COPT Defense Properties (CDP) 10-Q 30 Apr 15 2015 Q1 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	3 Months Ended
	Mar. 31, 2015	Apr. 17, 2015
Entity Information [Line Items]
Entity Registrant Name	CORPORATE OFFICE PROPERTIES TRUST
Entity Central Index Key	860546
Document Type	10-Q
Document Period End Date	31-Mar-15
Amendment Flag	FALSE
Current Fiscal Year End Date	-19
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		94,537,019
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q1
Corporate Office Properties, L.P.
Entity Information [Line Items]
Entity Registrant Name	CORPORATE OFFICE PROPERTIES, L.P.
Entity Central Index Key	1577966
Document Type	10-Q
Document Period End Date	31-Mar-15
Amendment Flag	FALSE
Current Fiscal Year End Date	-19
Entity Filer Category	Non-accelerated Filer
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q1

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Properties, net:
Operating properties, net	$2,888,534	$2,751,488
Projects in development or held for future development	489,618	545,426
Total properties, net	3,378,152	3,296,914
Assets held for sale, net	0	14,339
Cash and cash equivalents	4,429	6,077
Restricted cash and marketable securities	11,445	9,069
Accounts receivable (net of allowance for doubtful accounts of $784 and $717, respectively)	33,753	26,901
Deferred rent receivable (net of allowance of $1,695 and $1,418, respectively)	98,340	95,910
Intangible assets on real estate acquisitions, net	61,477	43,854
Deferred leasing and financing costs, net	65,245	64,797
Investing receivables	52,814	52,147
Prepaid expenses and other assets	71,500	60,249
Total assets	3,777,155	3,670,257
Liabilities:
Debt, net	1,999,622	1,920,057
Accounts payable and accrued expenses	138,214	123,035
Rents received in advance and security deposits	31,551	31,011
Dividends and distributions payable	30,174	29,862
Deferred revenue associated with operating leases	14,697	13,031
Interest rate derivatives	4,282	1,855
Other liabilities	9,990	12,105
Total liabilities	2,228,530	2,130,956
Commitments and contingencies
Redeemable noncontrolling interest	18,895	18,417
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 March 31, 2015 and December 31, 2014)	199,083	199,083
Common Shares of beneficial interest	945	933
Additional paid-in capital	1,999,708	1,969,968
Cumulative distributions in excess of net income	-733,459	-717,264
Accumulated other comprehensive loss	-3,947	-1,297
Total Corporate Office Properties Trust’s shareholders’ equity	1,462,330	1,451,423
Noncontrolling interests in subsidiaries:
Common units in COPLP	49,168	51,534
Preferred units in COPLP	8,800	8,800
Other consolidated entities	9,432	9,127
Noncontrolling interests in subsidiaries	67,400	69,461
Total equity	1,529,730	1,520,884
Total liabilities, redeemable noncontrolling interest and equity	3,777,155	3,670,257
Corporate Office Properties, L.P.
Properties, net:
Operating properties, net	2,888,534	2,751,488
Projects in development or held for future development	489,618	545,426
Total properties, net	3,378,152	3,296,914
Assets held for sale, net	0	14,339
Cash and cash equivalents	4,429	6,077
Restricted cash and marketable securities	5,509	3,187
Accounts receivable (net of allowance for doubtful accounts of $784 and $717, respectively)	33,753	26,901
Deferred rent receivable (net of allowance of $1,695 and $1,418, respectively)	98,340	95,910
Intangible assets on real estate acquisitions, net	61,477	43,854
Deferred leasing and financing costs, net	65,245	64,797
Investing receivables	52,814	52,147
Prepaid expenses and other assets	71,500	60,249
Total assets	3,771,219	3,664,375
Liabilities:
Debt, net	1,999,622	1,920,057
Accounts payable and accrued expenses	138,214	123,035
Rents received in advance and security deposits	31,551	31,011
Dividends and distributions payable	30,174	29,862
Deferred revenue associated with operating leases	14,697	13,031
Interest rate derivatives	4,282	1,855
Other liabilities	4,054	6,223
Total liabilities	2,222,594	2,125,074
Commitments and contingencies
Redeemable noncontrolling interest	18,895	18,417
Corporate Office Properties Trust’s shareholders’ equity:
Common Shares of beneficial interest	1,316,514	1,305,219
Accumulated other comprehensive loss	-4,137	-1,381
Total Corporate Office Properties Trust’s shareholders’ equity	1,520,260	1,511,721
Noncontrolling interests in subsidiaries:
Noncontrolling interests in subsidiaries	9,470	9,163
Total equity	1,529,730	1,520,884
Total liabilities, redeemable noncontrolling interest and equity	3,771,219	3,664,375
General Partner | Corporate Office Properties, L.P.
Corporate Office Properties Trust’s shareholders’ equity:
Preferred partners' capital accounts	199,083	199,083
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 $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, except Share data, unless otherwise specified
Allowance for doubtful accounts - AR	$784	$717
Allowance - Deferred rent receivable	1,695	1,418
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	7,431,667
Preferred Shares of beneficial interest, shares outstanding	7,431,667	7,431,667
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	94,536,269	93,255,284
Common Shares of beneficial interest, shares outstanding	94,536,269	93,255,284
Corporate Office Properties, L.P.
Allowance for doubtful accounts - AR	784	717
Allowance - Deferred rent receivable	$1,695	$1,418
Corporate Office Properties, L.P. | General Partner
Common Shares of beneficial interest, shares outstanding	94,536,269	93,255,284
Preferred Units, Outstanding	7,431,667	7,431,667
Corporate Office Properties, L.P. | Limited Partner
Common Shares of beneficial interest, shares outstanding	3,679,551	3,837,551
Preferred Units, Outstanding	352,000	352,000

Consolidated_Statements_of_Ope

Consolidated Statements of Operations (USD $)	3 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Revenues
Rental revenue	$98,238		$98,035
Tenant recoveries and other real estate operations revenue	24,472		26,842
Construction contract and other service revenues	38,324		21,790
Total revenues	161,034		146,667
Expenses
Property operating expenses	50,681		49,772
Depreciation and amortization associated with real estate operations	31,599		43,596
Construction contract and other service expenses	37,498		18,624
General, administrative and leasing expenses	7,891		8,143
Business development expenses and land carry costs	2,790		1,326
Total operating expenses	130,459		121,461
Operating income	30,575		25,206
Interest expense	-20,838		-20,827
Interest and other income	1,283		1,285
Loss on early extinguishment of debt	-3		0
Income from continuing operations before equity in income of unconsolidated entities and income taxes	11,017		5,664
Equity in income of unconsolidated entities	25		60
Income tax expense	-55		-64
Income from continuing operations	10,987		5,660
Discontinued operations	-238		11
Income before gain on sales of real estate	10,749		5,671
Gain on sales of real estate	3,986		0
Net income	14,735		5,671
Net income attributable to noncontrolling interests:
Common units in COPLP	-398		-16
Preferred units in COPLP	-165		-165
Other consolidated entities	-817		-749
Net income attributable to COPT	13,355		4,741
Preferred share/unit dividends/distributions	-3,552		-4,490
Net income attributable to COPT common shareholders	9,803		251
Net income attributable to COPT:
Income from continuing operations	13,581		4,728
Discontinued operations, net	-226		13
Net income attributable to COPT	13,355		4,741
Basic earnings per common share
Income from continuing operations (in dollars per share/unit)	$0.10	[1]	$0	[1]
Discontinued operations (in dollars per share/unit)	$0	[1]	$0	[1]
Net income attributable to COPT common shareholders (in dollars per share/unit)	$0.10	[1]	$0	[1]
Diluted earnings per common share
Income from continuing operations (in dollars per share/unit)	$0.10	[1]	$0	[1]
Discontinued operations (in dollars per share/unit)	$0	[1]	$0	[1]
Net income attributable to COPT common shareholders (in dollars per share/unit)	$0.10	[1]	$0	[1]
Dividends declared per common share/unit	$0.28		$0.28
Corporate Office Properties, L.P.
Revenues
Rental revenue	98,238		98,035
Tenant recoveries and other real estate operations revenue	24,472		26,842
Construction contract and other service revenues	38,324		21,790
Total revenues	161,034		146,667
Expenses
Property operating expenses	50,681		49,772
Depreciation and amortization associated with real estate operations	31,599		43,596
Construction contract and other service expenses	37,498		18,624
General, administrative and leasing expenses	7,891		8,143
Business development expenses and land carry costs	2,790		1,326
Total operating expenses	130,459		121,461
Operating income	30,575		25,206
Interest expense	-20,838		-20,827
Interest and other income	1,283		1,285
Loss on early extinguishment of debt	-3		0
Income from continuing operations before equity in income of unconsolidated entities and income taxes	11,017		5,664
Equity in income of unconsolidated entities	25		60
Income tax expense	-55		-64
Income from continuing operations	10,987		5,660
Discontinued operations	-238		11
Income before gain on sales of real estate	10,749		5,671
Gain on sales of real estate	3,986		0
Net income	14,735		5,671
Net income attributable to noncontrolling interests:
Net income attributable to noncontrolling interests in consolidated entities	-818		-737
Net income attributable to COPT	13,917		4,934
Preferred share/unit dividends/distributions	-3,717		-4,655
Net income attributable to COPT common shareholders	10,200		279
Net income attributable to COPT:
Income from continuing operations	14,152		4,921
Discontinued operations, net	-235		13
Net income attributable to COPT	$13,917		$4,934
Basic earnings per common share
Income from continuing operations (in dollars per share/unit)	$0.10	[2]	$0	[2]
Discontinued operations (in dollars per share/unit)	$0	[2]	$0	[2]
Net income attributable to COPT common shareholders (in dollars per share/unit)	$0.10	[2]	$0	[2]
Diluted earnings per common share
Income from continuing operations (in dollars per share/unit)	$0.10	[2]	$0	[2]
Discontinued operations (in dollars per share/unit)	$0	[2]	$0	[2]
Net income attributable to COPT common shareholders (in dollars per share/unit)	$0.10	[2]	$0	[2]
Dividends declared per common share/unit	$0.28		$0.28
[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
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Net income	$14,735	$5,671
Other comprehensive loss
Unrealized losses on interest rate derivatives	-3,474	-2,123
Other comprehensive loss	-2,701	-1,428
Comprehensive income	12,034	4,243
Comprehensive income attributable to noncontrolling interests	-1,329	-911
Comprehensive income attributable to COPT	10,705	3,332
Interest Expense [Member]
Other comprehensive loss
Losses on interest rate derivatives included in interest expense	773	695
Corporate Office Properties, L.P.
Net income	14,735	5,671
Other comprehensive loss
Unrealized losses on interest rate derivatives	-3,474	-2,123
Other comprehensive loss	-2,701	-1,428
Comprehensive income	12,034	4,243
Comprehensive income attributable to noncontrolling interests	-873	-782
Comprehensive income attributable to COPT	11,161	3,461
Corporate Office Properties, L.P. | Interest Expense [Member]
Other comprehensive loss
Losses on interest rate derivatives included in interest expense	$773	$695

