COPT Defense Properties (CDP) 10-Q 28 Oct 14 2014 Q3 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	9 Months Ended
	Sep. 30, 2014	Oct. 16, 2014
Entity Information [Line Items]	'	'
Entity Registrant Name	'CORPORATE OFFICE PROPERTIES TRUST	'
Entity Central Index Key	'0000860546	'
Document Type	'10-Q	'
Document Period End Date	30-Sep-14	'
Amendment Flag	'false	'
Current Fiscal Year End Date	'--12-31	'
Entity Filer Category	'Large Accelerated Filer	'
Entity Common Stock, Shares Outstanding	'	87,711,731
Document Fiscal Year Focus	'2014	'
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	'0001577966	'
Document Type	'10-Q	'
Document Period End Date	30-Sep-14	'
Amendment Flag	'false	'
Current Fiscal Year End Date	'--12-31	'
Entity Filer Category	'Non-accelerated Filer	'
Document Fiscal Year Focus	'2014	'
Document Fiscal Period Focus	'Q3	'

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Sep. 30, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Properties, net:	'	'
Operating properties, net	$2,757,207	$2,702,693
Projects in development or held for future development	513,949	511,608
Total properties, net	3,271,156	3,214,301
Cash and cash equivalents	40,018	54,373
Restricted cash and marketable securities	14,371	11,448
Accounts receivable (net of allowance for doubtful accounts of $750 and $2,976, respectively)	20,180	27,000
Deferred rent receivable (net of allowance of $1,426 and $2,126, respectively)	95,405	89,456
Intangible assets on real estate acquisitions, net	48,300	59,258
Deferred leasing and financing costs, net	65,009	66,267
Mortgage and other investing receivables	50,886	53,663
Prepaid expenses and other assets	74,863	54,186
Total assets	3,680,188	3,629,952
Liabilities:	'	'
Debt, net	2,049,992	1,927,703
Accounts payable and accrued expenses	123,893	98,785
Rents received in advance and security deposits	33,075	31,492
Dividends and distributions payable	28,344	29,080
Deferred revenue associated with operating leases	13,420	10,369
Interest rate derivatives	2,236	3,309
Other liabilities	13,288	14,207
Total liabilities	2,264,248	2,114,945
Commitments and contingencies (Note 17)	'	'
Redeemable noncontrolling interest	18,436	17,758
Corporate Office Properties Trust’s shareholders’ equity:	'	'
Preferred Shares of beneficial interest at liquidation preference ($0.01 par value; 25,000,000 shares authorized; issued and outstanding of 7,431,667 at September 30, 2014 and 9,431,667 at December 31, 2013)	199,083	249,083
Common Shares of beneficial interest	877	874
Additional paid-in capital	1,822,283	1,814,015
Cumulative distributions in excess of net income	-692,978	-641,868
Accumulated other comprehensive income	871	3,480
Total Corporate Office Properties Trust’s shareholders’ equity	1,330,136	1,425,584
Noncontrolling interests in subsidiaries:	'	'
Common units in COPLP	49,781	53,468
Preferred units in COPLP	8,800	8,800
Other consolidated entities	8,787	9,397
Noncontrolling interests in subsidiaries	67,368	71,665
Total equity	1,397,504	1,497,249
Total liabilities, redeemable noncontrolling interest and equity	3,680,188	3,629,952
Corporate Office Properties, L.P.	'	'
Properties, net:	'	'
Operating properties, net	2,757,207	2,702,693
Projects in development or held for future development	513,949	511,608
Total properties, net	3,271,156	3,214,301
Cash and cash equivalents	40,018	54,373
Restricted cash and marketable securities	6,871	3,981
Accounts receivable (net of allowance for doubtful accounts of $750 and $2,976, respectively)	20,180	27,000
Deferred rent receivable (net of allowance of $1,426 and $2,126, respectively)	95,405	89,456
Intangible assets on real estate acquisitions, net	48,300	59,258
Deferred leasing and financing costs, net	65,009	66,267
Mortgage and other investing receivables	50,886	53,663
Prepaid expenses and other assets	74,863	54,186
Total assets	3,672,688	3,622,485
Liabilities:	'	'
Debt, net	2,049,992	1,927,703
Accounts payable and accrued expenses	123,893	98,785
Rents received in advance and security deposits	33,075	31,492
Dividends and distributions payable	28,344	29,080
Deferred revenue associated with operating leases	13,420	10,369
Interest rate derivatives	2,236	3,309
Other liabilities	5,788	6,740
Total liabilities	2,256,748	2,107,478
Commitments and contingencies (Note 17)	'	'
Redeemable noncontrolling interest	18,436	17,758
Corporate Office Properties Trust’s shareholders’ equity:	'	'
Common Shares of beneficial interest	1,179,919	1,226,318
Accumulated other comprehensive income	879	3,605
Total Corporate Office Properties Trust’s shareholders’ equity	1,388,681	1,487,806
Noncontrolling interests in subsidiaries:	'	'
Noncontrolling interests in subsidiaries	8,823	9,443
Total equity	1,397,504	1,497,249
Total liabilities, redeemable noncontrolling interest and equity	3,672,688	3,622,485
General Partner | Corporate Office Properties, L.P.	'	'
Corporate Office Properties Trust’s shareholders’ equity:	'	'
Preferred partners' capital accounts	199,083	249,083
Limited Partner | Corporate Office Properties, L.P.	'	'
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, 2014	Dec. 31, 2013
In Thousands, except Share data, unless otherwise specified
Allowance for doubtful accounts - AR	$750	$2,976
Allowance - Deferred rent receivable	1,426	2,126
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	7,431,667	9,431,667
Preferred Shares of beneficial interest, shares outstanding	7,431,667	9,431,667
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 Stock, Shares, Issued	87,711,731	87,394,512
Common Shares of beneficial interest, shares outstanding	87,711,731	87,394,512
Corporate Office Properties, L.P.	'	'
Allowance for doubtful accounts - AR	750	2,976
Allowance - Deferred rent receivable	$1,426	$2,126
Corporate Office Properties, L.P. | General Partner	'	'
Common Shares of beneficial interest, shares outstanding	87,711,731	87,394,512
Preferred Units, Outstanding	7,431,667	9,431,667
Corporate Office Properties, L.P. | Limited Partner	'	'
Common Shares of beneficial interest, shares outstanding	3,860,551	3,977,700
Preferred Units, Outstanding	352,000	352,000

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, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Revenues	'		'		'		'
Rental revenue	$96,207		$94,641		$288,574		$280,911
Tenant recoveries and other real estate operations revenue	22,069		20,180		70,538		61,599
Construction contract and other service revenues	34,739		16,991		80,390		52,048
Total revenues	153,015		131,812		439,502		394,558
Expenses	'		'		'		'
Property operating expenses	43,056		41,361		136,600		123,082
Depreciation and amortization associated with real estate operations	30,237		28,205		104,728		82,888
Construction contract and other service expenses	33,593		16,306		75,353		49,165
Impairment losses	66		5,857		1,368		5,857
General, administrative and leasing expenses	7,211		8,027		22,882		22,430
Business development expenses and land carry costs	1,430		1,383		4,107		4,069
Total operating expenses	115,593		101,139		345,038		287,491
Operating income	37,422		30,673		94,464		107,067
Interest expense	-24,802		-19,342		-69,107		-60,734
Interest and other income (loss)	1,191		-3		3,775		2,949
Loss on early extinguishment of debt	-176		-374		-446		-27,028
Income from continuing operations before equity in income of unconsolidated entities and income taxes	13,635		10,954		28,686		22,254
Equity in income of unconsolidated entities	193		44		206		211
Income tax expense	-101		-24		-257		-61
Income from continuing operations	13,727		10,974		28,635		22,404
Discontinued operations	191		-12,974		4		-16,215
Income (loss) before gain on sales of real estate	13,918		-2,000		28,639		6,189
Gain on sales of real estate	10,630		0		10,630		2,683
Net income (loss)	24,548		-2,000		39,269		8,872
Net (income) loss attributable to noncontrolling interests:	'		'		'		'
Common units in COPLP	-768		232		-942		474
Preferred units in COPLP	-165		-165		-495		-495
Other consolidated entities	-895		-1,031		-2,481		-2,160
Net income (loss) attributable to COPT	22,720		-2,964		35,351		6,691
Preferred share/unit dividends/distributions	-3,553		-4,490		-12,387		-15,481
Issuance costs associated with redeemed preferred shares	0		0		-1,769		-2,904
Net income (loss) attributable to COPT common shareholders	19,167		-7,454		21,195		-11,694
Net income (loss) attributable to COPT:	'		'		'		'
Income from continuing operations	22,537		9,518		35,342		22,377
Discontinued operations, net	183		-12,482		9		-15,686
Net income (loss) attributable to COPT	22,720		-2,964		35,351		6,691
Basic earnings per common share	'		'		'		'
Income from continuing operations (in dollars per share/unit)	$0.22	[1]	$0.06	[1]	$0.24	[1]	$0.04	[1]
Discontinued operations (in dollars per share/unit)	$0	[1]	($0.15)	[1]	$0	[1]	($0.18)	[1]
Net income (loss) attributable to COPT common shareholders (in dollars per share/unit)	$0.22	[1]	($0.09)	[1]	$0.24	[1]	($0.14)	[1]
Diluted earnings per common share	'		'		'		'
Income from continuing operations (in dollars per share/unit)	$0.22	[1]	$0.06	[1]	$0.24	[1]	$0.04	[1]
Discontinued operations (in dollars per share/unit)	$0	[1]	($0.15)	[1]	$0	[1]	($0.18)	[1]
Net income (loss) attributable to COPT common shareholders (in dollars per share/unit)	$0.22	[1]	($0.09)	[1]	$0.24	[1]	($0.14)	[1]
Dividends declared per common share/unit	$0.28		$0.28		$0.83		$0.83
Corporate Office Properties, L.P.	'		'		'		'
Revenues	'		'		'		'
Rental revenue	96,207		94,641		288,574		280,911
Tenant recoveries and other real estate operations revenue	22,069		20,180		70,538		61,599
Construction contract and other service revenues	34,739		16,991		80,390		52,048
Total revenues	153,015		131,812		439,502		394,558
Expenses	'		'		'		'
Property operating expenses	43,056		41,361		136,600		123,082
Depreciation and amortization associated with real estate operations	30,237		28,205		104,728		82,888
Construction contract and other service expenses	33,593		16,306		75,353		49,165
Impairment losses	66		5,857		1,368		5,857
General, administrative and leasing expenses	7,211		8,027		22,882		22,430
Business development expenses and land carry costs	1,430		1,383		4,107		4,069
Total operating expenses	115,593		101,139		345,038		287,491
Operating income	37,422		30,673		94,464		107,067
Interest expense	-24,802		-19,342		-69,107		-60,734
Interest and other income (loss)	1,191		-3		3,775		2,949
Loss on early extinguishment of debt	-176		-374		-446		-27,028
Income from continuing operations before equity in income of unconsolidated entities and income taxes	13,635		10,954		28,686		22,254
Equity in income of unconsolidated entities	193		44		206		211
Income tax expense	-101		-24		-257		-61
Income from continuing operations	13,727		10,974		28,635		22,404
Discontinued operations	191		-12,974		4		-16,215
Income (loss) before gain on sales of real estate	13,918		-2,000		28,639		6,189
Gain on sales of real estate	10,630		0		10,630		2,683
Net income (loss)	24,548		-2,000		39,269		8,872
Net (income) loss attributable to noncontrolling interests:	'		'		'		'
Net income attributable to noncontrolling interests in consolidated entities	-897		-1,035		-2,471		-2,172
Net income (loss) attributable to COPT	23,651		-3,035		36,798		6,700
Preferred share/unit dividends/distributions	-3,718		-4,655		-12,882		-15,976
Issuance costs associated with redeemed preferred shares	0		0		-1,769		-2,904
Net income (loss) attributable to COPT common shareholders	19,933		-7,690		22,147		-12,180
Net income (loss) attributable to COPT:	'		'		'		'
Income from continuing operations	23,460		9,988		36,789		23,062
Discontinued operations, net	191		-13,023		9		-16,362
Net income (loss) attributable to COPT	$23,651		($3,035)		$36,798		$6,700
Basic earnings per common share	'		'		'		'
Income from continuing operations (in dollars per share/unit)	$0.22	[2]	$0.06	[2]	$0.24	[2]	$0.04	[2]
Discontinued operations (in dollars per share/unit)	$0	[2]	($0.15)	[2]	$0	[2]	($0.18)	[2]
Net income (loss) attributable to COPT common shareholders (in dollars per share/unit)	$0.22	[2]	($0.09)	[2]	$0.24	[2]	($0.14)	[2]
Diluted earnings per common share	'		'		'		'
Income from continuing operations (in dollars per share/unit)	$0.22	[2]	$0.06	[2]	$0.24	[2]	$0.04	[2]
Discontinued operations (in dollars per share/unit)	$0	[2]	($0.15)	[2]	$0	[2]	($0.18)	[2]
Net income (loss) attributable to COPT common shareholders (in dollars per share/unit)	$0.22	[2]	($0.09)	[2]	$0.24	[2]	($0.14)	[2]
Dividends 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, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Net income (loss)	$24,548	($2,000)	$39,269	$8,872
Other comprehensive income (loss)	'	'	'	'
Unrealized gains (losses) on interest rate derivatives	1,015	-2,482	-4,738	5,810
Losses on interest rate derivatives included in interest expense	756	689	2,170	2,021
Equity in other comprehensive income of an equity method investee	0	1,070	0	1,070
Other comprehensive income (loss)	1,771	-723	-2,568	8,901
Comprehensive income (loss)	26,319	-2,723	36,701	17,773
Comprehensive income attributable to noncontrolling interests	-1,968	-948	-3,960	-2,722
Comprehensive income (loss) attributable to COPT	24,351	-3,671	32,741	15,051
Corporate Office Properties, L.P.	'	'	'	'
Net income (loss)	24,548	-2,000	39,269	8,872
Other comprehensive income (loss)	'	'	'	'
Unrealized gains (losses) on interest rate derivatives	1,015	-2,482	-4,738	5,810
Losses on interest rate derivatives included in interest expense	756	689	2,170	2,021
Equity in other comprehensive income of an equity method investee	0	1,070	0	1,070
Other comprehensive income (loss)	1,771	-723	-2,568	8,901
Comprehensive income (loss)	26,319	-2,723	36,701	17,773
Comprehensive income attributable to noncontrolling interests	-964	-1,050	-2,630	-2,339
Comprehensive income (loss) attributable to COPT	$25,355	($3,773)	$34,071	$15,434

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.

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, 2012

$1,436,499,000

$333,833,000

$809,000

$1,653,672,000

($617,455,000)

($5,435,000)

$71,075,000

$1,436,499,000

$1,089,391,000

($5,708,000)

$10,183,000

'

$8,800,000

'

$333,833,000

Balance (in units/ shares) at Dec. 31, 2012

80,952,986

'

'

'

'

'

'

'

85,020,528

'

'

'

'

'

'

Balance (preferred units) at Dec. 31, 2012

'

'

'

'

'

'

'

'

'

'

'

'

352,000

'

12,821,667

Increase (Decrease) in Shareholders' Equity

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Redemption of preferred shares (in units/ shares)

'

'

'

'

'

'

'

'

'

'

'

'

'

'

-3,390,000

Redemption of preferred shares (in dollars)

-84,750,000

-84,750,000

'

2,904,000

-2,904,000

'

'

-84,750,000

'

'

'

'

'

'

-84,750,000

Conversion of common units to common shares (310,889 and 117,149 for the nine months ended September 30, 2013 and 2014, respectively)

0

'

3,000

3,988,000

'

'

-3,991,000

'

'

'

'

'

'

'

'

Shares issued to the public (in dollars)

117,961,000

'

45,000

117,916,000

'

'

'

117,961,000

117,961,000

'

'

'

'

'

'

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)

38,447,000

'

15,000

38,432,000

'

'

'

'

38,447,000

'

'

'

'

'

'

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)

1,369,000

'

'

'

'

'

'

'

3,899,000

'

-2,530,000

'

'

'

'

Acquisition of property and noncontrolling interest in other consolidated entity for COPLP common units

1,369,000

'

'

-1,296,000

'

'

2,665,000

'

'

'

'

'

'

'

'

Shares issued to the public (in units/ shares)

'

'

'

'

'

'

'

'

4,485,000

'

'

'

'

'

'

Exercise of share options (in units/shares)

32,756

'

'

'

'

'

'

'

32,756

'

'

'

'

'

'

Exercise of share options (in dollars)

642,000

'

'

642,000

'

'

'

642,000

642,000

'

'

'

'

'

'

Share-based compensation (in units/shares)

'

'

'

'

'

'

'

'

171,287

'

'

'

'

'

'

Share-based compensation

'

'

2,000

'

'

'

'

'

'

'

'

'

'

'

'

Share-based compensation

'

'

'

5,700,000

'

'

'

'

'

'

'

'

'

'

'

Share-based compensation

5,702,000

'

'

'

'

'

'

5,702,000

5,702,000

'

'

'

'

'

'

Restricted common units/shares redemptions (in units/shares)

-71,523

'

'

'

'

'

'

'

-71,523

'

'

'

'

'

'

Restricted common units/shares redemptions (in dollars)

-1,848,000

'

'

-1,848,000

'

'

'

-1,848,000

-1,848,000

'

'

'

'

'

'

Adjustments to noncontrolling interests resulting from changes in ownership of COPLP

0

'

'

-773,000

'

'

773,000

'

'

'

'

'

'

'

'

Comprehensive income (loss)

16,885,000

'

'

'

6,691,000

8,360,000

1,834,000

16,885,000

-9,276,000

8,734,000

1,451,000

'

495,000

'

