Percentile Rank		Earned PSUs Payout %
	75th or greater		200% of PSUs granted
	50th		100% of PSUs granted
	25th		50% of PSUs granted
	Below 25th		0% of PSUs granted
	If the percentile rank exceeds the 25th percentile and is between two of the percentile ranks set forth in the table above, then the percentage of the earned PSUs will be interpolated between the ranges set forth in the table above to reflect any performance between the listed percentiles.  At the end of the performance period, we, in settlement of the award, will issue a number of fully-vested COPT common shares equal to the sum of:
	•	the number of earned PSUs in settlement of the award plan; plus
	•	the aggregate dividends that would have been paid with respect to the common shares issued in settlement of the earned PSUs through the date of settlement had such shares been issued on the grant date, divided by the share price on such settlement date, as defined under the terms of the agreement.
	If a performance period ends due to a sale event or qualified termination, the number of earned PSUs is prorated based on the portion of the three-year performance period that has elapsed.  If employment is terminated by the employee or by us for cause, all PSUs are forfeited.  PSUs do not carry voting rights.
	We computed a grant date fair value of $26.84 per PSU using a Monte Carlo model, which included assumptions of, among other things, the following: baseline common share value of $25.85; expected volatility for COPT common shares of 29.5%; and a risk-free interest rate of 0.33%.  We are recognizing the grant date fair value in connection with these PSU awards over the performance period.
	The PSUs granted to our executives on March 1, 2012 and March 3, 2011, as described in COPT’s 2012 Annual Report on Form 10-K and our Current Report on Form 8-K dated July 25, 2013, were outstanding at September 30, 2013.
	Restricted Shares
	During the nine months ended September 30, 2013, certain employees as well as nonemployee members of our Board of Trustees were granted a total of 177,510 restricted shares with an aggregate grant date fair value of $4.7 million (weighted average of $26.24 per share).  Restricted shares granted to employees vest based on increments and over periods of time set forth under the terms of the respective awards provided that the employees remain employed by us.  The grants of restricted shares to nonemployee Trustees vest on the first anniversary of the grant date provided that the Trustee remains in his or her position. During the nine months ended September 30, 2013, forfeiture restrictions lapsed on 222,381 previously issued common shares; these shares had a weighted average grant date fair value of $31.54 per share, and the aggregate intrinsic value of the shares on the vesting dates was $5.9 million.
	Options
	During the nine months ended September 30, 2013, 32,756 options to purchase common shares (“options”) were exercised.  The weighted average exercise price of these options was $19.45 per share, and the aggregate intrinsic value of the options exercised was $237,000.

Income_Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2013
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes
	We own a taxable REIT subsidiary (“TRS”) that is subject to Federal and state income taxes.  Our TRS’s provision for income taxes consisted of the following (in thousands):
		For the Three Months Ended September 30,						For the Nine Months Ended September 30,
		2013			2012			2013			2012
	Deferred
	Federal	$	(20	)	$	(87	)	$	(50	)	$	(267	)
	State	(4		)	(19		)	(11		)	(60		)
	Total income tax expense	$	(24	)	$	(106	)	$	(61	)	$	(327	)
	Items in our TRS contributing to temporary differences that lead to deferred taxes include depreciation and amortization, share-based compensation, certain accrued compensation, compensation paid in the form of contributions to a deferred nonqualified compensation plan and net operating losses that are not deductible until future periods.
	Our TRS’s combined Federal and state effective tax rate was 36.7% for the three and nine months ended September 30, 2013 and 38.6% for the three and nine months ended September 30, 2012.

Discontinued_Operations_and_As

Discontinued Operations and Assets Held for Sale	9 Months Ended
	Sep. 30, 2013
Discontinued Operations and Disposal Groups [Abstract]
Discontinued Operations and Assets Held for Sale	Discontinued Operations and Assets Held for Sale
	Income from discontinued operations primarily includes revenues and expenses associated with the following:
	•	five properties in White Marsh, Maryland (in the Greater Baltimore region) that were sold on January 30, 2012;
	•	1101 Sentry Gateway in San Antonio that was sold on January 31, 2012;
	•	222 and 224 Schilling Circle in Greater Baltimore that were sold on February 10, 2012;
	•	15 and 45 West Gude Drive in Suburban Maryland that were sold on May 2, 2012;
	•	11800 Tech Road in Suburban Maryland that was sold on June 14, 2012;
	•	400 Professional Drive in Suburban Maryland for which the title to the property was transferred to the mortgage lender on July 2, 2012;
	•	23 operating properties in the Baltimore/Washington Corridor and Greater Baltimore regions that were sold on July 24, 2012;
	•	920 Elkridge Landing Road in the Baltimore/Washington Corridor that was sold on June 25, 2013; and
	•	16 operating properties in Colorado Springs and an operating property in Suburban Maryland classified as held for sale at September 30, 2013.
	The table below sets forth the components of discontinued operations reported on our consolidated statements of operations (in thousands):
		For the Three Months Ended September 30,						For the Nine Months Ended September 30,
		2013			2012			2013			2012
	Revenue from real estate operations	$	5,521		$	6,902		$	16,786		$	35,305
	Property operating expenses	(1,403		)	(2,225		)	(4,196		)	(12,191		)
	Depreciation and amortization	—			(2,020		)	(158		)	(8,744		)
	Impairment losses	(5,774		)	(9,733		)	(14,826		)	(23,510		)
	General, administrative and leasing expenses	—			(1		)	(1		)	(3		)
	Business development and land carry costs	—			—			—			(24		)
	Interest expense	(68		)	(127		)	(199		)	(2,107		)
	Gain on sales of real estate	—			16,913			—			20,948
	Gain on early extinguishment of debt	—			1,738			—			1,736
	Discontinued operations	$	(1,724	)	$	11,447		$	(2,594	)	$	11,410
	The table below sets forth the components of assets held for sale on our consolidated balance sheets (in thousands):
		September 30, 2013			December 31, 2012
	Properties, net	$	121,060		$	128,740
	Deferred rent receivable	4,951			4,068
	Intangible assets on real estate acquisitions, net	4,384			4,409
	Deferred leasing costs, net	3,511			2,923
	Lease incentives	78			89
	Assets held for sale, net	$	133,984		$	140,229

Earnings_Per_Share_EPS_and_Ear

Earnings Per Share ("EPS") and Earnings Per Unit ("EPU")	9 Months Ended
	Sep. 30, 2013
Earnings Per Share [Abstract]
Earnings Per Share (“EPS”) and Earnings Per Unit (“EPU”)	Earnings Per Share (“EPS”) and Earnings Per Unit (“EPU”)
	COPT and Subsidiaries EPS
	We present both basic and diluted EPS.  We compute basic EPS by dividing net income available to common shareholders allocable to unrestricted common shares under the two-class method by the weighted average number of unrestricted common shares outstanding during the period.  Our computation of diluted EPS is similar except that:
	•	the denominator is increased to include: (1) the weighted average number of potential additional common shares that would have been outstanding if securities that are convertible into our common shares were converted; and (2) the effect of dilutive potential common shares outstanding during the period attributable to share-based compensation using the treasury stock or if-converted methods; and
	•	the numerator is adjusted to add back any changes in income or loss that would result from the assumed conversion into common shares that we added to the denominator.
	Summaries of the numerator and denominator for purposes of basic and diluted EPS calculations are set forth below (in thousands, except per share data):
		For the Three Months Ended September 30,							For the Nine Months Ended September 30,
		2013				2012			2013			2012
	Numerator:
	(Loss) income from continuing operations	$	(276	)		$	(32,212	)	$	8,783		$	(10,100	)
	Gain on sales of real estate, net	—				—			2,683			21
	Preferred share dividends	(4,490		)		(6,546		)	(15,481		)	(14,738		)
	Issuance costs associated with redeemed preferred shares	—				(1,827		)	(2,904		)	(1,827		)
	(Loss) income from continuing operations attributable to noncontrolling interests	(990		)		2,249			(2,139		)	1,710
	Income from continuing operations attributable to restricted shares	(97		)		(111		)	(317		)	(357		)
	Numerator for basic and diluted EPS from continuing operations attributable to COPT common shareholders	$	(5,853	)		$	(38,447	)	$	(9,375	)	$	(25,291	)
	Discontinued operations	(1,724		)		11,447			(2,594		)	11,410
	Discontinued operations attributable to noncontrolling interests	26				(646		)	(42		)	(603		)
	Numerator for basic and diluted EPS on net (loss) income attributable to COPT common shareholders	$	(7,551	)		$	(27,646	)	$	(12,011	)	$	(14,484	)
	Denominator for basic and diluted EPS (common shares)	86,760				71,688			84,547			71,590
	Basic EPS:
	Loss from continuing operations attributable to COPT common shareholders	$	(0.07	)		$	(0.54	)	$	(0.11	)	$	(0.35	)
	Discontinued operations attributable to COPT common shareholders	(0.02		)		0.15			(0.03		)	0.15
	Net loss attributable to COPT common shareholders	$	(0.09	)		$	(0.39	)	$	(0.14	)	$	(0.20	)
	Diluted EPS:
	Loss from continuing operations attributable to COPT common shareholders	$	(0.07	)		$	(0.54	)	$	(0.11	)	$	(0.35	)
	Discontinued operations attributable to COPT common shareholders	(0.02		)		0.15			(0.03		)	0.15
	Net loss attributable to COPT common shareholders	$	(0.09	)		$	(0.39	)	$	(0.14	)	$	(0.20	)
	Our diluted EPS computations do not include the effects of the following securities since the conversions of such securities would increase diluted EPS for the respective periods (in thousands):
		Weighted Average Shares Excluded from Denominator
		For the Three Months Ended September 30,						For the Nine Months Ended September 30,
		2013			2012			2013		2012
	Conversion of common units	3,804			4,233			3,832		4,256
	Conversion of Series I Preferred Units	176			176			176		176
	Conversion of Series K Preferred Shares	434			434			434		434
	The following share-based compensation securities were excluded from the computation of diluted EPS because their effect was antidilutive:
	•	weighted average restricted shares for the three months ended September 30, 2013 and 2012 of 377,000 and 420,000, respectively, and for the nine months ended September 30, 2013 and 2012 of 388,000 and 475,000, respectively; and
	•	weighted average options for the three months ended September 30, 2013 and 2012 of 613,000 and 752,000, respectively, and for the nine months ended September 30, 2013 and 2012 of 571,000 and 780,000, respectively.
	As discussed in Note 8, we have outstanding senior notes that have an exchange settlement feature but did not affect our diluted EPS reported above since the weighted average closing price of our common shares during each of the periods was less than the exchange prices per common share applicable for such periods.
	COPLP and Subsidiaries EPU
	We present both basic and diluted EPU.  We compute basic EPU by dividing net income available to common unitholders allocable to unrestricted common units under the two-class method by the weighted average number of unrestricted common units outstanding during the period.  Our computation of diluted EPU is similar except that:
	•	the denominator is increased to include: (1) the weighted average number of potential additional common units that would have been outstanding if securities that are convertible into our common units were converted; and (2) the effect of dilutive potential common units outstanding during the period attributable to share-based compensation using the treasury stock or if-converted methods; and
	•	the numerator is adjusted to add back any changes in income or loss that would result from the assumed conversion into common units that we added to the denominator.
	Summaries of the numerator and denominator for purposes of basic and diluted EPU calculations are set forth below (in thousands, except per unit data):
		For the Three Months Ended September 30,							For the Nine Months Ended September 30,
		2013				2012			2013			2012
	Numerator:
	(Loss) income from continuing operations	$	(276	)		$	(32,212	)	$	8,783		$	(10,100	)
	Gain on sales of real estate, net	—				—			2,683			21
	Preferred unit distributions	(4,655		)		(6,711		)	(15,976		)	(15,233		)
	Issuance costs associated with redeemed preferred units	—				(1,827		)	(2,904		)	(1,827		)
	(Loss) income from continuing operations attributable to noncontrolling interests	(986		)		243			(2,025		)	829
	Income from continuing operations attributable to restricted units	(97		)		(111		)	(317		)	(357		)
	Numerator for basic and diluted EPU from continuing operations attributable to COPLP common unitholders	$	(6,014	)		$	(40,618	)	$	(9,756	)	$	(26,667	)
	Discontinued operations	(1,724		)		11,447			(2,594		)	11,410
	Discontinued operations attributable to noncontrolling interests	(49		)		(647		)	(147		)	(662		)
	Numerator for basic and diluted EPU on net income attributable to COPLP common unitholders	$	(7,787	)		$	(29,818	)	$	(12,497	)	$	(15,919	)
	Denominator for basic and diluted EPU (common units)	90,564				75,921			88,379			75,846
	Basic EPU:
	Loss from continuing operations attributable to COPLP common unitholders	$	(0.07	)		$	(0.54	)	$	(0.11	)	$	(0.35	)
	Discontinued operations attributable to COPLP common unitholders	(0.02		)		0.15			(0.03		)	0.14
	Net loss attributable to COPLP common unitholders	$	(0.09	)		$	(0.39	)	$	(0.14	)	$	(0.21	)
	Diluted EPU:
	Loss from continuing operations attributable to COPLP common unitholders	$	(0.07	)		$	(0.54	)	$	(0.11	)	$	(0.35	)
Discontinued operations attributable to COPLP common unitholders	(0.02		)		0.15			(0.03		)	0.14
Net loss attributable to COPLP common unitholders	$	(0.09	)		$	(0.39	)	$	(0.14	)	$	(0.21	)
Our diluted EPU computations do not include the effects of the following securities since the conversions of such securities would increase diluted EPU for the respective periods (in thousands):
	Weighted Average Units Excluded from Denominator
	For the Three Months Ended September 30,						For the Nine Months Ended September 30,
	2012			2011			2013		2012
Conversion of Series I Preferred Units	176			176			176		176
Conversion of Series K Preferred Units	434			434			434		434
The following share-based compensation securities were excluded from the computation of diluted EPU because their effect was antidilutive:
•	weighted average restricted units for the three months ended September 30, 2013 and 2012 of 377,000 and 420,000, respectively, and for the nine months ended September 30, 2013 and 2012 of 388,000 and 475,000, respectively; and
•	weighted average options for the three months ended September 30, 2013 and 2012 of 613,000 and 752,0000, respectively, and for the nine months ended September 30, 2013 and 2012 of 571,000 and 780,000, respectively.
As discussed in Note 8, we have outstanding senior notes that have an exchange settlement feature but did not affect our diluted EPU reported above since the weighted average closing price of COPT’s common shares during each of the periods was less than the exchange prices per common share applicable for such periods.