Consolidated_Statements_of_Equ

Consolidated Statements of Equity (USD $)	Total	Preferred Shares	Common Shares	Additional Paid-in Capital	Cumulative Distributions in Excess of Net Income	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interests	Corporate Office Properties, L.P.	Corporate Office Properties, L.P.	Corporate Office Properties, L.P.	Corporate Office Properties, L.P.	Corporate Office Properties, L.P.	Corporate Office Properties, L.P.	Corporate Office Properties, L.P.	Corporate Office Properties, L.P.
In Thousands, except Share data, unless otherwise specified	USD ($)	USD ($)	USD ($)	USD ($)	USD ($)	USD ($)	USD ($)	USD ($)	Common Shares	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interests	Limited Partner	Limited Partner	General Partner	General Partner
									USD ($)	USD ($)	USD ($)		Preferred Shares		Preferred Shares
													USD ($)		USD ($)
Balance (COPT: 87,394,512 shares and 93,255,284 shares as of December 31, 2013 and 2014, respectively) at Dec. 31, 2013	$1,497,249	$249,083	$874	$1,814,015	($641,868)	$3,480	$71,665	$1,497,249	$1,226,318	$3,605	$9,443		$8,800		$249,083
Balance (in units/ shares) at Dec. 31, 2013	87,394,512								91,372,212
Balance (preferred units) at Dec. 31, 2013													352,000		9,431,667
Increase (Decrease) in Shareholders' Equity
Conversion of common units to common shares (COPT: 48,498 shares and 158,000 shares for the three months ended March 31, 2014 and 2015, respectively)	0		0	651			-651
Exercise of share options (in units/shares)	26,614								26,614
Exercise of share options (COPT: 26,614 shares and 70,374 shares for the three months ended March 31, 2014 and 2015, respectively)	568			568				568	568
Share-based compensation issuance, net of redemptions (in units/shares)	125,307								125,307
Share-based compensation issuance, net of redemptions (COPT: 125,307 and 162,370 shares issued, net of redemptions for the three months ended March 31, 2014 and 2015, respectively)	1,856		2	1,854				1,856	1,856
Redemption of vested equity awards (in units/shares)									0
Redemption of vested equity awards	-1,092			-1,092				-1,092	-1,092
Adjustments to noncontrolling interests resulting from changes in ownership of COPLP	0			-69			69
Comprehensive income	3,741				4,741	-1,408	408	3,741	279	-1,472	279		165		4,490
Dividends / Distributions to owners of common and preferred units	-28,581				-28,581			-29,827	-25,172				-165		-4,490
Distributions to owners of common and preferred units in COPLP	-1,246						-1,246
Adjustment to arrive at fair value of redeemable noncontrolling interest	540			540				540	540
Balance (COPT: 87,594,931 shares and 94,536,269 shares as of March 31, 2014 and 2015, respectively) at Mar. 31, 2014	1,473,035	249,083	876	1,816,467	-665,708	2,072	70,245	1,473,035	1,203,297	2,133	9,722		8,800		249,083
Balance (in units/ shares) at Mar. 31, 2014	87,594,931								91,524,133
Balance (preferred units) at Mar. 31, 2014													352,000		9,431,667
Balance (COPT: 87,394,512 shares and 93,255,284 shares as of December 31, 2013 and 2014, respectively) at Dec. 31, 2014	1,520,884	199,083	933	1,969,968	-717,264	-1,297	69,461	1,520,884	1,305,219	-1,381	9,163		8,800		199,083
Balance (in units/ shares) at Dec. 31, 2014	93,255,284								97,092,835			3,837,551		93,255,284
Balance (preferred units) at Dec. 31, 2014												352,000	352,000	7,431,667	7,431,667
Increase (Decrease) in Shareholders' Equity
Conversion of common units to common shares (COPT: 48,498 shares and 158,000 shares for the three months ended March 31, 2014 and 2015, respectively)	0		2	2,120			-2,122
Issuance of common units resulting from common shares issued under at-the-market program (in units)									890,241
Issuance of common units resulting from common shares issued under at-the-market program (COPT: 890,241 shares for the three months ended March 31, 2015)	26,535		9	26,526				26,535	26,535
Exercise of share options (in units/shares)	70,374								70,374
Exercise of share options (COPT: 26,614 shares and 70,374 shares for the three months ended March 31, 2014 and 2015, respectively)	1,845			1,845				1,845	1,845
Share-based compensation issuance, net of redemptions (in units/shares)	162,370								162,370
Share-based compensation issuance, net of redemptions (COPT: 125,307 and 162,370 shares issued, net of redemptions for the three months ended March 31, 2014 and 2015, respectively)	1,829		1	1,828				1,829	1,829
Redemption of vested equity awards (in units/shares)									0
Redemption of vested equity awards	-2,031			-2,031				-2,031	-2,031
Adjustments to noncontrolling interests resulting from changes in ownership of COPLP	0			-475			475
Comprehensive income	11,472				13,355	-2,650	767	11,472	10,200	-2,756	311		165		3,552
Dividends / Distributions to owners of common and preferred units	-29,550				-29,550			-30,727	-27,010				-165		-3,552
Distributions to owners of common and preferred units in COPLP	-1,177						-1,177
Distributions to noncontrolling interests in subsidiaries	-4						-4	-4			-4
Adjustment to arrive at fair value of redeemable noncontrolling interest	-73			-73				-73	-73
Balance (COPT: 87,594,931 shares and 94,536,269 shares as of March 31, 2014 and 2015, respectively) at Mar. 31, 2015	$1,529,730	$199,083	$945	$1,999,708	($733,459)	($3,947)	$67,400	$1,529,730	$1,316,514	($4,137)	$9,470		$8,800		$199,083
Balance (in units/ shares) at Mar. 31, 2015	94,536,269								98,215,820			3,679,551		94,536,269
Balance (preferred units) at Mar. 31, 2015												352,000	352,000	7,431,667	7,431,667

Consolidated_Statements_of_Equ1

Consolidated Statements of Equity (Parenthetical)	3 Months Ended
	Mar. 31, 2015
Balance (in units/ shares)	94,536,269
Conversion of common units to common shares (in units/ shares)	158,000
Exercise of share options (in units/shares)	70,374
Share-based compensation issuance, net of redemptions (in units/shares)	162,370
Common Shares
Shares issued to the public (in units/ shares)	890,241

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Cash flows from operating activities
Revenues from real estate operations received	$117,521	$116,386
Construction contract and other service revenues received	19,968	17,289
Property operating expenses paid	-42,768	-42,739
Construction contract and other service expenses paid	-27,853	-11,397
General, administrative, leasing, business development and land carry costs paid	-12,728	-9,906
Interest expense paid	-12,795	-18,403
Payments in connection with early extinguishment of debt	-3	-101
Interest and other income received	556	217
Income taxes (paid) refunded	-8	192
Net cash provided by operating activities	41,890	51,538
Cash flows from investing activities
Acquisitions of operating properties	-56,622	0
Construction, development and redevelopment	-62,057	-42,625
Tenant improvements on operating properties	-5,520	-4,357
Other capital improvements on operating properties	-3,720	-9,115
Proceeds from dispositions of properties	17,424	220
Leasing costs paid	-1,935	-4,422
Increase in prepaid expenses and other assets associated with investing activities	-5,782	-450
Other	-174	-106
Net cash used in investing activities	-118,386	-60,855
Proceeds from debt
Revolving Credit Facility	150,000	0
Other debt proceeds	0	5,700
Repayments of debt
Revolving Credit Facility	-69,000	0
Scheduled principal amortization	-1,649	-1,855
Other debt repayments	-50	-50
Net proceeds from issuance of common shares/units	28,404	568
Common share/unit dividends/distributions paid	-25,646	-24,036
Preferred share/unit dividends/distributions paid	-3,552	-4,490
Distributions paid to noncontrolling interests in COPLP	-1,217	-1,253
Redemption of vested equity awards	-2,031	-1,092
Other	-411	-174
Net cash provided by (used in) financing activities	74,848	-26,682
Net decrease in cash and cash equivalents	-1,648	-35,999
Cash and cash equivalents
Beginning of period	6,077	54,373
End of period	4,429	18,374
Reconciliation of net income to net cash provided by operating activities:
Net income	14,735	5,671
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and other amortization	32,091	44,101
Impairment losses	233	1
Amortization of deferred financing costs	990	1,167
(Increase) decrease in deferred rent receivable	-1,746	398
Amortization of net debt discounts	264	171
(Gain) loss on sales of real estate	-3,986	4
Share-based compensation	1,552	1,555
Loss on early extinguishment of debt	0	-78
Other	-640	-1,032
Operating changes in assets and liabilities:
Increase in accounts receivable	-6,918	-1,769
(Increase) decrease in restricted cash and marketable securities	-1,577	283
Increase in prepaid expenses and other assets	-6,352	-494
Increase in accounts payable, accrued expenses and other liabilities	12,704	4,785
Increase (decrease) in rents received in advance and security deposits	540	-3,225
Net cash provided by operating activities	41,890	51,538
Supplemental schedule of non-cash investing and financing activities:
Decrease in accrued capital improvements, leasing and other investing activity costs	-3,897	-7,985
Liabilities assumed on acquisition of operating properties	5,265	0
Decrease in fair value of derivatives applied to accumulated other comprehensive loss and noncontrolling interests	-2,701	-1,443
Dividends/distributions payable	30,174	29,122
Decrease in noncontrolling interests and increase in shareholders’ equity in connection with the conversion of common units into common shares	2,122	651
Adjustments to noncontrolling interests resulting from changes in COPLP ownership	475	69
Increase (decrease) in redeemable noncontrolling interest and decrease (increase) in equity to carry redeemable noncontrolling interest at fair value	73	-540
Corporate Office Properties, L.P.
Cash flows from operating activities
Revenues from real estate operations received	117,521	116,386
Construction contract and other service revenues received	19,968	17,289
Property operating expenses paid	-42,768	-42,739
Construction contract and other service expenses paid	-27,853	-11,397
General, administrative, leasing, business development and land carry costs paid	-12,728	-9,906
Interest expense paid	-12,795	-18,403
Payments in connection with early extinguishment of debt	-3	-101
Interest and other income received	556	217
Income taxes (paid) refunded	-8	192
Net cash provided by operating activities	41,890	51,538
Cash flows from investing activities
Acquisitions of operating properties	-56,622	0
Construction, development and redevelopment	-62,057	-42,625
Tenant improvements on operating properties	-5,520	-4,357
Other capital improvements on operating properties	-3,720	-9,115
Proceeds from dispositions of properties	17,424	220
Leasing costs paid	-1,935	-4,422
Increase in prepaid expenses and other assets associated with investing activities	-5,782	-450
Other	-174	-106
Net cash used in investing activities	-118,386	-60,855
Proceeds from debt
Revolving Credit Facility	150,000	0
Other debt proceeds	0	5,700
Repayments of debt
Revolving Credit Facility	-69,000	0
Scheduled principal amortization	-1,649	-1,855
Other debt repayments	-50	-50
Net proceeds from issuance of common shares/units	28,404	568
Common share/unit dividends/distributions paid	-26,698	-25,124
Preferred share/unit dividends/distributions paid	-3,717	-4,655
Redemption of vested equity awards	-2,031	-1,092
Other	-411	-174
Net cash provided by (used in) financing activities	74,848	-26,682
Net decrease in cash and cash equivalents	-1,648	-35,999
Cash and cash equivalents
Beginning of period	6,077	54,373
End of period	4,429	18,374
Reconciliation of net income to net cash provided by operating activities:
Net income	14,735	5,671
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and other amortization	32,091	44,101
Impairment losses	233	1
Amortization of deferred financing costs	990	1,167
(Increase) decrease in deferred rent receivable	-1,746	398
Amortization of net debt discounts	264	171
(Gain) loss on sales of real estate	-3,986	4
Share-based compensation	1,552	1,555
Loss on early extinguishment of debt	0	-78
Other	-640	-1,032
Operating changes in assets and liabilities:
Increase in accounts receivable	-6,918	-1,769
(Increase) decrease in restricted cash and marketable securities	-1,523	381
Increase in prepaid expenses and other assets	-6,352	-494
Increase in accounts payable, accrued expenses and other liabilities	12,650	4,687
Increase (decrease) in rents received in advance and security deposits	540	-3,225
Net cash provided by operating activities	41,890	51,538
Supplemental schedule of non-cash investing and financing activities:
Decrease in accrued capital improvements, leasing and other investing activity costs	-3,897	-7,985
Liabilities assumed on acquisition of operating properties	5,265	0
Decrease in fair value of derivatives applied to accumulated other comprehensive loss and noncontrolling interests	-2,701	-1,443
Dividends/distributions payable	30,174	29,122
Increase (decrease) in redeemable noncontrolling interest and decrease (increase) in equity to carry redeemable noncontrolling interest at fair value	$73	($540)

Organization

Organization	3 Months Ended
	Mar. 31, 2015
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 their contractors, most of whom are engaged in national security and information technology 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 March 31, 2015, our properties included the following:
	•	178 operating office properties totaling 17.7 million square feet (excluding two properties serving as collateral for a nonrecourse mortgage loan 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);
	•	12 office properties under, or contractually committed for, construction or redevelopment that we estimate will total approximately 1.4 million square feet upon completion, including one partially operational property included above;
	•	1,402 acres of land we control that we believe are potentially developable into approximately 17.6 million square feet; and
	•	a partially operational, wholesale data center which upon completion and stabilization is expected to have a critical load of 19.25 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”).
	Equity interests in COPLP are in the form of common and preferred units. As of March 31, 2015, COPT owned 96.3% 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 is 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	3 Months Ended
	Mar. 31, 2015
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 we 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 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, 2014 included in our 2014 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 2014 Annual Report on Form 10-K.
	Recent Accounting Pronouncements
	In May 2014, the Financial Accounting Standards Board (“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, using one of two methods: retrospective restatement for each reporting period presented at the 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.
	In January 2015, the FASB issued guidance regarding the presentation of extraordinary and unusual items in statements of operations. This guidance eliminates the concept of extraordinary items. However, the presentation and disclosure requirements for items that are either unusual in nature or infrequent in occurrence remain and will be expanded to include items that are both unusual in nature and infrequent in occurrence. This guidance is effective for periods beginning after December 15, 2015. We expect that the application of this guidance will have no effect on our reported consolidated financial statements.
	In February 2015, the FASB issued guidance regarding amendments to the consolidation analysis. This guidance amends the criteria for determining which entities are considered variable interest entities (“VIE”), amends the criteria for determining if a service provider possesses a variable interest in a VIE and ends the deferral granted to investment companies for application of the VIE consolidation model. This guidance is effective for annual periods, and interim periods therein, beginning after December 15, 2015. We are currently assessing the financial impact of this guidance on our consolidated financial statements.
	In April 2015, the FASB issued guidance that changes the presentation of debt issuance costs in financial statements. This guidance requires an entity to present such costs in the balance sheet as a direct deduction from the related debt liability rather than as an asset. Amortization of the costs will continue to be reported as interest expense. This guidance is effective for annual reporting periods beginning after December 15, 2016. Early adoption is permitted. This guidance will be applied retrospectively to each prior period presented. We are currently in the process of evaluating the impact of this guidance on our consolidated financial statements.