15,481,000

Dividends/Distributions

-86,700,000

'

'

'

-86,700,000

'

'

-90,381,000

-74,405,000

'

'

'

-495,000

'

-15,481,000

Distributions to owners of common and preferred units in COPLP

-3,681,000

'

'

'

'

'

-3,681,000

'

'

'

'

'

'

'

'

Contributions from noncontrolling interests in other consolidated entities

85,000

'

'

'

'

'

85,000

85,000

'

'

85,000

'

'

'

'

Distributions to noncontrolling interests in other consolidated entities

-8,000

'

'

'

'

'

-8,000

-8,000

'

'

-8,000

'

'

'

'

Adjustment to arrive at fair value of redeemable noncontrolling interest

-6,414,000

'

'

-6,414,000

'

'

'

-6,414,000

-6,414,000

'

'

'

'

'

'

Tax loss from share-based compensation

-122,000

'

'

-122,000

'

'

'

-122,000

-122,000

'

'

'

'

'

'

Balance at Sep. 30, 2013

1,434,067,000

249,083,000

874,000

1,812,801,000

-700,368,000

2,925,000

68,752,000

1,434,067,000

1,163,977,000

3,026,000

9,181,000

'

8,800,000

'

249,083,000

Balance (in units/ shares) at Sep. 30, 2013

87,381,395

'

'

'

'

'

'

'

91,359,549

'

'

'

'

'

'

Balance (preferred units) at Sep. 30, 2013

'

'

'

'

'

'

'

'

'

'

'

'

352,000

'

9,431,667

Balance at Dec. 31, 2013

1,497,249,000

249,083,000

874,000

1,814,015,000

-641,868,000

3,480,000

71,665,000

1,497,249,000

1,226,318,000

3,605,000

9,443,000

'

8,800,000

'

249,083,000

Balance (in units/ shares) at Dec. 31, 2013

87,394,512

'

'

'

'

'

'

'

91,372,212

'

'

3,977,700

'

87,394,512

'

Balance (preferred units) at Dec. 31, 2013

'

'

'

'

'

'

'

'

'

'

'

352,000

352,000

9,431,667

9,431,667

Increase (Decrease) in Shareholders' Equity

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Redemption of preferred shares (in units/ shares)

'

'

'

'

'

'

'

'

'

'

'

'

'

'

-2,000,000

Redemption of preferred shares (in dollars)

-50,000,000

-50,000,000

'

1,769,000

-1,769,000

'

'

-50,000,000

'

'

'

'

'

'

-50,000,000

Conversion of common units to common shares (310,889 and 117,149 for the nine months ended September 30, 2013 and 2014, respectively)

0

'

1,000

1,544,000

'

'

-1,545,000

'

'

'

'

'

'

'

'

Shares issued to the public (in dollars)

-7,000

'

'

-7,000

'

'

'

-7,000

-7,000

'

'

'

'

'

'

Exercise of share options (in units/shares)

57,888

'

'

'

'

'

'

'

57,888

'

'

'

'

'

'

Exercise of share options (in dollars)

1,359,000

'

'

1,359,000

'

'

'

1,359,000

1,359,000

'

'

'

'

'

'

Share-based compensation (in units/shares)

'

'

'

'

'

'

'

'

193,863

'

'

'

'

'

'

Share-based compensation

'

'

2,000

'

'

'

'

'

'

'

'

'

'

'

'

Share-based compensation

'

'

'

5,247,000

'

'

'

'

'

'

'

'

'

'

'

Share-based compensation

5,249,000

'

'

'

'

'

'

5,249,000

5,249,000

'

'

'

'

'

'

Restricted common units/shares redemptions (in units/shares)

-51,681

'

'

'

'

'

'

'

-51,681

'

'

'

'

'

'

Restricted common units/shares redemptions (in dollars)

-1,389,000

'

'

-1,389,000

'

'

'

-1,389,000

-1,389,000

'

'

'

'

'

'

Adjustments to noncontrolling interests resulting from changes in ownership of COPLP

0

'

'

-248,000

'

'

248,000

'

'

'

'

'

'

'

'

Comprehensive income (loss)

35,055,000

'

'

'

35,351,000

-2,609,000

2,313,000

35,055,000

23,916,000

-2,726,000

983,000

'

495,000

'

12,387,000

Dividends/Distributions

-84,692,000

'

'

'

-84,692,000

'

'

-88,402,000

-75,520,000

'

'

'

-495,000

'

-12,387,000

Distributions to owners of common and preferred units in COPLP

-3,710,000

'

'

'

'

'

-3,710,000

'

'

'

'

'

'

'

'

Contributions from noncontrolling interests in other consolidated entities

3,000

'

'

'

'

'

3,000

3,000

'

'

3,000

'

'

'

'

Distributions to noncontrolling interests in other consolidated entities

-1,606,000

'

'

'

'

'

-1,606,000

-1,606,000

'

'

-1,606,000

'

'

'

'

Adjustment to arrive at fair value of redeemable noncontrolling interest

-7,000

'

'

-7,000

'

'

'

-7,000

-7,000

'

'

'

'

'

'

Balance at Sep. 30, 2014

$1,397,504,000

$199,083,000

$877,000

$1,822,283,000

($692,978,000)

$871,000

$67,368,000

$1,397,504,000

$1,179,919,000

$879,000

$8,823,000

'

$8,800,000

'

$199,083,000

Balance (in units/ shares) at Sep. 30, 2014

87,711,731

'

'

'

'

'

'

'

91,572,282

'

'

3,860,551

'

87,711,731

'

Balance (preferred units) at Sep. 30, 2014

'

'

'

'

'

'

'

'

'

'

'

352,000

352,000

7,431,667

7,431,667

Consolidated_Statements_of_Equ1

Consolidated Statements of Equity (Parenthetical)	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013
Balance (in units/ shares)	87,711,731	87,381,395
Conversion of common units to common shares (in units/ shares)	117,149	310,889
Exercise of share options (in units/shares)	57,888	32,756
Restricted common units/shares redemptions (in units/shares)	51,681	71,523
Common Shares	'	'
Shares issued to the public (in units/ shares)	'	4,485,000
Preferred Shares	'	'
Redemption of preferred shares (in units/ shares)	2,000,000	3,390,000

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (USD $)	9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013
Cash flows from operating activities	'	'
Revenues from real estate operations received	$358,212	$359,431
Construction contract and other service revenues received	62,170	48,877
Property operating expenses paid	-141,489	-133,799
Construction contract and other service expenses paid	-58,218	-53,191
General, administrative, leasing, business development and land carry costs paid	-22,288	-20,681
Interest expense paid	-54,683	-59,052
Previously accreted interest expense paid	0	-11,116
Proceeds from sale of trading marketable securities	0	298
Exit costs on property dispositions	0	-186
Payments in connection with early extinguishment of debt	-104	-23,969
Interest and other income received	448	391
Income taxes refunded	200	6
Net cash provided by operating activities	144,248	107,009
Cash flows from investing activities	'	'
Construction, development and redevelopment	-150,862	-156,820
Tenant improvements on operating properties	-17,754	-15,868
Other capital improvements on operating properties	-21,179	-17,528
Proceeds from dispositions of properties	57,973	12,447
Mortgage and other loan receivables funded	-3,610	-4,111
Mortgage and other loan receivables payments received	10,278	144
Leasing costs paid	-10,549	-9,366
Other	-1,343	4,257
Net cash used in investing activities	-137,046	-186,845
Proceeds from debt	'	'
Revolving Credit Facility	115,000	457,000
Unsecured senior notes	297,342	592,413
Other debt proceeds	11,569	82,886
Repayments of debt	'	'
Revolving Credit Facility	-115,000	-457,000
Scheduled principal amortization	-4,914	-7,229
Other debt repayments	-183,059	-542,532
Deferred financing costs paid	-694	-9,001
Net proceeds from issuance of common shares/units	1,352	157,307
Redemption of preferred shares	-50,000	-84,750
Common share/unit dividends/distributions paid	-72,217	-69,451
Preferred share/unit dividends/distributions paid	-13,179	-16,845
Distributions paid to noncontrolling interests in COPLP	-3,786	-3,705
Restricted share redemptions	-1,389	-1,848
Other	-2,582	-685
Net cash (used in) provided by financing activities	-21,557	96,560
Net (decrease) increase in cash and cash equivalents	-14,355	16,724
Cash and cash equivalents	'	'
Beginning of period	54,373	10,594
End of period	40,018	27,318
Reconciliation of net income to net cash provided by operating activities:	'	'
Net income	39,269	8,872
Adjustments to reconcile net income to net cash provided by operating activities:	'	'
Depreciation and other amortization	106,619	87,956
Impairment losses	1,371	30,940
Settlement of previously accreted interest expense	0	-11,116
Amortization of deferred financing costs	3,646	4,292
Increase in deferred rent receivable	-2,738	-7,995
Amortization of net debt discounts	659	1,207
Gain on sales of real estate	-10,654	-2,683
Share-based compensation	4,563	4,869
Loss on early extinguishment of debt	458	3,059
Other	-2,446	-2,284
Changes in operating assets and liabilities:	'	'
Decrease in accounts receivable	6,815	1,523
(Increase) decrease in restricted cash and marketable securities	-2,591	418
Increase in prepaid expenses and other assets	-26,553	-12,974
Increase (decrease) in accounts payable, accrued expenses and other liabilities	24,247	18
Increase in rents received in advance and security deposits	1,583	907
Net cash provided by operating activities	144,248	107,009
Supplemental schedule of non-cash investing and financing activities:	'	'
Decrease in accrued capital improvements, leasing and other investing activity costs	-174	-13,094
(Decrease) increase in fair value of derivatives applied to accumulated other comprehensive income (loss) and noncontrolling interests	-2,613	7,785
Equity in other comprehensive income of an equity method investee	0	1,070
Dividends and distributions payable	28,344	29,077
COPLP common units issued to acquire property and noncontrolling interest in other consolidated entity	0	5,194
Decrease in noncontrolling interests and increase in shareholders’ equity in connection with the conversion of common units into common shares	1,545	3,991
Adjustments to noncontrolling interests resulting from changes in COPLP ownership	248	773
Increase in redeemable noncontrolling interest and decrease in shareholders’ equity to carry redeemable noncontrolling interest at fair value	7	6,414
Corporate Office Properties, L.P.	'	'
Cash flows from operating activities	'	'
Revenues from real estate operations received	358,212	359,431
Construction contract and other service revenues received	62,170	48,877
Property operating expenses paid	-141,489	-133,799
Construction contract and other service expenses paid	-58,218	-53,191
General, administrative, leasing, business development and land carry costs paid	-22,288	-20,681
Interest expense paid	-54,683	-59,052
Previously accreted interest expense paid	0	-11,116
Proceeds from sale of trading marketable securities	0	298
Exit costs on property dispositions	0	-186
Payments in connection with early extinguishment of debt	-104	-23,969
Interest and other income received	448	391
Income taxes refunded	200	6
Net cash provided by operating activities	144,248	107,009
Cash flows from investing activities	'	'
Construction, development and redevelopment	-150,862	-156,820
Tenant improvements on operating properties	-17,754	-15,868
Other capital improvements on operating properties	-21,179	-17,528
Proceeds from dispositions of properties	57,973	12,447
Mortgage and other loan receivables funded	-3,610	-4,111
Mortgage and other loan receivables payments received	10,278	144
Leasing costs paid	-10,549	-9,366
Other	-1,343	4,257
Net cash used in investing activities	-137,046	-186,845
Proceeds from debt	'	'
Revolving Credit Facility	115,000	457,000
Unsecured senior notes	297,342	592,413
Other debt proceeds	11,569	82,886
Repayments of debt	'	'
Revolving Credit Facility	-115,000	-457,000
Scheduled principal amortization	-4,914	-7,229
Other debt repayments	-183,059	-542,532
Deferred financing costs paid	-694	-9,001
Net proceeds from issuance of common shares/units	1,352	157,307
Redemption of preferred shares	-50,000	-84,750
Common share/unit dividends/distributions paid	-75,508	-72,661
Preferred share/unit dividends/distributions paid	-13,674	-17,340
Restricted share redemptions	-1,389	-1,848
Other	-2,582	-685
Net cash (used in) provided by financing activities	-21,557	96,560
Net (decrease) increase in cash and cash equivalents	-14,355	16,724
Cash and cash equivalents	'	'
Beginning of period	54,373	10,594
End of period	40,018	27,318
Reconciliation of net income to net cash provided by operating activities:	'	'
Net income	39,269	8,872
Adjustments to reconcile net income to net cash provided by operating activities:	'	'
Depreciation and other amortization	106,619	87,956
Impairment losses	1,371	30,940
Settlement of previously accreted interest expense	0	-11,116
Amortization of deferred financing costs	3,646	4,292
Increase in deferred rent receivable	-2,738	-7,995
Amortization of net debt discounts	659	1,207
Gain on sales of real estate	-10,654	-2,683
Share-based compensation	4,563	4,869
Loss on early extinguishment of debt	458	3,059
Other	-2,446	-2,284
Changes in operating assets and liabilities:	'	'
Decrease in accounts receivable	6,815	1,523
(Increase) decrease in restricted cash and marketable securities	-2,558	822
Increase in prepaid expenses and other assets	-26,553	-12,974
Increase (decrease) in accounts payable, accrued expenses and other liabilities	24,214	-386
Increase in rents received in advance and security deposits	1,583	907
Net cash provided by operating activities	144,248	107,009
Supplemental schedule of non-cash investing and financing activities:	'	'
Decrease in accrued capital improvements, leasing and other investing activity costs	-174	-13,094
(Decrease) increase in fair value of derivatives applied to accumulated other comprehensive income (loss) and noncontrolling interests	-2,613	7,785
Equity in other comprehensive income of an equity method investee	0	1,070
Dividends and distributions payable	28,344	29,077
COPLP common units issued to acquire property and noncontrolling interest in other consolidated entity	0	5,194
Increase in redeemable noncontrolling interest and decrease in shareholders’ equity to carry redeemable noncontrolling interest at fair value	$7	$6,414

Organization

Organization	9 Months Ended
	Sep. 30, 2014
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, 2014, our properties included the following:
	•	174 operating office properties totaling 16.9 million square feet (excluding two properties serving as collateral for a nonrecourse mortgage loan which is in default, as discussed further in the Management’s Discussion and Analysis of Financial Condition and Results of Operations section of this Quarterly Report on Form 10-Q);
	•	10 office properties under, or contractually committed for, construction or redevelopment that we estimate will total approximately 1.1 million square feet upon completion;
	•	1,480 acres of land we control that we believe are potentially developable into approximately 18.7 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. Some of these services are performed by a taxable REIT subsidiary (“TRS”).
	Interests in COPLP are in the form of common and preferred units. As of September 30, 2014, COPT owned 95.8% of the outstanding COPLP common units (“common units”) and 95.5% of the outstanding COPLP preferred units (“preferred units”); the remaining common and preferred units in COPLP were owned by third parties. 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”) in COPT 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, 2014
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.
	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, 2013 included in our 2013 Annual Report on Form 10-K.  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.  The consolidated financial statements have been prepared using the accounting policies described in our 2013 Annual Report on Form 10-K.
	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.  These reclassifications occurred in conjunction with the transfer of properties to, and from, discontinued operations during 2013.
	Prior Out of Period Adjustment
	As previously disclosed in our 2013 Annual Report on Form 10-K, during the second quarter of 2013, we identified an error related to the estimated fair value of a redeemable noncontrolling interest in a real estate joint venture. Changes in such fair value are reported as changes in equity with no impact to net income or comprehensive income. The error resulted in an understatement of the line entitled “redeemable noncontrolling interest” in the mezzanine section of our consolidated balance sheet and an overstatement of the line entitled “additional paid-in capital” in the equity section of our consolidation balance sheet of $3.7 million as of December 31, 2012. We have determined that this adjustment was not material to our financial statements for 2012 or 2013. Accordingly, this change is reported as an out-of-period adjustment in the nine months ended September 30, 2013.
	Recent Accounting Pronouncements
	We adopted guidance issued by the Financial Accounting Standards Board (“FASB”) related to the reporting of discontinued operations and disclosures of disposals of components of an entity effective for the quarterly period ended June 30, 2014. This guidance defines a discontinued operation as a component or group of components disposed or classified as held for sale and represents a strategic shift that has (or will have) a major effect on an entity’s operations and financial results; the guidance states that a strategic shift could include a disposal of a major geographical area of operations, a major line of business, a major equity method investment or other major parts of an entity. The guidance also provides for additional disclosure requirements in connection with both discontinued operations and other dispositions not qualifying as discontinued operations. The guidance applies prospectively to new disposals and new classifications of disposal groups as held for sale after the effective date. Our adoption of the guidance will result in fewer disposed or held for sale properties being reported as discontinued operations in our results of operations (including operating properties sold during the current period) but will not otherwise materially affect our consolidated financial statements.
	In May 2014, the FASB issued guidance regarding the recognition of revenue from contracts with customers. Under this guidance, an entity will recognize revenue to depict the transfer of goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. Additionally, this guidance requires improved disclosures regarding the nature, amount, timing and uncertainty of revenue and cash flows arising from contracts with customers. We are required to adopt this guidance for our annual and interim periods beginning January 1, 2017, utilizing one of two methods: retrospective restatement for each reporting period presented at time of adoption, or retrospectively with the cumulative effect of initially applying this guidance recognized at the date of initial application. We are currently assessing the financial impact of this guidance on our consolidated financial statements.