Commitments_and_Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2013
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies
	Litigation
	In the normal course of business, we are involved in legal actions arising from our ownership and administration of properties.  We establish reserves for specific legal proceedings when we determine that the likelihood of an unfavorable outcome is probable and the amount of loss can be reasonably estimated.  Management does not anticipate that any liabilities that may result from such proceedings will have a materially adverse effect on our financial position, operations or liquidity.  Our assessment of the potential outcomes of these matters involves significant judgment and is subject to change based on future developments.
	Environmental
	We are subject to various Federal, state and local environmental regulations related to our property ownership and operation.  We have performed environmental assessments of our properties, the results of which have not revealed any environmental liability that we believe would have a materially adverse effect on our financial position, operations or liquidity.
	Joint Ventures
	In connection with our 2005 contribution of properties to an unconsolidated partnership in which we hold a partnership interest, we entered into standard nonrecourse loan guarantees (environmental indemnifications and guarantees against fraud and misrepresentation, and springing guarantees of partnership debt in the event of a voluntary bankruptcy of the partnership).  The maximum amount we could be required to pay under the guarantees as of September 30, 2013 totaled approximately $64 million for outstanding debt, plus interest and penalties associated with such debt.  We are entitled to recover 80% of any amounts paid under the guarantees from an affiliate of our partner pursuant to an indemnification agreement. In 2012, the holder of the mortgage debt encumbering all of the joint venture’s properties initiated foreclosure proceedings. Management considered this event and estimates that the aggregate fair value of the guarantees would not exceed the amounts included in distributions received in excess of investment in unconsolidated real estate joint venture reported on the consolidated balance sheets.
	We may be required to make our pro rata share of additional investments in our real estate joint ventures (generally based on our percentage ownership) in the event that additional funds are needed.  In the event that the other members of these joint ventures do not pay their share of investments when additional funds are needed, we may then deem it appropriate to make even larger investments in these joint ventures.
	Tax Incremental Financing Obligation
	In August 2010, Anne Arundel County, Maryland issued $30 million in tax incremental financing bonds to third-party investors in order to finance public improvements needed in connection with our project known as National Business Park North.  The real estate taxes on increases in assessed value of a development district encompassing National Business Park North are to be transferred to a special fund pledged to the repayment of the bonds.  We recognized a $2.5 million liability through September 30, 2013 representing the estimated fair value of our obligation to fund through a special tax any future shortfalls between debt service on the bonds and real estate taxes available to repay the bonds.
	Environmental Indemnity Agreement
	We agreed to provide certain environmental indemnifications in connection with a lease and subsequent sale of three New Jersey properties. The prior owner of the properties, a Fortune 100 company that is responsible for groundwater contamination at such properties, previously agreed to indemnify us for (1) direct losses incurred in connection with the contamination and (2) its failure to perform remediation activities required by the State of New Jersey, up to the point that the state declares the remediation to be complete.  Under the environmental indemnification agreement, we agreed to the following:
	•	to indemnify the tenant against losses covered under the prior owner’s indemnity agreement if the prior owner fails to indemnify the tenant for such losses.  This indemnification is capped at $5.0 million in perpetuity after the State of New Jersey declares the remediation to be complete;
	•	to indemnify the tenant for consequential damages (e.g., business interruption) at one of the buildings in perpetuity and another of the buildings through 2025.  This indemnification is limited to $12.5 million; and
	•	to pay 50% of additional costs related to construction and environmental regulatory activities incurred by the tenant as a result of the indemnified environmental condition of the properties.  This indemnification is limited to $300,000 annually and $1.5 million in the aggregate.

Summary_of_Significant_Account1

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2013
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation
	The COPT consolidated financial statements include the accounts of COPT, the Operating Partnership, their subsidiaries and other entities in which COPT has a majority voting interest and control.  The COPLP consolidated financial statements include the accounts of COPLP, its subsidiaries and other entities in which COPLP has a majority voting interest and control.  We also consolidate certain entities when control of such entities can be achieved through means other than voting rights (“variable interest entities” or “VIEs”) if they are deemed to be the primary beneficiary of such entities.  We eliminate all significant intercompany balances and transactions in consolidation.
	We use the equity method of accounting when we own an interest in an entity and can exert significant influence over the entity’s operations but cannot control the entity’s operations. We discontinue equity method accounting if our investment in an entity (and net advances) is reduced to zero unless we have guaranteed obligations of the entity or are otherwise committed to provide further financial support for the entity.
	We use the cost method of accounting when we own an interest in an entity and cannot exert significant influence over its operations.
	These interim financial statements should be read together with the consolidated financial statements and notes thereto as of and for the year ended December 31, 2012 for COPT (included in COPT’s 2012 Annual Report on Form 10-K) and COPLP (included in our Current Report on Form 8-K dated July 25, 2013).  The unaudited consolidated financial statements include all adjustments that are necessary, in the opinion of management, to fairly present our financial position and results of operations.  All adjustments are of a normal recurring nature except for the revisions and reclassifications noted below.  The consolidated financial statements have been prepared using the accounting policies described in COPT’s 2012 Annual Report on Form 10-K and our Current Report on Form 8-K dated July 25, 2013.
Reclassifications	Reclassifications
	We reclassified certain amounts from prior periods to conform to the current period presentation of our consolidated financial statements with no effect on previously reported net income or equity.
Revisions	Revisions
	As reported in COPT’s 2012 Annual Report on Form 10-K, the following errors were identified in 2012:
	•	the misapplication of accounting guidance related to the recognition of a deferred tax asset resulting from an impairment of assets in the fourth quarter of 2011 that failed to consider a partial reversal of that asset that would result from a cancellation of related inter-company debt in the first quarter of 2012.  The effect of this error was an overstatement of our income tax benefit and an understatement of our net loss for the year ended December 31, 2011 of $4.0 million.  Based on an evaluation against our projected annual net income at that time, this error was previously reported for COPT as an out-of-period adjustment in the three months ended March 31, 2012;
	•	an over-accrual of incentive compensation cost, the effect of which was an overstatement of general and administrative expenses and an overstatement of net loss for the calendar quarter and year ended December 31, 2011 of $711,000. Based on an evaluation against our projected annual net income at that time, this error was previously reported for COPT as an out-of-period adjustment in the three months ended March 31, 2012;
	•	the misapplication of accounting guidance requiring that we recognize loss allocations to a noncontrolling interest holder in a consolidated real estate joint venture associated with decreases in such holder’s claim on the book value of the joint venture’s assets, despite the fact that the real estate held by the joint venture was under development and the joint venture had no underlying losses. The effect of this error was an understatement for COPT of losses attributable to noncontrolling interests in other consolidated entities of $1.8 million for the nine months ended September 30, 2012 and $1.4 million for the year ended December 31, 2011; and
	•	the misapplication of accounting guidance pertaining to our reporting for a noncontrolling interest in a consolidated real estate joint venture formed in March 2010 for which the holder of such interest has the right to require us to acquire the interest at fair value. Accounting guidance requires that this noncontrolling interest be classified outside of permanent equity and reported at fair value as of the end of each reporting period, with changes in such fair value reported as equity transactions with no impact to net income or comprehensive income. This error resulted in an overstatement of equity and offsetting understatement of the line entitled “redeemable noncontrolling interest” in the mezzanine section of COPT’s consolidated balance sheet of $8.9 million as of December 31, 2011. This error had no effect on COPT’s consolidated statements of operations, including reported net income (losses) or earnings per share.
	With respect to the errors in the first two bullets above, we assessed the materiality of these errors on COPT’s consolidated financial statements in connection with previously filed periodic reports, in accordance with ASC 250 (SEC’s Staff Accounting Bulletin No. 99, “Materiality”), and concluded at such time that the errors were not material to any prior annual or interim periods. In assessing the cumulative effect of all such errors, we have considered ASC 250 (SEC Staff Accounting Bulletin No. 108, “Considering the Effects of Prior Year Misstatements when Quantifying Misstatements in Current Year Financial Statements”), and accordingly, the financial statements as of, and for the year ended, December 31, 2011 were revised in COPT’s 2012 Annual Report on Form 10-K. We revised COPT amounts pertaining to the first and second quarter of 2012 in COPT’s Form 10-Qs filed on April 30, 2013 and August 1, 2013, respectively, and amounts pertaining to the third quarter and first nine months of 2012 herein.
Recent Accounting Pronouncements	Recent Accounting Pronouncements
	We adopted guidance issued by the Financial Accounting Standards Board (“FASB”) effective January 1, 2013 related to the reporting of the effect of significant reclassifications from accumulated other comprehensive income. This guidance requires an entity to report, either parenthetically on the face of the financial statements or in a single footnote, changes in the components of accumulated other comprehensive income for the period. An entity is required to separately report the amount of such changes attributable to reclassifications (and the statements of operations line affected by such reclassifications) and the amount of such changes attributable to current period other comprehensive income. For amounts that are not required to be reclassified in their entirety to net income in the same reporting period, an entity is required to cross-reference other disclosures that provide additional detail about those amounts. Our adoption of this guidance did not affect our consolidated financial statements or disclosures.

Summary_of_Significant_Account2

Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2013
Accounting Policies [Abstract]
Schedule of adjustments to prior period financial statements	The following are selected line items from COPT’s consolidated financial statements as of, and for the three and nine months ended, September 30, 2012 illustrating the effect of adjustments pertaining to reclassifications and revisions (in thousands):
		Consolidated Balance Sheet as of September 30, 2012
		Per September 30, 2012 10-Q			As Revised			Change			Revisions
	Redeemable noncontrolling interest	$	—		$	9,932		$	9,932		$	9,932
	Additional paid-in capital	$	1,455,558		$	1,451,416		$	(4,142	)	$	(4,142	)
	Cumulative distributions in excess of net income	$	(610,659	)	$	(607,633	)	$	3,026		$	3,026
	Noncontrolling interests in common units in COPLP	$	48,973		$	49,157		$	184		$	184
	Noncontrolling interests in other consolidated entities	$	18,682		$	9,682		$	(9,000	)	$	(9,000	)
	Total equity	$	1,250,221		$	1,240,289		$	(9,932	)	$	(9,932	)
	Total liabilities, redeemable noncontrolling interest and equity	$	3,597,656		$	3,597,656		$	—		$	—
		Consolidated Statements of Operations for the
		Three Months Ended September 30, 2012
		Per September 30, 2012 10-Q			Per September 30, 2013 10-Q			Change			Discontinued Operations			Other Reclassifications			Revisions
	Total revenues	$	130,144		$	129,645		$	(499	)	$	(499	)	$	—		$	—
	Expenses
	Property operating expenses	$	42,799		$	41,474		$	(1,325	)	$	(9	)	$	(1,316	)	$	—
	Depreciation and amortization associated with real estate operations	28,698			28,604			(94		)	(94		)	—			—
	Construction contract and other service expenses	14,410			14,410			—			—			—			—
	Impairment losses	46,096			46,096			—			—			—			—
	General, administrative and leasing expenses	5,061			6,377			1,316			—			1,316			—
	Business development expenses and land carry costs	1,632			1,632			—			—			—			—
	Total operating expenses	$	138,696		$	138,593		$	(103	)	$	(103	)	$	—		$	—
	Operating loss	$	(8,552	)	$	(8,948	)	$	(396	)	$	(396	)	$	—		$	—
	Interest expense	$	(23,239	)	$	(23,239	)	$	—		$	—		$	—		$	—
	Income tax expense	$	(106	)	$	(106	)	$	—		$	—		$	—		$	—
	Loss from continuing operations	$	(31,816	)	$	(32,212	)	$	(396	)	$	(396	)	$	—		$	—
	Discontinued operations	$	11,051		$	11,447		$	396		$	396		$	—		$	—
	Net loss	$	(20,765	)	$	(20,765	)	$	—		$	—		$	—		$	—
	Net loss attributable to noncontrolling interests in common units in COPLP	$	1,569		$	1,533		$	(36	)	$	—		$	—		$	(36	)
	Net loss attributable to noncontrolling interests in other consolidated entities	$	(411	)	$	235		$	646		$	—		$	—		$	646
	Net loss attributable to COPT	$	(19,772	)	$	(19,162	)	$	610		$	—		$	—		$	610
	Basic and diluted earnings per common share:
	Loss from continuing operations	$	(0.54	)	$	(0.54	)	$	—		$	—		$	—		$	—
	Discontinued operations	0.15			0.15			—			—			—			—
	Net loss attributable to COPT common shareholders	$	(0.39	)	$	(0.39	)	$	—		$	—		$	—		$	—
		Consolidated Statements of Operations for the
		Nine Months Ended September 30, 2012
		Per September 30, 2012 10-Q			Per September 30, 2013 10-Q			Change			Discontinued Operations			Other Reclassifications			Revisions
	Total revenues	$	390,499		$	389,043		$	(1,456	)	$	(1,456	)	$	—		$	—
	Expenses
	Property operating expenses	$	126,339		$	122,102		$	(4,237	)	$	32		$	(4,269	)	$	—
	Depreciation and amortization associated with real estate operations	84,920			84,633			(287		)	(287		)	—			—
	Construction contract and other service expenses	51,302			51,302			—			—			—			—
	Impairment losses	41,260			41,260			—			—			—			—
	General, administrative and leasing expenses	19,820			24,797			4,977			(3		)	4,269			711
	Business development expenses and land carry costs	4,506			4,506			—			—			—			—
	Total operating expenses	$	328,147		$	328,600		$	453		$	(258	)	$	—		$	711
	Operating income	$	62,352		$	60,443		$	(1,909	)	$	(1,198	)	$	—		$	(711	)
	Interest expense	$	(71,909	)	$	(71,909	)	$	—		$	—		$	—		$	—
	Income tax expense	$	(4,296	)	$	(327	)	$	3,969		$	—		$	—		$	3,969
	Loss from continuing operations	$	(12,160	)	$	(10,100	)	$	2,060		$	(1,198	)	$	—		$	3,258
	Discontinued operations	$	10,212		$	11,410		$	1,198		$	1,198		$	—		$	—
	Net (loss) income	$	(1,927	)	$	1,331		$	3,258		$	—		$	—		$	3,258
	Net loss attributable to noncontrolling interests in common units in COPLP	$	1,020		$	738		$	(282	)	$	—		$	—		$	(282	)
	Net (income) loss attributable to noncontrolling interests in other consolidated entities	$	(939	)	$	864		$	1,803		$	—		$	—		$	1,803
	Net (loss) income attributable to COPT	$	(2,341	)	$	2,438		$	4,779		$	—		$	—		$	4,779
	Basic and diluted earnings per common share:
	Loss from continuing operations	$	(0.40	)	$	(0.35	)	$	0.05		$	(0.02	)	$	—		$	0.07
	Discontinued operations	0.13			0.15			0.02			0.02			—			—
	Net loss attributable to COPT common shareholders	$	(0.27	)	$	(0.20	)	$	0.07		$	—		$	—		$	0.07