Fair_Value_Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2015
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 2014 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 March 31, 2015, we used a discount rate of 15.5% which factored in risk appropriate to the level of future property development expected to be undertaken by the joint venture. A significant increase (decrease) in the discount rate used in determining the fair value would result in a significantly (lower) higher fair value. Given our reliance on the unobservable inputs, the valuations are classified in Level 3 of the fair value hierarchy.
	The carrying values of cash and cash equivalents, restricted cash, accounts receivable, other assets (excluding investing receivables) and accounts payable and accrued expenses are reasonable estimates of their fair values because of the short maturities of these instruments.  As discussed in Note 7, we estimated the fair values of our 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 9, 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 7 for investing receivables, Note 9 for debt and Note 10 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 March 31, 2015 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	$	5,826	$	—	$	—	$	5,826
	Other	110		—		—		110
	Warrants to purchase common stock (2)	—		89		—		89
	Total assets	$	5,936	$	89	$	—	$	6,025
	Liabilities:
	Deferred compensation plan liability (3)	$	—	$	5,936	$	—	$	5,936
	Interest rate derivatives	—		4,282		—		4,282
	Total liabilities	$	—	$	10,218	$	—	$	10,218
	Redeemable noncontrolling interest	$	—	$	—	$	18,895	$	18,895
	(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 March 31, 2015 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:
	Warrants to purchase common stock (1)	$	—	$	89	$	—	$	89
	Liabilities:
	Interest rate derivatives	$	—	$	4,282	$	—	$	4,282
	Redeemable noncontrolling interest	$	—	$	—	$	18,895	$	18,895
	(1) Included in the line entitled “prepaid expenses and other assets” on COPLP’s consolidated balance sheet.

Properties_net

Properties, net	3 Months Ended
	Mar. 31, 2015
Real Estate [Abstract]
Properties, net	Properties, Net
	Operating properties, net consisted of the following (in thousands):
		March 31,				December 31,
		2015				2014
	Land	$	455,639			$	439,355
	Buildings and improvements	3,157,434				3,015,216
	Less: Accumulated depreciation	(724,539		)		(703,083		)
	Operating properties, net	$	2,888,534			$	2,751,488
	During the three months ended March 31, 2014, we recognized $12.9 million in additional depreciation expense resulting from our revision of the useful life of a property in Greater Philadelphia, Pennsylvania (“Greater Philadelphia”) that was removed from service for redevelopment.
	Projects in development or held for future development consisted of the following (in thousands):
		March 31,				December 31,
		2015				2014
	Land	$	219,852			$	214,977
	Construction in progress, excluding land	269,766				330,449
	Projects in development or held for future development	$	489,618			$	545,426
	As of December 31, 2014, we had two land parcels in the Greater Baltimore region classified as held for sale with a cost basis of $14.3 million that were sold during the three months ended March 31, 2015.
	2015 Acquisition
	On March 19, 2015, we acquired 250 W. Pratt Street, a 367,000 square foot office property in Baltimore, Maryland that was 96.2% leased, for $61.9 million. The table below sets forth the allocation of the acquisition costs of this property (in thousands):
	Land, operating properties		$	8,057
	Building and improvements		34,740
	Intangible assets on real estate acquisitions		20,183
	Total assets		62,980
	Below-market leases		(1,093		)
	Total acquisition cost		$	61,887
	Intangible assets recorded in connection with the above acquisition included the following (dollars in thousands):
						Weighted Average Amortization Period (in Years)
	Tenant relationship value	$	7,252			11
	In-place lease value	10,218				7
	Above-market leases	2,713				4
		$	20,183			8
	We expensed $1.0 million in operating property acquisition costs during the three months ended March 31, 2015 that are included in business development expenses and land carry costs on our consolidated statements of operations.
	2015 Dispositions
	We sold land in the three months ended March 31, 2015 for $18.1 million and recognized gains of $4.0 million on the sales.
	2015 Construction Activities
	During the three months ended March 31, 2015, we placed into service an aggregate of 440,000 square feet in three newly constructed office properties located in Northern Virginia and San Antonio, Texas (“San Antonio”), and 111,000 square feet of a property undergoing redevelopment in Greater Philadelphia. As of March 31, 2015, we had seven office properties under construction, or for which we were contractually committed to construct, that we estimate will total 1.1 million square feet upon completion, including four in Northern Virginia, two in the Baltimore/Washington Corridor and one in Huntsville, Alabama (“Huntsville”). We also had five office properties under redevelopment (including one partially operational property) that we estimate will total 344,000 square feet upon completion, including three 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	3 Months Ended
	Mar. 31, 2015
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 March 31, 2015 (dollars in thousands):
				Nominal
				Ownership		March 31, 2015				-1
			Date	% as of		Total		Encumbered		Total
			Acquired	3/31/15	Nature of Activity	Assets		Assets		Liabilities
	LW Redstone Company, LLC		3/23/10	85%	Operates four buildings and developing others (2)	$	149,098	$	65,934	$	43,314
	M Square Associates, LLC		6/26/07	50%	Operates two buildings and developing others (3)	59,671		48,480		38,700
						$	208,769	$	114,414	$	82,014
	-1	Excludes amounts eliminated in consolidation.
	-2	This joint venture’s properties are in Huntsville, Alabama.
	-3	This joint venture’s properties are in College Park, Maryland (in the Baltimore/Washington Corridor).

Intangible_Assets_on_Real_Esta

Intangible Assets on Real Estate Acquisitions	3 Months Ended
	Mar. 31, 2015
Intangible Assets on Real Estate Acquisitions [Abstract]
Intangible Assets on Real Estate Acquisition	Intangible Assets on Real Estate Acquisitions, Net
	Intangible assets on real estate acquisitions consisted of the following (in thousands):
		March 31, 2015						December 31, 2014
		Gross Carrying Amount		Accumulated Amortization		Net		Gross Carrying Amount		Accumulated Amortization		Net
						Carrying Amount						Carrying Amount
	In-place lease value	$	133,395	$	102,019	$	31,376	$	123,759	$	101,040	$	22,719
	Tenant relationship value	49,552		29,265		20,287		42,301		28,492		13,809
	Below-market cost arrangements	12,415		6,154		6,261		12,415		5,984		6,431
	Above-market leases	11,372		8,211		3,161		8,659		8,159		500
	Market concentration premium	1,333		941		392		1,333		938		395
		$	208,067	$	146,590	$	61,477	$	188,467	$	144,613	$	43,854
	Amortization of the intangible asset categories set forth above totaled $2.6 million in the three months ended March 31, 2015 and $3.6 million in the three months ended March 31, 2014. The approximate weighted average amortization periods of the categories set forth above follow: in-place lease value: six years; tenant relationship value: eight years; below-market cost arrangements: 31 years; above-market leases: four years; and market concentration premium: 27 years. The approximate weighted average amortization period for all of the categories combined is ten years. Estimated amortization expense associated with the intangible asset categories set forth above through 2020 is: $9.4 million for the nine months ending December 31, 2015; $11.6 million for 2016; $9.4 million for 2017; $6.3 million for 2018; $5.1 million for 2019; and $3.8 million for 2020.

Investing_Receivables

Investing Receivables	3 Months Ended
	Mar. 31, 2015
Receivables [Abstract]
Investing Receivables	Investing Receivables
	Investing receivables, including accrued interest thereon, consisted of the following (in thousands):
		March 31,		December 31,
		2015		2014
	Notes receivable from the City of Huntsville	$	49,814	$	49,147
	Other investing loans receivable	3,000		3,000
		$	52,814	$	52,147
	Our notes receivable from the City of Huntsville funded infrastructure costs in connection with our LW Redstone Company, LLC joint venture (see Note 5) and carry an interest rate of 9.95%.
	We did not have an allowance for credit losses in connection with our investing receivables as of March 31, 2015 or December 31, 2014.  The fair value of these receivables approximated their carrying amounts as of March 31, 2015 and December 31, 2014.

Prepaid_Expenses_and_Other_Ass

Prepaid Expenses and Other Assets	3 Months Ended
	Mar. 31, 2015
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):
		March 31,		December 31,
		2015		2014
	Construction contract costs incurred in excess of billings	$	19,136	$	6,656
	Prepaid expenses	14,862		20,570
	Lease incentives, net	13,183		13,344
	Furniture, fixtures and equipment, net	6,349		6,637
	Deposit on acquisitions	5,066		516
	Deferred tax asset, net (1)	3,946		4,002
	Operating notes receivable	3,629		3,797
	Equity method investments	2,390		2,368
	Other assets	2,939		2,359
	Prepaid expenses and other assets	$	71,500	$	60,249
	(1) Includes a valuation allowance of $2.1 million.
	Operating notes receivable reported above includes amounts due from tenants with remaining terms exceeding one year totaling $3.6 million as of March 31, 2015 and December 31, 2014; we carried allowances for estimated losses for $281,000 of the March 31, 2015 balance and $252,000 of the December 31, 2014 balance.

Debt

Debt	3 Months Ended
	Mar. 31, 2015
Debt Disclosure [Abstract]
Debt	Debt
	Our debt consisted of the following (dollars in thousands):
		Maximum
		Availability at		Carrying Value at						Scheduled Maturity
		March 31,		March 31,		December 31,		Stated Interest Rates as of		as of
		2015		2015		2014		March 31, 2015		March 31, 2015
	Mortgage and Other Secured Loans:
	Fixed rate mortgage loans (1)			$	385,693	$	387,139	3.96% - 10.65% (2)		2015-2024
	Variable rate secured loan			36,668		36,877		LIBOR + 2.25% (3)		Nov-15
	Total mortgage and other secured loans			422,361		424,016
	Revolving Credit Facility	$	800,000	164,000		83,000		LIBOR + 0.975% to 1.75% (4)		Jul-17
	Term Loan Facilities	-5		520,000		520,000		LIBOR + 1.10% to 2.60% (6)		2015-2019
	Unsecured Senior Notes
	3.600% Senior Notes (7)			347,560		347,496		3.60%		May-23
	5.250% Senior Notes (8)			245,888		245,797		5.25%		Feb-24
	3.700% Senior Notes (9)			297,655		297,569		3.70%		Jun-21
	Unsecured notes payable			1,583		1,607		0% (10)		2026
	4.25% Exchangeable Senior Notes (11)			575		572		4.25%		-11
	Total debt, net			$	1,999,622	$	1,920,057
	(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 $36,000 as of March 31, 2015 and $42,000 as of December 31, 2014.
	-2	Includes a $150.0 million nonrecourse mortgage loan in default 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). 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. The maximum stated interest rate would be 7.87%, excluding the incremental additional interest rate associated with the default rate on the loan in default. The weighted average interest rate on our fixed rate mortgage loans was 8.11% as of March 31, 2015 (or 6.16% excluding the incremental additional interest rate associated with the default rate on the loan in default).
	(3)	The interest rate on the loan outstanding was 2.42% as of March 31, 2015.
	-4	The weighted average interest rate on the Revolving Credit Facility was 1.45% as of March 31, 2015.
	(5)	We have the ability to borrow an additional $330.0 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 1.82% as of March 31, 2015.
	-7	The carrying value of these notes included a principal amount of $350.0 million and an unamortized discount totaling $2.4 million as of March 31, 2015 and $2.5 million as of December 31, 2014.  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 $4.1 million as of March 31, 2015 and $4.2 million as of December 31, 2014.  The effective interest rate under the notes, including amortization of the issuance costs, was 5.49%.
	-9	The carrying value of these notes included a principal amount of $300.0 million and an unamortized discount totaling $2.3 million as of March 31, 2015 and $2.4 million as of December 31, 2014.  The effective interest rate under the notes, including amortization of the issuance costs, was 3.85%.
	(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 $628,000 as of March 31, 2015 and $654,000 as of December 31, 2014.
	(11)	As described further in our 2014 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 March 31, 2015 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 $3,000 as of December 31, 2014.  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 March 31, 2015 and December 31, 2014 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.  On April 20, 2015, we redeemed these notes at 100% of their principal amount.
	All debt is owed by the Operating Partnership. While COPT is not directly obligated by any debt, it has guaranteed the Operating Partnership’s Revolving Credit Facility, Term Loan Facilities, Unsecured Senior Notes and 4.25% Exchangeable Senior Notes.
	We capitalized interest costs of $2.1 million in the three months ended March 31, 2015 and $1.6 million in the three months ended March 31, 2014.
	The following table sets forth information pertaining to the fair value of our debt (in thousands):
		March 31, 2015				December 31, 2014
		Carrying		Estimated		Carrying		Estimated
		Amount		Fair Value		Amount		Fair Value
	Fixed-rate debt
	Unsecured Senior Notes	$	891,103	$	918,074	$	890,862	$	901,599
	4.25% Exchangeable Senior Notes	575		575		572		575
	Other fixed-rate debt	387,276		350,835		388,746		355,802
	Variable-rate debt	720,668		722,752		639,877		642,091
		$	1,999,622	$	1,992,236	$	1,920,057	$	1,900,067

Interest_Rate_Derivatives

Interest Rate Derivatives	3 Months Ended
	Mar. 31, 2015
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		March 31,			December 31,
												2015			2014
	$	100,000			0.83%		One-Month LIBOR		1/3/12	9/1/15		$	(267	)	$	(407	)
	100,000				0.83%		One-Month LIBOR		1/3/12	9/1/15		(267		)	(407		)
	36,668			-1	3.83%		One-Month LIBOR + 2.25%		11/2/10	11/2/15		(291		)	(400		)
	100,000				0.81%		One-Month LIBOR		9/2/14	9/1/16		(455		)	(317		)
	100,000				0.81%		One-Month LIBOR		9/2/14	9/1/16		(461		)	(324		)
	100,000				1.67%		One-Month LIBOR		9/1/15	8/1/19		(1,161		)	239
	100,000				1.73%		One-Month LIBOR		9/1/15	8/1/19		(1,380		)	35
												$	(4,282	)	$	(1,581	)
	-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 a cash flow hedge 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 sheets (in thousands):
			March 31, 2015						31-Dec-14
	Derivatives		Balance Sheet Location		Fair Value				Balance Sheet Location	Fair Value
	Interest rate swaps designated as cash flow hedges		Prepaid expenses and other assets		$	—			Prepaid expenses and other assets	$	274
	Interest rate swaps designated as cash flow hedges		Interest rate derivatives		(4,282		)		Interest rate derivatives	(1,855		)
	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 March 31,
			2015				2014
	Amount of losses recognized in accumulated other comprehensive loss (“AOCL”) (effective portion)		$	(3,474	)		$	(2,123	)
	Amount of losses reclassified from AOCL into interest expense (effective portion)		773				695
	Over the next 12 months, we estimate that approximately $3.2 million of losses will be reclassified from AOCL 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 March 31, 2015, the fair value of interest rate derivatives in a liability position related to these agreements was $4.3 million, excluding the effects of accrued interest. As of March 31, 2015, 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 $4.5 million.