Fair_Value_Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2014
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 our 2013 Annual Report on Form 10-K.
	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, 2014, we used a discount rate of 15.5%. The discount rate 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, 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 and other investing receivables, 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, 2014 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		$	7,187	$	—		$	—	$	7,187
	Common stocks		110		—			—		110
	Other		203		—			—		203
	Interest rate derivatives (2)		—		2,907			—		2,907
	Warrants to purchase common stock (2)		—		192			—		192
	Total Assets		$	7,500	$	3,099		$	—	$	10,599
	Liabilities:
	Deferred compensation plan liability (3)		$	—	$	7,500		$	—	$	7,500
	Interest rate derivatives		—		2,236			—		2,236
	Total Liabilities		$	—	$	9,736		$	—	$	9,736
	Redeemable noncontrolling interest		$	—	$	—		$	18,436	$	18,436
	(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, 2014 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:
	Interest rate derivatives (1)		$	—	$	2,907		$	—	$	2,907
	Warrants to purchase common stock (1)		—		192			—		192
	Total Assets		$	—	$	3,099		$	—	$	3,099
	Liabilities:
	Interest rate derivatives		$	—	$	2,236		$	—	$	2,236
	Redeemable noncontrolling interest		$	—	$	—		$	18,436	$	18,436
	(1) Included in the line entitled “prepaid expenses and other assets” on COPLP’s consolidated balance sheet.
	Nonrecurring Fair Value Measurements
	During the nine months ended September 30, 2014, we recognized impairment losses totaling $1.4 million primarily in connection with certain of our operating properties disposed in the current period.
	During the nine months ended September 30, 2013, we recognized the following impairment losses:
	•	for certain of our operating properties that served as collateral for a nonrecourse loan, we expected that the cash flows to be generated by the properties would be insufficient to fund debt service requirements on the loan. On December 23, 2013, we conveyed the properties to the lender to extinguish the loan. We recognized non-cash impairment losses of $10.4 million (all classified as discontinued operations) on these properties during the three months ended September 30, 2013 resulting primarily from the carrying amount of certain of these properties located in Colorado Springs, Colorado exceeding their fair value;
	•	$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, mostly in Colorado Springs; and
	•	$5.9 million during the three months ended September 30, 2013 on two properties in the Greater Baltimore region that Management concluded no longer met our strategic investment criteria. After shortening our expected holding period for these properties during the period, we determined that the carrying amount of the properties would 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 values of the properties (dollars in thousands):
			Fair Value of Properties Held as of September 30, 2013
			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		September 30, 2013		September 30, 2013
	Assets (2):
	Properties, net		$	—	$	—		$	142,702	$	142,702	$	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 as of September 30, 2013 (dollars in thousands):
	Valuation Technique		Fair Value on		Unobservable Input		Range (Weighted Average)
			Measurement Date
	Contract of sale		$	110,499	Contract price (1)		-1
	Discounted cash flow		$	32,203	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 the properties.
(2) Only one value applied for this unobservable input.

Properties_net

Properties, net	9 Months Ended
	Sep. 30, 2014
Real Estate [Abstract]	'
Properties, net	'
	Properties, net
	Operating properties, net consisted of the following (in thousands):
		September 30,				December 31,
		2014				2013
	Land	$	439,355			$	430,472
	Buildings and improvements	2,997,450				2,869,870
	Less: accumulated depreciation	(679,598		)		(597,649		)
	Operating properties, net	$	2,757,207			$	2,702,693
	During the nine months ended September 30, 2014, we recognized $12.9 million in additional depreciation expense resulting from our revision of the useful life of a property in Greater Philadelphia that was removed from service for redevelopment.
	Projects in development or held for future development consisted of the following (in thousands):
		September 30,				December 31,
		2014				2013
	Land	$	229,286			$	245,676
	Construction in progress, excluding land	284,663				265,932
	Projects in development or held for future development	$	513,949			$	511,608
	2014 Dispositions
	During the nine months ended September 30, 2014, we completed dispositions of the following operating properties (dollars in thousands):
	Project Name		Location		Date of Sale		Number of Buildings		Total Rentable Square Feet	Transaction Value		Gain on Disposition
	4969 and 4979 Mercantile Road		White Marsh, MD		7/14/14		2		96,721	$	5,960	$	2,126
	9930 and 9940 Franklin Square		White Marsh, MD		7/30/14		2		71,992	10,475		2,303
	5020, 5022, 5024 and 5026 Campbell Boulevard		White Marsh, MD		8/4/14		4		134,245	12,400		666
							8		302,958	$	28,835	$	5,095
	We also sold land in the third quarter of 2014 for $28.3 million and recognized a $5.5 million gain on the sales.
	2014 Construction Activities
	During the nine months ended September 30, 2014, we placed into service an aggregate of 692,000 square feet in five newly constructed office properties located in the Baltimore/Washington Corridor, Northern Virginia and Huntsville, Alabama. As of September 30, 2014, we had six office properties under construction, or for which we were contractually committed to construct, that we estimate will total 861,000 square feet upon completion, including two in Northern Virginia, two in the Baltimore/Washington Corridor, one in San Antonio and one in Huntsville. We also had four office properties under redevelopment that we estimate will total 276,000 square feet upon completion, including two in the Baltimore/Washington Corridor, one in Greater Philadelphia and one in St. Mary’s County, Maryland.

Real_Estate_Joint_Ventures

Real Estate Joint Ventures	9 Months Ended
	Sep. 30, 2014
Equity Method Investments and Joint Ventures [Abstract]	'
Real Estate Joint Ventures	'
	Real Estate Joint Ventures
	The table below sets forth information pertaining to our investments in consolidated real estate joint ventures as of September 30, 2014 (dollars in thousands):
				Nominal
				Ownership		September 30, 2014				-1
			Date	% as of		Total		Encumbered		Total
			Acquired	9/30/14	Nature of Activity	Assets		Assets		Liabilities
	LW Redstone Company, LLC		3/23/10	85%	Operates four buildings and developing others (2)	$	139,314	$	67,100	$	42,134
	M Square Associates, LLC		6/26/07	50%	Operates two buildings and developing others (3)	60,115		47,815		39,810
						$	199,429	$	114,915	$	81,944
	-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 Baltimore/Washington Corridor).
	During the periods included herein, we also owned an investment in COPT-FD Indian Head, LLC, a joint venture owning property in Charles County, Maryland. On August 7, 2014, the joint venture’s property was repurchased by Charles County under the terms of a development agreement for $6.4 million, after which the proceeds were distributed to the partners. The joint venture has no remaining business operations or assets.
	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, 2014
Receivables [Abstract]	'
Mortgage and Other Investing Receivables	'
	Mortgage and Other Investing Receivables
	Mortgage and other investing receivables, including accrued interest thereon, consisted of the following (in thousands):
		September 30,		December 31,
		2014		2013
	Notes receivable from the City of Huntsville	$	47,878	$	44,055
	Other investing loans receivable	3,008		9,608
		$	50,886	$	53,663
	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 mortgage and other investing receivables as of September 30, 2014 or December 31, 2013.  The fair value of these receivables approximated their carrying amounts as of September 30, 2014 and December 31, 2013.

Prepaid_Expenses_and_Other_Ass

Prepaid Expenses and Other Assets	9 Months Ended
	Sep. 30, 2014
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,
		2014		2013
	Prepaid expenses	$	24,497	$	19,308
	Construction contract costs incurred in excess of billings	17,962		2,462
	Lease incentives, net	12,939		8,435
	Furniture, fixtures and equipment, net	6,527		6,556
	Deferred tax asset, net (1)	4,058		4,305
	Interest rate derivatives	2,907		6,594
	Other equity method investments	2,240		2,258
	Other assets	3,733		4,268
	Prepaid expenses and other assets	$	74,863	$	54,186
	(1) Includes a valuation allowance of $2.1 million.
	Other assets, as reported above, include operating notes receivable due from tenants with terms exceeding one year totaling $1.7 million as of September 30, 2014 and December 31, 2013; we carried allowances for estimated losses for $207,000 of the September 30, 2014 balance and $87,000 of the December 31, 2013 balance.

Debt

Debt	9 Months Ended
	Sep. 30, 2014
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 as of		as of
		2014		2014		2013		September 30, 2014		September 30, 2014
	Mortgage and Other Secured Loans:
	Fixed rate mortgage loans (1)			$	600,082	$	675,060	3.96% - 10.65% (2)		2014-2024
	Variable rate secured loan			37,087		37,691		LIBOR + 2.25% (3)		Nov-15
	Total mortgage and other secured loans			637,169		712,751
	Revolving Credit Facility (4)	$	800,000	—		—		LIBOR + 0.975% to 1.75%		Jul-17
	Term Loan Facilities	-5		520,000		620,000		LIBOR + 1.10% to 2.60% (6)		2015-2019
	Unsecured Senior Notes
	3.600% Senior Notes (7)			347,432		347,244		3.60%		May-23
	5.250% Senior Notes (8)			245,707		245,445		5.25%		Feb-24
	3.700% Senior Notes (9)			297,483		—		3.70%		Jun-21
	Unsecured notes payable			1,631		1,700		0% (10)		2026
	4.25% Exchangeable Senior Notes (11)			570		563		4.25%		Apr-30
	Total debt, net			$	2,049,992	$	1,927,703
	(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 $49,000 as of September 30, 2014 and $69,000 as of December 31, 2013.
	-2	The weighted average interest rate on these loans was 7.20% as of September 30, 2014.
	(3)	The interest rate on the loan outstanding was 2.41% as of September 30, 2014.
	-4	No borrowings were outstanding on this facility as of the end of the respective periods.
	(5)	We have the ability to borrow an additional $180.0 million in the aggregate under these term loan facilities, provided that there is no default under the facilities and subject to the approval of the lenders.
	(6)	The weighted average interest rate on these loans was 1.80% as of September 30, 2014.
	-7	The carrying value of these notes included a principal amount of $350.0 million and an unamortized discount totaling $2.6 million as of September 30, 2014 and $2.8 million as of December 31, 2013.  The effective interest rate under the notes, including amortization of the issuance costs, was 3.70%.
	-8	The carrying value of these notes included a principal amount of $250.0 million and an unamortized discount totaling $4.3 million as of September 30, 2014 and $4.6 million as of December 31, 2013.  The effective interest rate under the notes, including amortization of the issuance costs, was 5.49%.
	-9	Refer to the paragraph below for disclosure pertaining to these notes.
	(10)	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 $680,000 as of September 30, 2014 and $761,000 as of December 31, 2013.
	(11)	As described further in our 2013 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, 2014 and is equivalent to an exchange price of $47.96 per common share).  The carrying value of these notes included a principal amount of $575,000 and an unamortized discount totaling $5,000 as of September 30, 2014 and $12,000 as of December 31, 2013.  The effective interest rate under the notes, including amortization of the issuance costs, was 6.05%.  Because the closing price of our common shares as of September 30, 2014 and December 31, 2013 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,
		2014		2013		2014		2013
	Interest expense at stated interest rate	$	6	$	6	$	18	$	4,201
	Interest expense associated with amortization of discount	2		2		7		1,613
	Total	$	8	$	8	$	25	$	5,814
	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.
	In April 2014, a wholly owned subsidiary defaulted on the payment terms of a $150.0 million nonrecourse mortgage loan secured by two operating properties in Northern Virginia with an aggregate estimated fair value that was less than the loan balance. This loan has an interest rate of 10.65% (including the effect of default interest) and was originally scheduled to mature in 2017. In July 2014, the lender accelerated the loan’s maturity date to July 2014. Additional disclosure regarding this loan is provided in the Management’s Discussion and Analysis of Financial Condition and Results of Operations section of this Quarterly Report on Form 10-Q.
	On May 14, 2014, we issued a $300.0 million aggregate principal amount of 3.700% Senior Notes at an initial offering price of 99.739% of their face value. The proceeds from the offering, after deducting underwriting discounts, but before other offering expenses, were approximately $297.3 million. The notes mature on June 15, 2021. We may redeem the 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 or (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 25 basis points, plus, in each case, accrued and unpaid interest thereon to the date of redemption. The notes are unconditionally guaranteed by COPT. The carrying value of these notes reflects an unamortized discount totaling $2.5 million at September 30, 2014. The effective interest rate under the notes, including amortization of the issuance costs, was 3.85%.
	We capitalized interest costs of $1.3 million in the three months ended September 30, 2014, $2.2 million in the three months ended September 30, 2013, $4.3 million in the nine months ended September 30, 2014 and $6.7 million in the nine months ended September 30, 2013.
	The following table sets forth information pertaining to the fair value of our debt (in thousands):
		September 30, 2014				December 31, 2013
		Carrying		Estimated		Carrying		Estimated
		Amount		Fair Value		Amount		Fair Value
	Fixed-rate debt
	Unsecured Senior Notes	$	890,622	$	901,022	$	592,689	$	575,374
	4.25% Exchangeable Senior Notes	570		575		563		575
	Other fixed-rate debt	601,713		578,116		676,760		650,997
	Variable-rate debt	557,087		558,665		657,691		657,527
		$	2,049,992	$	2,038,378	$	1,927,703	$	1,884,473

Interest_Rate_Derivatives

Interest Rate Derivatives	9 Months Ended
	Sep. 30, 2014
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,
													2014			2013
	$	100,000			0.83%		One-Month LIBOR		1/3/12		9/1/15		$	(556	)	$	(861	)
	100,000				0.83%		One-Month LIBOR		1/3/12		9/1/15		(556		)	(861		)
	37,087			-1	3.83%		One-Month LIBOR + 2.25%		11/2/10		11/2/15		(515		)	(832		)
	100,000				0.81%		One-Month LIBOR		9/2/14		9/1/16		(300		)	(94		)
	100,000				0.81%		One-Month LIBOR		9/2/14		9/1/16		(309		)	(105		)
	100,000				1.67%		One-Month LIBOR		9/1/15		8/1/19		1,551			3,377
	100,000				1.73%		One-Month LIBOR		9/1/15		8/1/19		1,356			3,217
	100,000				0.61%		One-Month LIBOR		1/3/12		9/1/14		—			(279		)
	100,000				0.61%		One-Month LIBOR		1/3/12		9/1/14		—			(277		)
													$	671		$	3,285
	-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, 2014						31-Dec-13
	Derivatives		Balance Sheet Location		Fair Value				Balance Sheet Location		Fair Value
	Interest rate swaps designated as cash flow hedges		Prepaid expenses and other assets		$	2,907			Prepaid expenses and other assets		$	6,594
	Interest rate swaps designated as cash flow hedges		Interest rate derivatives		(2,236		)		Interest rate derivatives		(3,309		)
	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,
		2014				2013				2014				2013
	Amount of gains (losses) recognized in accumulated other comprehensive income (loss) (“AOCI”) (effective portion)	$	1,015			$	(2,482	)		$	(4,738	)		$	5,810
	Amount of losses reclassified from AOCI into interest expense (effective portion)	756				689				2,170				2,021
	Over the next 12 months, we estimate that approximately $2.9 million of losses will be reclassified from AOCI as an increase 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 defined levels 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, 2014, the fair value of interest rate derivatives in a liability position related to these agreements was $2.2 million, excluding the effects of accrued interest. As of September 30, 2014, 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 $2.5 million.

Redeemable_Noncontrolling_Inte

Redeemable Noncontrolling Interest	9 Months Ended
	Sep. 30, 2014
Noncontrolling Interest [Abstract]	'
Redeemable Noncontrolling Interest	'
	Redeemable Noncontrolling Interest
	The table below sets forth activity in a redeemable noncontrolling interest in a consolidated real estate joint venture (in thousands):
		For the Nine Months Ended September 30,
		2014			2013
	Beginning balance	$	17,758		$	10,298
	Distributions	(976		)	(811		)
	Net income attributable to noncontrolling interest	1,647			888
	Adjustment to arrive at fair value of interest	7			6,414
	Ending balance	$	18,436		$	16,789