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2013
Fair Value Disclosures [Abstract]
Schedule of fair value assets and liabilities measured on recurring basis	The table below sets forth financial assets and liabilities of COPT and its subsidiaries that are accounted for at fair value on a recurring basis as of September 30, 2013 and the hierarchy level of inputs used in measuring their respective fair values under applicable accounting standards (in thousands):
	Description	Quoted Prices in		Significant Other			Significant		Total
		Active Markets for		Observable Inputs(Level 2)			Unobservable Inputs(Level 3)
		Identical Assets(Level 1)
	Assets:
	Marketable securities in deferred compensation plan (1)
	Mutual funds	$	6,753	$	—		$	—	$	6,753
	Common stocks	217		—			—		217
	Other	211		—			—		211
	Common stock (1)	520		—			—		520
	Interest rate derivatives (2)	—		5,195			—		5,195
	Warrants to purchase common stock (2)	—		301			—		301
	Total Assets	$	7,701	$	5,496		$	—	$	13,197
	Liabilities:
	Deferred compensation plan liability (3)	$	7,181	$	—		$	—	$	7,181
	Interest rate derivatives	—		3,595			—		3,595
	Total Liabilities	$	7,181	$	3,595		$	—	$	10,776
	Redeemable noncontrolling interest	$	—	$	—		$	16,789	$	16,789
	(1) Included in the line entitled “restricted cash and marketable securities” on COPT’s consolidated balance sheet.
	(2) Included in the line entitled “prepaid expenses and other assets” on COPT’s consolidated balance sheet.
	(3) Included in the line entitled “other liabilities” on COPT’s consolidated balance sheet.
	The table below sets forth financial assets and liabilities of COPLP and its subsidiaries that are accounted for at fair value on a recurring basis as of September 30, 2013 and the hierarchy level of inputs used in measuring their respective fair values under applicable accounting standards (in thousands):
	Description	Quoted Prices in		Significant Other			Significant		Total
		Active Markets for		Observable Inputs(Level 2)			Unobservable Inputs(Level 3)
		Identical Assets(Level 1)
	Assets:
	Common stock (1)	$	520	$	—		$	—	$	520
	Interest rate derivatives (2)	—		5,195			—		5,195
	Warrants to purchase common stock (2)	—		301			—		301
	Total Assets	$	520	$	5,496		$	—	$	6,016
	Liabilities:
	Interest rate derivatives	$	—	$	3,595		$	—	$	3,595
	Redeemable noncontrolling interest	$	—	$	—		$	16,789	$	16,789
	(1) Included in the line entitled “restricted cash and marketable securities” on COPLP’s consolidated balance sheet.
	(2) Included in the line entitled “prepaid expenses and other assets” on COPLP’s consolidated balance sheet.
Schedule of fair value hierarchy of impaired properties and other assets associated with such properties	The table below sets forth the fair value hierarchy of the valuation techniques used by us in determining such fair values (dollars in thousands):
		Quoted Prices in					Significant				Impairment Losses Recognized (1)
		Active Markets for		Significant Other			Unobservable				Three Months		Nine Months
		Identical Assets		Observable Inputs			Inputs				Ended		Ended
	Description	(Level 1)		(Level 2)			(Level 3)		Total		30-Sep-12		30-Sep-12
	Assets (2):
	Properties, net	$	—	$	—		$	369,312	$	369,312	$	52,900	60,593
	(1) Represents impairment losses, excluding exit costs incurred of $2.9 million for the three months ended September 30, 2012 and $4.2 million for the nine months ended September 30, 2012.
	(2) Reflects balance sheet classifications of assets at time of fair value measurement, excluding the effect of held for sale classifications.
	The table below sets forth the fair value hierarchy of the valuation technique used by us in determining the fair value of the properties (dollars in thousands):
		Quoted Prices in					Significant				Impairment Losses Recognized (1)
		Active Markets for		Significant Other			Unobservable				Three Months		Nine Months
		Identical Assets		Observable Inputs			Inputs				Ended		Ended
	Description	(Level 1)		(Level 2)			(Level 3)		Total		30-Sep-13		30-Sep-13
	Assets (2):
	Properties, net	$	—	$	—		$	245,535	$	245,535	$	21,888	$	30,940
	(1) Represents impairment losses, excluding exit costs incurred of $186,000 for the three months and nine months ended September 30, 2013.
	(2) Reflects balance sheet classifications of assets at time of fair value measurement, excluding the effect of held for sale classifications.
Schedule of quantitative information about significant unobservable inputs used for Level 3 fair value measurements	The table below sets forth quantitative information about significant unobservable inputs used for the Level 3 fair value measurements reported above (dollars in thousands):
	Description	Fair Value on		Valuation Technique		Unobservable Input	Range (Weighted Average)
		Measurement Date
	Properties on which impairment losses were recognized	$	369,312	Bid for properties indicative of value		Indicative bid (1)	-1
				Contract of sale		Contract price (1)	-1
				Discounted cash flow		Discount rate	10.1% to 11.0% (10.4%)
						Terminal capitalization rate	8.7% to 10.0% (8.9%)
						Market rent growth rate	3.0% (2)
						Expense growth rate	3.0% (2)
				Yield Analysis		Yield	12% (2)
						Market rent rate	$8.50 per square foot(2)
						Leasing costs	$20.00 per square foot (2)
	(1) These fair value measurements were developed as a result of negotiations between us and potential, or actual, purchasers of properties.
	(2) Only one value applied for this unobservable input.
	The table below sets forth quantitative information about significant unobservable inputs used for the Level 3 fair value measurements reported above (dollars in thousands):
	Description	Fair Value on		Valuation Technique		Unobservable Input	Range (Weighted Average)
		Measurement Date
	Properties on which impairment losses were recognized	$	245,535	Bids for property indicative of value		Indicative bids (1)	-1
				Contract of sale		Contract price (1)	-1
				Discounted cash flow		Discount rate	10.0% to 11.0% (10.9%)
						Terminal capitalization rate	9.5% to 10.0% (9.7%)
						Market rent growth rate	3.0% (2)
						Expense growth rate	3.0% (2)
	(1) These fair value measurements were developed as a result of negotiations between us and potential, or actual, purchasers of properties.
	(2) Only one value applied for this unobservable input.

Properties_net_Tables

Properties, net (Tables)	9 Months Ended
	Sep. 30, 2013
Real Estate [Abstract]
Schedule of operating properties, net	Operating properties, net consisted of the following (in thousands):
		September 30,			December 31,
		2013			2012
	Land	$	436,565		$	427,766
	Buildings and improvements	2,889,383			2,725,875
	Less: accumulated depreciation	(612,369		)	(555,975		)
	Operating properties, net	$	2,713,579		$	2,597,666
Schedule of projects in development or held for future development	Projects we had in development or held for future development consisted of the following (in thousands):
		September 30,			December 31,
		2013			2012
	Land	$	253,810		$	236,324
	Construction in progress, excluding land	272,357			329,054
	Projects in development or held for future development	$	526,167		$	565,378

Real_Estate_Joint_Ventures_Tab

Real Estate Joint Ventures (Tables)	9 Months Ended
	Sep. 30, 2013
Equity Method Investments and Joint Ventures [Abstract]
Schedule of information related to the investment in unconsolidated real estate joint venture accounted for using the equity method of accounting	Information pertaining to this investment is set forth below (dollars in thousands):
	Investment Balance at (1)								Date				Nature of		Maximum Exposure
	September 30, 2013				31-Dec-12				Acquired		Ownership		Activity		to Loss (2)
	$	(6,420	)		$	(6,420	)		9/29/05		20%		Operates 16 Buildings		$	—
	(1)   The carrying amount of our investment in this joint venture was lower than our share of the equity in the joint venture by $3.0 million at September 30, 2013 and $4.5 million at December 31, 2012 due to our deferral of gain on the contribution by us of real estate into the joint venture upon its formation and our discontinuance of loss recognition under the equity method effective October 2012, as discussed below.  A difference will continue to exist to the extent the nature of our continuing involvement in the joint venture remains the same and we continue to no longer recognize income or losses under the equity method.
	(2)   Derived from the sum of our investment balance and maximum additional unilateral capital contributions or loans required from us.  Not reported above are additional amounts that we and our partner are required to fund when needed by this joint venture; these funding requirements are proportional to our respective ownership percentages.  Also not reported above are additional unilateral contributions or loans from us, the amounts of which are uncertain, that we would be required to make if certain contingent events occur (see Note 17).
Schedule of condensed balance sheets for unconsolidated real estate joint venture	The following table sets forth condensed balance sheets for this unconsolidated real estate joint venture (in thousands):
		September 30,				December 31,
		2013				2012
	Properties, net	$	57,222			$	58,460
	Other assets	6,248				4,376
	Total assets	$	63,470			$	62,836
	Liabilities (primarily debt)	$	80,424			$	72,693
	Owners’ equity	(16,954		)		(9,857		)
	Total liabilities and owners’ equity	$	63,470			$	62,836
Schedule of condensed statements of operations for unconsolidated real estate joint venture	The following table sets forth condensed statements of operations for this unconsolidated real estate joint venture (in thousands):
		For the Three Months Ended September 30,								For the Nine Months Ended September 30,
		2013				2012				2013				2012
	Revenues	$	1,739			$	1,778			$	5,342			$	5,497
	Property operating expenses	(755		)		(758		)		(2,250		)		(2,219		)
	Interest expense	(2,966		)		(1,138		)		(8,521		)		(3,291		)
	Depreciation and amortization expense	(575		)		(568		)		(1,668		)		(1,732		)
	Net loss	$	(2,557	)		$	(686	)		$	(7,097	)		$	(1,745	)
Schedule of information related to investments in consolidated real estate joint ventures	The table below sets forth information pertaining to our investments in consolidated real estate joint ventures at September 30, 2013 (dollars in thousands):
					Ownership				September 30, 2013								-1
			Date		% at				Total				Pledged				Total
			Acquired		9/30/13		Nature of Activity		Assets				Assets				Liabilities
	LW Redstone Company, LLC		3/23/10		85%		Developing business park (2)		$	114,141			$	45,209			$	22,141
	M Square Associates, LLC		6/26/07		50%		Operating two buildings and developing others (3)		60,879				47,854				41,463
	COPT-FD Indian Head, LLC		10/23/06		75%		Holding land parcel (4)		6,447				—				—
	MOR Forbes 2 LLC		12/24/02		50%		Operating one building (5)		4,132				—				90
									$	185,599			$	93,063			$	63,694
	(1) Excludes amounts eliminated in consolidation.
	(2) This joint venture’s property is in Huntsville, Alabama.
	(3) This joint venture’s properties are in College Park, Maryland (in the Suburban Maryland region).
	(4) This joint venture’s property is in Charles County, Maryland. In 2012, the joint venture exercised its option under a development agreement to require Charles County to repurchase the land parcel at its original acquisition cost. Under the terms of the agreement with Charles County, the repurchase is expected to occur by August 2014.
	(5) This joint venture’s property is in Lanham, Maryland (in the Suburban Maryland region).

Mortgage_and_Other_Investing_R1

Mortgage and Other Investing Receivables (Tables)	9 Months Ended
	Sep. 30, 2013
Receivables [Abstract]
Mortgage and other investing receivables	Mortgage and other investing receivables consisted of the following (in thousands):
		September 30,		December 31,
		2013		2012
	Notes receivable from City of Huntsville	$	40,321	$	33,252
	Mortgage loan receivable	—		144
		$	40,321	$	33,396

Prepaid_Expenses_and_Other_Ass1

Prepaid Expenses and Other Assets (Tables)	9 Months Ended
	Sep. 30, 2013
Prepaid Expense and Other Assets [Abstract]
Schedule of prepaid expenses and other assets	Prepaid expenses and other assets consisted of the following (in thousands):
		September 30,		December 31,
		2013		2012
	Prepaid expenses	$	23,552	$	19,270
	Lease incentives	8,751		5,578
	Furniture, fixtures and equipment, net	6,705		7,991
	Deferred tax asset	6,222		6,612
	Construction contract costs incurred in excess of billings	5,465		—
	Interest rate derivatives	5,195		—
	Other equity method investments	4,928		2,425
	Other assets	3,257		2,183
	Prepaid expenses and other assets	$	64,075	$	44,059

Debt_Tables

Debt (Tables)	9 Months Ended
	Sep. 30, 2013
Debt Disclosure [Abstract]
Schedule of debt	Our debt consisted of the following (dollars in thousands):
		Maximum
		Availability at			Carrying Value at						Scheduled Maturity
		September 30,			September 30,		December 31,		Stated Interest Rates at		Dates at
		2013			2013		2012		September 30, 2013		September 30, 2013
	Mortgage and Other Secured Loans:
	Fixed rate mortgage loans (1)	N/A			$	882,311	$	948,414	3.96% - 7.87% (2)		2014-2034
	Variable rate secured loan	N/A			37,894		38,475		LIBOR + 2.25% (3)		11/2/15
	Other construction loan facilities	$	26,150		—		29,557		N/A		N/A
	Total mortgage and other secured loans				920,205		1,016,446
	Revolving Credit Facility (4)	800,000			—		—		LIBOR + 0.975% to 1.75% (5)		7/1/17
	Term Loan Facilities (6)	(6		)	620,000		770,000		LIBOR + 1.10% to 2.60% (7)		2015-2019
	Unsecured Senior Notes (8)
	3.600% Senior Notes	N/A			347,183		—		3.60%		5/15/23
	5.250% Senior Notes	N/A			245,360		—		5.25%		2/15/24
	Unsecured notes payable	N/A			1,723		1,788		0% (9)		5/1/26
	4.25% Exchangeable Senior Notes (10)	N/A			560		230,934		4.25%		4/15/30
	Total debt, net				$	2,135,031	$	2,019,168
	(1)	Several of the fixed rate mortgages carry interest rates that were above or below market rates upon assumption and therefore were recorded at their fair value based on applicable effective interest rates.  The carrying values of these loans reflect net unamortized premiums totaling $465,000 at September 30, 2013 and $1.3 million at December 31, 2012.
	-2	The weighted average interest rate on these loans was 5.98% at September 30, 2013.
	(3)	The interest rate on the loan outstanding was 2.43% at September 30, 2013.
	-4	Refer to the paragraph below for disclosure pertaining to the Revolving Credit Facility.
	-5	The weighted average interest rate on the Revolving Credit Facility was 1.45% at September 30, 2013.
	(6)	We have the ability to borrow an aggregate of an additional $180.0 million under these term loan facilities, provided that there is no default under the facilities and subject to the approval of the lenders.
	(7)	The weighted average interest rate on these loans was 1.80% at September 30, 2013.
	-8	Refer to the paragraphs below for disclosure pertaining to these notes.
	(9)	These notes carry interest rates that were below market rates upon assumption and therefore were recorded at their fair value based on applicable effective interest rates.  The carrying value of these notes reflects an unamortized discount totaling $788,000 at September 30, 2013 and $873,000 at December 31, 2012.
	(10)	As described further in our 2012 Annual Report on Form 10-K, these notes have an exchange settlement feature that provides that the notes may, under certain circumstances, be exchangeable for cash and, at COPLP’s discretion, COPT common shares at an exchange rate (subject to adjustment) of 20.8513 shares per one thousand dollar principal amount of the notes (exchange rate is as of September 30, 2013 and is equivalent to an exchange price of $47.96 per common share).  During the nine months ended September 30, 2013, we repaid $239.4 million principal amount of these notes and recognized a $25.9 million loss on early extinguishment of debt. The carrying value of these notes included a principal amount of $575,000 and an unamortized discount totaling $15,000 at September 30, 2013 and a principal amount of $240.0 million and an unamortized discount totaling $9.1 million at December 31, 2012.  The effective interest rate under the notes, including amortization of the issuance costs, was 6.05%.  Because the closing price of our common shares at September 30, 2013 and December 31, 2012 was less than the exchange price per common share applicable to these notes, the if-converted value of the notes did not exceed the principal amount.  The table below sets forth interest expense recognized on these notes before deductions for amounts capitalized (in thousands):
		For the Three Months Ended September 30,					For the Nine Months Ended September 30,
		2013			2012		2013		2012
	Interest expense at stated interest rate	$	6		$	2,550	$	4,201	$	7,650
	Interest expense associated with amortization of discount	2			919		1,613		2,717
	Total	$	8		$	3,469	$	5,814	$	10,367
Schedule of the fair value of debt	The following table sets forth information pertaining to the fair value of our debt (in thousands):
		September 30, 2013					December 31, 2012
		Carrying			Estimated		Carrying		Estimated
		Amount			Fair Value		Amount		Fair Value
	Fixed-rate debt
	Unsecured Senior Notes	$	592,543		$	579,654	$	—	$	—
	4.25% Exchangeable Senior Notes	560			575		230,934		240,282
	Other fixed-rate debt	884,034			812,246		950,202		968,180
	Variable-rate debt	657,894			661,593		838,032		845,558
		$	2,135,031		$	2,054,068	$	2,019,168	$	2,054,020

Interest_Rate_Derivatives_Tabl

Interest Rate Derivatives (Tables)	9 Months Ended
	Sep. 30, 2013
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule of key terms and fair values of interest rate swap derivatives	The following table sets forth the key terms and fair values of our interest rate swap derivatives (dollars in thousands):
													Fair Value at
	Notional Amount				Fixed Rate		Floating Rate Index		Effective Date		Expiration Date		September 30,				December 31,
													2013				2012
	$	100,000			0.61%		One-Month LIBOR		1/3/12		9/1/14		$	(365	)		$	(594	)
	100,000				0.61%		One-Month LIBOR		1/3/12		9/1/14		(362		)		(591		)
	100,000				0.83%		One-Month LIBOR		1/3/12		9/1/15		(963		)		(1,313		)
	100,000				0.83%		One-Month LIBOR		1/3/12		9/1/15		(963		)		(1,313		)
	37,894			-1	3.83%		One-Month LIBOR + 2.25%		11/2/10		11/2/15		(933		)		(1,268		)
	100,000				0.81%		One-Month LIBOR		9/2/14		9/1/16		1				(263		)
	100,000				0.81%		One-Month LIBOR		9/2/14		9/1/16		(9		)		(272		)
	100,000				1.67%		One-Month LIBOR		9/1/15		8/1/19		2,685				(154		)
	100,000				1.73%		One-Month LIBOR		9/1/15		8/1/19		2,509				(417		)
													$	1,600			$	(6,185	)
	-1	The notional amount of this instrument is scheduled to amortize to $36.2 million.
Schedule of fair value and balance sheet classification of interest rate derivatives	The table below sets forth the fair value of our interest rate derivatives as well as their classification on our consolidated balance sheet (in thousands):
			September 30, 2013						31-Dec-12
	Derivatives		Balance Sheet Location		Fair Value				Balance Sheet Location		Fair Value
	Interest rate swaps designated as cash flow hedges		Prepaid expenses and other assets		$	5,195			Prepaid expenses and other assets		$	—
	Interest rate swaps designated as cash flow hedges		Interest rate derivatives		(3,595		)		Interest rate derivatives		(6,185		)
Schedule of effect of interest rate derivatives on consolidated statements of operations and comprehensive income	The table below presents the effect of our interest rate derivatives on our consolidated statements of operations and comprehensive income (in thousands):
		For the Three Months Ended September 30,								For the Nine Months Ended September 30,
		2013				2012				2013				2012
	Amount of (loss) gain recognized in accumulated other comprehensive income (loss) (“AOCI”) (effective portion)	$	(2,482	)		$	(2,760	)		$	5,810			$	(7,386	)
	Amount of loss reclassified from AOCI into interest expense (effective portion)	689				632				2,021				3,034