Redeemable_Noncontrolling_Inte

Redeemable Noncontrolling Interest	3 Months Ended
	Mar. 31, 2015
Noncontrolling Interest [Abstract]
Redeemable Noncontrolling Interest	Redeemable Noncontrolling Interest
	The table below sets forth the activity in a redeemable noncontrolling interest in a consolidated real estate joint venture (in thousands):
		For the Three Months Ended March 31,
		2015			2014
	Beginning balance	$	18,417		$	17,758
	Distribution to noncontrolling interest	(157		)	(68		)
	Net income attributable to noncontrolling interest	562			504
	Adjustment to arrive at fair value of interest	73			(540		)
	Ending balance	$	18,895		$	17,654

Equity

Equity	3 Months Ended
	Mar. 31, 2015
Equity [Abstract]
Equity	Equity
	During the three months ended March 31, 2015, COPT issued 890,241 common shares at a weighted average price of $30.29 per share under its at-the-market (“ATM”) stock offering program established in October 2012. Net proceeds from the shares issued totaled $26.6 million, after payment of $0.4 million in commissions to sales agents. These net proceeds were contributed to COPLP in exchange for 890,241 common units. COPT’s remaining capacity under the ATM Plan is an aggregate gross sales price of $84.0 million in common share sales.
	During the three months ended March 31, 2015, certain COPLP limited partners redeemed 158,000 common units in COPLP for an equal number of common shares in COPT.
	See Note 14 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	3 Months Ended
	Mar. 31, 2015
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). In our 2015 quarterly reports on Form 10-Q, our Colorado Springs segment is, and will be, included in our Other segment as it is insignificant in the 2014 and 2015 reporting periods. 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		Other		Operating		Total
		Washington				Virginia				Antonio					DC - Capitol		King George		Baltimore		Philadelphia				Wholesale
		Corridor													Riverfront		Counties								Data Center
	Three Months Ended March 31, 2015
	Revenues from real estate operations	$	61,792			$	21,749			$	9,171		$	2,446	$	3,364	$	3,901	$	11,485	$	3,224	$	2,543	$	3,035	$	122,710
	Property operating expenses	23,583				9,121				4,975			829		1,814		1,584		4,997		1,375		196		2,212		50,686
	NOI from real estate operations	$	38,209			$	12,628			$	4,196		$	1,617	$	1,550	$	2,317	$	6,488	$	1,849	$	2,347	$	823	$	72,024
	Additions to long-lived assets	$	3,445			$	2,821			$	21		$	83	$	393	$	1,200	$	63,604	$	294	$	80	$	30	$	71,971
	Transfers from non-operating properties	$	12,930			$	58,571			$	31,091		$	2,997	$	—	$	—	$	—	$	11,498	$	8	$	177	$	117,272
	Segment assets at March 31, 2015	$	1,280,742			$	696,276			$	147,078		$	99,335	$	94,400	$	100,849	$	334,271	$	117,919	$	77,340	$	162,170	$	3,110,380
	Three Months Ended March 31, 2014
	Revenues from real estate operations	$	61,113			$	24,968			$	8,479		$	2,555	$	3,634	$	4,316	$	11,496	$	3,340	$	2,594	$	2,401	$	124,896
	Property operating expenses	23,597				8,973				4,474			653		1,765		1,504		5,476		1,300		322		1,688		49,752
	NOI from real estate operations	$	37,516			$	15,995			$	4,005		$	1,902	$	1,869	$	2,812	$	6,020	$	2,040	$	2,272	$	713	$	75,144
	Additions to long-lived assets	$	5,741			$	3,495			$	(6	)	$	2,507	$	63	$	839	$	1,012	$	11	$	55	$	12	$	13,729
	Transfers from non-operating properties	$	6,911			$	26,588			$	—		$	20,102	$	—	$	—	$	74	$	3,176	$	11	$	78	$	56,940
	Segment assets at March 31, 2014	$	1,244,876			$	636,895			$	117,812		$	99,378	$	97,843	$	95,645	$	300,619	$	94,428	$	79,542	$	165,711	$	2,932,749
	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 March 31,
			2015				2014
	Segment revenues from real estate operations		$	122,710			$	124,896
	Construction contract and other service revenues		38,324				21,790
	Less: Revenues from discontinued operations		—				(19		)
	Total revenues		$	161,034			$	146,667
	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 March 31,
			2015				2014
	Segment property operating expenses		$	50,686			$	49,752
	Less: Property operating expenses from discontinued operations		(5		)		20
	Total property operating expenses		$	50,681			$	49,772
	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 March 31,
			2015				2014
	Construction contract and other service revenues		$	38,324			$	21,790
	Construction contract and other service expenses		(37,498		)		(18,624		)
	NOI from service operations		$	826			$	3,166
	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 March 31,
			2015				2014
	NOI from real estate operations		$	72,024			$	75,144
	NOI from service operations		826				3,166
	Interest and other income		1,283				1,285
	Equity in income of unconsolidated entities		25				60
	Income tax expense		(55		)		(64		)
	Other adjustments:		—
	Depreciation and other amortization associated with real estate operations		(31,599		)		(43,596		)
	General, administrative and leasing expenses		(7,891		)		(8,143		)
	Business development expenses and land carry costs		(2,790		)		(1,326		)
	Interest expense on continuing operations		(20,838		)		(20,827		)
	NOI from discontinued operations		5				(39		)
	Loss on early extinguishment of debt		(3		)		—
	Income from continuing operations		$	10,987			$	5,660
	The following table reconciles our segment assets to the consolidated total assets of COPT and subsidiaries (in thousands):
		March 31,				March 31,
		2015				2014
	Segment assets	$	3,110,380			$	2,932,749
	Non-operating property assets	496,930				503,030
	Other assets	169,845				170,118
	Total COPT consolidated assets	$	3,777,155			$	3,605,897
	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, 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	3 Months Ended
	Mar. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-Based Compensation	Share-Based Compensation
	Performance Share Units (“PSUs”)
	On March 5, 2015, our Board of Trustees granted 45,656 PSUs with an aggregate grant date fair value of $1.7 million to executives.  The PSUs have a performance period beginning on January 1, 2015 and concluding on the earlier of December 31, 2017 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 service period ends due to a sale event or qualified termination, the number of earned PSUs is prorated based on the portion of the service 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 $36.76 per PSU using a Monte Carlo model, which included assumptions of, among other things, the following: baseline common share value of $29.28; expected volatility for COPT common shares of 19.9%; and a risk-free interest rate of 0.99%.  We are recognizing the grant date fair value in connection with these PSU awards over the period commencing on March 6, 2015 and ending on December 31, 2017.
	With regard to the PSUs granted to our executives in prior years that were outstanding as of December 31, 2014 as described in our 2014 Annual Report on Form 10-K:
	•	the performance period for the PSUs granted to executives on March 1, 2012 ended on December 31, 2014. Based on COPT’s total shareholder return during the performance period relative to its peer group of companies, we issued 40,309 common shares in settlement of the PSUs on March 5, 2015; and
	•	we issued 15,289 common shares on March 5, 2015 to Mr. Stephen E. Riffee, our former Chief Financial Officer, upon his departure on February 3, 2015, in settlement of PSUs granted on March 1, 2013 and March 6, 2014.
	Restricted Shares
	During the three months ended March 31, 2015, certain employees were granted a total of 177,674 restricted common shares with an aggregate grant date fair value of $5.2 million (weighted average of $29.46 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.  During the three months ended March 31, 2015, forfeiture restrictions lapsed on 127,444 previously issued common shares; these shares had a weighted average grant date fair value of $26.01 per share, and the aggregate intrinsic value of the shares on the vesting dates was $3.7 million.
	Options
	During the three months ended March 31, 2015, 70,374 options to purchase common shares (“options”) were exercised.  The weighted average exercise price of these options was $26.23 per share, and the aggregate intrinsic value of the options exercised was $291,000.

Income_Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2015
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 March 31,
		2015		2014
	Deferred
	Federal	$	45	$	53
	State	10		11
	Total income tax expense	$	55	$	64
	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 37.8% for the three months ended March 31, 2015 and 38.1% for the three months ended March 31, 2014.

Earnings_Per_Share_EPS_and_Ear

Earnings Per Share ("EPS") and Earnings Per Unit ("EPU")	3 Months Ended
	Mar. 31, 2015
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 March 31,
			2015				2014
	Numerator:
	Income from continuing operations		$	10,987			$	5,660
	Gain on sales of real estate, net		3,986				—
	Preferred share dividends		(3,552		)		(4,490		)
	Income from continuing operations attributable to noncontrolling interests		(1,392		)		(932		)
	Income from continuing operations attributable to restricted shares		(122		)		(121		)
	Numerator for basic and diluted EPS from continuing operations attributable to COPT common shareholders		$	9,907			$	117
	Discontinued operations		(238		)		11
	Discontinued operations attributable to noncontrolling interests		12				2
	Numerator for basic and diluted EPS on net income attributable to COPT common shareholders		$	9,681			$	130
	Denominator (all weighted averages):
	Denominator for basic EPS (common shares)		93,199				87,080
	Dilutive effect of share-based compensation awards		198				112
	Denominator for diluted EPS (common shares)		93,397				87,192
	Basic EPS:
	Income from continuing operations attributable to COPT common shareholders		$	0.1			$	0
	Discontinued operations attributable to COPT common shareholders		0				0
	Net income attributable to COPT common shareholders		$	0.1			$	0
	Diluted EPS:
	Income from continuing operations attributable to COPT common shareholders		$	0.1			$	0
	Discontinued operations attributable to COPT common shareholders		0				0
	Net income attributable to COPT common shareholders		$	0.1			$	0
	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 March 31,
			2015			2014
	Conversion of common units		3,732			3,958
	Conversion of Series I Preferred Units		176			176
	Conversion of Series K Preferred Shares		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 March 31, 2015 and 2014 of 400,000 and 390,000, respectively; and
	•	weighted average options for the three months ended March 31, 2015 and 2014 of 474,000 and 588,000, respectively.
	As discussed in Note 9, as of March 31, 2015, we had outstanding senior notes with 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 March 31,
			2015				2014
	Numerator:
	Income from continuing operations		$	10,987			$	5,660
	Gain on sales of real estate, net		3,986				—
	Preferred unit distributions		(3,717		)		(4,655		)
	Income from continuing operations attributable to noncontrolling interests		(821		)		(739		)
	Income from continuing operations attributable to restricted units		(122		)		(121		)
	Numerator for basic and diluted EPU from continuing operations attributable to COPLP common unitholders		$	10,313			$	145
	Discontinued operations		(238		)		11
	Discontinued operations attributable to noncontrolling interests		3				2
	Numerator for basic and diluted EPU on net income attributable to COPLP common unitholders		$	10,078			$	158
	Denominator (all weighted averages):
	Denominator for basic EPU (common units)		96,931				91,038
	Dilutive effect of share-based compensation awards		198				112
	Denominator for basic and diluted EPU (common units)		97,129				91,150
	Basic EPU:
	Income from continuing operations attributable to COPLP common unitholders		$	0.1			$	0
Discontinued operations attributable to COPLP common unitholders		0				0
Net income attributable to COPLP common unitholders		$	0.1			$	0
Diluted EPU:
Income from continuing operations attributable to COPLP common unitholders		$	0.1			$	0
Discontinued operations attributable to COPLP common unitholders		0				0
Net income attributable to COPLP common unitholders		$	0.1			$	0
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 March 31,
		2015			2014
Conversion of Series I preferred units		176			176
Conversion of Series K preferred units		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 March 31, 2015 and 2014 of 400,000 and 390,000, respectively; and
•	weighted average options for the three months ended March 31, 2015 and 2014 of 474,000 and 588,000, respectively.
As discussed in Note 9, as of March 31, 2015, we had outstanding senior notes with 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	3 Months Ended
	Mar. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies
	Acquisition
	As of March 31, 2015, we were under contract to acquire 2600 Park Tower Drive, a 240,000 square foot office property in Vienna, Virginia that was 100% leased, for approximately $83 million. We completed this acquisition on April 15, 2015.
	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.
	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 $1.4 million liability through March 31, 2015 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)	3 Months Ended
	Mar. 31, 2015
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 we 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 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, 2014 included in our 2014 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 2014 Annual Report on Form 10-K.
Recent Accounting Pronouncement	Recent Accounting Pronouncements
	In May 2014, the Financial Accounting Standards Board (“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, using one of two methods: retrospective restatement for each reporting period presented at the 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.
	In January 2015, the FASB issued guidance regarding the presentation of extraordinary and unusual items in statements of operations. This guidance eliminates the concept of extraordinary items. However, the presentation and disclosure requirements for items that are either unusual in nature or infrequent in occurrence remain and will be expanded to include items that are both unusual in nature and infrequent in occurrence. This guidance is effective for periods beginning after December 15, 2015. We expect that the application of this guidance will have no effect on our reported consolidated financial statements.
	In February 2015, the FASB issued guidance regarding amendments to the consolidation analysis. This guidance amends the criteria for determining which entities are considered variable interest entities (“VIE”), amends the criteria for determining if a service provider possesses a variable interest in a VIE and ends the deferral granted to investment companies for application of the VIE consolidation model. This guidance is effective for annual periods, and interim periods therein, beginning after December 15, 2015. We are currently assessing the financial impact of this guidance on our consolidated financial statements.
	In April 2015, the FASB issued guidance that changes the presentation of debt issuance costs in financial statements. This guidance requires an entity to present such costs in the balance sheet as a direct deduction from the related debt liability rather than as an asset. Amortization of the costs will continue to be reported as interest expense. This guidance is effective for annual reporting periods beginning after December 15, 2016. Early adoption is permitted. This guidance will be applied retrospectively to each prior period presented. We are currently in the process of evaluating the impact of this guidance on our consolidated financial statements.