Equity

Equity	9 Months Ended
	Sep. 30, 2014
Equity [Abstract]	'
Equity	'
	Equity
	On June 16, 2014, COPT redeemed all of its outstanding 7.5% Series H Preferred Shares of beneficial interest (the “Series H Preferred Shares”) at a price of $25.00 per share, or $50.0 million in the aggregate, plus accrued and unpaid dividends thereon through the date of redemption. Concurrently, COPLP redeemed the Series H Preferred Units previously owned by COPT that carried terms substantially the same as the Series H Preferred Shares. At the time of the redemption, we recognized a $1.8 million decrease to net income available to common shareholders/unitholders pertaining to the original issuance costs incurred on these securities.
	During the nine months ended September 30, 2014, certain COPLP limited partners redeemed 117,149 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, 2014
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; Huntsville; Washington, DC — Capitol Riverfront; St. Mary’s and King George Counties; Greater Baltimore; Greater Philadelphia; Colorado Springs; 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			Huntsville			Washington,		St. Mary’s &		Greater		Greater		Colorado			Other			Operating			Total
		Washington			Virginia			Antonio						DC - Capitol		King George		Baltimore		Philadelphia		Springs						Wholesale
		Corridor												Riverfront		Counties												Data Center
	Three Months Ended September 30, 2014
	Revenues from real estate operations	$	58,883		$	21,369		$	9,031		$	2,471		$	3,524	$	4,158	$	10,436	$	2,951	$	—		$	2,541		$	2,876		$	118,240
	Property operating expenses	19,457			7,500			5,100			763			1,824		1,277		3,810		837		3			257			2,053			42,881
	NOI from real estate operations	$	39,426		$	13,869		$	3,931		$	1,708		$	1,700	$	2,881	$	6,626	$	2,114	$	(3	)	$	2,284		$	823		$	75,359
	Additions to long-lived assets	$	7,248		$	5,898		$	—		$	455		$	458	$	5,189	$	3,021	$	625	$	—		$	(125	)	$	24		$	22,793
	Transfers from non-operating properties	$	22,680		$	15,403		$	—		$	1,496		$	—	$	—	$	495	$	2,506	$	—		$	—		$	222		$	42,802
	Three Months Ended September 30, 2013										—
	Revenues from real estate operations	$	59,558		$	23,062		$	7,898		$	1,360		$	4,295	$	4,270	$	10,703	$	3,258	$	6,571		$	2,510		$	1,076		$	124,561
	Property operating expenses	19,642			7,999			4,061			382			1,946		1,299		4,143		924		2,361			282			1,846			44,885
	NOI from real estate operations	$	39,916		$	15,063		$	3,837		$	978		$	2,349	$	2,971	$	6,560	$	2,334	$	4,210		$	2,228		$	(770	)	$	79,676
	Additions to long-lived assets	$	7,292		$	2,459		$	251		$	4		$	86	$	630	$	1,707	$	102	$	1,413		$	(88	)	$	86		$	13,942
	Transfers from non-operating properties	$	8,523		$	26,355		$	—		$	(33	)	$	—	$	—	$	9	$	3,414	$	1,860		$	—		$	2,714		$	42,842
	Nine Months Ended September 30, 2014
	Revenues from real estate operations	$	177,452		$	67,235		$	26,268		$	7,430		$	10,989	$	12,676	$	32,956	$	8,657	$	6		$	7,662		$	7,769		$	359,100
	Property operating expenses	62,402			24,124			14,391			2,275			5,343		4,070		13,786		3,281		5			1,190			5,622			136,489
	NOI from real estate operations	$	115,050		$	43,111		$	11,877		$	5,155		$	5,646	$	8,606	$	19,170	$	5,376	$	1		$	6,472		$	2,147		$	222,611
	Additions to long-lived assets	$	19,278		$	14,198		$	(6	)	$	3,296		$	999	$	6,971	$	5,275	$	724	$	—		$	(163	)	$	46		$	50,618
	Transfers from non-operating properties	$	50,303		$	42,674		$	—		$	21,821		$	—	$	—	$	3,522	$	15,880	$	30		$	—		$	897		$	135,127
	Segment assets at September 30, 2014	$	1,278,713		$	648,248		$	116,837		$	98,334		$	96,131	$	100,009	$	274,931	$	107,051	$	—		$	78,240		$	164,192		$	2,962,686
	Nine Months Ended September 30, 2013
	Revenues from real estate operations	$	177,858		$	68,992		$	24,019		$	3,459		$	12,716	$	12,355	$	32,246	$	8,529	$	19,722		$	7,571		$	4,446		$	371,913
	Property operating expenses	59,423			24,020			12,427			874			5,769		3,715		12,408		2,482		6,983			824			4,680			133,605
	NOI from real estate operations	$	118,435		$	44,972		$	11,592		$	2,585		$	6,947	$	8,640	$	19,838	$	6,047	$	12,739		$	6,747		$	(234	)	$	238,308
	Additions to long-lived assets	$	14,172		$	6,699		$	268		$	3,501		$	484	$	1,634	$	1,761	$	387	$	2,323		$	87		$	262		$	31,578
	Transfers from non-operating properties	$	37,607		$	37,209		$	—		$	20,924		$	—	$	12	$	113	$	28,031	$	4,539		$	2		$	69,163		$	197,600
	Segment assets at September 30, 2013	$	1,275,972		$	593,335		$	118,701		$	50,737		$	99,852	$	96,679	$	305,947	$	105,112	$	158,690		$	80,749		$	168,039		$	3,053,813
	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,
		2014			2013			2014			2013
	Segment revenues from real estate operations	$	118,240		$	124,561		$	359,100		$	371,913
	Construction contract and other service revenues	34,739			16,991			80,390			52,048
	Less: Revenues from discontinued operations (Note 15)	36			(9,740		)	12			(29,403		)
	Total revenues	$	153,015		$	131,812		$	439,502		$	394,558
	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,
		2014			2013			2014			2013
	Segment property operating expenses	$	42,881		$	44,885		$	136,489		$	133,605
	Less: Property operating expenses from discontinued operations (Note 15)	175			(3,524		)	111			(10,523		)
	Total property operating expenses	$	43,056		$	41,361		$	136,600		$	123,082
	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,
		2014			2013			2014			2013
	Construction contract and other service revenues	$	34,739		$	16,991		$	80,390		$	52,048
	Construction contract and other service expenses	(33,593		)	(16,306		)	(75,353		)	(49,165		)
	NOI from service operations	$	1,146		$	685		$	5,037		$	2,883
	The following table reconciles our NOI from real estate operations for reportable segments and NOI from service operations to 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,
		2014			2013			2014			2013
	NOI from real estate operations	$	75,359		$	79,676		$	222,611		$	238,308
	NOI from service operations	1,146			685			5,037			2,883
	Interest and other income (loss)	1,191			(3		)	3,775			2,949
	Equity in income of unconsolidated entities	193			44			206			211
	Income tax expense	(101		)	(24		)	(257		)	(61		)
	Other adjustments:	—						—
	Depreciation and other amortization associated with real estate operations	(30,237		)	(28,205		)	(104,728		)	(82,888		)
	Impairment losses	(66		)	(5,857		)	(1,368		)	(5,857		)
	General, administrative and leasing expenses	(7,211		)	(8,027		)	(22,882		)	(22,430		)
Business development expenses and land carry costs	(1,430		)	(1,383		)	(4,107		)	(4,069		)
Interest expense on continuing operations	(24,802		)	(19,342		)	(69,107		)	(60,734		)
NOI from discontinued operations	(139		)	(6,216		)	(99		)	(18,880		)
Loss on early extinguishment of debt	(176		)	(374		)	(446		)	(27,028		)
Income from continuing operations	$	13,727		$	10,974		$	28,635		$	22,404
The following table reconciles our segment assets to the consolidated total assets of COPT and subsidiaries (in thousands):
	September 30,			September 30,
	2014			2013
Segment assets	$	2,962,686		$	3,053,813
Non-operating property assets	518,951			530,389
Other assets	198,551			171,386
Total COPT consolidated assets	$	3,680,188		$	3,755,588
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, impairment losses, loss on early extinguishment of debt and gain on sales of real estate 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 income 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, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]	'
Share-Based Compensation	'
	Share-Based Compensation
	Performance Share Units (“PSUs”)
	On March 6, 2014, our Board of Trustees granted 49,103 PSUs with an aggregate grant date fair value of $1.7 million to executives.  The PSUs have a performance period beginning on January 1, 2014 and concluding on the earlier of December 31, 2016 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 or greater		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 $35.09 per PSU using a Monte Carlo model, which included assumptions of, among other things, the following: baseline common share value of $26.52; expected volatility for COPT common shares of 28.6%; and a risk-free interest rate of 0.66%.  We are recognizing the grant date fair value in connection with these PSU awards over the period commencing on March 6, 2014 and ending on December 31, 2016.
	With regard to the PSUs granted to our executives in prior years that were outstanding as of December 31, 2013 as described in our 2013 Annual Report on Form 10-K:
	•	the performance period for the PSUs granted to executives on March 3, 2011 ended on March 2, 2014. Based on COPT’s total shareholder return during the performance period relative to its peer group of companies, there was no payout value in connection with the termination of the PSUs; and
	•	the PSUs granted to executives on March 1, 2012 and March 1, 2013 were outstanding as of September 30, 2014.
	Restricted Shares
	During the nine months ended September 30, 2014, certain employees, as well as nonemployee members of our Board of Trustees, were granted a total of 211,982 restricted common shares with an aggregate grant date fair value of $5.7 million (weighted average of $26.71 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, 2014, forfeiture restrictions lapsed on 165,076 previously issued common shares; these shares had a weighted average grant date fair value of $29.01 per share, and the aggregate intrinsic value of the shares on the vesting dates was $4.4 million.
	Options
	During the nine months ended September 30, 2014, 57,888 options to purchase common shares (“options”) were exercised.  The weighted average exercise price of these options was $23.50 per share, and the aggregate intrinsic value of the options exercised was $214,000.

Income_Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2014
Income Tax Disclosure [Abstract]	'
Income Taxes	'
	Income Taxes
	We own a 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,
		2014			2013			2014			2013
	Deferred
	Federal	$	(83	)	$	(20	)	$	(215	)	$	(50	)
	State	(18		)	(4		)	(42		)	(11		)
	Total income tax expense	$	(101	)	$	(24	)	$	(257	)	$	(61	)
	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 37.7% for the three and nine months ended September 30, 2014 and 36.7% for the three and nine months ended September 30, 2013.

Discontinued_Operations

Discontinued Operations	9 Months Ended
	Sep. 30, 2014
Discontinued Operations and Disposal Groups [Abstract]	'
Discontinued Operations	'
	Discontinued Operations
	Income from discontinued operations primarily includes revenues and expenses associated with the following:
	•	920 Elkridge Landing Road in the Baltimore/Washington Corridor that was sold on June 25, 2013;
	•	4230 Forbes Boulevard in the Baltimore/Washington Corridor that was sold on December 11, 2013;
	•	15 operating properties in Colorado Springs that were sold on December 12, 2013; and
	•	nine operating properties in the Baltimore/Washington Corridor and five operating properties in Colorado Springs for which the title to the properties was transferred to the mortgage lender on December 23, 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,
		2014			2013			2014			2013
	Revenue from real estate operations	$	(36	)	$	9,740		$	(12	)	$	29,403
	Property operating expenses	175			(3,524		)	111			(10,523		)
	Depreciation and amortization	—			(1,005		)	—			(3,509		)
	Impairment recoveries (losses)	24			(16,217		)	(3		)	(25,269		)
	General, administrative and leasing expenses	—			—			—			(1		)
	Interest expense	—			(1,968		)	—			(6,316		)
	Gain on sales of real estate	28			—			24			—
	Loss on early extinguishment of debt	—			—			(116		)	—
	Discontinued operations	$	191		$	(12,974	)	$	4		$	(16,215	)

Earnings_Per_Share_EPS_and_Ear

Earnings Per Share ("EPS") and Earnings Per Unit ("EPU")	9 Months Ended
	Sep. 30, 2014
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 COPT 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,
		2014				2013			2014			2013
	Numerator:
	Income from continuing operations	$	13,727			$	10,974		$	28,635		$	22,404
	Gain on sales of real estate, net	10,630				—			10,630			2,683
	Preferred share dividends	(3,553		)		(4,490		)	(12,387		)	(15,481		)
	Issuance costs associated with redeemed preferred shares	—				—			(1,769		)	(2,904		)
	Income from continuing operations attributable to noncontrolling interests	(1,820		)		(1,456		)	(3,923		)	(2,710		)
	Income from continuing operations attributable to restricted shares	(103		)		(97		)	(332		)	(317		)
	Numerator for basic and diluted EPS from continuing operations attributable to COPT common shareholders	$	18,881			$	4,931		$	20,854		$	3,675
	Discontinued operations	191				(12,974		)	4			(16,215		)
	Discontinued operations attributable to noncontrolling interests	(8		)		492			5			529
	Numerator for basic and diluted EPS on net income (loss) attributable to COPT common shareholders	$	19,064			$	(7,551	)	$	20,863		$	(12,011	)
	Denominator (all weighted averages):
	Denominator for basic EPS (common shares)	87,290				86,760			87,196			84,547
	Dilutive effect of share-based compensation awards	195				—			169			—
	Denominator for diluted EPS (common shares)	87,485				86,760			87,365			84,547
	Basic EPS:
	Income from continuing operations attributable to COPT common shareholders	$	0.22			$	0.06		$	0.24		$	0.04
	Discontinued operations attributable to COPT common shareholders	0				(0.15		)	0			(0.18		)
	Net income (loss) attributable to COPT common shareholders	$	0.22			$	(0.09	)	$	0.24		$	(0.14	)
	Diluted EPS:
	Income from continuing operations attributable to COPT common shareholders	$	0.22			$	0.06		$	0.24		$	0.04
	Discontinued operations attributable to COPT common shareholders	0				(0.15		)	0			(0.18		)
	Net income (loss) attributable to COPT common shareholders	$	0.22			$	(0.09	)	$	0.24		$	(0.14	)
	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,
		2014			2013			2014		2013
	Conversion of common units	3,876			3,804			3,915		3,832
	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, 2014 and 2013 of 401,000 and 377,000, respectively, and for the nine months ended September 30, 2014 and 2013 of 404,000 and 388,000, respectively; and
	•	weighted average options for the three months ended September 30, 2014 and 2013 of 490,000 and 613,000, respectively, and for the nine months ended September 30, 2014 and 2013 of 495,000 and 571,000, respectively.
	As discussed in Note 8, we have outstanding senior notes that have an exchange settlement feature, but such notes did not affect our diluted EPS 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.
	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,
		2014				2013			2014			2013
	Numerator:
	Income from continuing operations	$	13,727			$	10,974		$	28,635		$	22,404
	Gain on sales of real estate, net	10,630				—			10,630			2,683
	Preferred unit distributions	(3,718		)		(4,655		)	(12,882		)	(15,976		)
	Issuance costs associated with redeemed preferred units	—				—			(1,769		)	(2,904		)
	Income from continuing operations attributable to noncontrolling interests	(897		)		(986		)	(2,476		)	(2,025		)
	Income from continuing operations attributable to restricted units	(103		)		(97		)	(332		)	(317		)
	Numerator for basic and diluted EPU from continuing operations attributable to COPLP common unitholders	$	19,639			$	5,236		$	21,806		$	3,865
	Discontinued operations	191				(12,974		)	4			(16,215		)
	Discontinued operations attributable to noncontrolling interests	—				(49		)	5			(147		)
	Numerator for basic and diluted EPU on net income attributable to COPLP common unitholders	$	19,830			$	(7,787	)	$	21,815		$	(12,497	)
	Denominator (all weighted averages):
	Denominator for basic EPU (common units)	91,166				90,564			91,111			88,379
	Dilutive effect of share-based compensation awards	195				—			169			—
Denominator for basic and diluted EPU (common units)	91,361				90,564			91,280			88,379
Basic EPU:
Income from continuing operations attributable to COPLP common unitholders	$	0.22			$	0.06		$	0.24		$	0.04
Discontinued operations attributable to COPLP common unitholders	0				(0.15		)	0			(0.18		)
Net income (loss) attributable to COPLP common unitholders	$	0.22			$	(0.09	)	$	0.24		$	(0.14	)
Diluted EPU:
Income from continuing operations attributable to COPLP common unitholders	$	0.22			$	0.06		$	0.24		$	0.04
Discontinued operations attributable to COPLP common unitholders	0				(0.15		)	0			(0.18		)
Net income (loss) attributable to COPLP common unitholders	$	0.22			$	(0.09	)	$	0.24		$	(0.14	)
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,
	2014			2013			2014		2013
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, 2014 and 2013 of 401,000 and 377,000, respectively, and for the nine months ended September 30, 2014 and 2013 of 404,000 and 388,000, respectively; and
•	weighted average options for the three months ended September 30, 2014 and 2013 of 490,000 and 613,000, respectively, and for the nine months ended September 30, 2014 and 2013 of 495,000 and 571,000, respectively.
As discussed in Note 8, we have outstanding senior notes that have an exchange settlement feature, but such notes 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, 2014
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 joint venture 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).  On December 6, 2013, the holder of mortgage debt encumbering all of the joint venture’s properties foreclosed on the properties. As a result, title to the properties was transferred to the mortgage lender and the joint venture was relieved of the debt obligation plus accrued interest and penalties. The joint venture still had $5.6 million in nonrecourse mezzanine debt as of September 30, 2014; however, the joint venture no longer holds any property and has ceased all business operations. Management estimates there to be no fair value to the guarantees as of September 30, 2014 because the actions that would trigger performance are all within our control.
	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 $1.1 million liability through September 30, 2014 representing our estimated 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, 2014
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.
	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, 2013 included in our 2013 Annual Report on Form 10-K.  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.  The consolidated financial statements have been prepared using the accounting policies described in our 2013 Annual Report on Form 10-K.
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.  These reclassifications occurred in conjunction with the transfer of properties to, and from, discontinued operations during 2013.
Prior Out of Period Adjustment	'
	Prior Out of Period Adjustment
	As previously disclosed in our 2013 Annual Report on Form 10-K, during the second quarter of 2013, we identified an error related to the estimated fair value of a redeemable noncontrolling interest in a real estate joint venture. Changes in such fair value are reported as changes in equity with no impact to net income or comprehensive income. The error resulted in an understatement of the line entitled “redeemable noncontrolling interest” in the mezzanine section of our consolidated balance sheet and an overstatement of the line entitled “additional paid-in capital” in the equity section of our consolidation balance sheet of $3.7 million as of December 31, 2012. We have determined that this adjustment was not material to our financial statements for 2012 or 2013. Accordingly, this change is reported as an out-of-period adjustment in the nine months ended September 30, 2013.
Recent Accounting Pronouncement	'
	Recent Accounting Pronouncements
	We adopted guidance issued by the Financial Accounting Standards Board (“FASB”) related to the reporting of discontinued operations and disclosures of disposals of components of an entity effective for the quarterly period ended June 30, 2014. This guidance defines a discontinued operation as a component or group of components disposed or classified as held for sale and represents a strategic shift that has (or will have) a major effect on an entity’s operations and financial results; the guidance states that a strategic shift could include a disposal of a major geographical area of operations, a major line of business, a major equity method investment or other major parts of an entity. The guidance also provides for additional disclosure requirements in connection with both discontinued operations and other dispositions not qualifying as discontinued operations. The guidance applies prospectively to new disposals and new classifications of disposal groups as held for sale after the effective date. Our adoption of the guidance will result in fewer disposed or held for sale properties being reported as discontinued operations in our results of operations (including operating properties sold during the current period) but will not otherwise materially affect our consolidated financial statements.
	In May 2014, the FASB issued guidance regarding the recognition of revenue from contracts with customers. Under this guidance, an entity will recognize revenue to depict the transfer of goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. Additionally, this guidance requires improved disclosures regarding the nature, amount, timing and uncertainty of revenue and cash flows arising from contracts with customers. We are required to adopt this guidance for our annual and interim periods beginning January 1, 2017, utilizing one of two methods: retrospective restatement for each reporting period presented at time of adoption, or retrospectively with the cumulative effect of initially applying this guidance recognized at the date of initial application. We are currently assessing the financial impact of this guidance on our consolidated financial statements.