Redeemable_Noncontrolling_Inte1

Redeemable Noncontrolling Interest (Tables)	9 Months Ended
	Sep. 30, 2013
Noncontrolling Interest [Abstract]
Schedule of activity for redeemable noncontrolling interest	The table below sets forth activity in our redeemable noncontrolling interest (in thousands):
		Nine Months Ended September 30,
		2013			2012
	Beginning balance	$	10,298		$	8,908
	Distribution to noncontrolling interest	(811		)	—
	Net income (loss) attributable to noncontrolling interest	888			(1,803		)
	Adjustment to arrive at fair value of interest	6,414			2,827
	Ending balance	$	16,789		$	9,932

Information_by_Business_Segmen1

Information by Business Segment (Tables)	9 Months Ended
	Sep. 30, 2013
Segment Reporting [Abstract]
Schedule of segment financial information for real estate operations	The table below reports segment financial information for our reportable segments (in thousands).  We measure the performance of our segments through the measure we define as NOI from real estate operations, which is derived by subtracting property operating expenses from revenues from real estate operations.
		Operating Office Property Segments
		Baltimore/			Northern			San			Washington,			St. Mary’s &		Greater		Suburban			Colorado		Greater		Other			Operating			Total
		Washington			Virginia			Antonio			DC - Capitol			King George		Baltimore		Maryland			Springs		Philadelphia					Wholesale
		Corridor									Riverfront			Counties														Data Center
	Three Months Ended September 30, 2013
	Revenues from real estate operations	$	57,226		$	23,062		$	7,898		$	4,295		$	4,270	$	10,703	$	2,332		$	6,622	$	3,258	$	3,819		$	1,076		$	124,561
	Property operating expenses	18,780			7,999			4,061			1,946			1,299		4,143		862			2,371		924		654			1,846			44,885
	NOI from real estate operations	$	38,446		$	15,063		$	3,837		$	2,349		$	2,971	$	6,560	$	1,470		$	4,251	$	2,334	$	3,165		$	(770	)	$	79,676
	Additions to long-lived assets	$	7,204		$	2,459		$	251		$	86		$	630	$	1,707	$	88		$	1,413	$	102	$	(84	)	$	86		$	13,942
	Transfers from non-operating properties	$	8,706		$	26,355		$	—		$	—		$	—	$	9	$	(183	)	$	1,860	$	3,414	$	(33	)	$	2,714		$	42,842
	Three Months Ended September 30, 2012
	Revenues from real estate operations	$	55,799		$	20,363		$	8,125		$	4,389		$	4,085	$	11,918	$	2,371		$	6,278	$	2,541	$	3,589		$	1,806		$	121,264
	Property operating expenses	18,534			7,115			4,272			1,924			1,241		4,539		1,041			2,432		663		686			1,252			43,699
	NOI from real estate operations	$	37,265		$	13,248		$	3,853		$	2,465		$	2,844	$	7,379	$	1,330		$	3,846	$	1,878	$	2,903		$	554		$	77,565
	Additions to long-lived assets	$	4,454		$	54,781		$	—		$	414		$	355	$	1,055	$	36		$	548	$	195	$	(35	)	$	—		$	61,803
	Transfers from non-operating properties	$	12,431		$	12,979		$	—		$	—		$	24	$	99	$	(88	)	$	315	$	1,843	$	548		$	(5	)	$	28,146
	Nine Months Ended September 30, 2013
	Revenues from real estate operations	$	171,049		$	68,992		$	24,019		$	12,716		$	12,355	$	32,246	$	6,809		$	19,874	$	8,529	$	10,878		$	4,446		$	371,913
	Property operating expenses	57,030			24,020			12,427			5,769			3,715		12,408		2,393			7,014		2,482		1,667			4,680			133,605
	NOI from real estate operations	$	114,019		$	44,972		$	11,592		$	6,947		$	8,640	$	19,838	$	4,416		$	12,860	$	6,047	$	9,211		$	(234	)	$	238,308
	Additions to long-lived assets	$	14,029		$	6,699		$	268		$	484		$	1,634	$	1,761	$	143		$	2,323	$	387	$	3,588		$	262		$	31,578
	Transfers from non-operating properties	$	37,270		$	37,209		$	—		$	—		$	12	$	113	$	337		$	4,540	$	28,031	$	20,925		$	69,163		$	197,600
	Segment assets at September 30, 2013	$	1,223,246		$	593,335		$	118,701		$	99,852		$	96,679	$	305,947	$	52,726		$	159,578	$	105,112	$	130,598		$	168,039		$	3,053,813
	Nine Months Ended September 30, 2012
	Revenues from real estate operations	$	167,726		$	57,974		$	23,563		$	12,515		$	12,436	$	41,954	$	12,680		$	18,880	$	7,171	$	10,977		$	4,660		$	370,536
	Property operating expenses	56,677			21,270			11,998			5,485			3,524		15,911		5,357			6,754		1,851		2,088			3,378			134,293
	NOI from real estate operations	$	111,049		$	36,704		$	11,565		$	7,030		$	8,912	$	26,043	$	7,323		$	12,126	$	5,320	$	8,889		$	1,282		$	236,243
	Additions to long-lived assets	$	9,393		$	57,161		$	259		$	116		$	815	$	5,390	$	1,219		$	1,334	$	285	$	124		$	11		$	76,107
	Transfers from non-operating properties	$	42,488		$	12,979		$	464		$	—		$	218	$	493	$	793		$	2,611	$	11,233	$	394		$	57,675		$	129,348
	Segment assets at September 30, 2012	$	1,187,700		$	535,521		$	119,752		$	106,090		$	97,977	$	283,996	$	53,395		$	173,918	$	66,301	$	111,426		$	100,471		$	2,836,547
Schedule of reconciliation of segment revenues to total revenues	The following table reconciles our segment revenues to total revenues as reported on our consolidated statements of operations (in thousands):
		For the Three Months Ended September 30,						For the Nine Months Ended September 30,
		2013			2012			2013			2012
	Segment revenues from real estate operations	$	124,561		$	121,264		$	371,913		$	370,536
	Construction contract and other service revenues	16,991			15,283			52,048			53,812
	Less: Revenues from discontinued operations (Note 15)	(5,521		)	(6,902		)	(16,786		)	(35,305		)
	Total revenues	$	136,031		$	129,645		$	407,175		$	389,043
Schedule of reconciliation of segment property operating expenses to total property operating expenses	The following table reconciles our segment property operating expenses to property operating expenses as reported on our consolidated statements of operations (in thousands):
		For the Three Months Ended September 30,						For the Nine Months Ended September 30,
		2013			2012			2013			2012
	Segment property operating expenses	$	44,885		$	43,699		$	133,605		$	134,293
	Less: Property operating expenses from discontinued operations (Note 15)	(1,403		)	(2,225		)	(4,196		)	(12,191		)
	Total property operating expenses	$	43,482		$	41,474		$	129,409		$	122,102
Schedule of computation of net operating income from service operations	The table below sets forth the computation of our NOI from service operations (in thousands):
		For the Three Months Ended September 30,						For the Nine Months Ended September 30,
		2013			2012			2013			2012
	Construction contract and other service revenues	$	16,991		$	15,283		$	52,048		$	53,812
	Construction contract and other service expenses	(16,306		)	(14,410		)	(49,165		)	(51,302		)
	NOI from service operations	$	685		$	873		$	2,883		$	2,510
Schedule of reconciliation of net operating income from real estate operations and service operations to (loss) income from continuing operations	The following table reconciles our NOI from real estate operations for reportable segments and NOI from service operations to (loss) income from continuing operations as reported on our consolidated statements of operations (in thousands):
		For the Three Months Ended September 30,						For the Nine Months Ended September 30,
		2013			2012			2013			2012
	NOI from real estate operations	$	79,676		$	77,565		$	238,308		$	236,243
	NOI from service operations	685			873			2,883			2,510
	Interest and other (loss) income	(3		)	1,095			2,949			3,152
	Equity in income (loss) of unconsolidated entities	44			(246		)	211			(522		)
	Income tax expense	(24		)	(106		)	(61		)	(327		)
	Other adjustments:	—						—
	Depreciation and other amortization associated with real estate operations	(29,210		)	(28,604		)	(86,239		)	(84,633		)
	Impairment losses	(16,300		)	(46,096		)	(16,300		)	(41,260		)
	General, administrative and leasing expenses	(8,027		)	(6,377		)	(22,430		)	(24,797		)
	Business development expenses and land carry costs	(1,383		)	(1,632		)	(4,069		)	(4,506		)
	Interest expense on continuing operations	(21,242		)	(23,239		)	(66,851		)	(71,909		)
	NOI from discontinued operations	(4,118		)	(4,677		)	(12,590		)	(23,114		)
	Loss on early extinguishment of debt	(374		)	(768		)	(27,028		)	(937		)
	(Loss) income from continuing operations	$	(276	)	$	(32,212	)	$	8,783		$	(10,100	)
Schedule of reconciliation of segment assets to total assets	The following table reconciles our segment assets to COPT’s consolidated total assets (in thousands):
		September 30,			September 30,
		2013			2012
	Segment assets	$	3,053,813		$	2,836,547
	Non-operating property assets	530,389			616,526
	Other assets	171,386			144,583
	Total COPT consolidated assets	$	3,755,588		$	3,597,656

ShareBased_Compensation_Tables

Share-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2013
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of payouts for defined performance under performance-based awards of share-based compensation	The number of PSUs earned (“earned PSUs”) at the end of the performance period will be determined based on the percentile rank of COPT’s total shareholder return relative to a peer group of companies, as set forth in the following schedule:
	Percentile Rank	Earned PSUs Payout %
	75th or greater	200% of PSUs granted
	50th	100% of PSUs granted
	25th	50% of PSUs granted
	Below 25th	0% of PSUs granted

Income_Taxes_Tables

Income Taxes (Tables)	9 Months Ended
	Sep. 30, 2013
Income Tax Disclosure [Abstract]
Components of provision for income tax	Our TRS’s provision for income taxes consisted of the following (in thousands):
		For the Three Months Ended September 30,						For the Nine Months Ended September 30,
		2013			2012			2013			2012
	Deferred
	Federal	$	(20	)	$	(87	)	$	(50	)	$	(267	)
	State	(4		)	(19		)	(11		)	(60		)
	Total income tax expense	$	(24	)	$	(106	)	$	(61	)	$	(327	)

Discontinued_Operations_and_As1

Discontinued Operations and Assets Held for Sale (Tables)	9 Months Ended
	Sep. 30, 2013
Discontinued Operations and Disposal Groups [Abstract]
Components of discontinued operations	The table below sets forth the components of discontinued operations reported on our consolidated statements of operations (in thousands):
		For the Three Months Ended September 30,						For the Nine Months Ended September 30,
		2013			2012			2013			2012
	Revenue from real estate operations	$	5,521		$	6,902		$	16,786		$	35,305
	Property operating expenses	(1,403		)	(2,225		)	(4,196		)	(12,191		)
	Depreciation and amortization	—			(2,020		)	(158		)	(8,744		)
	Impairment losses	(5,774		)	(9,733		)	(14,826		)	(23,510		)
	General, administrative and leasing expenses	—			(1		)	(1		)	(3		)
	Business development and land carry costs	—			—			—			(24		)
	Interest expense	(68		)	(127		)	(199		)	(2,107		)
	Gain on sales of real estate	—			16,913			—			20,948
	Gain on early extinguishment of debt	—			1,738			—			1,736
	Discontinued operations	$	(1,724	)	$	11,447		$	(2,594	)	$	11,410
Components of assets held for sale	The table below sets forth the components of assets held for sale on our consolidated balance sheets (in thousands):
		September 30, 2013			December 31, 2012
	Properties, net	$	121,060		$	128,740
	Deferred rent receivable	4,951			4,068
	Intangible assets on real estate acquisitions, net	4,384			4,409
	Deferred leasing costs, net	3,511			2,923
	Lease incentives	78			89
	Assets held for sale, net	$	133,984		$	140,229

Earnings_Per_Share_EPS_and_Ear1

Earnings Per Share ("EPS") and Earnings Per Unit ("EPU") (Tables)	9 Months Ended
	Sep. 30, 2013
Earnings Per Share [Line Items]
Summary of calculation of numerator and denominator in basic and diluted earnings per share	Summaries of the numerator and denominator for purposes of basic and diluted EPS calculations are set forth below (in thousands, except per share data):
		For the Three Months Ended September 30,							For the Nine Months Ended September 30,
		2013				2012			2013			2012
	Numerator:
	(Loss) income from continuing operations	$	(276	)		$	(32,212	)	$	8,783		$	(10,100	)
	Gain on sales of real estate, net	—				—			2,683			21
	Preferred share dividends	(4,490		)		(6,546		)	(15,481		)	(14,738		)
	Issuance costs associated with redeemed preferred shares	—				(1,827		)	(2,904		)	(1,827		)
	(Loss) income from continuing operations attributable to noncontrolling interests	(990		)		2,249			(2,139		)	1,710
	Income from continuing operations attributable to restricted shares	(97		)		(111		)	(317		)	(357		)
	Numerator for basic and diluted EPS from continuing operations attributable to COPT common shareholders	$	(5,853	)		$	(38,447	)	$	(9,375	)	$	(25,291	)
	Discontinued operations	(1,724		)		11,447			(2,594		)	11,410
	Discontinued operations attributable to noncontrolling interests	26				(646		)	(42		)	(603		)
	Numerator for basic and diluted EPS on net (loss) income attributable to COPT common shareholders	$	(7,551	)		$	(27,646	)	$	(12,011	)	$	(14,484	)
	Denominator for basic and diluted EPS (common shares)	86,760				71,688			84,547			71,590
	Basic EPS:
	Loss from continuing operations attributable to COPT common shareholders	$	(0.07	)		$	(0.54	)	$	(0.11	)	$	(0.35	)
	Discontinued operations attributable to COPT common shareholders	(0.02		)		0.15			(0.03		)	0.15
	Net loss attributable to COPT common shareholders	$	(0.09	)		$	(0.39	)	$	(0.14	)	$	(0.20	)
	Diluted EPS:
	Loss from continuing operations attributable to COPT common shareholders	$	(0.07	)		$	(0.54	)	$	(0.11	)	$	(0.35	)
	Discontinued operations attributable to COPT common shareholders	(0.02		)		0.15			(0.03		)	0.15
	Net loss attributable to COPT common shareholders	$	(0.09	)		$	(0.39	)	$	(0.14	)	$	(0.20	)
Schedule of securities excluded from computation of diluted earnings per share	Our diluted EPS computations do not include the effects of the following securities since the conversions of such securities would increase diluted EPS for the respective periods (in thousands):
		Weighted Average Shares Excluded from Denominator
		For the Three Months Ended September 30,						For the Nine Months Ended September 30,
		2013			2012			2013		2012
	Conversion of common units	3,804			4,233			3,832		4,256
	Conversion of Series I Preferred Units	176			176			176		176
	Conversion of Series K Preferred Shares	434			434			434		434
Corporate Office Properties, L.P.
Earnings Per Share [Line Items]
Summary of calculation of numerator and denominator in basic and diluted earnings per share	Summaries of the numerator and denominator for purposes of basic and diluted EPU calculations are set forth below (in thousands, except per unit data):
		For the Three Months Ended September 30,							For the Nine Months Ended September 30,
		2013				2012			2013			2012
	Numerator:
	(Loss) income from continuing operations	$	(276	)		$	(32,212	)	$	8,783		$	(10,100	)
	Gain on sales of real estate, net	—				—			2,683			21
	Preferred unit distributions	(4,655		)		(6,711		)	(15,976		)	(15,233		)
	Issuance costs associated with redeemed preferred units	—				(1,827		)	(2,904		)	(1,827		)
	(Loss) income from continuing operations attributable to noncontrolling interests	(986		)		243			(2,025		)	829
	Income from continuing operations attributable to restricted units	(97		)		(111		)	(317		)	(357		)
	Numerator for basic and diluted EPU from continuing operations attributable to COPLP common unitholders	$	(6,014	)		$	(40,618	)	$	(9,756	)	$	(26,667	)
	Discontinued operations	(1,724		)		11,447			(2,594		)	11,410
	Discontinued operations attributable to noncontrolling interests	(49		)		(647		)	(147		)	(662		)
	Numerator for basic and diluted EPU on net income attributable to COPLP common unitholders	$	(7,787	)		$	(29,818	)	$	(12,497	)	$	(15,919	)
	Denominator for basic and diluted EPU (common units)	90,564				75,921			88,379			75,846
	Basic EPU:
	Loss from continuing operations attributable to COPLP common unitholders	$	(0.07	)		$	(0.54	)	$	(0.11	)	$	(0.35	)
	Discontinued operations attributable to COPLP common unitholders	(0.02		)		0.15			(0.03		)	0.14
	Net loss attributable to COPLP common unitholders	$	(0.09	)		$	(0.39	)	$	(0.14	)	$	(0.21	)
	Diluted EPU:
	Loss from continuing operations attributable to COPLP common unitholders	$	(0.07	)		$	(0.54	)	$	(0.11	)	$	(0.35	)
	Discontinued operations attributable to COPLP common unitholders	(0.02		)		0.15			(0.03		)	0.14
	Net loss attributable to COPLP common unitholders	$	(0.09	)		$	(0.39	)	$	(0.14	)	$	(0.21	)
Schedule of securities excluded from computation of diluted earnings per share	Our diluted EPU computations do not include the effects of the following securities since the conversions of such securities would increase diluted EPU for the respective periods (in thousands):
		Weighted Average Units Excluded from Denominator
		For the Three Months Ended September 30,						For the Nine Months Ended September 30,
		2012			2011			2013		2012
	Conversion of Series I Preferred Units	176			176			176		176
	Conversion of Series K Preferred Units	434			434			434		434