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2015
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 March 31, 2015 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	$	5,826	$	—	$	—	$	5,826
	Other	110		—		—		110
	Warrants to purchase common stock (2)	—		89		—		89
	Total assets	$	5,936	$	89	$	—	$	6,025
	Liabilities:
	Deferred compensation plan liability (3)	$	—	$	5,936	$	—	$	5,936
	Interest rate derivatives	—		4,282		—		4,282
	Total liabilities	$	—	$	10,218	$	—	$	10,218
	Redeemable noncontrolling interest	$	—	$	—	$	18,895	$	18,895
	(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 March 31, 2015 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:
	Warrants to purchase common stock (1)	$	—	$	89	$	—	$	89
	Liabilities:
	Interest rate derivatives	$	—	$	4,282	$	—	$	4,282
	Redeemable noncontrolling interest	$	—	$	—	$	18,895	$	18,895
	(1) Included in the line entitled “prepaid expenses and other assets” on COPLP’s consolidated balance sheet.

Properties_net_Tables

Properties, net (Tables)	3 Months Ended
	Mar. 31, 2015
Real Estate [Abstract]
Schedule of operating properties, net	Operating properties, net consisted of the following (in thousands):
		March 31,				December 31,
		2015				2014
	Land	$	455,639			$	439,355
	Buildings and improvements	3,157,434				3,015,216
	Less: Accumulated depreciation	(724,539		)		(703,083		)
	Operating properties, net	$	2,888,534			$	2,751,488
Schedule of projects in development or held for future development	Projects in development or held for future development consisted of the following (in thousands):
		March 31,				December 31,
		2015				2014
	Land	$	219,852			$	214,977
	Construction in progress, excluding land	269,766				330,449
	Projects in development or held for future development	$	489,618			$	545,426
Schedule of allocation of acquisition costs	The table below sets forth the allocation of the acquisition costs of this property (in thousands):
	Land, operating properties		$	8,057
	Building and improvements		34,740
	Intangible assets on real estate acquisitions		20,183
	Total assets		62,980
	Below-market leases		(1,093		)
	Total acquisition cost		$	61,887
Schedule of intangible assets recorded in connection with acquisition	Intangible assets recorded in connection with the above acquisition included the following (dollars in thousands):
						Weighted Average Amortization Period (in Years)
	Tenant relationship value	$	7,252			11
	In-place lease value	10,218				7
	Above-market leases	2,713				4
		$	20,183			8

Real_Estate_Joint_Ventures_Tab

Real Estate Joint Ventures (Tables)	3 Months Ended
	Mar. 31, 2015
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 March 31, 2015 (dollars in thousands):
				Nominal
				Ownership		March 31, 2015				-1
			Date	% as of		Total		Encumbered		Total
			Acquired	3/31/15	Nature of Activity	Assets		Assets		Liabilities
	LW Redstone Company, LLC		3/23/10	85%	Operates four buildings and developing others (2)	$	149,098	$	65,934	$	43,314
	M Square Associates, LLC		6/26/07	50%	Operates two buildings and developing others (3)	59,671		48,480		38,700
						$	208,769	$	114,414	$	82,014
	-1	Excludes amounts eliminated in consolidation.
	-2	This joint venture’s properties are in Huntsville, Alabama.
	-3	This joint venture’s properties are in College Park, Maryland (in the Baltimore/Washington Corridor).

Intangible_Assets_on_Real_Esta1

Intangible Assets on Real Estate Acquisitions (Tables)	3 Months Ended
	Mar. 31, 2015
Intangible Assets on Real Estate Acquisitions [Abstract]
Schedule of intangible assets on real estate acquisitions	Intangible assets on real estate acquisitions consisted of the following (in thousands):
		March 31, 2015						December 31, 2014
		Gross Carrying Amount		Accumulated Amortization		Net		Gross Carrying Amount		Accumulated Amortization		Net
						Carrying Amount						Carrying Amount
	In-place lease value	$	133,395	$	102,019	$	31,376	$	123,759	$	101,040	$	22,719
	Tenant relationship value	49,552		29,265		20,287		42,301		28,492		13,809
	Below-market cost arrangements	12,415		6,154		6,261		12,415		5,984		6,431
	Above-market leases	11,372		8,211		3,161		8,659		8,159		500
	Market concentration premium	1,333		941		392		1,333		938		395
		$	208,067	$	146,590	$	61,477	$	188,467	$	144,613	$	43,854

Investing_Receivables_Tables

Investing Receivables (Tables)	3 Months Ended
	Mar. 31, 2015
Receivables [Abstract]
Schedule of investing receivables	Investing receivables, including accrued interest thereon, consisted of the following (in thousands):
		March 31,		December 31,
		2015		2014
	Notes receivable from the City of Huntsville	$	49,814	$	49,147
	Other investing loans receivable	3,000		3,000
		$	52,814	$	52,147

Prepaid_Expenses_and_Other_Ass1

Prepaid Expenses and Other Assets (Tables)	3 Months Ended
	Mar. 31, 2015
Prepaid Expense and Other Assets [Abstract]
Schedule of prepaid expenses and other assets	Prepaid expenses and other assets consisted of the following (in thousands):
		March 31,		December 31,
		2015		2014
	Construction contract costs incurred in excess of billings	$	19,136	$	6,656
	Prepaid expenses	14,862		20,570
	Lease incentives, net	13,183		13,344
	Furniture, fixtures and equipment, net	6,349		6,637
	Deposit on acquisitions	5,066		516
	Deferred tax asset, net (1)	3,946		4,002
	Operating notes receivable	3,629		3,797
	Equity method investments	2,390		2,368
	Other assets	2,939		2,359
	Prepaid expenses and other assets	$	71,500	$	60,249
	(1) Includes a valuation allowance of $2.1 million.

Debt_Tables

Debt (Tables)	3 Months Ended
	Mar. 31, 2015
Debt Disclosure [Abstract]
Schedule of debt	Our debt consisted of the following (dollars in thousands):
		Maximum
		Availability at		Carrying Value at						Scheduled Maturity
		March 31,		March 31,		December 31,		Stated Interest Rates as of		as of
		2015		2015		2014		March 31, 2015		March 31, 2015
	Mortgage and Other Secured Loans:
	Fixed rate mortgage loans (1)			$	385,693	$	387,139	3.96% - 10.65% (2)		2015-2024
	Variable rate secured loan			36,668		36,877		LIBOR + 2.25% (3)		Nov-15
	Total mortgage and other secured loans			422,361		424,016
	Revolving Credit Facility	$	800,000	164,000		83,000		LIBOR + 0.975% to 1.75% (4)		Jul-17
	Term Loan Facilities	-5		520,000		520,000		LIBOR + 1.10% to 2.60% (6)		2015-2019
	Unsecured Senior Notes
	3.600% Senior Notes (7)			347,560		347,496		3.60%		May-23
	5.250% Senior Notes (8)			245,888		245,797		5.25%		Feb-24
	3.700% Senior Notes (9)			297,655		297,569		3.70%		Jun-21
	Unsecured notes payable			1,583		1,607		0% (10)		2026
	4.25% Exchangeable Senior Notes (11)			575		572		4.25%		-11
	Total debt, net			$	1,999,622	$	1,920,057
	(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 $36,000 as of March 31, 2015 and $42,000 as of December 31, 2014.
	-2	Includes a $150.0 million nonrecourse mortgage loan in default 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). 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. The maximum stated interest rate would be 7.87%, excluding the incremental additional interest rate associated with the default rate on the loan in default. The weighted average interest rate on our fixed rate mortgage loans was 8.11% as of March 31, 2015 (or 6.16% excluding the incremental additional interest rate associated with the default rate on the loan in default).
	(3)	The interest rate on the loan outstanding was 2.42% as of March 31, 2015.
	-4	The weighted average interest rate on the Revolving Credit Facility was 1.45% as of March 31, 2015.
	(5)	We have the ability to borrow an additional $330.0 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 1.82% as of March 31, 2015.
	-7	The carrying value of these notes included a principal amount of $350.0 million and an unamortized discount totaling $2.4 million as of March 31, 2015 and $2.5 million as of December 31, 2014.  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 $4.1 million as of March 31, 2015 and $4.2 million as of December 31, 2014.  The effective interest rate under the notes, including amortization of the issuance costs, was 5.49%.
	-9	The carrying value of these notes included a principal amount of $300.0 million and an unamortized discount totaling $2.3 million as of March 31, 2015 and $2.4 million as of December 31, 2014.  The effective interest rate under the notes, including amortization of the issuance costs, was 3.85%.
	(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 $628,000 as of March 31, 2015 and $654,000 as of December 31, 2014.
	(11)	As described further in our 2014 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 March 31, 2015 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 $3,000 as of December 31, 2014.  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 March 31, 2015 and December 31, 2014 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.  On April 20, 2015, we redeemed these notes at 100% of their principal amount.
Schedule of the fair value of debt	The following table sets forth information pertaining to the fair value of our debt (in thousands):
		March 31, 2015				December 31, 2014
		Carrying		Estimated		Carrying		Estimated
		Amount		Fair Value		Amount		Fair Value
	Fixed-rate debt
	Unsecured Senior Notes	$	891,103	$	918,074	$	890,862	$	901,599
	4.25% Exchangeable Senior Notes	575		575		572		575
	Other fixed-rate debt	387,276		350,835		388,746		355,802
	Variable-rate debt	720,668		722,752		639,877		642,091
		$	1,999,622	$	1,992,236	$	1,920,057	$	1,900,067

Interest_Rate_Derivatives_Tabl

Interest Rate Derivatives (Tables)	3 Months Ended
	Mar. 31, 2015
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		March 31,			December 31,
												2015			2014
	$	100,000			0.83%		One-Month LIBOR		1/3/12	9/1/15		$	(267	)	$	(407	)
	100,000				0.83%		One-Month LIBOR		1/3/12	9/1/15		(267		)	(407		)
	36,668			-1	3.83%		One-Month LIBOR + 2.25%		11/2/10	11/2/15		(291		)	(400		)
	100,000				0.81%		One-Month LIBOR		9/2/14	9/1/16		(455		)	(317		)
	100,000				0.81%		One-Month LIBOR		9/2/14	9/1/16		(461		)	(324		)
	100,000				1.67%		One-Month LIBOR		9/1/15	8/1/19		(1,161		)	239
	100,000				1.73%		One-Month LIBOR		9/1/15	8/1/19		(1,380		)	35
												$	(4,282	)	$	(1,581	)
	-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 sheets (in thousands):
			March 31, 2015						31-Dec-14
	Derivatives		Balance Sheet Location		Fair Value				Balance Sheet Location	Fair Value
	Interest rate swaps designated as cash flow hedges		Prepaid expenses and other assets		$	—			Prepaid expenses and other assets	$	274
	Interest rate swaps designated as cash flow hedges		Interest rate derivatives		(4,282		)		Interest rate derivatives	(1,855		)
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 March 31,
			2015				2014
	Amount of losses recognized in accumulated other comprehensive loss (“AOCL”) (effective portion)		$	(3,474	)		$	(2,123	)
	Amount of losses reclassified from AOCL into interest expense (effective portion)		773				695