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2014
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, 2014 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	$	7,187	$	—		$	—	$	7,187
	Common stocks	110		—			—		110
	Other	203		—			—		203
	Interest rate derivatives (2)	—		2,907			—		2,907
	Warrants to purchase common stock (2)	—		192			—		192
	Total Assets	$	7,500	$	3,099		$	—	$	10,599
	Liabilities:
	Deferred compensation plan liability (3)	$	—	$	7,500		$	—	$	7,500
	Interest rate derivatives	—		2,236			—		2,236
	Total Liabilities	$	—	$	9,736		$	—	$	9,736
	Redeemable noncontrolling interest	$	—	$	—		$	18,436	$	18,436
	(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, 2014 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:
	Interest rate derivatives (1)	$	—	$	2,907		$	—	$	2,907
	Warrants to purchase common stock (1)	—		192			—		192
	Total Assets	$	—	$	3,099		$	—	$	3,099
	Liabilities:
	Interest rate derivatives	$	—	$	2,236		$	—	$	2,236
	Redeemable noncontrolling interest	$	—	$	—		$	18,436	$	18,436
	(1) 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 technique used by us in determining the fair values of the properties (dollars in thousands):
		Fair Value of Properties Held as of September 30, 2013
		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		September 30, 2013		September 30, 2013
	Assets (2):
	Properties, net	$	—	$	—		$	142,702	$	142,702	$	21,888	$	30,940
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 as of September 30, 2013 (dollars in thousands):
	Valuation Technique	Fair Value on		Unobservable Input		Range (Weighted Average)
		Measurement Date
	Contract of sale	$	110,499	Contract price (1)		-1
	Discounted cash flow	$	32,203	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 the properties.
	(2) Only one value applied for this unobservable input.

Properties_net_Tables

Properties, net (Tables)	9 Months Ended
	Sep. 30, 2014
Real Estate [Abstract]	'
Schedule of operating properties, net	'
	Operating properties, net consisted of the following (in thousands):
		September 30,				December 31,
		2014				2013
	Land	$	439,355			$	430,472
	Buildings and improvements	2,997,450				2,869,870
	Less: accumulated depreciation	(679,598		)		(597,649		)
	Operating properties, net	$	2,757,207			$	2,702,693
Schedule of projects in development or held for future development	'
	Projects in development or held for future development consisted of the following (in thousands):
		September 30,				December 31,
		2014				2013
	Land	$	229,286			$	245,676
	Construction in progress, excluding land	284,663				265,932
	Projects in development or held for future development	$	513,949			$	511,608
Schedule of operating property dispositions	'
	During the nine months ended September 30, 2014, we completed dispositions of the following operating properties (dollars in thousands):
	Project Name		Location		Date of Sale		Number of Buildings		Total Rentable Square Feet	Transaction Value		Gain on Disposition
	4969 and 4979 Mercantile Road		White Marsh, MD		7/14/14		2		96,721	$	5,960	$	2,126
	9930 and 9940 Franklin Square		White Marsh, MD		7/30/14		2		71,992	10,475		2,303
	5020, 5022, 5024 and 5026 Campbell Boulevard		White Marsh, MD		8/4/14		4		134,245	12,400		666
							8		302,958	$	28,835	$	5,095

Real_Estate_Joint_Ventures_Tab

Real Estate Joint Ventures (Tables)	9 Months Ended
	Sep. 30, 2014
Equity Method Investments and Joint Ventures [Abstract]	'
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 as of September 30, 2014 (dollars in thousands):
				Nominal
				Ownership		September 30, 2014				-1
			Date	% as of		Total		Encumbered		Total
			Acquired	9/30/14	Nature of Activity	Assets		Assets		Liabilities
	LW Redstone Company, LLC		3/23/10	85%	Operates four buildings and developing others (2)	$	139,314	$	67,100	$	42,134
	M Square Associates, LLC		6/26/07	50%	Operates two buildings and developing others (3)	60,115		47,815		39,810
						$	199,429	$	114,915	$	81,944
	-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 Baltimore/Washington Corridor).

Mortgage_and_Other_Investing_R1

Mortgage and Other Investing Receivables (Tables)	9 Months Ended
	Sep. 30, 2014
Receivables [Abstract]	'
Mortgage and other investing receivables	'
	Mortgage and other investing receivables, including accrued interest thereon, consisted of the following (in thousands):
		September 30,		December 31,
		2014		2013
	Notes receivable from the City of Huntsville	$	47,878	$	44,055
	Other investing loans receivable	3,008		9,608
		$	50,886	$	53,663

Prepaid_Expenses_and_Other_Ass1

Prepaid Expenses and Other Assets (Tables)	9 Months Ended
	Sep. 30, 2014
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,
		2014		2013
	Prepaid expenses	$	24,497	$	19,308
	Construction contract costs incurred in excess of billings	17,962		2,462
	Lease incentives, net	12,939		8,435
	Furniture, fixtures and equipment, net	6,527		6,556
	Deferred tax asset, net (1)	4,058		4,305
	Interest rate derivatives	2,907		6,594
	Other equity method investments	2,240		2,258
	Other assets	3,733		4,268
	Prepaid expenses and other assets	$	74,863	$	54,186
	(1) Includes a valuation allowance of $2.1 million.

Debt_Tables

Debt (Tables)	9 Months Ended
	Sep. 30, 2014
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 as of		as of
		2014		2014		2013		September 30, 2014		September 30, 2014
	Mortgage and Other Secured Loans:
	Fixed rate mortgage loans (1)			$	600,082	$	675,060	3.96% - 10.65% (2)		2014-2024
	Variable rate secured loan			37,087		37,691		LIBOR + 2.25% (3)		Nov-15
	Total mortgage and other secured loans			637,169		712,751
	Revolving Credit Facility (4)	$	800,000	—		—		LIBOR + 0.975% to 1.75%		Jul-17
	Term Loan Facilities	-5		520,000		620,000		LIBOR + 1.10% to 2.60% (6)		2015-2019
	Unsecured Senior Notes
	3.600% Senior Notes (7)			347,432		347,244		3.60%		May-23
	5.250% Senior Notes (8)			245,707		245,445		5.25%		Feb-24
	3.700% Senior Notes (9)			297,483		—		3.70%		Jun-21
	Unsecured notes payable			1,631		1,700		0% (10)		2026
	4.25% Exchangeable Senior Notes (11)			570		563		4.25%		Apr-30
	Total debt, net			$	2,049,992	$	1,927,703
	(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 $49,000 as of September 30, 2014 and $69,000 as of December 31, 2013.
	-2	The weighted average interest rate on these loans was 7.20% as of September 30, 2014.
	(3)	The interest rate on the loan outstanding was 2.41% as of September 30, 2014.
	-4	No borrowings were outstanding on this facility as of the end of the respective periods.
	(5)	We have the ability to borrow an additional $180.0 million in the aggregate under these term loan facilities, provided that there is no default under the facilities and subject to the approval of the lenders.
	(6)	The weighted average interest rate on these loans was 1.80% as of September 30, 2014.
	-7	The carrying value of these notes included a principal amount of $350.0 million and an unamortized discount totaling $2.6 million as of September 30, 2014 and $2.8 million as of December 31, 2013.  The effective interest rate under the notes, including amortization of the issuance costs, was 3.70%.
	-8	The carrying value of these notes included a principal amount of $250.0 million and an unamortized discount totaling $4.3 million as of September 30, 2014 and $4.6 million as of December 31, 2013.  The effective interest rate under the notes, including amortization of the issuance costs, was 5.49%.
	-9	Refer to the paragraph below for disclosure pertaining to these notes.
	(10)	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 $680,000 as of September 30, 2014 and $761,000 as of December 31, 2013.
	(11)	As described further in our 2013 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, 2014 and is equivalent to an exchange price of $47.96 per common share).  The carrying value of these notes included a principal amount of $575,000 and an unamortized discount totaling $5,000 as of September 30, 2014 and $12,000 as of December 31, 2013.  The effective interest rate under the notes, including amortization of the issuance costs, was 6.05%.  Because the closing price of our common shares as of September 30, 2014 and December 31, 2013 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,
		2014		2013		2014		2013
	Interest expense at stated interest rate	$	6	$	6	$	18	$	4,201
	Interest expense associated with amortization of discount	2		2		7		1,613
	Total	$	8	$	8	$	25	$	5,814
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, 2014				December 31, 2013
		Carrying		Estimated		Carrying		Estimated
		Amount		Fair Value		Amount		Fair Value
	Fixed-rate debt
	Unsecured Senior Notes	$	890,622	$	901,022	$	592,689	$	575,374
	4.25% Exchangeable Senior Notes	570		575		563		575
	Other fixed-rate debt	601,713		578,116		676,760		650,997
	Variable-rate debt	557,087		558,665		657,691		657,527
		$	2,049,992	$	2,038,378	$	1,927,703	$	1,884,473

Interest_Rate_Derivatives_Tabl

Interest Rate Derivatives (Tables)	9 Months Ended
	Sep. 30, 2014
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,
													2014			2013
	$	100,000			0.83%		One-Month LIBOR		1/3/12		9/1/15		$	(556	)	$	(861	)
	100,000				0.83%		One-Month LIBOR		1/3/12		9/1/15		(556		)	(861		)
	37,087			-1	3.83%		One-Month LIBOR + 2.25%		11/2/10		11/2/15		(515		)	(832		)
	100,000				0.81%		One-Month LIBOR		9/2/14		9/1/16		(300		)	(94		)
	100,000				0.81%		One-Month LIBOR		9/2/14		9/1/16		(309		)	(105		)
	100,000				1.67%		One-Month LIBOR		9/1/15		8/1/19		1,551			3,377
	100,000				1.73%		One-Month LIBOR		9/1/15		8/1/19		1,356			3,217
	100,000				0.61%		One-Month LIBOR		1/3/12		9/1/14		—			(279		)
	100,000				0.61%		One-Month LIBOR		1/3/12		9/1/14		—			(277		)
													$	671		$	3,285
	-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, 2014						31-Dec-13
	Derivatives		Balance Sheet Location		Fair Value				Balance Sheet Location		Fair Value
	Interest rate swaps designated as cash flow hedges		Prepaid expenses and other assets		$	2,907			Prepaid expenses and other assets		$	6,594
	Interest rate swaps designated as cash flow hedges		Interest rate derivatives		(2,236		)		Interest rate derivatives		(3,309		)
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,
		2014				2013				2014				2013
	Amount of gains (losses) recognized in accumulated other comprehensive income (loss) (“AOCI”) (effective portion)	$	1,015			$	(2,482	)		$	(4,738	)		$	5,810
	Amount of losses reclassified from AOCI into interest expense (effective portion)	756				689				2,170				2,021

Redeemable_Noncontrolling_Inte1

Redeemable Noncontrolling Interest (Tables)	9 Months Ended
	Sep. 30, 2014
Noncontrolling Interest [Abstract]	'
Schedule of activity for redeemable noncontrolling interest	'
	The table below sets forth activity in a redeemable noncontrolling interest in a consolidated real estate joint venture (in thousands):
		For the Nine Months Ended September 30,
		2014			2013
	Beginning balance	$	17,758		$	10,298
	Distributions	(976		)	(811		)
	Net income attributable to noncontrolling interest	1,647			888
	Adjustment to arrive at fair value of interest	7			6,414
	Ending balance	$	18,436		$	16,789

Information_by_Business_Segmen1

Information by Business Segment (Tables)	9 Months Ended
	Sep. 30, 2014
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			Huntsville			Washington,		St. Mary’s &		Greater		Greater		Colorado			Other			Operating			Total
		Washington			Virginia			Antonio						DC - Capitol		King George		Baltimore		Philadelphia		Springs						Wholesale
		Corridor												Riverfront		Counties												Data Center
	Three Months Ended September 30, 2014
	Revenues from real estate operations	$	58,883		$	21,369		$	9,031		$	2,471		$	3,524	$	4,158	$	10,436	$	2,951	$	—		$	2,541		$	2,876		$	118,240
	Property operating expenses	19,457			7,500			5,100			763			1,824		1,277		3,810		837		3			257			2,053			42,881
	NOI from real estate operations	$	39,426		$	13,869		$	3,931		$	1,708		$	1,700	$	2,881	$	6,626	$	2,114	$	(3	)	$	2,284		$	823		$	75,359
	Additions to long-lived assets	$	7,248		$	5,898		$	—		$	455		$	458	$	5,189	$	3,021	$	625	$	—		$	(125	)	$	24		$	22,793
	Transfers from non-operating properties	$	22,680		$	15,403		$	—		$	1,496		$	—	$	—	$	495	$	2,506	$	—		$	—		$	222		$	42,802
	Three Months Ended September 30, 2013										—
	Revenues from real estate operations	$	59,558		$	23,062		$	7,898		$	1,360		$	4,295	$	4,270	$	10,703	$	3,258	$	6,571		$	2,510		$	1,076		$	124,561
	Property operating expenses	19,642			7,999			4,061			382			1,946		1,299		4,143		924		2,361			282			1,846			44,885
	NOI from real estate operations	$	39,916		$	15,063		$	3,837		$	978		$	2,349	$	2,971	$	6,560	$	2,334	$	4,210		$	2,228		$	(770	)	$	79,676
	Additions to long-lived assets	$	7,292		$	2,459		$	251		$	4		$	86	$	630	$	1,707	$	102	$	1,413		$	(88	)	$	86		$	13,942
	Transfers from non-operating properties	$	8,523		$	26,355		$	—		$	(33	)	$	—	$	—	$	9	$	3,414	$	1,860		$	—		$	2,714		$	42,842
	Nine Months Ended September 30, 2014
	Revenues from real estate operations	$	177,452		$	67,235		$	26,268		$	7,430		$	10,989	$	12,676	$	32,956	$	8,657	$	6		$	7,662		$	7,769		$	359,100
	Property operating expenses	62,402			24,124			14,391			2,275			5,343		4,070		13,786		3,281		5			1,190			5,622			136,489
	NOI from real estate operations	$	115,050		$	43,111		$	11,877		$	5,155		$	5,646	$	8,606	$	19,170	$	5,376	$	1		$	6,472		$	2,147		$	222,611
	Additions to long-lived assets	$	19,278		$	14,198		$	(6	)	$	3,296		$	999	$	6,971	$	5,275	$	724	$	—		$	(163	)	$	46		$	50,618
	Transfers from non-operating properties	$	50,303		$	42,674		$	—		$	21,821		$	—	$	—	$	3,522	$	15,880	$	30		$	—		$	897		$	135,127
	Segment assets at September 30, 2014	$	1,278,713		$	648,248		$	116,837		$	98,334		$	96,131	$	100,009	$	274,931	$	107,051	$	—		$	78,240		$	164,192		$	2,962,686
	Nine Months Ended September 30, 2013
	Revenues from real estate operations	$	177,858		$	68,992		$	24,019		$	3,459		$	12,716	$	12,355	$	32,246	$	8,529	$	19,722		$	7,571		$	4,446		$	371,913
	Property operating expenses	59,423			24,020			12,427			874			5,769		3,715		12,408		2,482		6,983			824			4,680			133,605
	NOI from real estate operations	$	118,435		$	44,972		$	11,592		$	2,585		$	6,947	$	8,640	$	19,838	$	6,047	$	12,739		$	6,747		$	(234	)	$	238,308
	Additions to long-lived assets	$	14,172		$	6,699		$	268		$	3,501		$	484	$	1,634	$	1,761	$	387	$	2,323		$	87		$	262		$	31,578
	Transfers from non-operating properties	$	37,607		$	37,209		$	—		$	20,924		$	—	$	12	$	113	$	28,031	$	4,539		$	2		$	69,163		$	197,600
	Segment assets at September 30, 2013	$	1,275,972		$	593,335		$	118,701		$	50,737		$	99,852	$	96,679	$	305,947	$	105,112	$	158,690		$	80,749		$	168,039		$	3,053,813
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,
		2014			2013			2014			2013
	Segment revenues from real estate operations	$	118,240		$	124,561		$	359,100		$	371,913
	Construction contract and other service revenues	34,739			16,991			80,390			52,048
	Less: Revenues from discontinued operations (Note 15)	36			(9,740		)	12			(29,403		)
	Total revenues	$	153,015		$	131,812		$	439,502		$	394,558
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,
		2014			2013			2014			2013
	Segment property operating expenses	$	42,881		$	44,885		$	136,489		$	133,605
	Less: Property operating expenses from discontinued operations (Note 15)	175			(3,524		)	111			(10,523		)
	Total property operating expenses	$	43,056		$	41,361		$	136,600		$	123,082
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,
		2014			2013			2014			2013
	Construction contract and other service revenues	$	34,739		$	16,991		$	80,390		$	52,048
	Construction contract and other service expenses	(33,593		)	(16,306		)	(75,353		)	(49,165		)
	NOI from service operations	$	1,146		$	685		$	5,037		$	2,883
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 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,
		2014			2013			2014			2013
	NOI from real estate operations	$	75,359		$	79,676		$	222,611		$	238,308
	NOI from service operations	1,146			685			5,037			2,883
	Interest and other income (loss)	1,191			(3		)	3,775			2,949
	Equity in income of unconsolidated entities	193			44			206			211
	Income tax expense	(101		)	(24		)	(257		)	(61		)
	Other adjustments:	—						—
	Depreciation and other amortization associated with real estate operations	(30,237		)	(28,205		)	(104,728		)	(82,888		)
	Impairment losses	(66		)	(5,857		)	(1,368		)	(5,857		)
	General, administrative and leasing expenses	(7,211		)	(8,027		)	(22,882		)	(22,430		)
	Business development expenses and land carry costs	(1,430		)	(1,383		)	(4,107		)	(4,069		)
	Interest expense on continuing operations	(24,802		)	(19,342		)	(69,107		)	(60,734		)
	NOI from discontinued operations	(139		)	(6,216		)	(99		)	(18,880		)
	Loss on early extinguishment of debt	(176		)	(374		)	(446		)	(27,028		)
	Income from continuing operations	$	13,727		$	10,974		$	28,635		$	22,404
Schedule of reconciliation of segment assets to total assets	'
	The following table reconciles our segment assets to the consolidated total assets of COPT and subsidiaries (in thousands):
		September 30,			September 30,
		2014			2013
	Segment assets	$	2,962,686		$	3,053,813
	Non-operating property assets	518,951			530,389
	Other assets	198,551			171,386
	Total COPT consolidated assets	$	3,680,188		$	3,755,588