Organization_Details

Organization (Details)	9 Months Ended
	Sep. 30, 2013
	sqft
	acre
Operating office properties
Investments in real estate
Number of real estate properties	210
Square footage of real estate properties (in square feet)	19,200,000
Office propertiesunder, or contractually committed for, construction or approved for redevelopment
Investments in real estate
Number of real estate properties	11
Square footage of real estate properties (in square feet)	1,500,000
Office properties under construction or redevelopment which are partially operational
Investments in real estate
Number of real estate properties	1
Land held or under pre-construction
Investments in real estate
Area of land held or under pre-construction (in acres)	1,721
Area of land parcels controlled but not owned (in acres)	56
Developable square footage of real estate properties (in square feet)	20,400,000
Partially operational, wholesale data center
Investments in real estate
The expected stabilization critical load after completion of development (in megawatts)	18

Organization_Details_2

Organization (Details 2) (Corporate Office Properties, L.P.)	9 Months Ended
	Sep. 30, 2013
	Trustee
Common Units
Forms of ownership in Operating Partnership and ownership percentage by the entity
Percentage ownership in operating partnership	95.60%
Number of trustees controlling additional common units	3
Additional percentage of common units controlled by trustees (as a percent)	3.40%
Preferred Units
Forms of ownership in Operating Partnership and ownership percentage by the entity
Percentage ownership in operating partnership	96.40%

Summary_of_Significant_Account3

Summary of Significant Accounting Policies (Details) (USD $)

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

12 Months Ended

9 Months Ended

12 Months Ended

In Thousands, unless otherwise specified

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Dec. 31, 2012

Sep. 30, 2012

Sep. 30, 2012

Dec. 31, 2011

Dec. 31, 2011

Sep. 30, 2012

Dec. 31, 2011

Dec. 31, 2011

Scenario, Previously Reported

Scenario, Previously Reported

Misapplication Of Accounting Guidance Related To Recognition Of Deferred Tax Asset

Over accrual Of Incentive Compensation Cost

Misapplication of Accounting Guidance Related to Recognization of Losses Related to Consolidated Real Estate Joint Ventures [Member]

Misapplication of Accounting Guidance Related to Recognization of Losses Related to Consolidated Real Estate Joint Ventures [Member]

Misapplication of Accounting Guidance Related to Reporting for a Noncontrolling Interest in a Consolidated Real Estate Joint Venture [Member]

Scenario, Previously Reported

Scenario, Previously Reported

Scenario, Previously Reported

Scenario, Previously Reported

Scenario, Previously Reported

Net (loss) income

($2,000)

($20,765)

$8,872

$1,331

($20,765)

($1,927)

($4,000)

$711

($1,800)

($1,400)

Redeemable noncontrolling interest

$16,789

$9,932

$16,789

$9,932

$10,298

$0

$0

$8,900

Summary_of_Significant_Account4

Summary of Significant Accounting Policies 2 (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Dec. 31, 2011
Redeemable noncontrolling interest	$16,789	$9,932	$16,789	$9,932	$10,298
Additional paid-in capital	1,812,801	1,451,416	1,812,801	1,451,416	1,653,672
Cumulative distributions in excess of net income	-700,368	-607,633	-700,368	-607,633	-617,455
Noncontrolling interests in common units in COPLP	50,815	49,157	50,815	49,157	52,122
Noncontrolling interests in other consolidated entities	9,137	9,682	9,137	9,682	10,153
Total equity	1,434,067	1,240,289	1,434,067	1,240,289	1,436,499	1,205,899
Total liabilities, redeemable noncontrolling interest and equity	3,755,588	3,597,656	3,755,588	3,597,656	3,653,759
Total revenues	136,031	129,645	407,175	389,043
Expenses
Property operating expenses	43,482	41,474	129,409	122,102
Depreciation and amortization associated with real estate operations	29,210	28,604	86,239	84,633
Construction contract and other service expenses	16,306	14,410	49,165	51,302
Impairment losses	16,300	46,096	16,300	41,260
General, administrative and leasing expenses		6,377		24,797
Business development expenses and land carry costs	1,383	1,632	4,069	4,506
Total operating expenses	114,708	138,593	307,612	328,600
Operating income (loss)	21,323	-8,948	99,563	60,443
Interest expense	-21,242	-23,239	-66,851	-71,909
Income tax expense	-24	-106	-61	-327
Loss from continuing operations	-276	-32,212	8,783	-10,100
Discontinued operations	-1,724	11,447	-2,594	11,410
Net (loss) income	-2,000	-20,765	8,872	1,331
Net loss attributable to noncontrolling interests in common units in COPLP	232	1,533	474	738
Net loss attributable to noncontrolling interests in other consolidated entities	-1,031	235	-2,160	864
Net loss attributable to COPT	-2,964	-19,162	6,691	2,438
Basic and diluted earnings per common share:
Income from continuing operations (in dollars per share)		($0.54)		($0.35)
Income from discontinued operations (in dollars per share)		$0.15		$0.15
Net income attributable to COPT common shareholders (in dollars per share)		($0.39)		($0.20)
Scenario, Previously Reported
Redeemable noncontrolling interest		0		0
Additional paid-in capital		1,455,558		1,455,558
Cumulative distributions in excess of net income		-610,659		-610,659
Noncontrolling interests in common units in COPLP		48,973		48,973
Noncontrolling interests in other consolidated entities		18,682		18,682
Total equity		1,250,221		1,250,221
Total liabilities, redeemable noncontrolling interest and equity		3,597,656		3,597,656
Total revenues		130,144		390,499
Expenses
Property operating expenses		42,799		126,339
Depreciation and amortization associated with real estate operations		28,698		84,920
Construction contract and other service expenses		14,410		51,302
Impairment losses		46,096		41,260
General, administrative and leasing expenses		5,061		19,820
Business development expenses and land carry costs		1,632		4,506
Total operating expenses		138,696		328,147
Operating income (loss)		-8,552		62,352
Interest expense		-23,239		-71,909
Income tax expense		-106		-4,296
Loss from continuing operations		-31,816		-12,160
Discontinued operations		11,051		10,212
Net (loss) income		-20,765		-1,927
Net loss attributable to noncontrolling interests in common units in COPLP		1,569		1,020
Net loss attributable to noncontrolling interests in other consolidated entities		-411		-939
Net loss attributable to COPT		-19,772		-2,341
Basic and diluted earnings per common share:
Income from continuing operations (in dollars per share)		($0.54)		($0.40)
Income from discontinued operations (in dollars per share)		$0.15		$0.13
Net income attributable to COPT common shareholders (in dollars per share)		($0.39)		($0.27)
Change
Redeemable noncontrolling interest		9,932		9,932
Additional paid-in capital		-4,142		-4,142
Cumulative distributions in excess of net income		3,026		3,026
Noncontrolling interests in common units in COPLP		184		184
Noncontrolling interests in other consolidated entities		-9,000		-9,000
Total equity		-9,932		-9,932
Total revenues		-499		-1,456
Expenses
Property operating expenses		-1,325		-4,237
Depreciation and amortization associated with real estate operations		-94		-287
General, administrative and leasing expenses		1,316		4,977
Total operating expenses		-103		453
Operating income (loss)		-396		-1,909
Income tax expense				3,969
Loss from continuing operations		-396		2,060
Discontinued operations		396		1,198
Net (loss) income				3,258
Net loss attributable to noncontrolling interests in common units in COPLP		-36		-282
Net loss attributable to noncontrolling interests in other consolidated entities		646		1,803
Net loss attributable to COPT		610		4,779
Basic and diluted earnings per common share:
Income from continuing operations (in dollars per share)				$0.05
Income from discontinued operations (in dollars per share)				$0.02
Net income attributable to COPT common shareholders (in dollars per share)				$0.07
Revisions
Redeemable noncontrolling interest		9,932		9,932
Additional paid-in capital		-4,142		-4,142
Cumulative distributions in excess of net income		3,026		3,026
Noncontrolling interests in common units in COPLP		184		184
Noncontrolling interests in other consolidated entities		-9,000		-9,000
Total equity		-9,932		-9,932
Continuing Operations | Reclassifications
Expenses
Property operating expenses		-1,316		-4,269
General, administrative and leasing expenses		1,316		4,269
Continuing Operations | Revisions
Expenses
General, administrative and leasing expenses				711
Total operating expenses				711
Operating income (loss)				-711
Income tax expense				3,969
Loss from continuing operations				3,258
Net (loss) income				3,258
Net loss attributable to noncontrolling interests in common units in COPLP		-36		-282
Net loss attributable to noncontrolling interests in other consolidated entities		646		1,803
Net loss attributable to COPT		610		4,779
Basic and diluted earnings per common share:
Income from continuing operations (in dollars per share)				$0.07
Net income attributable to COPT common shareholders (in dollars per share)				$0.07
Discontinued Operations | Reclassifications
Total revenues		-499		-1,456
Expenses
Property operating expenses		-9		32
Depreciation and amortization associated with real estate operations		-94		-287
General, administrative and leasing expenses				-3
Total operating expenses		-103		-258
Operating income (loss)		-396		-1,198
Loss from continuing operations		-396		-1,198
Discontinued operations		$396		$1,198
Basic and diluted earnings per common share:
Income from continuing operations (in dollars per share)				($0.02)
Income from discontinued operations (in dollars per share)				$0.02

Fair_Value_Measurements_Detail

Fair Value Measurements (Details) (USD $)

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

In Thousands, unless otherwise specified

Corporate Office Properties, L.P.

Corporate Office Properties, L.P.

Fair value measurement on a recurring basis

Fair value measurement on a recurring basis

Fair value measurement on a recurring basis

Fair value measurement on a recurring basis

Fair value measurement on a recurring basis

Fair value measurement on a recurring basis

Fair value measurement on a recurring basis

Fair value measurement on a recurring basis

Fair value measurement on a recurring basis

Fair value measurement on a recurring basis

Fair value measurement on a recurring basis

Fair value measurement on a recurring basis

Fair value measurement on a recurring basis

Fair value measurement on a recurring basis

Redeemable Noncontrolling Interest

Redeemable Noncontrolling Interest

Redeemable Noncontrolling Interest

Quoted Prices in Active Markets for Identical Assets (Level 1)

Quoted Prices in Active Markets for Identical Assets (Level 1)

Quoted Prices in Active Markets for Identical Assets (Level 1)

Quoted Prices in Active Markets for Identical Assets (Level 1)

Quoted Prices in Active Markets for Identical Assets (Level 1)

Significant Other Observable Inputs (Level 2)

Significant Other Observable Inputs (Level 2)

Significant Unobservable Inputs (Level 3)

Significant Unobservable Inputs (Level 3)

Total Estimated Fair Value

Total Estimated Fair Value

Total Estimated Fair Value

Total Estimated Fair Value

Total Estimated Fair Value

Minimum

Maximum

Weighted Average

Corporate Office Properties, L.P.

Mutual funds

Common Shares

Other

Corporate Office Properties, L.P.

Corporate Office Properties, L.P.

Corporate Office Properties, L.P.

Mutual funds

Common Shares

Other

Fair value measurement on a recurring basis

Fair value measurement on a recurring basis

Fair value measurement on a recurring basis

Significant Unobservable Inputs (Level 3)

Significant Unobservable Inputs (Level 3)

Significant Unobservable Inputs (Level 3)

Assets:

Marketable securities in deferred compensation plan

$6,753

[1]

$217

[1]

$211

[1]

$6,753

[1]

$217

[1]

$211

[1]

Common stock

520

[1]

520

[2]

520

[1]

520

[2]

Interest rate derivatives

1,600

-6,185

5,195

[3]

5,195

[4]

5,195

[3]

5,195

[4]

Warrants to purchase common stock

301

[3]

301

[4]

301

[3]

301

[4]

Assets

7,701

520

5,496

5,496

0

0

13,197

6,016

Liabilities:

Deferred compensation plan liability

7,181

[5]

7,181

[5]

Interest rate derivatives

3,595

6,185

3,595

6,185

3,595

3,595

3,595

3,595

Liabilities

7,181

3,595

10,776

Redeemable noncontrolling interest

$16,789

$10,298

$9,932

$16,789

$10,298

$16,789

$16,789

$16,789

$16,789

Discount rate

10.00%

20.00%

15.30%

[1]

Included in the line entitled “restricted cash and marketable securities” on COPT’s consolidated balance sheet.

[2]

Included in the line entitled “restricted cash and marketable securities” on COPLP’s consolidated balance sheet.

[3]

Included in the line entitled “prepaid expenses and other assets” on COPT’s consolidated balance sheet.

[4]

Included in the line entitled “prepaid expenses and other assets” on COPLP’s consolidated balance sheet.

[5]

Included in the line entitled “other liabilities” on COPT’s consolidated balance sheet.