Redeemable_Noncontrolling_Inte1

Redeemable Noncontrolling Interest (Tables)	3 Months Ended
	Mar. 31, 2015
Noncontrolling Interest [Abstract]
Schedule of activity for redeemable noncontrolling interest	The table below sets forth the activity in a redeemable noncontrolling interest in a consolidated real estate joint venture (in thousands):
		For the Three Months Ended March 31,
		2015			2014
	Beginning balance	$	18,417		$	17,758
	Distribution to noncontrolling interest	(157		)	(68		)
	Net income attributable to noncontrolling interest	562			504
	Adjustment to arrive at fair value of interest	73			(540		)
	Ending balance	$	18,895		$	17,654

Information_by_Business_Segmen1

Information by Business Segment (Tables)	3 Months Ended
	Mar. 31, 2015
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		Other		Operating		Total
		Washington				Virginia				Antonio					DC - Capitol		King George		Baltimore		Philadelphia				Wholesale
		Corridor													Riverfront		Counties								Data Center
	Three Months Ended March 31, 2015
	Revenues from real estate operations	$	61,792			$	21,749			$	9,171		$	2,446	$	3,364	$	3,901	$	11,485	$	3,224	$	2,543	$	3,035	$	122,710
	Property operating expenses	23,583				9,121				4,975			829		1,814		1,584		4,997		1,375		196		2,212		50,686
	NOI from real estate operations	$	38,209			$	12,628			$	4,196		$	1,617	$	1,550	$	2,317	$	6,488	$	1,849	$	2,347	$	823	$	72,024
	Additions to long-lived assets	$	3,445			$	2,821			$	21		$	83	$	393	$	1,200	$	63,604	$	294	$	80	$	30	$	71,971
	Transfers from non-operating properties	$	12,930			$	58,571			$	31,091		$	2,997	$	—	$	—	$	—	$	11,498	$	8	$	177	$	117,272
	Segment assets at March 31, 2015	$	1,280,742			$	696,276			$	147,078		$	99,335	$	94,400	$	100,849	$	334,271	$	117,919	$	77,340	$	162,170	$	3,110,380
	Three Months Ended March 31, 2014
	Revenues from real estate operations	$	61,113			$	24,968			$	8,479		$	2,555	$	3,634	$	4,316	$	11,496	$	3,340	$	2,594	$	2,401	$	124,896
	Property operating expenses	23,597				8,973				4,474			653		1,765		1,504		5,476		1,300		322		1,688		49,752
	NOI from real estate operations	$	37,516			$	15,995			$	4,005		$	1,902	$	1,869	$	2,812	$	6,020	$	2,040	$	2,272	$	713	$	75,144
	Additions to long-lived assets	$	5,741			$	3,495			$	(6	)	$	2,507	$	63	$	839	$	1,012	$	11	$	55	$	12	$	13,729
	Transfers from non-operating properties	$	6,911			$	26,588			$	—		$	20,102	$	—	$	—	$	74	$	3,176	$	11	$	78	$	56,940
	Segment assets at March 31, 2014	$	1,244,876			$	636,895			$	117,812		$	99,378	$	97,843	$	95,645	$	300,619	$	94,428	$	79,542	$	165,711	$	2,932,749
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 March 31,
			2015				2014
	Segment revenues from real estate operations		$	122,710			$	124,896
	Construction contract and other service revenues		38,324				21,790
	Less: Revenues from discontinued operations		—				(19		)
	Total revenues		$	161,034			$	146,667
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 March 31,
			2015				2014
	Segment property operating expenses		$	50,686			$	49,752
	Less: Property operating expenses from discontinued operations		(5		)		20
	Total property operating expenses		$	50,681			$	49,772
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 March 31,
			2015				2014
	Construction contract and other service revenues		$	38,324			$	21,790
	Construction contract and other service expenses		(37,498		)		(18,624		)
	NOI from service operations		$	826			$	3,166
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 March 31,
			2015				2014
	NOI from real estate operations		$	72,024			$	75,144
	NOI from service operations		826				3,166
	Interest and other income		1,283				1,285
	Equity in income of unconsolidated entities		25				60
	Income tax expense		(55		)		(64		)
	Other adjustments:		—
	Depreciation and other amortization associated with real estate operations		(31,599		)		(43,596		)
	General, administrative and leasing expenses		(7,891		)		(8,143		)
	Business development expenses and land carry costs		(2,790		)		(1,326		)
	Interest expense on continuing operations		(20,838		)		(20,827		)
	NOI from discontinued operations		5				(39		)
	Loss on early extinguishment of debt		(3		)		—
	Income from continuing operations		$	10,987			$	5,660
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):
		March 31,				March 31,
		2015				2014
	Segment assets	$	3,110,380			$	2,932,749
	Non-operating property assets	496,930				503,030
	Other assets	169,845				170,118
	Total COPT consolidated assets	$	3,777,155			$	3,605,897

ShareBased_Compensation_Tables

Share-Based Compensation (Tables)	3 Months Ended
	Mar. 31, 2015
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)	3 Months Ended
	Mar. 31, 2015
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 March 31,
		2015		2014
	Deferred
	Federal	$	45	$	53
	State	10		11
	Total income tax expense	$	55	$	64

Earnings_Per_Share_EPS_and_Ear1

Earnings Per Share ("EPS") and Earnings Per Unit ("EPU") (Tables)	3 Months Ended
	Mar. 31, 2015
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 March 31,
			2015				2014
	Numerator:
	Income from continuing operations		$	10,987			$	5,660
	Gain on sales of real estate, net		3,986				—
	Preferred share dividends		(3,552		)		(4,490		)
	Income from continuing operations attributable to noncontrolling interests		(1,392		)		(932		)
	Income from continuing operations attributable to restricted shares		(122		)		(121		)
	Numerator for basic and diluted EPS from continuing operations attributable to COPT common shareholders		$	9,907			$	117
	Discontinued operations		(238		)		11
	Discontinued operations attributable to noncontrolling interests		12				2
	Numerator for basic and diluted EPS on net income attributable to COPT common shareholders		$	9,681			$	130
	Denominator (all weighted averages):
	Denominator for basic EPS (common shares)		93,199				87,080
	Dilutive effect of share-based compensation awards		198				112
	Denominator for diluted EPS (common shares)		93,397				87,192
	Basic EPS:
	Income from continuing operations attributable to COPT common shareholders		$	0.1			$	0
	Discontinued operations attributable to COPT common shareholders		0				0
	Net income attributable to COPT common shareholders		$	0.1			$	0
	Diluted EPS:
	Income from continuing operations attributable to COPT common shareholders		$	0.1			$	0
	Discontinued operations attributable to COPT common shareholders		0				0
	Net income attributable to COPT common shareholders		$	0.1			$	0
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 March 31,
			2015			2014
	Conversion of common units		3,732			3,958
	Conversion of Series I Preferred Units		176			176
	Conversion of Series K Preferred Shares		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 March 31,
			2015				2014
	Numerator:
	Income from continuing operations		$	10,987			$	5,660
	Gain on sales of real estate, net		3,986				—
	Preferred unit distributions		(3,717		)		(4,655		)
	Income from continuing operations attributable to noncontrolling interests		(821		)		(739		)
	Income from continuing operations attributable to restricted units		(122		)		(121		)
	Numerator for basic and diluted EPU from continuing operations attributable to COPLP common unitholders		$	10,313			$	145
	Discontinued operations		(238		)		11
	Discontinued operations attributable to noncontrolling interests		3				2
	Numerator for basic and diluted EPU on net income attributable to COPLP common unitholders		$	10,078			$	158
	Denominator (all weighted averages):
	Denominator for basic EPU (common units)		96,931				91,038
	Dilutive effect of share-based compensation awards		198				112
	Denominator for basic and diluted EPU (common units)		97,129				91,150
	Basic EPU:
	Income from continuing operations attributable to COPLP common unitholders		$	0.1			$	0
	Discontinued operations attributable to COPLP common unitholders		0				0
	Net income attributable to COPLP common unitholders		$	0.1			$	0
	Diluted EPU:
	Income from continuing operations attributable to COPLP common unitholders		$	0.1			$	0
	Discontinued operations attributable to COPLP common unitholders		0				0
	Net income attributable to COPLP common unitholders		$	0.1			$	0
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 March 31,
			2015			2014
	Conversion of Series I preferred units		176			176
	Conversion of Series K preferred units		434			434

Organization_Details

Organization (Details)	Mar. 31, 2015
	Property
Debt in default of payment terms | Northern Virginia | Wholly owned subsidiary
Investments in real estate
Number of real estate properties	2
Operating office properties
Investments in real estate
Number of real estate properties	178
Square footage of real estate properties (in square feet)	17,700,000
Office properties under, or contractually committed for, construction or approved for redevelopment
Investments in real estate
Number of real estate properties	12
Square footage of real estate properties (in square feet)	1,400,000
Partially operational properties
Investments in real estate
Number of real estate properties	1
Land controlled for future development
Investments in real estate
Square footage of real estate properties (in square feet)	1,402
Developable square feet	17,600,000
Partially operational, wholesale data center
Investments in real estate
The expected stabilization critical load after completion of development (in megawatts)	19.25

Organization_Details_2

Organization (Details 2) (Corporate Office Properties, L.P.)	3 Months Ended
	Mar. 31, 2015
Common Units
Forms of ownership in Operating Partnership and ownership percentage by the entity
Percentage ownership in operating partnership	96.30%
Preferred Units
Forms of ownership in Operating Partnership and ownership percentage by the entity
Percentage ownership in operating partnership	95.50%

Fair_Value_Measurements_Detail

Fair Value Measurements (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Liabilities:
Interest rate derivatives	$4,282	$1,855
Redeemable noncontrolling interest	18,895	18,417	17,654	17,758
Corporate Office Properties, L.P.
Liabilities:
Interest rate derivatives	4,282	1,855
Fair value measurement on a recurring basis
Assets:
Warrants to purchase common stock	89
Assets	6,025
Liabilities:
Deferred compensation plan liability	5,936
Interest rate derivatives	4,282
Liabilities	10,218
Redeemable noncontrolling interest	18,895
Fair value measurement on a recurring basis | Mutual funds
Assets:
Marketable securities in deferred compensation plan	5,826
Fair value measurement on a recurring basis | Other
Assets:
Marketable securities in deferred compensation plan	110
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Assets	5,936
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	5,826
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	110
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2)
Assets:
Warrants to purchase common stock	89
Assets	89
Liabilities:
Deferred compensation plan liability	5,936
Interest rate derivatives	4,282
Liabilities	10,218
Fair value measurement on a recurring basis | Significant Unobservable Inputs (Level 3)
Liabilities:
Redeemable noncontrolling interest	18,895
Fair value measurement on a recurring basis | Corporate Office Properties, L.P.
Assets:
Warrants to purchase common stock	89
Liabilities:
Interest rate derivatives	4,282
Redeemable noncontrolling interest	18,895
Fair value measurement on a recurring basis | Corporate Office Properties, L.P. | Significant Other Observable Inputs (Level 2)
Assets:
Warrants to purchase common stock	89
Liabilities:
Interest rate derivatives	4,282
Fair value measurement on a recurring basis | Corporate Office Properties, L.P. | Significant Unobservable Inputs (Level 3)
Liabilities:
Redeemable noncontrolling interest	$18,895

Fair_Value_Measurements_Detail1

Fair Value Measurements (Details 2) (Redeemable Non-Controlling Interest, Fair value measurement on a recurring basis, Significant Unobservable Inputs (Level 3))	3 Months Ended
	Mar. 31, 2015
Redeemable Non-Controlling Interest | Fair value measurement on a recurring basis | Significant Unobservable Inputs (Level 3)
Assets and liabilities measured at fair value on a non-recurring basis
Fair value inputs, discount rate	15.50%

Properties_net_Details

Properties, net (Details) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2015	Dec. 31, 2014
Investments in real estate
Operating properties, net		$2,888,534,000	$2,751,488,000
Greater Philadelphia
Investments in real estate
Depreciation expense on reclassified assets	12,900,000
Operating properties, net
Investments in real estate
Less: accumulated depreciation		-724,539,000	-703,083,000
Operating properties, net | Land
Investments in real estate
Gross		455,639,000	439,355,000
Operating properties, net | Buildings and improvements
Investments in real estate
Gross		$3,157,434,000	$3,015,216,000

Properties_net_Details_2

Properties, net (Details 2) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Properties
Projects in development or held for future development	$489,618	$545,426
Assets held for sale, net	0	14,339
Greater Baltimore
Properties
Number of properties held-for-sale		2
Assets held for sale, net		14,300
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	219,852	214,977
Projects in development or held for future development | Construction in progress, excluding land
Properties
Projects in development or held for future development	$269,766	$330,449