ShareBased_Compensation_Tables

Share-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2014
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 or greater	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, 2014
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,
		2014			2013			2014			2013
	Deferred
	Federal	$	(83	)	$	(20	)	$	(215	)	$	(50	)
	State	(18		)	(4		)	(42		)	(11		)
	Total income tax expense	$	(101	)	$	(24	)	$	(257	)	$	(61	)

Discontinued_Operations_Tables

Discontinued Operations (Tables)	9 Months Ended
	Sep. 30, 2014
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,
		2014			2013			2014			2013
	Revenue from real estate operations	$	(36	)	$	9,740		$	(12	)	$	29,403
	Property operating expenses	175			(3,524		)	111			(10,523		)
	Depreciation and amortization	—			(1,005		)	—			(3,509		)
	Impairment recoveries (losses)	24			(16,217		)	(3		)	(25,269		)
	General, administrative and leasing expenses	—			—			—			(1		)
	Interest expense	—			(1,968		)	—			(6,316		)
	Gain on sales of real estate	28			—			24			—
	Loss on early extinguishment of debt	—			—			(116		)	—
	Discontinued operations	$	191		$	(12,974	)	$	4		$	(16,215	)

Earnings_Per_Share_EPS_and_Ear1

Earnings Per Share ("EPS") and Earnings Per Unit ("EPU") (Tables)	9 Months Ended
	Sep. 30, 2014
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,
		2014				2013			2014			2013
	Numerator:
	Income from continuing operations	$	13,727			$	10,974		$	28,635		$	22,404
	Gain on sales of real estate, net	10,630				—			10,630			2,683
	Preferred share dividends	(3,553		)		(4,490		)	(12,387		)	(15,481		)
	Issuance costs associated with redeemed preferred shares	—				—			(1,769		)	(2,904		)
	Income from continuing operations attributable to noncontrolling interests	(1,820		)		(1,456		)	(3,923		)	(2,710		)
	Income from continuing operations attributable to restricted shares	(103		)		(97		)	(332		)	(317		)
	Numerator for basic and diluted EPS from continuing operations attributable to COPT common shareholders	$	18,881			$	4,931		$	20,854		$	3,675
	Discontinued operations	191				(12,974		)	4			(16,215		)
	Discontinued operations attributable to noncontrolling interests	(8		)		492			5			529
	Numerator for basic and diluted EPS on net income (loss) attributable to COPT common shareholders	$	19,064			$	(7,551	)	$	20,863		$	(12,011	)
	Denominator (all weighted averages):
	Denominator for basic EPS (common shares)	87,290				86,760			87,196			84,547
	Dilutive effect of share-based compensation awards	195				—			169			—
	Denominator for diluted EPS (common shares)	87,485				86,760			87,365			84,547
	Basic EPS:
	Income from continuing operations attributable to COPT common shareholders	$	0.22			$	0.06		$	0.24		$	0.04
	Discontinued operations attributable to COPT common shareholders	0				(0.15		)	0			(0.18		)
	Net income (loss) attributable to COPT common shareholders	$	0.22			$	(0.09	)	$	0.24		$	(0.14	)
	Diluted EPS:
	Income from continuing operations attributable to COPT common shareholders	$	0.22			$	0.06		$	0.24		$	0.04
	Discontinued operations attributable to COPT common shareholders	0				(0.15		)	0			(0.18		)
	Net income (loss) attributable to COPT common shareholders	$	0.22			$	(0.09	)	$	0.24		$	(0.14	)
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,
		2014			2013			2014		2013
	Conversion of common units	3,876			3,804			3,915		3,832
	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,
		2014				2013			2014			2013
	Numerator:
	Income from continuing operations	$	13,727			$	10,974		$	28,635		$	22,404
	Gain on sales of real estate, net	10,630				—			10,630			2,683
	Preferred unit distributions	(3,718		)		(4,655		)	(12,882		)	(15,976		)
	Issuance costs associated with redeemed preferred units	—				—			(1,769		)	(2,904		)
	Income from continuing operations attributable to noncontrolling interests	(897		)		(986		)	(2,476		)	(2,025		)
	Income from continuing operations attributable to restricted units	(103		)		(97		)	(332		)	(317		)
	Numerator for basic and diluted EPU from continuing operations attributable to COPLP common unitholders	$	19,639			$	5,236		$	21,806		$	3,865
	Discontinued operations	191				(12,974		)	4			(16,215		)
	Discontinued operations attributable to noncontrolling interests	—				(49		)	5			(147		)
	Numerator for basic and diluted EPU on net income attributable to COPLP common unitholders	$	19,830			$	(7,787	)	$	21,815		$	(12,497	)
	Denominator (all weighted averages):
	Denominator for basic EPU (common units)	91,166				90,564			91,111			88,379
	Dilutive effect of share-based compensation awards	195				—			169			—
	Denominator for basic and diluted EPU (common units)	91,361				90,564			91,280			88,379
	Basic EPU:
	Income from continuing operations attributable to COPLP common unitholders	$	0.22			$	0.06		$	0.24		$	0.04
	Discontinued operations attributable to COPLP common unitholders	0				(0.15		)	0			(0.18		)
	Net income (loss) attributable to COPLP common unitholders	$	0.22			$	(0.09	)	$	0.24		$	(0.14	)
	Diluted EPU:
	Income from continuing operations attributable to COPLP common unitholders	$	0.22			$	0.06		$	0.24		$	0.04
	Discontinued operations attributable to COPLP common unitholders	0				(0.15		)	0			(0.18		)
	Net income (loss) attributable to COPLP common unitholders	$	0.22			$	(0.09	)	$	0.24		$	(0.14	)
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,
		2014			2013			2014		2013
	Conversion of Series I preferred units	176			176			176		176
	Conversion of Series K preferred units	434			434			434		434

Organization_Details

Organization (Details)	Apr. 01, 2014	Sep. 30, 2014	Sep. 30, 2014	Sep. 30, 2014	Sep. 30, 2014
	Debt in default of payment terms	Operating office properties	Office properties under, or contractually committed for, construction or approved for redevelopment	Land controlled for future development	Partially operational, wholesale data center
	Northern Virginia	Property	sqft	acre	MW
	Wholly owned subsidiary	sqft	Property	sqft
	Property
Investments in real estate	'	'	'	'	'
Number of real estate properties	2	174	10	'	'
Square footage of real estate properties (in square feet)	'	16,900,000	1,100,000	'	'
Area of land held or under pre-construction (in acres)	'	'	'	1,480	'
Developable square footage of real estate properties (in square feet)	'	'	'	18,700,000	'
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, 2014
Common Units	'
Forms of ownership in Operating Partnership and ownership percentage by the entity	'
Percentage ownership in operating partnership	95.80%
Preferred Units	'
Forms of ownership in Operating Partnership and ownership percentage by the entity	'
Percentage ownership in operating partnership	95.50%

Summary_of_Significant_Account2

Summary of Significant Accounting Policies (Details) (USD $)	Sep. 30, 2014	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified			Misapplication of Fair Value Assumptions
			Restatement Adjustment
Quantifying Misstatement in Current Year Financial Statements [Line Items]	'	'	'
Redeemable noncontrolling interest	$18,436	$17,758	$3,700
Additional paid-in capital	$1,822,283	$1,814,015	($3,700)

Fair_Value_Measurements_Detail

Fair Value Measurements (Details) (USD $)	Sep. 30, 2014		Dec. 31, 2013	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Liabilities:	'		'	'	'
Interest rate derivatives	$2,236		$3,309	'	'
Redeemable noncontrolling interest	18,436		17,758	16,789	10,298
Corporate Office Properties, L.P.	'		'	'	'
Liabilities:	'		'	'	'
Interest rate derivatives	2,236		3,309	'	'
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)	'		'	'	'
Assets:	'		'	'	'
Assets	7,500		'	'	'
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Mutual funds	'		'	'	'
Assets:	'		'	'	'
Marketable securities in deferred compensation plan	7,187	[1]	'	'	'
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Common Shares	'		'	'	'
Assets:	'		'	'	'
Marketable securities in deferred compensation plan	110	[1]	'	'	'
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other	'		'	'	'
Assets:	'		'	'	'
Marketable securities in deferred compensation plan	203	[1]	'	'	'
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2)	'		'	'	'
Assets:	'		'	'	'
Interest rate derivatives	2,907	[2]	'	'	'
Warrants to purchase common stock	192	[2]	'	'	'
Assets	3,099		'	'	'
Liabilities:	'		'	'	'
Deferred compensation plan liability	7,500	[3]	'	'	'
Interest rate derivatives	2,236		'	'	'
Liabilities	9,736		'	'	'
Fair value measurement on a recurring basis | Total Estimated Fair Value	'		'	'	'
Assets:	'		'	'	'
Interest rate derivatives	2,907	[2]	'	'	'
Warrants to purchase common stock	192	[2]	'	'	'
Assets	10,599		'	'	'
Liabilities:	'		'	'	'
Deferred compensation plan liability	7,500	[3]	'	'	'
Interest rate derivatives	2,236		'	'	'
Liabilities	9,736		'	'	'
Redeemable noncontrolling interest	18,436		'	'	'
Fair value measurement on a recurring basis | Total Estimated Fair Value | Mutual funds	'		'	'	'
Assets:	'		'	'	'
Marketable securities in deferred compensation plan	7,187	[1]	'	'	'
Fair value measurement on a recurring basis | Total Estimated Fair Value | Common Shares	'		'	'	'
Assets:	'		'	'	'
Marketable securities in deferred compensation plan	110	[1]	'	'	'
Fair value measurement on a recurring basis | Total Estimated Fair Value | Other	'		'	'	'
Assets:	'		'	'	'
Marketable securities in deferred compensation plan	203	[1]	'	'	'
Fair value measurement on a recurring basis | Corporate Office Properties, L.P. | Significant Other Observable Inputs (Level 2)	'		'	'	'
Assets:	'		'	'	'
Interest rate derivatives	2,907	[4]	'	'	'
Warrants to purchase common stock	192	[4]	'	'	'
Assets	3,099		'	'	'
Liabilities:	'		'	'	'
Interest rate derivatives	2,236		'	'	'
Fair value measurement on a recurring basis | Corporate Office Properties, L.P. | Significant Unobservable Inputs (Level 3)	'		'	'	'
Liabilities:	'		'	'	'
Redeemable noncontrolling interest	18,436		'	'	'
Fair value measurement on a recurring basis | Corporate Office Properties, L.P. | Total Estimated Fair Value	'		'	'	'
Assets:	'		'	'	'
Interest rate derivatives	2,907	[4]	'	'	'
Warrants to purchase common stock	192	[4]	'	'	'
Assets	3,099		'	'	'
Liabilities:	'		'	'	'
Interest rate derivatives	2,236		'	'	'
Redeemable noncontrolling interest	$18,436		'	'	'
[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.
[4]	Included in the line entitled “prepaid expenses and other assets” on COPLP’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, 2014	Sep. 30, 2013		Sep. 30, 2014	Sep. 30, 2013
Assets and liabilities measured at fair value on a non-recurring basis	'	'		'	'
Impairment losses (recoveries)	$66	$5,857		$1,368	$5,857
Fair value measurement on a nonrecurring basis | Properties, net	'	'		'	'
Assets and liabilities measured at fair value on a non-recurring basis	'	'		'	'
Impairment losses (recoveries)	'	21,888	[1]	1,371	30,940	[1]
Fair value measurement on a nonrecurring basis | Properties, net | Significant Unobservable Inputs (Level 3)	'	'		'	'
Assets and liabilities measured at fair value on a non-recurring basis	'	'		'	'
Assets	'	142,702	[2]	'	142,702	[2]
Fair value measurement on a nonrecurring basis | Properties, net | Total Estimated Fair Value	'	'		'	'
Assets and liabilities measured at fair value on a non-recurring basis	'	'		'	'
Assets	'	$142,702	[2]	'	$142,702	[2]
[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.

Fair_Value_Measurements_Detail2

Fair Value Measurements (Details 3) (USD $)	3 Months Ended			9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013		Sep. 30, 2014	Sep. 30, 2013
Assets and liabilities measured at fair value on a non-recurring basis	'	'		'	'
Impairment losses	$66	$5,857		$1,368	$5,857
Exit costs on property dispositions	'	'		0	186
Properties, net | Fair value measurement on a nonrecurring basis	'	'		'	'
Assets and liabilities measured at fair value on a non-recurring basis	'	'		'	'
Impairment losses	'	21,888	[1]	1,371	30,940	[1]
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	'	'		'	'
Exit costs on property dispositions	'	186		'	186
Property Conveyances | Fair value measurement on a nonrecurring basis	'	'		'	'
Assets and liabilities measured at fair value on a non-recurring basis	'	'		'	'
Impairment losses	'	10,400		'	'
Strategic Reallocation Plan | Fair value measurement on a nonrecurring basis	'	'		'	'
Assets and liabilities measured at fair value on a non-recurring basis	'	'		'	'
Exit costs on property dispositions	'	'		'	186
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	'	'		'	'
Number of properties with a shortened holding period (in properties)	'	2		'	'
Greater Baltimore | 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	'	5,900		'	'
Discontinued Operations | Strategic Reallocation Plan | Fair value measurement on a nonrecurring basis	'	'		'	'
Assets and liabilities measured at fair value on a non-recurring basis	'	'		'	'
Impairment losses	'	'		'	$14,800
[1]	Represents impairment losses, excluding exit costs incurred of $186,000 for the three months and nine months ended September 30, 2013.

Fair_Value_Measurements_Detail3

Fair Value Measurements (Details 4) (USD $)

9 Months Ended

9 Months Ended

In Thousands, unless otherwise specified

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2014

Discounted Cash Flow Valuation Technique

Properties, net

Properties, net

Properties, net

Minimum

Maximum

Weighted Average

Redeemable Non-Controlling Interest

Fair value measurement on a nonrecurring basis

Fair value measurement on a nonrecurring basis

Contract of sale

Discounted Cash Flow Valuation Technique

Discounted Cash Flow Valuation Technique

Discounted Cash Flow Valuation Technique

Discounted Cash Flow Valuation Technique

Fair value measurement on a recurring basis

Significant Unobservable Inputs (Level 3)

Fair value measurement on a nonrecurring basis

Fair value measurement on a nonrecurring basis

Fair value measurement on a nonrecurring basis

Fair value measurement on a nonrecurring basis

Fair value measurement on a nonrecurring basis

Significant Unobservable Inputs (Level 3)

Significant Unobservable Inputs (Level 3)

Significant Unobservable Inputs (Level 3)

Assets and liabilities measured at fair value on a non-recurring basis

'

'

'

'

'

'

'

'

Fair value inputs, discount rate

'

'

'

'

10.00%

11.00%

10.90%

15.50%

Assets

'

$142,702

[1]

$110,499

[2]

$32,203

[3]

'

'

'

'

Fair Value Inputs Terminal Capitalization Rate

'

'

'

'

9.50%

10.00%

9.70%

'

Fair Value Inputs Market Rent Growth Rate

3.00%

[3]

'

'

'

'

'

'

'

Fair Value Inputs Expense Growth Rate

3.00%

[3]

'

'

'

'

'

'

'

[1]

Reflects balance sheet classifications of assets at time of fair value measurement, excluding the effect of held for sale classifications.

[2]

These fair value measurements were developed as a result of negotiations between us and potential, or actual, purchasers of the properties.

[3]

Only one value applied for this unobservable input.