Fair_Value_Measurements_Detail1

Fair Value Measurements (Details 2) (USD $)	3 Months Ended				9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Assets and liabilities measured at fair value on a non-recurring basis
Impairment losses (recoveries)	$16,300		$46,096		$16,300		$41,260
Exit costs on property dispositions					186		4,066
Fair value measurement on a nonrecurring basis | Discontinued Operations
Assets and liabilities measured at fair value on a non-recurring basis
Impairment losses (recoveries)					14,800
Exit costs on property dispositions					186
Fair value measurement on a nonrecurring basis | Significant Unobservable Inputs (Level 3)
Assets and liabilities measured at fair value on a non-recurring basis
Exit costs on property dispositions	186				186
Properties Expected to be Conveyed | Fair value measurement on a nonrecurring basis
Assets and liabilities measured at fair value on a non-recurring basis
Impairment losses (recoveries)	10,400
Other Properties with Shortened Holding Periods | Fair value measurement on a nonrecurring basis
Assets and liabilities measured at fair value on a non-recurring basis
Impairment losses (recoveries)	5,900
Number of Properties, Shortened Holding Period	2
Properties,net | Fair value measurement on a nonrecurring basis
Assets and liabilities measured at fair value on a non-recurring basis
Impairment losses (recoveries)	21,888	[1],[2]	52,900	[1],[3]	30,940	[1],[2]	60,593	[1],[3]
Properties,net | Fair value measurement on a nonrecurring basis | Significant Unobservable Inputs (Level 3)
Assets and liabilities measured at fair value on a non-recurring basis
Assets	245,535	[1],[4]	369,312	[1],[4]	245,535	[1],[4]	369,312	[1],[4]
Exit costs on property dispositions			2,900				4,200
Properties,net | Fair value measurement on a nonrecurring basis | Total Estimated Fair Value
Assets and liabilities measured at fair value on a non-recurring basis
Assets	$245,535	[1]	$369,312	[1]	$245,535	[1]	$369,312	[1]
[1]	Reflects balance sheet classifications of assets at time of fair value measurement, excluding the effect of held for sale classifications.
[2]	Represents impairment losses, excluding exit costs incurred of $186,000 for the three months and nine months ended September 30, 2013.
[3]	Represents impairment losses, excluding exit costs incurred of $2.9 million for the three months ended September 30, 2012 and $4.2 million for the nine months ended September 30, 2012.
[4]	These fair value measurements were developed as a result of negotiations between us and potential, or actual, purchasers of properties.

Fair_Value_Measurements_Detail2

Fair Value Measurements (Details 3) (USD $)	9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013		Sep. 30, 2012
Discounted Cash Flow Valuation Technique [Member]
Quantitative information for significant unobservable inputs used for Level 3 fair value measurements
Discount rate	10.90%		10.40%
Terminal capitalization rate	9.70%		8.90%
Market rent growth rate	3.00%	[1]	3.00%	[1]
Expense growth rate	3.00%	[1]	3.00%	[1]
Discounted Cash Flow Valuation Technique [Member] | Minimum
Quantitative information for significant unobservable inputs used for Level 3 fair value measurements
Discount rate	10.00%		10.10%
Terminal capitalization rate	9.50%		8.70%
Discounted Cash Flow Valuation Technique [Member] | Maximum
Quantitative information for significant unobservable inputs used for Level 3 fair value measurements
Discount rate	11.00%		11.00%
Terminal capitalization rate	10.00%		10.00%
Yield Analysis Valuation Technique [Member]
Quantitative information for significant unobservable inputs used for Level 3 fair value measurements
Yield			12.00%	[1]
Market rent rate (in dollars per square foot)			8.5	[1]
Leasing costs (in dollars per square foot)			20	[1]
Fair value measurement on a nonrecurring basis | Significant Unobservable Inputs (Level 3) | Properties,net
Quantitative information for significant unobservable inputs used for Level 3 fair value measurements
Fair value on measurement date	245,535	[2],[3]	369,312	[2],[3]
[1]	Only one value applied for this unobservable input.
[2]	Reflects balance sheet classifications of assets at time of fair value measurement, excluding the effect of held for sale classifications.
[3]	These fair value measurements were developed as a result of negotiations between us and potential, or actual, purchasers of properties.

Properties_net_Details

Properties, net (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Properties
Operating properties, net	$2,713,579	$2,597,666
Operating properties, net
Properties
Less: accumulated depreciation	-612,369	-555,975
Operating properties, net | Land
Properties
Gross	436,565	427,766
Operating properties, net | Buildings and improvements
Properties
Gross	$2,889,383	$2,725,875

Properties_net_Details_2

Properties, net (Details 2) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Properties
Projects in development or held for future development	$526,167	$565,378
Projects in development or held for future development | Land in development or held for future development
Properties
Projects in development or held for future development	253,810	236,324
Projects in development or held for future development | Construction in progress, excluding land
Properties
Projects in development or held for future development	$272,357	$329,054

Properties_net_Details_3

Properties, net (Details 3) (Linthicum Maryland, USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013
	sqft
Linthicum Maryland
Dispositions [Line Items]
Square footage of real estate properties (in square feet)	103,000
Sales Price of Operating Properties Sold	$6.90

Properties_net_Details_4

Properties, net (Details 4)	9 Months Ended
	Sep. 30, 2013
	Property
	sqft
Newly constructed properties placed in service
Construction and Redevelopment Activities
Square Feet of Properties Placed in Service	392,000
Number of Real Estate Properties Placed In Service	4
Properties under construction or contractually committed for construction
Construction and Redevelopment Activities
Number of real estate properties	9
Square footage of real estate properties (in square feet)	1,300,000
Properties under construction or contractually committed for construction | Northern Virginia
Construction and Redevelopment Activities
Number of real estate properties	4
Properties under construction or contractually committed for construction | Baltimore and Washington Corridor
Construction and Redevelopment Activities
Number of real estate properties	2
Properties under construction or contractually committed for construction | Huntsville
Construction and Redevelopment Activities
Number of real estate properties	3
Properties under or approved for redevelopment
Construction and Redevelopment Activities
Number of real estate properties	2
Square footage of real estate properties (in square feet)	235,000
Properties under or approved for redevelopment | Baltimore and Washington Corridor
Construction and Redevelopment Activities
Number of real estate properties	1
Properties under or approved for redevelopment | Greater Philadelphia
Construction and Redevelopment Activities
Number of real estate properties	1

Real_Estate_Joint_Ventures_Det

Real Estate Joint Ventures (Details) (USD $)	3 Months Ended			9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012	Sep. 30, 2013		Sep. 30, 2012	Dec. 31, 2012
	Venture			Property
				Venture
Investments in real estate joint ventures
Investment Balance	($6,420,000)			($6,420,000)			($6,420,000)
Corporate Office Properties, L.P.
Investments in real estate joint ventures
Investment Balance	-6,420,000			-6,420,000			-6,420,000
Investments in consolidated real estate joint ventures
Acquisition of property and noncontrolling interest in other consolidated entity for COPLP common units (in dollars)				1,369,000
Unconsolidated real estate joint ventures
Investments in real estate joint ventures
Number of unconsolidated real estate joint ventures in which the entity has an interest	1			1
Investment Balance	-6,420,000	[1]		-6,420,000	[1]		-6,420,000	[1]
Ownership (as a percent)				20.00%
Number of buildings operated by the joint venture				16
Maximum exposure to loss	0	[2]		0	[2]
Difference between carrying amount of investment and share of the equity in the joint venture	3,000,000			3,000,000			4,500,000
Condensed balance sheets for unconsolidated real estate joint venture
Properties, net	57,222,000			57,222,000			58,460,000
Other assets	6,248,000			6,248,000			4,376,000
Total assets	63,470,000			63,470,000			62,836,000
Liabilities (primarily debt)	80,424,000			80,424,000			72,693,000
Owners' equity	-16,954,000			-16,954,000			-9,857,000
Total liabilities and owners' equity	63,470,000			63,470,000			62,836,000
Condensed statements of operations for unconsolidated real estate joint venture
Revenues	1,739,000		1,778,000	5,342,000		5,497,000
Property operating expenses	-755,000		-758,000	-2,250,000		-2,219,000
Interest expense	-2,966,000		-1,138,000	-8,521,000		-3,291,000
Depreciation and amortization expense	-575,000		-568,000	-1,668,000		-1,732,000
Net loss	-2,557,000		-686,000	-7,097,000		-1,745,000
Consolidated real estate joint ventures
Investments in consolidated real estate joint ventures
Total Assets	185,599,000	[3]		185,599,000	[3]
Pledged Assets	93,063,000	[3]		93,063,000	[3]
Total Liabilities	63,694,000	[3]		63,694,000	[3]
LW Redstone Company, LLC
Investments in real estate joint ventures
Ownership (as a percent)				85.00%
Investments in consolidated real estate joint ventures
Total Assets	114,141,000	[3],[4]		114,141,000	[3],[4]
Pledged Assets	45,209,000	[3],[4]		45,209,000	[3],[4]
Total Liabilities	22,141,000	[3],[4]		22,141,000	[3],[4]
M Square Associates, LLC
Investments in real estate joint ventures
Ownership (as a percent)				50.00%
Number of buildings operated by the joint venture				2
Investments in consolidated real estate joint ventures
Total Assets	60,879,000	[3],[5]		60,879,000	[3],[5]
Pledged Assets	47,854,000	[3],[5]		47,854,000	[3],[5]
Total Liabilities	41,463,000	[3],[5]		41,463,000	[3],[5]
COPT-FD Indian Head, LLC
Investments in real estate joint ventures
Ownership (as a percent)				75.00%
Investments in consolidated real estate joint ventures
Total Assets	6,447,000	[3],[6]		6,447,000	[3],[6]
MOR Forbes 2 LLC
Investments in real estate joint ventures
Ownership (as a percent)				50.00%
Number of buildings operated by the joint venture				1
Investments in consolidated real estate joint ventures
Total Assets	4,132,000	[3],[7]		4,132,000	[3],[7]
Total Liabilities	90,000	[3],[7]		90,000	[3],[7]
Common Shares | Corporate Office Properties, L.P.
Investments in consolidated real estate joint ventures
Acquisition of property and noncontrolling interest in other consolidated entity for COPLP common units (in units)				221,501
Acquisition of property and noncontrolling interest in other consolidated entity for COPLP common units (in dollars)				3,899,000
Common Shares | Arundel Preserve LLC
Investments in consolidated real estate joint ventures
Acquisition of property and noncontrolling interest in other consolidated entity for COPLP common units (in units)				221,501
Acquisition of property and noncontrolling interest in other consolidated entity for COPLP common units (in dollars)				$5,200,000
[1]	The carrying amount of our investment in this joint venture was lower than our share of the equity in the joint venture by $3.0 million at September 30, 2013 and $4.5 million at December 31, 2012 due to our deferral of gain on the contribution by us of real estate into the joint venture upon its formation and our discontinuance of loss recognition under the equity method effective October 2012, as discussed below. A difference will continue to exist to the extent the nature of our continuing involvement in the joint venture remains the same and we continue to no longer recognize income or losses under the equity method.
[2]	Derived from the sum of our investment balance and maximum additional unilateral capital contributions or loans required from us. Not reported above are additional amounts that we and our partner are required to fund when needed by this joint venture; these funding requirements are proportional to our respective ownership percentages. Also not reported above are additional unilateral contributions or loans from us, the amounts of which are uncertain, that we would be required to make if certain contingent events occur (see Note 17).
[3]	Excludes amounts eliminated in consolidation.
[4]	This joint venture’s property is in Huntsville, Alabama.
[5]	This joint venture’s properties are in College Park, Maryland (in the Suburban Maryland region).
[6]	This joint venture’s property is in Charles County, Maryland. In 2012, the joint venture exercised its option under a development agreement to require Charles County to repurchase the land parcel at its original acquisition cost. Under the terms of the agreement with Charles County, the repurchase is expected to occur by August 2014.
[7]	This joint venture’s property is in Lanham, Maryland (in the Suburban Maryland region).

Mortgage_and_Other_Investing_R2

Mortgage and Other Investing Receivables (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage and other investing receivables	$40,321	$33,396
Notes Receivable from City of Huntsville
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage and other investing receivables	40,321	33,252
Mortgage Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage and other investing receivables	$0	$144

Prepaid_Expenses_and_Other_Ass2

Prepaid Expenses and Other Assets (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Mortgage and Other Investing Receivables [Line Items]
Prepaid expenses	$23,552	$19,270
Lease incentives	8,751	5,578
Furniture, fixtures and equipment, net	6,705	7,991
Deferred tax asset	6,222	6,612
Construction contract costs incurred in excess of billings	5,465	0
Interest rate derivatives	5,195	0
Other equity method investments	4,928	2,425
Other assets	3,257	2,183
Prepaid expenses and other assets	64,075	44,059
Long-term Operating Notes Receivable
Mortgage and Other Investing Receivables [Line Items]
Operating Notes Receivable Long-term	$186	$271

Debt_Details

Debt (Details) (USD $)

3 Months Ended

9 Months Ended

9 Months Ended

9 Months Ended

9 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

9 Months Ended

0 Months Ended

0 Months Ended

0 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

3 Months Ended

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Dec. 31, 2012

Sep. 30, 2013

6-May-13

Sep. 30, 2013

Dec. 31, 2012

Sep. 15, 2013

Sep. 30, 2013

Sep. 16, 2013

Dec. 31, 2012

Sep. 30, 2013

Sep. 30, 2013

Jul. 16, 2013

Jul. 16, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Mortgage and Other Secured Loans:

Mortgage and Other Secured Loans:

Fixed rate mortgage loans

Fixed rate mortgage loans

Variable rate secured loans

Variable rate secured loans

Other construction loan facilities

Other construction loan facilities

Revolving Credit Facility

Revolving Credit Facility

Term Loan Facility

Term Loan Facility

Unsecured notes payable

Unsecured notes payable

4.25% Exchangeable Senior Notes

4.25% Exchangeable Senior Notes

4.25% Exchangeable Senior Notes

4.25% Exchangeable Senior Notes

4.25% Exchangeable Senior Notes

Unsecured senior notes

Unsecured senior notes

Unsecured senior notes

Unsecured senior notes

Unsecured senior notes

Unsecured senior notes

Unsecured senior notes

Unsecured senior notes

Adjusted Treasury

Adjusted Treasury

Amendment Number 1

Amendment Number 2

Minimum

Minimum

Minimum

Minimum

Maximum

Maximum

Maximum

Maximum

3.60% Senior Notes

3.60% Senior Notes

3.60% Senior Notes

5.250% Senior Notes

5.250% Senior Notes

5.250% Senior Notes

5.250% Senior Notes

Unsecured senior notes

Unsecured senior notes

Revolving Credit Facility

London Interbank Offered Rate (LIBOR)

Revolving Credit Facility

Term Loan Facility

Amendment Number 1

Amendment Number 2

Revolving Credit Facility

Term Loan Facility

Amendment Number 1

Amendment Number 2

3.60% Senior Notes

5.250% Senior Notes

Unsecured notes payable

Revolving Credit Facility

London Interbank Offered Rate (LIBOR)

Revolving Credit Facility

London Interbank Offered Rate (LIBOR)

Term Loan Facility

London Interbank Offered Rate (LIBOR)

Term Loan Facility

London Interbank Offered Rate (LIBOR)

Term Loan Facility

Debt

Maximum Availability

$26,150,000

$800,000,000

[1]

Carrying Value

2,135,031,000

2,135,031,000

2,019,168,000

920,205,000

1,016,446,000

882,311,000

[2]

948,414,000

[2]

37,894,000

38,475,000

0

29,557,000

0

[1]

0

[1]

620,000,000

[3]

770,000,000

[3]

1,723,000

[4]

1,788,000

[4]

560,000

[5]

560,000

[5]

230,934,000

[5]

347,183,000

[4]

0

[4]

245,360,000

[4]

0

[4]

Stated interest rate (as a percent)

0.00%

[6]

4.25%

4.25%

3.60%

3.60%

5.25%

5.25%

Stated Interest Rates, low end of range (as a percent)

3.96%

[7]

Stated Interest Rates, high end of range (as a percent)

7.87%

[7]

Description of variable rate basis

LIBOR

[8]

LIBOR

[9]

LIBOR

[10]

Variable rate, spread (as a percent)

2.25%

[8]

0.98%

[9]

1.10%

[10]

0.98%

1.10%

1.75%

[9]

2.60%

[10]

1.75%

2.00%

Unamortized premium included in carrying value

465,000

1,300,000

Weighted average interest rate (as a percent)

5.98%

1.45%

1.80%

Interest rate on debt (as a percent)

2.43%

Additional borrowing capacity available provided there is no default under the agreement

180,000,000

Convertible debt principal amount, basis for exchange

1,000

1,000

Exchange rate per $1000 principal amount (in common shares)

20.8513

Exchange price per common share based on exchange rate (in dollars per share)

$47.96

$47.96

Amount of debt repaid

239,400,000

Loss on early extinguishment of debt

374,000

768,000

27,028,000

937,000

25,900,000

Principal amount of debt

575,000

575,000

240,000,000

Effective interest rate, including amortization of issuance costs (as a percent)

6.05%

6.05%

3.70%

5.49%

Number of Months Prior to Loan Maturity Requiring Redemption at Principal Value

3 months

Redemption Percentage Within Three Months of Maturity

100.00%

Interest expense at stated interest rate

6,000

2,550,000

4,201,000

7,650,000

Interest expense associated with amortization of discount

1,207,000

2,357,000

2,000

919,000

1,613,000

2,717,000

Total interest expenses

8,000

3,469,000

5,814,000

10,367,000

Debt instrument extension option, term

1 year

Extension fee percentage

0.15%

Debt instrument, face amount

350,000,000

250,000,000

550,000,000

Proceeds from issuance of debt

347,100,000

245,300,000

Debt issuance as a percentage of principal amount

99.82%

98.78%

Basis points used in determining redemption price prior to maturity

30.00%

40.00%

Unamortized discount included in carrying value

788,000

873,000

15,000

15,000

9,100,000

2,800,000

4,600,000

Capitalized interest costs

$2,200,000

$3,400,000

$6,700,000

$10,800,000

[1]

Refer to the paragraph below for disclosure pertaining to the Revolving Credit Facility.