Properties_net_Details_3

Properties, net (Details 3) (USD $)	0 Months Ended	3 Months Ended
	Mar. 19, 2015	Mar. 31, 2015	Mar. 19, 2015
			sqft
Business Acquisition [Line Items]
Weighted average useful life		10 years
Tenant relationship value
Business Acquisition [Line Items]
Weighted average useful life		8 years
250 W Pratt St | Baltimore, Maryland
Business Acquisition [Line Items]
Square footage of real estate properties (in square feet)	367,000		367,000
Acquired property percentage leased	96.20%		96.20%
Total acquisition cost	$61,900,000
Land	8,057,000		8,057,000
Building and improvements	34,740,000		34,740,000
Intangible assets on real estate acquisitions	20,183,000		20,183,000
Total assets	62,980,000		62,980,000
Below-market leases	-1,093,000		-1,093,000
Total acquisition cost	61,887,000		61,887,000
Finite-lived intangible assets acquired	20,183,000
Weighted average useful life	8 years
Operating property acquisition costs		1,000,000
250 W Pratt St | Baltimore, Maryland | Tenant relationship value
Business Acquisition [Line Items]
Finite-lived intangible assets acquired	7,252,000
Weighted average useful life	11 years
250 W Pratt St | Baltimore, Maryland | In-place lease value
Business Acquisition [Line Items]
Finite-lived intangible assets acquired	10,218,000
Weighted average useful life	7 years
250 W Pratt St | Baltimore, Maryland | Above market leases
Business Acquisition [Line Items]
Finite-lived intangible assets acquired	$2,713,000
Weighted average useful life	4 years

Properties_net_Details_4

Properties, net (Details 4) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015
	sqft
	Property
Newly constructed properties placed in service
Construction and Redevelopment Activities
Square feet of properties placed in service	440,000
Number of real estate properties placed in service	3
Properties under construction or contractually committed for construction
Construction and Redevelopment Activities
Number of real estate properties	7
Square footage of real estate properties (in square feet)	1,100,000
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	4
Properties under construction or contractually committed for construction | Huntsville
Construction and Redevelopment Activities
Number of real estate properties	1
Properties redeveloped
Construction and Redevelopment Activities
Square feet of properties placed in service	111,000
Properties under or approved for redevelopment
Construction and Redevelopment Activities
Number of real estate properties	5
Square footage of real estate properties (in square feet)	344,000
Properties under or approved for redevelopment | Baltimore and Washington Corridor
Construction and Redevelopment Activities
Number of real estate properties	3
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
Non-operating properties
Construction and Redevelopment Activities
Sale price of land	18.1
Gain on sale	4
Partially operational properties
Construction and Redevelopment Activities
Number of real estate properties	1

Real_Estate_Joint_Ventures_Det

Real Estate Joint Ventures (Details) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015
Consolidated real estate joint ventures
Investments in consolidated real estate joint ventures
Total Assets	$208,769
Encumbered Assets	114,414
Total Liabilities	82,014
LW Redstone Company, LLC
Investments in consolidated real estate joint ventures
Buildings Operated by Joint Ventures Number	4
LW Redstone Company, LLC | Variable Interest Entity, Primary Beneficiary [Member]
Investments in consolidated real estate joint ventures
Ownership (as a percent)	85.00%
Total Assets	149,098
Encumbered Assets	65,934
Total Liabilities	43,314
M Square Associates, LLC
Investments in consolidated real estate joint ventures
Buildings Operated by Joint Ventures Number	2
M Square Associates, LLC | Variable Interest Entity, Primary Beneficiary [Member]
Investments in consolidated real estate joint ventures
Ownership (as a percent)	50.00%
Total Assets	59,671
Encumbered Assets	48,480
Total Liabilities	$38,700

Intangible_Assets_on_Real_Esta2

Intangible Assets on Real Estate Acquisitions (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Intangible assets on real estate acquisitions
Gross Carrying Amount	$208,067,000		$188,467,000
Accumulated Amortization	146,590,000		144,613,000
Net Carrying Amount	61,477,000		43,854,000
Amortization of intangible assets	2,600,000	3,600,000
Weighted average useful life	10 years
Estimated future amortization expense associated with the intangible asset categories for the next five years
Remainder of fiscal year	9,400,000
2016	11,600,000
2017	9,400,000
2018	6,300,000
2019	5,100,000
2020	3,800,000
In-place lease value
Intangible assets on real estate acquisitions
Gross Carrying Amount	133,395,000		123,759,000
Accumulated Amortization	102,019,000		101,040,000
Net Carrying Amount	31,376,000		22,719,000
Weighted average useful life	6 years
Tenant relationship value
Intangible assets on real estate acquisitions
Gross Carrying Amount	49,552,000		42,301,000
Accumulated Amortization	29,265,000		28,492,000
Net Carrying Amount	20,287,000		13,809,000
Weighted average useful life	8 years
Below-market cost arrangements
Intangible assets on real estate acquisitions
Gross Carrying Amount	12,415,000		12,415,000
Accumulated Amortization	6,154,000		5,984,000
Net Carrying Amount	6,261,000		6,431,000
Weighted average useful life	31 years
Above-market leases
Intangible assets on real estate acquisitions
Gross Carrying Amount	11,372,000		8,659,000
Accumulated Amortization	8,211,000		8,159,000
Net Carrying Amount	3,161,000		500,000
Weighted average useful life	4 years
Market concentration premium
Intangible assets on real estate acquisitions
Gross Carrying Amount	1,333,000		1,333,000
Accumulated Amortization	941,000		938,000
Net Carrying Amount	$392,000		$395,000
Weighted average useful life	27 years

Investing_Receivables_Details

Investing Receivables (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Accounts, Notes, Loans and Financing Receivable [Line Items]
Investing receivables	$52,814	$52,147
Notes Receivable from City of Huntsville
Accounts, Notes, Loans and Financing Receivable [Line Items]
Investing receivables	49,814	49,147
Notes Receivable from City of Huntsville | LW Redstone Company, LLC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Stated interest rate (as a percent)	9.95%
Other investing loans receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Investing receivables	$3,000	$3,000

Prepaid_Expenses_and_Other_Ass2

Prepaid Expenses and Other Assets (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Prepaid Expense and Other Assets [Abstract]
Construction contract costs incurred in excess of billings	$19,136,000	$6,656,000
Prepaid expenses	14,862,000	20,570,000
Lease incentives, net	13,183,000	13,344,000
Furniture, fixtures and equipment, net	6,349,000	6,637,000
Deposit on acquisitions	5,066,000	516,000
Deferred tax asset, net	3,946,000	4,002,000
Operating notes receivable	3,629,000	3,797,000
Equity method investments	2,390,000	2,368,000
Other assets	2,939,000	2,359,000
Prepaid expenses and other assets	71,500,000	60,249,000
Long-term Operating Notes Receivable
Mortgage and Other Investing Receivables [Line Items]
Operating notes receivable	3,600,000	3,600,000
Allowances for estimated losses	281,000	252,000
Taxable REIT Subsidiary
Mortgage and Other Investing Receivables [Line Items]
Deferred tax assets, valuation allowance	$2,100,000	$2,100,000

Debt_Details

Debt (Details) (USD $)	3 Months Ended		0 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Apr. 20, 2015	Dec. 31, 2014
Debt
Carrying Value	$1,999,622,000			$1,920,057,000
Interest costs capitalized	2,100,000	1,600,000
Wholly owned subsidiary
Debt
Debt Instrument, Debt Default, Amount	150,000,000
Mortgage and Other Secured Loans:
Debt
Carrying Value	422,361,000			424,016,000
Fixed rate mortgage loans
Debt
Carrying Value	385,693,000			387,139,000
Stated interest rates, low end of range (as a percent)	3.96%
Stated interest rates, high end of range (as a percent)	10.65%
Unamortized premium included in carrying value	36,000			42,000
Weighted average interest rate (as a percent)	8.11%
Debt, Weighted Average Interest Rate, Excluding Incremental Rate On Default Rate	6.16%
Fixed rate mortgage loans | Maximum
Debt
Interest rate excluding incremental rate on default rate (percent)	7.87%
Variable rate secured loans
Debt
Carrying Value	36,668,000			36,877,000
Description of variable rate basis	LIBOR
Stated interest rate (as a percent)	2.42%
Variable rate secured loans | London Interbank Offered Rate (LIBOR) [Member]
Debt
Variable rate, spread (as a percent)	2.25%
Variable rate secured loans | Revolving Credit Facility
Debt
Weighted average interest rate (as a percent)	1.45%
Revolving Credit Facility
Debt
Carrying Value	164,000,000			83,000,000
Description of variable rate basis	LIBOR
Revolving Credit Facility | Minimum | London Interbank Offered Rate (LIBOR) [Member]
Debt
Variable rate, spread (as a percent)	0.98%
Revolving Credit Facility | Maximum | London Interbank Offered Rate (LIBOR) [Member]
Debt
Variable rate, spread (as a percent)	1.75%
Revolving Credit Facility | Revolving Credit Facility
Debt
Maximum Availability	800,000,000
Term Loan Facilities
Debt
Carrying Value	520,000,000			520,000,000
Description of variable rate basis	LIBOR
Weighted average interest rate (as a percent)	1.82%
Additional borrowing capacity available provided there is no default under the agreement	330,000,000
Term Loan Facilities | Minimum | London Interbank Offered Rate (LIBOR) [Member]
Debt
Variable rate, spread (as a percent)	1.10%
Term Loan Facilities | Maximum | London Interbank Offered Rate (LIBOR) [Member]
Debt
Variable rate, spread (as a percent)	2.60%
3.60% Senior Notes | Unsecured senior notes
Debt
Carrying Value	347,560,000			347,496,000
Stated interest rate (as a percent)	3.60%
Interest rate on debt (as a percent)	3.70%
Debt instrument, face amount	350,000,000
Unamortized discount included in carrying value	2,400,000			2,500,000
5.250% Senior Notes | Unsecured senior notes
Debt
Carrying Value	245,888,000			245,797,000
Stated interest rate (as a percent)	5.25%
Interest rate on debt (as a percent)	5.49%
Debt instrument, face amount	250,000,000
Unamortized discount included in carrying value	4,100,000			4,200,000
3.70% Senior Notes | Unsecured senior notes
Debt
Carrying Value	297,655,000			297,569,000
Stated interest rate (as a percent)	3.70%
Interest rate on debt (as a percent)	3.85%
Debt instrument, face amount	300,000,000
Unamortized discount included in carrying value	2,300,000			2,400,000
Unsecured notes payable
Debt
Carrying Value	1,583,000			1,607,000
Stated interest rate (as a percent)	0.00%
Unamortized discount included in carrying value	628,000			654,000
4.25% Exchangeable Senior Notes
Debt
Carrying Value	575,000			572,000
Stated interest rate (as a percent)	4.25%
Interest rate on debt (as a percent)	6.05%
Unamortized discount included in carrying value				3,000
Exchange rate per $1000 principal amount (in common shares)	20.8513
Convertible debt principal amount, basis for exchange	1,000
Exchange price per common share based on exchange rate (in dollars per share)	$47.96
Principal amount of debt				$575,000
4.25% Exchangeable Senior Notes | Subsequent Event
Debt
Debt Instrument, Redemption Price, Percentage of Principal Amount Redeemed			100.00%
Debt in default of payment terms | Wholly owned subsidiary
Debt
Stated interest rate (as a percent)	10.65%
Debt in default of payment terms | Wholly owned subsidiary | Northern Virginia
Debt
Number of real estate properties	2

Debt_Details_2

Debt (Details 2) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Carrying amount and estimated fair value of debt
Carrying Value	$1,999,622	$1,920,057
4.25% Exchangeable Senior Notes
Carrying amount and estimated fair value of debt
Carrying Value	575	572
Carrying Amount
Carrying amount and estimated fair value of debt
Variable-rate debt	720,668	639,877
Carrying Value	1,999,622	1,920,057
Carrying Amount | Unsecured senior notes
Carrying amount and estimated fair value of debt
Fixed-rate debt	891,103	890,862
Carrying Amount | 4.25% Exchangeable Senior Notes
Carrying amount and estimated fair value of debt
Fixed-rate debt	575	572
Carrying Amount | Other fixed-rate debt
Carrying amount and estimated fair value of debt
Fixed-rate debt	387,276	388,746
Total Estimated Fair Value
Carrying amount and estimated fair value of debt
Variable-rate debt	722,752	642,091
Carrying Value	1,992,236	1,900,067
Total Estimated Fair Value | Unsecured senior notes
Carrying amount and estimated fair value of debt
Fixed-rate debt	918,074	901,599
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	$350,835	$355,802