Properties_net_Details

Properties, net (Details) (USD $)	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014	Dec. 31, 2013
			Greater Philadelphia	Operating properties, net	Operating properties, net	Operating properties, net	Operating properties, net	Operating properties, net	Operating properties, net
						Land	Land	Buildings and improvements	Buildings and improvements
Investments in real estate	'	'	'	'	'	'	'	'	'
Gross	'	'	'	'	'	$439,355,000	$430,472,000	$2,997,450,000	$2,869,870,000
Less: accumulated depreciation	'	'	'	-679,598,000	-597,649,000	'	'	'	'
Operating properties, net	2,757,207,000	2,702,693,000	'	'	'	'	'	'	'
Depreciation expense on reclassified assets	'	'	$12,900,000	'	'	'	'	'	'

Properties_net_Details_2

Properties, net (Details 2) (USD $)	Sep. 30, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Properties	'	'
Projects in development or held for future development	$513,949	$511,608
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	229,286	245,676
Projects in development or held for future development | Construction in progress, excluding land	'	'
Properties	'	'
Projects in development or held for future development	$284,663	$265,932

Properties_net_Details_3

Properties, net (Details 3)	9 Months Ended
	Sep. 30, 2014
	Property
	sqft
Newly constructed properties placed in service	'
Construction and Redevelopment Activities	'
Square feet of properties placed in service	692,000
Number of real estate properties placed in service	5
Properties under construction or contractually committed for construction	'
Construction and Redevelopment Activities	'
Number of real estate properties	6
Square footage of real estate properties (in square feet)	861,000
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 | Northern Virginia	'
Construction and Redevelopment Activities	'
Number of real estate properties	2
Properties under construction or contractually committed for construction | San Antonio	'
Construction and Redevelopment Activities	'
Number of real estate properties	1
Properties under construction or contractually committed for construction | Huntsville	'
Construction and Redevelopment Activities	'
Number of real estate properties	1
Properties under or approved for redevelopment	'
Construction and Redevelopment Activities	'
Number of real estate properties	4
Square footage of real estate properties (in square feet)	276,000
Properties under or approved for redevelopment | Baltimore and Washington Corridor	'
Construction and Redevelopment Activities	'
Number of real estate properties	2
Properties under or approved for redevelopment | Greater Philadelphia	'
Construction and Redevelopment Activities	'
Number of real estate properties	1
Properties under or approved for redevelopment | St Marys County	'
Construction and Redevelopment Activities	'
Number of real estate properties	1

Properties_net_Details_4

Properties, net (Details 4) (USD $)	9 Months Ended				3 Months Ended
	Sep. 30, 2014	Sep. 30, 2014	Sep. 30, 2014	Sep. 30, 2014	Sep. 30, 2014
	Operating properties, net	Operating properties, net	Operating properties, net	Operating properties, net	Non-operating Properties
	buillding	4969 and 4979 Mercantile Rd	9930 and 9940 Franklin Square	5020, 5022, 5024 and 5026 Campbell Boulevard
	sqft	buillding	buillding	buillding
		sqft	sqft	sqft
Investments in real estate	'	'	'	'	'
Number of Properties Disposed	8	2	2	4	'
Total Rentable Square Feet	302,958	96,721	71,992	134,245	'
Transaction Value of Operating Property Dispositions	$28,835,000	$5,960,000	$10,475,000	$12,400,000	'
Gain on Sale	5,095,000	2,126,000	2,303,000	666,000	5,500,000
Transaction Value of Non-operating Property Dispositions	'	'	'	'	$28,300,000

Real_Estate_Joint_Ventures_Det

Real Estate Joint Ventures (Details) (USD $)	Sep. 30, 2014		Sep. 30, 2014		Sep. 30, 2014		Sep. 30, 2014		Sep. 30, 2014		Aug. 07, 2014
	Consolidated real estate joint ventures		LW Redstone Company, LLC		LW Redstone Company, LLC		M Square Associates, LLC		M Square Associates, LLC		COPT-FD Indian Head, LLC
			buillding		Variable Interest Entity, Primary Beneficiary [Member]		buillding		Variable Interest Entity, Primary Beneficiary [Member]		Variable Interest Entity, Primary Beneficiary [Member]
Investments in consolidated real estate joint ventures	'		'		'		'		'		'
Ownership (as a percent)	'		'		85.00%		'		50.00%		'
Total Assets	$199,429,000	[1]	'		$139,314,000	[1],[2]	'		$60,115,000	[1],[3]	'
Encumbered Assets	114,915,000	[1]	'		67,100,000	[1],[2]	'		47,815,000	[1],[3]	'
Total Liabilities	81,944,000	[1]	'		42,134,000	[1],[2]	'		39,810,000	[1],[3]	'
Buildings Operated by Joint Ventures Number	'		4	[2]	'		2	[3]	'		'
Proceeds from the repurchase of real estate	'		'		'		'		'		$6,400,000
[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 Baltimore/Washington Corridor).

Mortgage_and_Other_Investing_R2

Mortgage and Other Investing Receivables (Details) (USD $)	Sep. 30, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Accounts, Notes, Loans and Financing Receivable [Line Items]	'	'
Mortgage and other investing receivables	$50,886	$53,663
Notes Receivable from City of Huntsville	'	'
Accounts, Notes, Loans and Financing Receivable [Line Items]	'	'
Mortgage and other investing receivables	47,878	44,055
Other investing loans receivable	'	'
Accounts, Notes, Loans and Financing Receivable [Line Items]	'	'
Mortgage and other investing receivables	$3,008	$9,608

Prepaid_Expenses_and_Other_Ass2

Prepaid Expenses and Other Assets (Details) (USD $)	Sep. 30, 2014		Dec. 31, 2013
Prepaid Expense and Other Assets [Abstract]	'		'
Prepaid expenses	$24,497,000		$19,308,000
Construction contract costs incurred in excess of billings	17,962,000		2,462,000
Lease incentives	12,939,000		8,435,000
Furniture, fixtures and equipment, net	6,527,000		6,556,000
Deferred tax asset, net	4,058,000	[1]	4,305,000	[1]
Interest rate derivatives	2,907,000		6,594,000
Other equity method investments	2,240,000		2,258,000
Other assets	3,733,000		4,268,000
Prepaid expenses and other assets	74,863,000		54,186,000
Long-term Operating Notes Receivable	'		'
Mortgage and Other Investing Receivables [Line Items]	'		'
Operating notes receivable	1,700,000		1,700,000
Allowances for estimated losses	207,000		87,000
Taxable REIT Subsidiary	'		'
Mortgage and Other Investing Receivables [Line Items]	'		'
Deferred tax assets, valuation allowance	$2,100,000		$2,100,000
[1]	Includes a valuation allowance of $2.1 million.

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

0 Months Ended

9 Months Ended

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Dec. 31, 2013

Sep. 30, 2014

Dec. 31, 2013

Sep. 30, 2014

Dec. 31, 2013

Sep. 30, 2014

Dec. 31, 2013

Sep. 30, 2014

Dec. 31, 2013

Sep. 30, 2014

Dec. 31, 2013

Sep. 30, 2014

Dec. 31, 2013

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Dec. 31, 2013

Apr. 01, 2014

Apr. 01, 2014

Sep. 30, 2014

Dec. 31, 2013

Sep. 30, 2014

Dec. 31, 2013

14-May-14

Sep. 30, 2014

14-May-14

Dec. 31, 2013

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

14-May-14

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

Revolving Credit Facility

Revolving Credit Facility

Term Loan Facilities

Term Loan Facilities

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

Debt in default of payment terms

Debt in default of payment terms

Unsecured senior notes

Unsecured senior notes

Unsecured senior notes

Unsecured senior notes

Unsecured senior notes

Unsecured senior notes

Unsecured senior notes

Unsecured senior notes

Revolving Credit Facility

Minimum

Minimum

Maximum

Maximum

Adjusted Treasury

Wholly owned subsidiary

Wholly owned subsidiary

3.60% Senior Notes

3.60% Senior Notes

5.250% Senior Notes

5.250% Senior Notes

3.70% Senior Notes

3.70% Senior Notes

3.70% Senior Notes

3.70% Senior Notes

Revolving Credit Facility

Term Loan Facilities

Revolving Credit Facility

Term Loan Facilities

Unsecured senior notes

Northern Virginia

3.70% Senior Notes

Property

Debt

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Maximum Availability

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

$800,000,000

[1]

'

'

'

'

'

Carrying Value

2,049,992,000

'

2,049,992,000

'

1,927,703,000

637,169,000

712,751,000

600,082,000

[2]

675,060,000

[2]

37,087,000

37,691,000

0

[1]

0

[1]

520,000,000

[3]

620,000,000

[3]

1,631,000

1,700,000

570,000

[4]

'

570,000

[4]

'

563,000

[4]

150,000,000

'

347,432,000

[5]

347,244,000

[5]

245,707,000

[6]

245,445,000

[6]

'

297,483,000

[7]

'

0

[7]

'

'

'

'

'

'

Stated interest rates, low end of range (as a percent)

'

'

'

'

'

'

'

3.96%

[8]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Stated interest rates, high end of range (as a percent)

'

'

'

'

'

'

'

10.65%

[8]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Description of variable rate basis

'

'

'

'

'

'

'

'

'

'LIBOR

[9]

'

'LIBOR

[1]

'

'LIBOR

[10]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Variable rate, spread (as a percent)

'

'

'

'

'

'

'

'

'

2.25%

[9]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

0.98%

[1]

1.10%

[10]

1.75%

[1]

2.60%

[10]

'

Stated interest rate (as a percent)

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

0.00%

[11]

'

4.25%

[4]

'

4.25%

[4]

'

'

10.65%

'

3.60%

[5]

'

5.25%

[6]

'

'

3.70%

[7]

3.70%

'

'

'

'

'

'

'

Number of real estate properties

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

2

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Debt issuance as a percentage of principal amount

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

99.74%

'

'

'

'

'

'

'

'

'

Proceeds from issuance of debt

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

297,300,000

'

'

'

'

'

'

'

'

'

Unamortized premium included in carrying value

'

'

'

'

'

'

'

49,000

69,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Weighted average interest rate (as a percent)

'

'

'

'

'

'

'

7.20%

'

'

'

'

'

1.80%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Interest rate on debt (as a percent)

'

'

'

'

'

'

'

'

'

2.41%

'

'

'

'

'

'

'

6.05%

'

6.05%

'

'

'

'

3.70%

'

5.49%

'

'

'

3.85%

'

'

'

'

'

'

'

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

'

'

'

'

'

'

'

'

'

'

'

'

'

180,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Debt instrument, face amount

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

350,000,000

'

250,000,000

'

'

'

300,000,000

'

'

'

'

'

'

'

Unamortized discount included in carrying value

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

680,000

761,000

5,000

'

5,000

'

12,000

'

'

2,600,000

2,800,000

4,300,000

4,600,000

'

2,500,000

'

'

'

'

'

'

'

'

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

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

20.8513

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Convertible debt principal amount, basis for exchange

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

1,000

'

1,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

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

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

$47.96

'

$47.96

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Principal amount of debt

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

575,000

'

575,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Interest expense at stated interest rate

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

6,000

6,000

18,000

4,201,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Interest expense associated with amortization of discount

'

'

659,000

1,207,000

'

'

'

'

'

'

'

'

'

'

'

'

'

2,000

2,000

7,000

1,613,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Total interest expenses

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

8,000

8,000

25,000

5,814,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Basis points used in determining redemption price prior to maturity

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

0.25%

Interest costs capitalized

$1,300,000

$2,200,000

$4,300,000

$6,700,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

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'

'

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'

'

'

'

'

'

'

'

[1]

No borrowings were outstanding on this facility as of the end of the respective periods.

[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 $49,000 as of September 30, 2014 and $69,000 as of December 31, 2013.

[3]

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

[4]

As described further in our 2013 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, 2014 and is equivalent to an exchange price of $47.96 per common share). The carrying value of these notes included a principal amount of $575,000 and an unamortized discount totaling $5,000 as of September 30, 2014 and $12,000 as of December 31, 2013. The effective interest rate under the notes, including amortization of the issuance costs, was 6.05%. Because the closing price of our common shares as of September 30, 2014 and December 31, 2013 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, 2014 2013 2014 2013Interest expense at stated interest rate$6 $6 $18 $4,201Interest expense associated with amortization of discount2 2 7 1,613Total$8 $8 $25 $5,814

[5]

The carrying value of these notes included a principal amount of $350.0 million and an unamortized discount totaling $2.6 million as of September 30, 2014 and $2.8 million as of December 31, 2013. The effective interest rate under the notes, including amortization of the issuance costs, was 3.70%.

[6]

The carrying value of these notes included a principal amount of $250.0 million and an unamortized discount totaling $4.3 million as of September 30, 2014 and $4.6 million as of December 31, 2013. The effective interest rate under the notes, including amortization of the issuance costs, was 5.49%.

[7]

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

[8]

The weighted average interest rate on these loans was 7.20% as of September 30, 2014.

[9]

The interest rate on the loan outstanding was 2.41% as of September 30, 2014.

[10]

The weighted average interest rate on these loans was 1.80% as of September 30, 2014.

[11]

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 $680,000 as of September 30, 2014 and $761,000 as of December 31, 2013.

Debt_Details_2

Debt (Details 2) (USD $)	Sep. 30, 2014		Dec. 31, 2013
In Thousands, unless otherwise specified
Carrying amount and estimated fair value of debt	'		'
Carrying Value	$2,049,992		$1,927,703
4.25% Exchangeable Senior Notes	'		'
Carrying amount and estimated fair value of debt	'		'
Carrying Value	570	[1]	563	[1]
Carrying Amount	'		'
Carrying amount and estimated fair value of debt	'		'
Variable-rate debt	557,087		657,691
Carrying Value	2,049,992		1,927,703
Carrying Amount | Unsecured senior notes	'		'
Carrying amount and estimated fair value of debt	'		'
Fixed-rate debt	890,622		592,689
Carrying Amount | 4.25% Exchangeable Senior Notes	'		'
Carrying amount and estimated fair value of debt	'		'
Fixed-rate debt	570		563
Carrying Amount | Other fixed-rate debt	'		'
Carrying amount and estimated fair value of debt	'		'
Fixed-rate debt	601,713		676,760
Total Estimated Fair Value	'		'
Carrying amount and estimated fair value of debt	'		'
Variable-rate debt	558,665		657,527
Carrying Value	2,038,378		1,884,473
Total Estimated Fair Value | Unsecured senior notes	'		'
Carrying amount and estimated fair value of debt	'		'
Fixed-rate debt	901,022		575,374
Total Estimated Fair Value | 4.25% Exchangeable Senior Notes	'		'
Carrying amount and estimated fair value of debt	'		'
Fixed-rate debt	575		575
Total Estimated Fair Value | Other fixed-rate debt	'		'
Carrying amount and estimated fair value of debt	'		'
Fixed-rate debt	$578,116		$650,997
[1]	As described further in our 2013 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, 2014 and is equivalent to an exchange price of $47.96 per common share). The carrying value of these notes included a principal amount of $575,000 and an unamortized discount totaling $5,000 as of September 30, 2014 and $12,000 as of December 31, 2013. The effective interest rate under the notes, including amortization of the issuance costs, was 6.05%. Because the closing price of our common shares as of September 30, 2014 and December 31, 2013 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, 2014 2013 2014 2013Interest expense at stated interest rate$6 $6 $18 $4,201Interest expense associated with amortization of discount2 2 7 1,613Total$8 $8 $25 $5,814

Interest_Rate_Derivatives_Deta

Interest Rate Derivatives (Details) (USD $)	3 Months Ended			9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013	Sep. 30, 2014		Sep. 30, 2013	Dec. 31, 2013
Fair value of interest rate derivatives and balance sheet classification	'		'	'		'	'
Interest rate derivatives	$2,907,000		'	$2,907,000		'	$6,594,000
Effect of interest rate derivatives on consolidated statements of operations and comprehensive income	'		'	'		'	'
Amount of gains (losses) recognized in accumulated other comprehensive income (loss) (“AOCI”) (effective portion)	1,015,000		-2,482,000	-4,738,000		5,810,000	'
Amount of losses reclassified from AOCI into interest expense (effective portion)	756,000		689,000	2,170,000		2,021,000	'
Interest rate swaps	'		'	'		'	'
Effect of interest rate derivatives on consolidated statements of operations and comprehensive income	'		'	'		'	'
Approximate loss amount to be reclassified from AOCI to interest expense over the next 12 months	-2,900,000		'	-2,900,000		'	'
Interest rate derivatives in liability position, fair value	2,200,000		'	2,200,000		'	'
Termination value to settle obligations under interest rate derivative agreements	2,500,000		'	2,500,000		'	'
Interest rate swaps | Prepaid expenses and other current assets	'		'	'		'	'
Fair value of interest rate derivatives and balance sheet classification	'		'	'		'	'
Interest rate derivatives	2,907,000		'	2,907,000		'	6,594,000
Interest rate swaps | Interest rate derivatives	'		'	'		'	'
Fair value of interest rate derivatives and balance sheet classification	'		'	'		'	'
Fair value of Interest rate swaps classified as interest rate derivatives	-2,236,000		'	-2,236,000		'	-3,309,000
Designated	'		'	'		'	'
Fair values of interest rate swap derivatives	'		'	'		'	'
Fair value of interest rate swaps	671,000		'	671,000		'	3,285,000
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%		'	'
Fair value of interest rate swaps	0		'	0		'	-279,000
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%		'	'
Fair value of interest rate swaps	0		'	0		'	-277,000
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%		'	'
Fair value of interest rate swaps	-556,000		'	-556,000		'	-861,000
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%		'	'
Fair value of interest rate swaps	-556,000		'	-556,000		'	-861,000
Designated | Interest rate swap, effective date November 2, 2010	'		'	'		'	'
Fair values of interest rate swap derivatives	'		'	'		'	'
Notional Amount	37,087,000	[1]	'	37,087,000	[1]	'	'
Fixed rate (as a percent)	3.83%		'	3.83%		'	'
Derivative, basis spread on variable rate	2.25%		'	2.25%		'	'
Fair value of interest rate swaps	-515,000		'	-515,000		'	-832,000
Notional amount of interest rate derivatives after scheduled amortization	36,200,000		'	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%		'	'
Fair value of interest rate swaps	-300,000		'	-300,000		'	-94,000
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%		'	'
Fair value of interest rate swaps	-309,000		'	-309,000		'	-105,000
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%		'	'
Fair value of interest rate swaps	1,551,000		'	1,551,000		'	3,377,000
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%		'	'
Fair value of interest rate swaps	$1,356,000		'	$1,356,000		'	$3,217,000
[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, 2014	Sep. 30, 2013
Redeemable Noncontrolling Interest [Roll Forward]	'	'
Beginning balance	$17,758	$10,298
Distributions	-976	-811
Net income attributable to noncontrolling interest	1,647	888
Adjustment to arrive at fair value of interest	7	6,414
Ending balance	$18,436	$16,789