[2]

Several of the fixed rate mortgages carry interest rates that were above or below market rates upon assumption and therefore were recorded at their fair value based on applicable effective interest rates. The carrying values of these loans reflect net unamortized premiums totaling $465,000 at September 30, 2013 and $1.3 million at December 31, 2012.

[3]

We have the ability to borrow an aggregate of an additional $180.0 million under these term loan facilities, provided that there is no default under the facilities and subject to the approval of the lenders.

[4]

Refer to the paragraphs below for disclosure pertaining to these notes.

[5]

As described further in our 2012 Annual Report on Form 10-K, these notes have an exchange settlement feature that provides that the notes may, under certain circumstances, be exchangeable for cash and, at COPLP’s discretion, COPT common shares at an exchange rate (subject to adjustment) of 20.8513 shares per one thousand dollar principal amount of the notes (exchange rate is as of September 30, 2013 and is equivalent to an exchange price of $47.96 per common share). During the nine months ended September 30, 2013, we repaid $239.4 million principal amount of these notes and recognized a $25.9 million loss on early extinguishment of debt. The carrying value of these notes included a principal amount of $575,000 and an unamortized discount totaling $15,000 at September 30, 2013 and a principal amount of $240.0 million and an unamortized discount totaling $9.1 million at December 31, 2012. The effective interest rate under the notes, including amortization of the issuance costs, was 6.05%. Because the closing price of our common shares at September 30, 2013 and December 31, 2012 was less than the exchange price per common share applicable to these notes, the if-converted value of the notes did not exceed the principal amount. The table below sets forth interest expense recognized on these notes before deductions for amounts capitalized (in thousands): For the Three Months Ended September 30, For the Nine Months Ended September 30, 2013 2012 2013 2012Interest expense at stated interest rate$6 $2,550 $4,201 $7,650Interest expense associated with amortization of discount2 919 1,613 2,717Total$8 $3,469 $5,814 $10,367

[6]

These notes carry interest rates that were below market rates upon assumption and therefore were recorded at their fair value based on applicable effective interest rates. The carrying value of these notes reflects an unamortized discount totaling $788,000 at September 30, 2013 and $873,000 at December 31, 2012.

[7]

The weighted average interest rate on these loans was 5.98% at September 30, 2013.

[8]

The interest rate on the loan outstanding was 2.43% at September 30, 2013.

[9]

The weighted average interest rate on the Revolving Credit Facility was 1.45% at September 30, 2013.

[10]

The weighted average interest rate on these loans was 1.80% at September 30, 2013.

Debt_Details_2

Debt (Details 2) (USD $)	Sep. 30, 2013		Dec. 31, 2012
In Thousands, unless otherwise specified
Carrying amount and estimated fair value of debt
Carrying Value	$2,135,031		$2,019,168
4.25% Exchangeable Senior Notes
Carrying amount and estimated fair value of debt
Carrying Value	560	[1]	230,934	[1]
Carrying Amount
Carrying amount and estimated fair value of debt
Variable-rate debt	657,894		838,032
Carrying Value	2,135,031		2,019,168
Carrying Amount | Unsecured senior notes
Carrying amount and estimated fair value of debt
Fixed-rate debt	592,543		0
Carrying Amount | 4.25% Exchangeable Senior Notes
Carrying amount and estimated fair value of debt
Fixed-rate debt	560		230,934
Carrying Amount | Other fixed-rate debt
Carrying amount and estimated fair value of debt
Fixed-rate debt	884,034		950,202
Total Estimated Fair Value
Carrying amount and estimated fair value of debt
Variable-rate debt	661,593		845,558
Carrying Value	2,054,068		2,054,020
Total Estimated Fair Value | Unsecured senior notes
Carrying amount and estimated fair value of debt
Fixed-rate debt	579,654		0
Total Estimated Fair Value | 4.25% Exchangeable Senior Notes
Carrying amount and estimated fair value of debt
Fixed-rate debt	575		240,282
Total Estimated Fair Value | Other fixed-rate debt
Carrying amount and estimated fair value of debt
Fixed-rate debt	$812,246		$968,180
[1]	As described further in our 2012 Annual Report on Form 10-K, these notes have an exchange settlement feature that provides that the notes may, under certain circumstances, be exchangeable for cash and, at COPLP’s discretion, COPT common shares at an exchange rate (subject to adjustment) of 20.8513 shares per one thousand dollar principal amount of the notes (exchange rate is as of September 30, 2013 and is equivalent to an exchange price of $47.96 per common share). During the nine months ended September 30, 2013, we repaid $239.4 million principal amount of these notes and recognized a $25.9 million loss on early extinguishment of debt. The carrying value of these notes included a principal amount of $575,000 and an unamortized discount totaling $15,000 at September 30, 2013 and a principal amount of $240.0 million and an unamortized discount totaling $9.1 million at December 31, 2012. The effective interest rate under the notes, including amortization of the issuance costs, was 6.05%. Because the closing price of our common shares at September 30, 2013 and December 31, 2012 was less than the exchange price per common share applicable to these notes, the if-converted value of the notes did not exceed the principal amount. The table below sets forth interest expense recognized on these notes before deductions for amounts capitalized (in thousands): For the Three Months Ended September 30, For the Nine Months Ended September 30, 2013 2012 2013 2012Interest expense at stated interest rate$6 $2,550 $4,201 $7,650Interest expense associated with amortization of discount2 919 1,613 2,717Total$8 $3,469 $5,814 $10,367

Interest_Rate_Derivatives_Deta

Interest Rate Derivatives (Details) (USD $)	3 Months Ended			9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012	Sep. 30, 2013		Sep. 30, 2012	Dec. 31, 2012
Fair values of interest rate swap derivatives
Fair value of interest rate swaps	$1,600,000			$1,600,000			($6,185,000)
Fair value of interest rate derivatives and balance sheet classification
Interest rate derivatives	5,195,000			5,195,000			0
Effect of interest rate derivatives on consolidated statements of operations and comprehensive income
Amount of (loss) gain recognized in accumulated other comprehensive income (loss) (“AOCI”) (effective portion)	-2,482,000		-2,760,000	5,810,000		-7,386,000
Amount of loss reclassified from AOCI into interest expense (effective portion)	689,000		632,000	2,021,000		3,034,000
Interest rate swaps
Fair value of interest rate derivatives and balance sheet classification
Interest rate derivatives	5,195,000			5,195,000			0
Fair value of Interest rate swaps classified as interest rate derivatives	-3,595,000			-3,595,000			-6,185,000
Effect of interest rate derivatives on consolidated statements of operations and comprehensive income
Approximate amount to be reclassified from AOCI to interest expense over the next 12 months	2,600,000			2,600,000
Interest rate derivatives in liability position, fair value	3,600,000			3,600,000
Termination value to settle obligations under interest rate derivative agreements	3,800,000			3,800,000
Designated | Interest rate swap, effective date January 3, 2012, swap one
Fair values of interest rate swap derivatives
Notional Amount	100,000,000			100,000,000
Fixed rate (as a percent)	0.61%			0.61%
Floating rate index				One-Month LIBOR
Fair value of interest rate swaps	-365,000			-365,000			-594,000
Designated | Interest rate swap, effective date January 3, 2012, swap two
Fair values of interest rate swap derivatives
Notional Amount	100,000,000			100,000,000
Fixed rate (as a percent)	0.61%			0.61%
Floating rate index				One-Month LIBOR
Fair value of interest rate swaps	-362,000			-362,000			-591,000
Designated | Interest rate swap, effective date January 3, 2012, swap three
Fair values of interest rate swap derivatives
Notional Amount	100,000,000			100,000,000
Fixed rate (as a percent)	0.83%			0.83%
Floating rate index				One-Month LIBOR
Fair value of interest rate swaps	-963,000			-963,000			-1,313,000
Designated | Interest rate swap, effective date January 3, 2012, swap four
Fair values of interest rate swap derivatives
Notional Amount	100,000,000			100,000,000
Fixed rate (as a percent)	0.83%			0.83%
Floating rate index				One-Month LIBOR
Fair value of interest rate swaps	-963,000			-963,000			-1,313,000
Designated | Interest rate swap, effective date November 2, 2010
Fair values of interest rate swap derivatives
Notional Amount	37,894,000	[1]		37,894,000	[1]
Fixed rate (as a percent)	3.83%			3.83%
Floating rate index				One-Month LIBOR + 2.25%
Derivative, Basis Spread on Variable Rate	2.25%			2.25%
Fair value of interest rate swaps	-933,000			-933,000			-1,268,000
Notional amount of interest rate derivatives after scheduled amortization				36,200,000
Designated | Interest rate swap, effective date September 2, 2014, swap one
Fair values of interest rate swap derivatives
Notional Amount	100,000,000			100,000,000
Fixed rate (as a percent)	0.81%			0.81%
Floating rate index				One-Month LIBOR
Fair value of interest rate swaps	1,000			1,000			-263,000
Designated | Interest rate swap, effective date September 2, 2014, swap two
Fair values of interest rate swap derivatives
Notional Amount	100,000,000			100,000,000
Fixed rate (as a percent)	0.81%			0.81%
Floating rate index				One-Month LIBOR
Fair value of interest rate swaps	-9,000			-9,000			-272,000
Designated | Interest rate swap, effective date September 1, 2015, swap one
Fair values of interest rate swap derivatives
Notional Amount	100,000,000			100,000,000
Fixed rate (as a percent)	1.67%			1.67%
Floating rate index				One-Month LIBOR
Fair value of interest rate swaps	2,685,000			2,685,000			-154,000
Designated | Interest rate swap, effective date September 1, 2015, swap two
Fair values of interest rate swap derivatives
Notional Amount	100,000,000			100,000,000
Fixed rate (as a percent)	1.73%			1.73%
Floating rate index				One-Month LIBOR
Fair value of interest rate swaps	$2,509,000			$2,509,000			($417,000)
[1]	The notional amount of this instrument is scheduled to amortize to $36.2 million.

Redeemable_Noncontrolling_Inte2

Redeemable Noncontrolling Interests (Details) (USD $)	9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
Redeemable Noncontrolling Interest [Roll Forward]
Beginning balance	$10,298	$8,908
Distribution to noncontrolling interest	-811	0
Net income (loss) attributable to noncontrolling interest	888	-1,803
Adjustment to arrive at fair value of interest	6,414	2,827
Ending balance	$16,789	$9,932

Equity_Details

Equity (Details) (USD $)	0 Months Ended	9 Months Ended		0 Months Ended		9 Months Ended		0 Months Ended
	Mar. 19, 2013	Sep. 30, 2013	Sep. 30, 2012	Apr. 22, 2013	Mar. 19, 2013	Sep. 30, 2013	Sep. 30, 2012	Jul. 16, 2013	Jul. 16, 2013
				Series J Preferred Stock	Corporate Office Properties, L.P.	Corporate Office Properties, L.P.	Corporate Office Properties, L.P.	Common shares issued to public under at-the-market program	Common shares issued to public under at-the-market program
									Corporate Office Properties, L.P.
Class of Stock [Line Items]
Dividend rate				7.63%
Redemption price per share				$25
Stock redeemed or called during period, value		$84,750,000	$55,000,000	$84,800,000		$84,750,000	$55,000,000
Issuance costs associated with redeemed preferred shares				2,900,000
Shares issued to the public	4,485,000				4,485,000			1,500,000	1,500,000
Shares issued to the public (in dollars per share)	$26.34
Shares issued to the public (in dollars)	118,100,000	117,961,000	165,662,000			117,961,000	165,662,000	38,500,000
Issuance of stock, weighted average price per share								$26.05
Payments of Stock Issuance Costs								586,000
At Market Stock, Offering Program Established, Remaining Capacity (in dollars)								$110,900,000
Number of operating partnerships units converted into common shares (in units)		310,889	94,550
Number of common shares for each converted common unit (in shares)		1

Information_by_Business_Segmen2

Information by Business Segment (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
	segment		segment
Segment Reporting [Abstract]
Number of primary office property segments	10		10
Segment financial information for real estate operations
Revenues from real estate operations	$124,561	$121,264	$371,913	$370,536
Property operating expenses	44,885	43,699	133,605	134,293
NOI from real estate operations	79,676	77,565	238,308	236,243
Additions to long-lived assets	13,942	61,803
Transfers from non-operating properties	42,842	28,146
Segment assets	3,755,588	3,597,656	3,755,588	3,597,656	3,653,759
Operating Segment Total
Segment financial information for real estate operations
Revenues from real estate operations			371,913	370,536
Property operating expenses			133,605	134,293
NOI from real estate operations			238,308	236,243
Additions to long-lived assets			31,578	76,107
Transfers from non-operating properties			197,600	129,348
Segment assets	3,053,813	2,836,547	3,053,813	2,836,547
Baltimore and Washington Corridor
Segment financial information for real estate operations
Revenues from real estate operations	57,226	55,799	171,049	167,726
Property operating expenses	18,780	18,534	57,030	56,677
NOI from real estate operations	38,446	37,265	114,019	111,049
Additions to long-lived assets	7,204	4,454	14,029	9,393
Transfers from non-operating properties	8,706	12,431	37,270	42,488
Segment assets	1,223,246	1,187,700	1,223,246	1,187,700
Northern Virginia
Segment financial information for real estate operations
Revenues from real estate operations	23,062	20,363	68,992	57,974
Property operating expenses	7,999	7,115	24,020	21,270
NOI from real estate operations	15,063	13,248	44,972	36,704
Additions to long-lived assets	2,459	54,781	6,699	57,161
Transfers from non-operating properties	26,355	12,979	37,209	12,979
Segment assets	593,335	535,521	593,335	535,521
San Antonio
Segment financial information for real estate operations
Revenues from real estate operations	7,898	8,125	24,019	23,563
Property operating expenses	4,061	4,272	12,427	11,998
NOI from real estate operations	3,837	3,853	11,592	11,565
Additions to long-lived assets	251	0	268	259
Transfers from non-operating properties	0	0	0	464
Segment assets	118,701	119,752	118,701	119,752
Washington, DC - Capitol Riverfront
Segment financial information for real estate operations
Revenues from real estate operations	4,295	4,389	12,716	12,515
Property operating expenses	1,946	1,924	5,769	5,485
NOI from real estate operations	2,349	2,465	6,947	7,030
Additions to long-lived assets	86	414	484	116
Transfers from non-operating properties	0	0	0	0
Segment assets	99,852	106,090	99,852	106,090
St. Mary's and King George Counties
Segment financial information for real estate operations
Revenues from real estate operations	4,270	4,085	12,355	12,436
Property operating expenses	1,299	1,241	3,715	3,524
NOI from real estate operations	2,971	2,844	8,640	8,912
Additions to long-lived assets	630	355	1,634	815
Transfers from non-operating properties	0	24	12	218
Segment assets	96,679	97,977	96,679	97,977
Greater Baltimore
Segment financial information for real estate operations
Revenues from real estate operations	10,703	11,918	32,246	41,954
Property operating expenses	4,143	4,539	12,408	15,911
NOI from real estate operations	6,560	7,379	19,838	26,043
Additions to long-lived assets	1,707	1,055	1,761	5,390
Transfers from non-operating properties	9	99	113	493
Segment assets	305,947	283,996	305,947	283,996
Suburban Maryland
Segment financial information for real estate operations
Revenues from real estate operations	2,332	2,371	6,809	12,680
Property operating expenses	862	1,041	2,393	5,357
NOI from real estate operations	1,470	1,330	4,416	7,323
Additions to long-lived assets	88	36	143	1,219
Transfers from non-operating properties	-183	-88	337	793
Segment assets	52,726	53,395	52,726	53,395
Colorado Springs
Segment financial information for real estate operations
Revenues from real estate operations	6,622	6,278	19,874	18,880
Property operating expenses	2,371	2,432	7,014	6,754
NOI from real estate operations	4,251	3,846	12,860	12,126
Additions to long-lived assets	1,413	548	2,323	1,334
Transfers from non-operating properties	1,860	315	4,540	2,611
Segment assets	159,578	173,918	159,578	173,918
Greater Philadelphia
Segment financial information for real estate operations
Revenues from real estate operations	3,258	2,541	8,529	7,171
Property operating expenses	924	663	2,482	1,851
NOI from real estate operations	2,334	1,878	6,047	5,320
Additions to long-lived assets	102	195	387	285
Transfers from non-operating properties	3,414	1,843	28,031	11,233
Segment assets	105,112	66,301	105,112	66,301
Other
Segment financial information for real estate operations
Revenues from real estate operations	3,819	3,589	10,878	10,977
Property operating expenses	654	686	1,667	2,088
NOI from real estate operations	3,165	2,903	9,211	8,889
Additions to long-lived assets	-84	-35	3,588	124
Transfers from non-operating properties	-33	548	20,925	394
Segment assets	130,598	111,426	130,598	111,426
Wholesale Data Center
Segment financial information for real estate operations
Revenues from real estate operations	1,076	1,806	4,446	4,660
Property operating expenses	1,846	1,252	4,680	3,378
NOI from real estate operations	-770	554	-234	1,282
Additions to long-lived assets	86	0	262	11
Transfers from non-operating properties	2,714	-5	69,163	57,675
Segment assets	$168,039	$100,471	$168,039	$100,471

Information_by_Business_Segmen3

Information by Business Segment (Details 2) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Reconciliation of segment revenues to total revenues
Segment revenues from real estate operations	$124,561	$121,264	$371,913	$370,536
Construction contract and other service revenues	16,991	15,283	52,048	53,812
Less: Revenues from discontinued operations (Note 15)	-5,521	-6,902	-16,786	-35,305
Total revenues	136,031	129,645	407,175	389,043
Reconciliation of segment property operating expenses to property operating expenses
Segment property operating expenses	44,885	43,699	133,605	134,293
Less: Property operating expenses from discontinued operations (Note 15)	-1,403	-2,225	-4,196	-12,191
Property operating expenses	43,482	41,474	129,409	122,102
Computation of net operating income from service operations
Construction contract and other service revenues	16,991	15,283	52,048	53,812
Construction contract and other service expenses	-16,306	-14,410	-49,165	-51,302
NOI from service operations	685	873	2,883	2,510
Reconciliation of NOI from real estate operations and NOI from service operations to (loss) income from continuing operations
NOI from real estate operations	79,676	77,565	238,308	236,243
NOI from service operations	685	873	2,883	2,510
Interest and other (loss) income	-3	1,095	2,949	3,152
Equity in income (loss) of unconsolidated entities	44	-246	211	-522
Income tax expense	-24	-106	-61	-327
Other adjustments:
Depreciation and other amortization associated with real estate operations	-29,210	-28,604	-86,239	-84,633
Impairment losses	-16,300	-46,096	-16,300	-41,260
General, administrative and leasing expenses	-8,027	-6,377	-22,430	-24,797
Business development expenses and land carry costs	-1,383	-1,632	-4,069	-4,506
Interest expense on continuing operations	-21,242	-23,239	-66,851	-71,909
NOI from discontinued operations	-4,118	-4,677	-12,590	-23,114
Loss on early extinguishment of debt	-374	-768	-27,028	-937
(Loss) income from continuing operations	($276)	($32,212)	$8,783	($10,100)

Information_by_Business_Segmen4

Information by Business Segment (Details 3) (USD $)	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
In Thousands, unless otherwise specified
Reconciliation of segment assets to total assets
Assets	$3,755,588	$3,653,759	$3,597,656
Segment assets
Reconciliation of segment assets to total assets
Assets	3,053,813		2,836,547
Non-operating property assets
Reconciliation of segment assets to total assets
Assets	530,389		616,526
Other assets
Reconciliation of segment assets to total assets
Assets	$171,386		$144,583

ShareBased_Compensation_Detail

Share-Based Compensation (Details) (USD $)	9 Months Ended		0 Months Ended	9 Months Ended			0 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Mar. 01, 2013	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	Mar. 01, 2013
			Performance share units	Performance share units	Restricted shares	Options	2013 PSU Grants
				Percentile_Rank			Performance share units
Potential earned PSUs payout for defined levels of performance under awards
Earned PSUs payout (as a percent of PSUs granted) on 75th or greater percentile rank				200.00%
Earned PSUs payout (as a percent of PSUs granted) on 50th percentile rank				100.00%
Earned PSUs payout (as a percent of PSUs granted) on 25th percentile rank				50.00%
Performance share units granted on percentile rank below 25th (as a percent)				0.00%
The number of percentile ranks to fall between to earn interpolated PSUs between such percentile ranks, conditioned on the percentile rank exceeding 25%				2
Performance period of the award (in years)				3 years
Assumptions used to value stock awards
Baseline value per common share (in dollars per share)				$25.85
Expected volatility of common shares (as a percent)				29.50%
Risk-free interest rate (as a percent)				0.33%
Shares
Stock awards granted (in shares or units)					177,510		69,579
Weighted Average Grant Date Fair Value
Aggregate grant date fair value			$1,900,000		$4,700,000
Weighted average grant date fair value (in dollars per share)				$26.84	$26.24
Exercised (in shares)	32,756	44,624				32,756
Weighted average exercise price (in dollars per share)						$19.45
Aggregate intrinsic value of options exercised (in dollars)						237,000
Other Share-based Compensation Additional Disclosures
Shares vested (in shares)					222,381
Weighted average fair value of shares vested (in dollars per share)					$31.54
Aggregate intrinsic value of restricted common shares, forfeiture restrictions lapsed					$5,900,000

Income_Taxes_Details

Income Taxes (Details) (Taxable REIT Subsidiary [Member], USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Taxable REIT Subsidiary [Member]
Deferred income tax (expense) benefit
Federal	($20)	($87)	($50)	($267)
State	-4	-19	-11	-60
Total income tax expense	($24)	($106)	($61)	($327)
Effective tax rate (as a percent)	36.70%	38.60%	36.70%	38.60%

Discontinued_Operations_and_As2

Discontinued Operations and Assets Held for Sale (Details) (USD $)	3 Months Ended		9 Months Ended					0 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013	Dec. 31, 2012	Jan. 30, 2012	Jul. 24, 2012	Sep. 30, 2013
						Assets Held-for-sale	Assets Held-for-sale	White Marsh Portfolio Disposition Property	July 2012 Portfolio Disposition	Colorado Springs
								Property	Property	Assets Held-for-sale
										Property
Properties
Number of properties sold								5	23
Number of operating properties classified as held for sale										16
Revenue from real estate operations	$5,521	$6,902	$16,786	$35,305
Property operating expenses	-1,403	-2,225	-4,196	-12,191
Depreciation and amortization	0	-2,020	-158	-8,744
Impairment losses	-5,774	-9,733	-14,826	-23,510
General, administrative and leasing expenses	0	-1	-1	-3
Business development and land carry costs	0	0	0	-24
Interest expense	-68	-127	-199	-2,107
Gain on sales of real estate	0	16,913	0	20,948
Gain on early extinguishment of debt	0	1,738	0	1,736
Discontinued operations	-1,724	11,447	-2,594	11,410
Properties, net	3,239,746		3,239,746		3,163,044	121,060	128,740
Deferred rent receivable	90,104		90,104		85,802	4,951	4,068
Intangible assets on real estate acquisitions, net	64,372		64,372		75,879	4,384	4,409
Deferred leasing, net						3,511	2,923
Lease incentives	8,751		8,751		5,578	78	89
Assets held for sale, net	$133,984		$133,984		$140,229	$133,984	$140,229

Earnings_Per_Share_EPS_and_Ear2

Earnings Per Share ("EPS") and Earnings Per Unit ("EPU") (Details) (USD $)	3 Months Ended				9 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Numerator:
(Loss) income from continuing operations	($276)		($32,212)		$8,783		($10,100)
Gain on sales of real estate, net	0		0		2,683		21
Preferred share/unit dividends/distributions	-4,490		-6,546		-15,481		-14,738
Issuance costs associated with redeemed preferred shares/units	0		-1,827		-2,904		-1,827
(Loss) income from continuing operations attributable to noncontrolling interests	-990		2,249		-2,139		1,710
Income from continuing operations attributable to restricted shares	-97		-111		-317		-357
Numerator for basic and diluted EPS from continuing operations attributable to COPT common shareholders	-5,853		-38,447		-9,375		-25,291
Discontinued operations	-1,724		11,447		-2,594		11,410
Discontinued operations attributable to noncontrolling interests	26		-646		-42		-603
Numerator for basic and diluted EPS on net (loss) income attributable to COPT common shareholders	-7,551		-27,646		-12,011		-14,484
Denominator (all weighted averages):
Denominator for basic and diluted EPS (in shares/units)	86,760		71,688		84,547		71,590
Basic EPS:
Loss from continuing operations	($0.07)	[1]	($0.54)	[1]	($0.11)	[1]	($0.35)	[1]
Discontinued operations	($0.02)	[1]	$0.15	[1]	($0.03)	[1]	$0.15	[1]
Net income attributable to COPT common shareholders (in dollars per share/unit)	($0.09)	[1]	($0.39)	[1]	($0.14)	[1]	($0.20)	[1]
Diluted EPS:
Loss from continuing operations	($0.07)	[1]	($0.54)	[1]	($0.11)	[1]	($0.35)	[1]
Discontinued operations	($0.02)	[1]	$0.15	[1]	($0.03)	[1]	$0.15	[1]
Net income attributable to COPT common shareholders (in dollars per share/unit)	($0.09)	[1]	($0.39)	[1]	($0.14)	[1]	($0.20)	[1]
Corporate Office Properties, L.P.
Numerator:
(Loss) income from continuing operations	-276		-32,212		8,783		-10,100
Gain on sales of real estate, net	0		0		2,683		21
Preferred share/unit dividends/distributions	-4,655		-6,711		-15,976		-15,233
Issuance costs associated with redeemed preferred shares/units	0		-1,827		-2,904		-1,827
(Loss) income from continuing operations attributable to noncontrolling interests	-986		243		-2,025		829
Income from continuing operations attributable to restricted shares	-97		-111		-317		-357
Numerator for basic and diluted EPU from continuing operations attributable to COPLP common unitholders	-6,014		-40,618		-9,756		-26,667
Discontinued operations	-1,724		11,447		-2,594		11,410
Discontinued operations attributable to noncontrolling interests	-49		-647		-147		-662
Numerator for basic and diluted EPU on net income attributable to COPLP common unitholders	($7,787)		($29,818)		($12,497)		($15,919)
Denominator (all weighted averages):
Denominator for basic and diluted EPS (in shares/units)	90,564		75,921		88,379		75,846
Basic EPS:
Loss from continuing operations	($0.07)	[2]	($0.54)	[2]	($0.11)	[2]	($0.35)	[2]
Discontinued operations	($0.02)	[2]	$0.15	[2]	($0.03)	[2]	$0.14	[2]
Net income attributable to COPT common shareholders (in dollars per share/unit)	($0.09)	[2]	($0.39)	[2]	($0.14)	[2]	($0.21)	[2]
Diluted EPS:
Loss from continuing operations	($0.07)	[2]	($0.54)	[2]	($0.11)	[2]	($0.35)	[2]
Discontinued operations	($0.02)	[2]	$0.15	[2]	($0.03)	[2]	$0.14	[2]
Net income attributable to COPT common shareholders (in dollars per share/unit)	($0.09)	[2]	($0.39)	[2]	($0.14)	[2]	($0.21)	[2]
[1]	Basic and diluted earnings per common share are calculated based on amounts attributable to common shareholders of Corporate Office Properties Trust.
[2]	Basic and diluted earnings per common unit are calculated based on amounts attributable to common unitholders of Corporate Office Properties, L.P.

Earnings_Per_Share_EPS_Details

Earnings Per Share ("EPS") (Details 2)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Conversion of common units
Antidilutive securities
Antidilutive securities excluded from computation of diluted earnings per share (in shares)	3,804	4,233	3,832	4,256
Conversion of Series I Preferred Units
Antidilutive securities
Antidilutive securities excluded from computation of diluted earnings per share (in shares)	176	176	176	176
Conversion of Series K Preferred Shares
Antidilutive securities
Antidilutive securities excluded from computation of diluted earnings per share (in shares)	434	434	434	434
Weighted average restricted shares
Antidilutive securities
Antidilutive securities excluded from computation of diluted earnings per share (in shares)	377	420	388	475
Weighted average options
Antidilutive securities
Antidilutive securities excluded from computation of diluted earnings per share (in shares)	613	752	571	780
Corporate Office Properties, L.P. | Weighted average restricted shares
Antidilutive securities
Antidilutive securities excluded from computation of diluted earnings per share (in shares)	377	420	388	475
Corporate Office Properties, L.P. | Weighted average options
Antidilutive securities
Antidilutive securities excluded from computation of diluted earnings per share (in shares)	613	752	571	780
Corporate Office Properties, L.P. | Conversion of Series I Preferred Units
Antidilutive securities
Antidilutive securities excluded from computation of diluted earnings per share (in shares)	176	176	176	176
Corporate Office Properties, L.P. | Conversion of Series K Preferred Units
Antidilutive securities
Antidilutive securities excluded from computation of diluted earnings per share (in shares)	434	434	434	434

Commitments_and_Contingencies_

Commitments and Contingencies (Details) (USD $)	1 Months Ended
	Aug. 31, 2010	Sep. 30, 2013
		Property
Tax incremental financing obligation
The amount of tax incremental financing bonds issued by Anne Arundel County, Maryland	$30,000,000
Liability recognized with regard to tax incremental financing obligation at end of current period		2,500,000
Environmental Indemnity Agreement
Number of lease properties which were provided environmental indemnifications		3
Environmental indemnification to the tenant against losses covered under prior owner's indemnity agreement		5,000,000
Maximum environmental indemnification to the tenant against consequential damages after acquisition of property		12,500,000
Additional costs agreed to be paid by the entity related to construction and environmental regulatory activities (as a percent)		50.00%
Maximum annual additional costs agreed to be paid by the entity related to construction and environmental regulatory activities		300,000
Maximum additional costs agreed to be paid by the entity related to construction and environmental regulatory activities		1,500,000
Nonrecourse loan guarantees of unconsolidated partnership
Joint Ventures
Maximum approximate amount required to be paid under the guarantees for outstanding debt, excluding interest and penalties		$64,000,000
Recovery percentage of amount paid under the guarantee		80.00%