Equity_Details

Equity (Details) (USD $)	9 Months Ended		0 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Jun. 16, 2014	Jun. 16, 2014
			Series H Preferred Stock	Series H Preferred Stock
Class of Stock [Line Items]	'	'	'	'
Preferred stock, dividend rate	'	'	7.50%	'
Preferred stock, redemption price per share	'	'	'	$25
Stock redeemed or called during period (in units/ shares)	$50,000,000	$84,750,000	$50,000,000	'
Issuance costs associated with redeemed preferred shares	'	'	$1,800,000	'
Number of operating partnerships units converted into common shares (in units)	117,149	310,889	'	'
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, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
	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	$118,240	$124,561	$359,100	$371,913	'
Property operating expenses	42,881	44,885	136,489	133,605	'
NOI from real estate operations	75,359	79,676	222,611	238,308	'
Additions to long-lived assets	22,793	13,942	'	'	'
Transfers from non-operating properties	42,802	42,842	'	'	'
Segment assets	3,680,188	3,755,588	3,680,188	3,755,588	3,629,952
Operating Segment Total	'	'	'	'	'
Segment financial information for real estate operations	'	'	'	'	'
Revenues from real estate operations	'	'	359,100	371,913	'
Property operating expenses	'	'	136,489	133,605	'
NOI from real estate operations	'	'	222,611	238,308	'
Additions to long-lived assets	'	'	50,618	31,578	'
Transfers from non-operating properties	'	'	135,127	197,600	'
Segment assets	2,962,686	3,053,813	2,962,686	3,053,813	'
Baltimore and Washington Corridor	'	'	'	'	'
Segment financial information for real estate operations	'	'	'	'	'
Revenues from real estate operations	58,883	59,558	177,452	177,858	'
Property operating expenses	19,457	19,642	62,402	59,423	'
NOI from real estate operations	39,426	39,916	115,050	118,435	'
Additions to long-lived assets	7,248	7,292	19,278	14,172	'
Transfers from non-operating properties	22,680	8,523	50,303	37,607	'
Segment assets	1,278,713	1,275,972	1,278,713	1,275,972	'
Northern Virginia	'	'	'	'	'
Segment financial information for real estate operations	'	'	'	'	'
Revenues from real estate operations	21,369	23,062	67,235	68,992	'
Property operating expenses	7,500	7,999	24,124	24,020	'
NOI from real estate operations	13,869	15,063	43,111	44,972	'
Additions to long-lived assets	5,898	2,459	14,198	6,699	'
Transfers from non-operating properties	15,403	26,355	42,674	37,209	'
Segment assets	648,248	593,335	648,248	593,335	'
San Antonio	'	'	'	'	'
Segment financial information for real estate operations	'	'	'	'	'
Revenues from real estate operations	9,031	7,898	26,268	24,019	'
Property operating expenses	5,100	4,061	14,391	12,427	'
NOI from real estate operations	3,931	3,837	11,877	11,592	'
Additions to long-lived assets	0	251	-6	268	'
Transfers from non-operating properties	0	0	0	0	'
Segment assets	116,837	118,701	116,837	118,701	'
Huntsville	'	'	'	'	'
Segment financial information for real estate operations	'	'	'	'	'
Revenues from real estate operations	2,471	1,360	7,430	3,459	'
Property operating expenses	763	382	2,275	874	'
NOI from real estate operations	1,708	978	5,155	2,585	'
Additions to long-lived assets	455	4	3,296	3,501	'
Transfers from non-operating properties	1,496	-33	21,821	20,924	'
Segment assets	98,334	50,737	98,334	50,737	'
Washington, DC - Capitol Riverfront	'	'	'	'	'
Segment financial information for real estate operations	'	'	'	'	'
Revenues from real estate operations	3,524	4,295	10,989	12,716	'
Property operating expenses	1,824	1,946	5,343	5,769	'
NOI from real estate operations	1,700	2,349	5,646	6,947	'
Additions to long-lived assets	458	86	999	484	'
Transfers from non-operating properties	0	0	0	0	'
Segment assets	96,131	99,852	96,131	99,852	'
St. Mary's and King George Counties	'	'	'	'	'
Segment financial information for real estate operations	'	'	'	'	'
Revenues from real estate operations	4,158	4,270	12,676	12,355	'
Property operating expenses	1,277	1,299	4,070	3,715	'
NOI from real estate operations	2,881	2,971	8,606	8,640	'
Additions to long-lived assets	5,189	630	6,971	1,634	'
Transfers from non-operating properties	0	0	0	12	'
Segment assets	100,009	96,679	100,009	96,679	'
Greater Baltimore	'	'	'	'	'
Segment financial information for real estate operations	'	'	'	'	'
Revenues from real estate operations	10,436	10,703	32,956	32,246	'
Property operating expenses	3,810	4,143	13,786	12,408	'
NOI from real estate operations	6,626	6,560	19,170	19,838	'
Additions to long-lived assets	3,021	1,707	5,275	1,761	'
Transfers from non-operating properties	495	9	3,522	113	'
Segment assets	274,931	305,947	274,931	305,947	'
Greater Philadelphia	'	'	'	'	'
Segment financial information for real estate operations	'	'	'	'	'
Revenues from real estate operations	2,951	3,258	8,657	8,529	'
Property operating expenses	837	924	3,281	2,482	'
NOI from real estate operations	2,114	2,334	5,376	6,047	'
Additions to long-lived assets	625	102	724	387	'
Transfers from non-operating properties	2,506	3,414	15,880	28,031	'
Segment assets	107,051	105,112	107,051	105,112	'
Colorado Springs	'	'	'	'	'
Segment financial information for real estate operations	'	'	'	'	'
Revenues from real estate operations	0	6,571	6	19,722	'
Property operating expenses	3	2,361	5	6,983	'
NOI from real estate operations	-3	4,210	1	12,739	'
Additions to long-lived assets	0	1,413	0	2,323	'
Transfers from non-operating properties	0	1,860	30	4,539	'
Segment assets	0	158,690	0	158,690	'
Other	'	'	'	'	'
Segment financial information for real estate operations	'	'	'	'	'
Revenues from real estate operations	2,541	2,510	7,662	7,571	'
Property operating expenses	257	282	1,190	824	'
NOI from real estate operations	2,284	2,228	6,472	6,747	'
Additions to long-lived assets	-125	-88	-163	87	'
Transfers from non-operating properties	0	0	0	2	'
Segment assets	78,240	80,749	78,240	80,749	'
Wholesale Data Center	'	'	'	'	'
Segment financial information for real estate operations	'	'	'	'	'
Revenues from real estate operations	2,876	1,076	7,769	4,446	'
Property operating expenses	2,053	1,846	5,622	4,680	'
NOI from real estate operations	823	-770	2,147	-234	'
Additions to long-lived assets	24	86	46	262	'
Transfers from non-operating properties	222	2,714	897	69,163	'
Segment assets	$164,192	$168,039	$164,192	$168,039	'

Information_by_Business_Segmen3

Information by Business Segment (Details 2) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Reconciliation of segment revenues to total revenues	'	'	'	'
Segment revenues from real estate operations	$118,240	$124,561	$359,100	$371,913
Construction contract and other service revenues	34,739	16,991	80,390	52,048
Less: Revenues from discontinued operations (Note 15)	36	-9,740	12	-29,403
Total revenues	153,015	131,812	439,502	394,558
Reconciliation of segment property operating expenses to property operating expenses	'	'	'	'
Segment property operating expenses	42,881	44,885	136,489	133,605
Less: Property operating expenses from discontinued operations (Note 15)	175	-3,524	111	-10,523
Total property operating expenses	43,056	41,361	136,600	123,082
Computation of net operating income from service operations	'	'	'	'
Construction contract and other service revenues	34,739	16,991	80,390	52,048
Construction contract and other service expenses	-33,593	-16,306	-75,353	-49,165
NOI from service operations	1,146	685	5,037	2,883
Reconciliation of NOI from real estate operations and NOI from service operations to (loss) income from continuing operations	'	'	'	'
NOI from real estate operations	75,359	79,676	222,611	238,308
NOI from service operations	1,146	685	5,037	2,883
Interest and other income (loss)	1,191	-3	3,775	2,949
Equity in income of unconsolidated entities	193	44	206	211
Income tax expense	-101	-24	-257	-61
Other adjustments:	'	'	'	'
Depreciation and other amortization associated with real estate operations	-30,237	-28,205	-104,728	-82,888
Impairment losses	-66	-5,857	-1,368	-5,857
General, administrative and leasing expenses	-7,211	-8,027	-22,882	-22,430
Business development expenses and land carry costs	-1,430	-1,383	-4,107	-4,069
Interest expense on continuing operations	-24,802	-19,342	-69,107	-60,734
NOI from discontinued operations	-139	-6,216	-99	-18,880
Loss on early extinguishment of debt	-176	-374	-446	-27,028
Income from continuing operations	$13,727	$10,974	$28,635	$22,404

Information_by_Business_Segmen4

Information by Business Segment (Details 3) (USD $)	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013
In Thousands, unless otherwise specified
Reconciliation of segment assets to total assets	'	'	'
Assets	$3,680,188	$3,629,952	$3,755,588
Segment assets	'	'	'
Reconciliation of segment assets to total assets	'	'	'
Assets	2,962,686	'	3,053,813
Non-operating property assets	'	'	'
Reconciliation of segment assets to total assets	'	'	'
Assets	518,951	'	530,389
Other assets	'	'	'
Reconciliation of segment assets to total assets	'	'	'
Assets	$198,551	'	$171,386

ShareBased_Compensation_Detail

Share-Based Compensation (Details) (USD $)	9 Months Ended				0 Months Ended	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2014	Mar. 06, 2014	Sep. 30, 2014
			Restricted shares	Options	2014 PSU Grants	2014 PSU Grants
					Performance share units	Performance share units
						Percentile_Rank
Share-Based Compensation	'	'	'	'	'	'
Stock awards granted (in shares or units)	'	'	211,982	'	49,103	'
Aggregate grant date fair value	'	'	$5,700,000	'	$1,700,000	'
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	'	'	'	'	'	'
Weighted average grant date fair value (in dollars per share)	'	'	$26.71	'	'	$35.09
Baseline value per common share (in dollars per share)	'	'	'	'	'	$26.52
Expected volatility of common shares (as a percent)	'	'	'	'	'	28.60%
Risk-free interest rate (as a percent)	'	'	'	'	'	0.66%
Other Share-based Compensation Additional Disclosures	'	'	'	'	'	'
Shares vested (in shares)	'	'	165,076	'	'	'
Weighted average fair value of shares vested (in dollars per share)	'	'	$29.01	'	'	'
Aggregate intrinsic value of restricted common shares, forfeiture restrictions lapsed	'	'	4,400,000	'	'	'
Options	'	'	'	'	'	'
Exercised (in shares)	57,888	32,756	'	57,888	'	'
Weighted average exercise price (in dollars per share)	'	'	'	$23.50	'	'
Aggregate intrinsic value of options exercised (in dollars)	'	'	'	$214,000	'	'

Income_Taxes_Details

Income Taxes (Details) (Taxable REIT Subsidiary, USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Taxable REIT Subsidiary	'	'	'	'
Deferred income tax (expense) benefit	'	'	'	'
Federal	($83)	($20)	($215)	($50)
State	-18	-4	-42	-11
Total income tax expense	($101)	($24)	($257)	($61)
Effective tax rate (as a percent)	37.70%	36.70%	37.70%	36.70%

Discontinued_Operations_Detail

Discontinued Operations (Details) (USD $)	3 Months Ended		9 Months Ended		0 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 12, 2013	Dec. 23, 2013	Dec. 23, 2013
					December 2013 Colorado Springs Portfolio Disposition	Baltimore and Washington Corridor	Colorado Springs
					Property	Property	Property
Properties	'	'	'	'	'	'	'
Number of properties sold	'	'	'	'	15	'	'
Number of properties transferred	'	'	'	'	'	9	5
Revenue from real estate operations	($36)	$9,740	($12)	$29,403	'	'	'
Property operating expenses	175	-3,524	111	-10,523	'	'	'
Depreciation and amortization	0	-1,005	0	-3,509	'	'	'
Impairment recoveries (losses)	24	-16,217	-3	-25,269	'	'	'
General, administrative and leasing expenses	0	0	0	-1	'	'	'
Interest expense	0	-1,968	0	-6,316	'	'	'
Gain on sales of real estate	28	0	24	0	'	'	'
Loss on early extinguishment of debt	0	0	-116	0	'	'	'
Discontinued operations	$191	($12,974)	$4	($16,215)	'	'	'

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, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Numerator:	'		'		'		'
Income from continuing operations	$13,727		$10,974		$28,635		$22,404
Gain on sales of real estate, net	10,630		0		10,630		2,683
Preferred share/unit dividends/distributions	-3,553		-4,490		-12,387		-15,481
Issuance costs associated with redeemed preferred shares	0		0		-1,769		-2,904
Income from continuing operations attributable to noncontrolling interests	-1,820		-1,456		-3,923		-2,710
Income from continuing operations attributable to restricted shares	-103		-97		-332		-317
Numerator for basic and diluted EPS from continuing operations attributable to COPT common shareholders	18,881		4,931		20,854		3,675
Discontinued operations	191		-12,974		4		-16,215
Discontinued operations attributable to noncontrolling interests	-8		492		5		529
Numerator for basic and diluted EPS on net income (loss) attributable to COPT common shareholders	19,064		-7,551		20,863		-12,011
Denominator (all weighted averages):	'		'		'		'
Denominator for basic EPS (common shares)	87,290		86,760		87,196		84,547
Dilutive effect of share-based compensation awards	195		0		169		0
Denominator for diluted EPS (common shares)	87,485		86,760		87,365		84,547
Basic EPS:	'		'		'		'
Income from continuing operations (in dollars per share/unit)	$0.22	[1]	$0.06	[1]	$0.24	[1]	$0.04	[1]
Discontinued operations (in dollars per share/unit)	$0	[1]	($0.15)	[1]	$0	[1]	($0.18)	[1]
Net income (loss) attributable to COPT common shareholders (in dollars per share/unit)	$0.22	[1]	($0.09)	[1]	$0.24	[1]	($0.14)	[1]
Diluted EPS:	'		'		'		'
Income from continuing operations (in dollars per share/unit)	$0.22	[1]	$0.06	[1]	$0.24	[1]	$0.04	[1]
Discontinued operations (in dollars per share/unit)	$0	[1]	($0.15)	[1]	$0	[1]	($0.18)	[1]
Net income (loss) attributable to COPT common shareholders (in dollars per share/unit)	$0.22	[1]	($0.09)	[1]	$0.24	[1]	($0.14)	[1]
Corporate Office Properties, L.P.	'		'		'		'
Numerator:	'		'		'		'
Income from continuing operations	13,727		10,974		28,635		22,404
Gain on sales of real estate, net	10,630		0		10,630		2,683
Preferred share/unit dividends/distributions	-3,718		-4,655		-12,882		-15,976
Issuance costs associated with redeemed preferred shares	0		0		-1,769		-2,904
Income from continuing operations attributable to noncontrolling interests	-897		-986		-2,476		-2,025
Income from continuing operations attributable to restricted shares	-103		-97		-332		-317
Numerator for basic and diluted EPS from continuing operations attributable to COPT common shareholders	19,639		5,236		21,806		3,865
Discontinued operations	191		-12,974		4		-16,215
Discontinued operations attributable to noncontrolling interests	0		-49		5		-147
Numerator for basic and diluted EPS on net income (loss) attributable to COPT common shareholders	$19,830		($7,787)		$21,815		($12,497)
Denominator (all weighted averages):	'		'		'		'
Denominator for basic EPS (common shares)	91,166		90,564		91,111		88,379
Dilutive effect of share-based compensation awards	195		0		169		0
Denominator for diluted EPS (common shares)	91,361		90,564		91,280		88,379
Basic EPS:	'		'		'		'
Income from continuing operations (in dollars per share/unit)	$0.22	[2]	$0.06	[2]	$0.24	[2]	$0.04	[2]
Discontinued operations (in dollars per share/unit)	$0	[2]	($0.15)	[2]	$0	[2]	($0.18)	[2]
Net income (loss) attributable to COPT common shareholders (in dollars per share/unit)	$0.22	[2]	($0.09)	[2]	$0.24	[2]	($0.14)	[2]
Diluted EPS:	'		'		'		'
Income from continuing operations (in dollars per share/unit)	$0.22	[2]	$0.06	[2]	$0.24	[2]	$0.04	[2]
Discontinued operations (in dollars per share/unit)	$0	[2]	($0.15)	[2]	$0	[2]	($0.18)	[2]
Net income (loss) attributable to COPT common shareholders (in dollars per share/unit)	$0.22	[2]	($0.09)	[2]	$0.24	[2]	($0.14)	[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, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Conversion of common units	'	'	'	'
Antidilutive securities	'	'	'	'
Antidilutive securities excluded from computation of diluted earnings per share (in shares)	3,876	3,804	3,915	3,832
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)	401	377	404	388
Weighted average options	'	'	'	'
Antidilutive securities	'	'	'	'
Antidilutive securities excluded from computation of diluted earnings per share (in shares)	490	613	495	571
Corporate Office Properties, L.P. | Weighted average restricted shares	'	'	'	'
Antidilutive securities	'	'	'	'
Antidilutive securities excluded from computation of diluted earnings per share (in shares)	401	377	404	388
Corporate Office Properties, L.P. | Weighted average options	'	'	'	'
Antidilutive securities	'	'	'	'
Antidilutive securities excluded from computation of diluted earnings per share (in shares)	490	613	495	571
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, 2014
		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	'	1,100,000
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	'	$5,600,000