Interest_Rate_Derivatives_Deta

Interest Rate Derivatives (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Effect of interest rate derivatives on consolidated statements of operations and comprehensive income
Amount of losses recognized in accumulated other comprehensive loss (“AOCL”) (effective portion)	($3,474,000)	($2,123,000)
Interest Expense [Member]
Effect of interest rate derivatives on consolidated statements of operations and comprehensive income
Amount of losses reclassified from AOCL into interest expense (effective portion)	773,000	695,000
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	-3,200,000
Interest rate derivatives in liability position, fair value	4,300,000
Termination value to settle obligations under interest rate derivative agreements	4,500,000
Interest rate swaps | Prepaid expenses and other current assets
Fair value of interest rate derivatives and balance sheet classification
Interest rate derivatives	0		274,000
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	-4,282,000		-1,855,000
Designated
Fair values of interest rate swap derivatives
Fair value of interest rate swaps	-4,282,000		-1,581,000
Designated | Interest rate swap, effective date January 3, 2012, swap three
Fair values of interest rate swap derivatives
Notional Amount	100,000,000
Fixed rate (as a percent)	0.83%
Fair value of interest rate swaps	-267,000		-407,000
Designated | Interest rate swap, effective date January 3, 2012, swap four
Fair values of interest rate swap derivatives
Notional Amount	100,000,000
Fixed rate (as a percent)	0.83%
Fair value of interest rate swaps	-267,000		-407,000
Designated | Interest rate swap, effective date November 2, 2010
Fair values of interest rate swap derivatives
Notional Amount	36,668,000
Fixed rate (as a percent)	3.83%
Fair value of interest rate swaps	-291,000		-400,000
Notional amount of interest rate derivatives after scheduled amortization	36,200,000
Designated | Interest rate swap, effective date November 2, 2010 | London Interbank Offered Rate (LIBOR) [Member]
Fair values of interest rate swap derivatives
Derivative, basis spread on variable rate	2.25%
Designated | Interest rate swap, effective date September 2, 2014, swap one
Fair values of interest rate swap derivatives
Notional Amount	100,000,000
Fixed rate (as a percent)	0.81%
Fair value of interest rate swaps	-455,000		-317,000
Designated | Interest rate swap, effective date September 2, 2014, swap two
Fair values of interest rate swap derivatives
Notional Amount	100,000,000
Fixed rate (as a percent)	0.81%
Fair value of interest rate swaps	-461,000		-324,000
Designated | Interest rate swap, effective date September 1, 2015, swap one
Fair values of interest rate swap derivatives
Notional Amount	100,000,000
Fixed rate (as a percent)	1.67%
Fair value of interest rate swaps	-1,161,000		239,000
Designated | Interest rate swap, effective date September 1, 2015, swap two
Fair values of interest rate swap derivatives
Notional Amount	100,000,000
Fixed rate (as a percent)	1.73%
Fair value of interest rate swaps	($1,380,000)		$35,000

Redeemable_Noncontrolling_Inte2

Redeemable Noncontrolling Interests (Details) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Redeemable Noncontrolling Interest [Roll Forward]
Beginning balance	$18,417	$17,758
Distribution to noncontrolling interest	-157	-68
Net income attributable to noncontrolling interest	562	504
Adjustment to arrive at fair value of interest	73	-540
Ending balance	$18,895	$17,654

Equity_Details

Equity (Details) (USD $)	3 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Class of Stock [Line Items]
Number of operating partnerships units converted into common shares (in units)	158,000	48,498
Common Shares
Class of Stock [Line Items]
Shares issued to the public (in units/ shares)	890,241
Weighted average price per share of stock issued	30.29
Net proceeds of stock issuance	26.6
Commission costs	0.4
Remaining capacity under ATM Plan	84
Corporate Office Properties, L.P. | Common Shares
Class of Stock [Line Items]
Shares issued to the public (in units/ shares)	890,241

Information_by_Business_Segmen2

Information by Business Segment (Details) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
	segment
Segment Reporting [Abstract]
Number of primary office property segments	10
Segment financial information for real estate operations
Revenues from real estate operations	$122,710	$124,896
Property operating expenses	50,686	49,752
NOI from real estate operations	72,024	75,144
Segment assets	3,777,155	3,605,897	3,670,257
Operating Segment Total
Segment financial information for real estate operations
Revenues from real estate operations	122,710	124,896
Property operating expenses	50,686	49,752
NOI from real estate operations	72,024	75,144
Additions to long-lived assets	71,971	13,729
Transfers from non-operating properties	117,272	56,940
Segment assets	3,110,380	2,932,749
Baltimore and Washington Corridor
Segment financial information for real estate operations
Revenues from real estate operations	61,792	61,113
Property operating expenses	23,583	23,597
NOI from real estate operations	38,209	37,516
Additions to long-lived assets	3,445	5,741
Transfers from non-operating properties	12,930	6,911
Segment assets	1,280,742	1,244,876
Northern Virginia
Segment financial information for real estate operations
Revenues from real estate operations	21,749	24,968
Property operating expenses	9,121	8,973
NOI from real estate operations	12,628	15,995
Additions to long-lived assets	2,821	3,495
Transfers from non-operating properties	58,571	26,588
Segment assets	696,276	636,895
San Antonio
Segment financial information for real estate operations
Revenues from real estate operations	9,171	8,479
Property operating expenses	4,975	4,474
NOI from real estate operations	4,196	4,005
Additions to long-lived assets	21	-6
Transfers from non-operating properties	31,091	0
Segment assets	147,078	117,812
Huntsville
Segment financial information for real estate operations
Revenues from real estate operations	2,446	2,555
Property operating expenses	829	653
NOI from real estate operations	1,617	1,902
Additions to long-lived assets	83	2,507
Transfers from non-operating properties	2,997	20,102
Segment assets	99,335	99,378
Washington, DC - Capitol Riverfront
Segment financial information for real estate operations
Revenues from real estate operations	3,364	3,634
Property operating expenses	1,814	1,765
NOI from real estate operations	1,550	1,869
Additions to long-lived assets	393	63
Transfers from non-operating properties	0	0
Segment assets	94,400	97,843
St. Mary's and King George Counties
Segment financial information for real estate operations
Revenues from real estate operations	3,901	4,316
Property operating expenses	1,584	1,504
NOI from real estate operations	2,317	2,812
Additions to long-lived assets	1,200	839
Transfers from non-operating properties	0	0
Segment assets	100,849	95,645
Greater Baltimore
Segment financial information for real estate operations
Revenues from real estate operations	11,485	11,496
Property operating expenses	4,997	5,476
NOI from real estate operations	6,488	6,020
Additions to long-lived assets	63,604	1,012
Transfers from non-operating properties	0	74
Segment assets	334,271	300,619
Greater Philadelphia
Segment financial information for real estate operations
Revenues from real estate operations	3,224	3,340
Property operating expenses	1,375	1,300
NOI from real estate operations	1,849	2,040
Additions to long-lived assets	294	11
Transfers from non-operating properties	11,498	3,176
Segment assets	117,919	94,428
Other
Segment financial information for real estate operations
Revenues from real estate operations	2,543	2,594
Property operating expenses	196	322
NOI from real estate operations	2,347	2,272
Additions to long-lived assets	80	55
Transfers from non-operating properties	8	11
Segment assets	77,340	79,542
Wholesale Data Center
Segment financial information for real estate operations
Revenues from real estate operations	3,035	2,401
Property operating expenses	2,212	1,688
NOI from real estate operations	823	713
Additions to long-lived assets	30	12
Transfers from non-operating properties	177	78
Segment assets	$162,170	$165,711

Information_by_Business_Segmen3

Information by Business Segment (Details 2) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Reconciliation of segment revenues to total revenues
Segment revenues from real estate operations	$122,710	$124,896
Construction contract and other service revenues	38,324	21,790
Less: Revenues from discontinued operations	0	-19
Total revenues	161,034	146,667
Reconciliation of segment property operating expenses to property operating expenses
Segment property operating expenses	50,686	49,752
Less: Property operating expenses from discontinued operations	-5	20
Total property operating expenses	50,681	49,772
Computation of net operating income from service operations
Construction contract and other service revenues	38,324	21,790
Construction contract and other service expenses	-37,498	-18,624
NOI from service operations	826	3,166
Reconciliation of NOI from real estate operations and NOI from service operations to (loss) income from continuing operations
NOI from real estate operations	72,024	75,144
NOI from service operations	826	3,166
Interest and other income	1,283	1,285
Equity in income of unconsolidated entities	25	60
Income tax expense	-55	-64
Other adjustments:
Depreciation and other amortization associated with real estate operations	-31,599	-43,596
General, administrative and leasing expenses	-7,891	-8,143
Business development expenses and land carry costs	-2,790	-1,326
Interest expense on continuing operations	-20,838	-20,827
NOI from discontinued operations	5	-39
Loss on early extinguishment of debt	-3	0
Income from continuing operations	$10,987	$5,660

Information_by_Business_Segmen4

Information by Business Segment (Details 3) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
In Thousands, unless otherwise specified
Reconciliation of segment assets to total assets
Assets	$3,777,155	$3,670,257	$3,605,897
Segment assets
Reconciliation of segment assets to total assets
Assets	3,110,380		2,932,749
Non-operating property assets
Reconciliation of segment assets to total assets
Assets	496,930		503,030
Other assets
Reconciliation of segment assets to total assets
Assets	$169,845		$170,118

ShareBased_Compensation_Detail

Share-Based Compensation (Details) (USD $)	3 Months Ended		0 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 05, 2015
Options
Exercised (in shares)	70,374	26,614
Performance share units
Assumptions used to value stock awards
Weighted average grant date fair value (in dollars per share)	36.76
Baseline value per common share (in dollars per share)	29.28
Expected volatility of common shares (as a percent)	19.90%
Risk-free interest rate (as a percent)	0.99%
Restricted shares
Share-Based Compensation
Stock awards granted (in shares or units)	177,674
Aggregate grant date fair value	5,200,000
Assumptions used to value stock awards
Weighted average grant date fair value (in dollars per share)	29.46
Other Share-based Compensation Additional Disclosures
Shares vested (in shares)	127,444
Weighted average fair value of shares vested (in dollars per share)	26.01
Aggregate intrinsic value of restricted common shares, forfeiture restrictions lapsed	3,700,000
Options
Options
Exercised (in shares)	70,374
Weighted average exercise price (in dollars per share)	26.23
Aggregate intrinsic value of options exercised (in dollars)	291,000
2014 PSU Grants | Performance share units
Share-Based Compensation
Stock awards granted (in shares or units)			45,656
Aggregate grant date fair value			$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
A2012PSUGrants [Member] | Performance share units
Assumptions used to value stock awards
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Shares issued for Awards Vested in Period			40,309
2013 PSU Grants [Member] | Performance share units | Former Chief Financial Officer [Member]
Assumptions used to value stock awards
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Shares issued for Awards Vested in Period			15,289

Income_Taxes_Details

Income Taxes (Details) (Taxable REIT Subsidiary, USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Taxable REIT Subsidiary
Deferred income tax (expense) benefit
Federal	$45	$53
State	10	11
Total income tax expense	$55	$64
Effective tax rate (as a percent)	37.80%	38.10%

Earnings_Per_Share_EPS_and_Ear2

Earnings Per Share ("EPS") and Earnings Per Unit ("EPU") (Details) (USD $)	3 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Numerator:
Income from continuing operations	$10,987		$5,660
Gain on sales of real estate, net	3,986		0
Preferred share/unit dividends/distributions	-3,552		-4,490
Income from continuing operations attributable to noncontrolling interests	-1,392		-932
Income from continuing operations attributable to restricted shares	-122		-121
Numerator for basic and diluted EPS from continuing operations attributable to COPT common shareholders	9,907		117
Discontinued operations	-238		11
Discontinued operations attributable to noncontrolling interests	12		2
Numerator for basic and diluted EPS on net income attributable to COPT common shareholders	9,681		130
Denominator (all weighted averages):
Denominator for basic EPS (common shares)	93,199		87,080
Dilutive effect of share-based compensation awards	198		112
Denominator for diluted EPS (common shares)	93,397		87,192
Basic EPS:
Income from continuing operations (in dollars per share/unit)	$0.10	[1]	$0	[1]
Discontinued operations (in dollars per share/unit)	$0	[1]	$0	[1]
Net income attributable to COPT common shareholders (in dollars per share/unit)	$0.10	[1]	$0	[1]
Diluted EPS:
Income from continuing operations (in dollars per share/unit)	$0.10	[1]	$0	[1]
Discontinued operations (in dollars per share/unit)	$0	[1]	$0	[1]
Net income attributable to COPT common shareholders (in dollars per share/unit)	$0.10	[1]	$0	[1]
Corporate Office Properties, L.P.
Numerator:
Income from continuing operations	10,987		5,660
Gain on sales of real estate, net	3,986		0
Preferred share/unit dividends/distributions	-3,717		-4,655
Income from continuing operations attributable to noncontrolling interests	-821		-739
Income from continuing operations attributable to restricted shares	-122		-121
Numerator for basic and diluted EPS from continuing operations attributable to COPT common shareholders	10,313		145
Discontinued operations	-238		11
Discontinued operations attributable to noncontrolling interests	3		2
Numerator for basic and diluted EPS on net income attributable to COPT common shareholders	$10,078		$158
Denominator (all weighted averages):
Denominator for basic EPS (common shares)	96,931		91,038
Dilutive effect of share-based compensation awards	198		112
Denominator for diluted EPS (common shares)	97,129		91,150
Basic EPS:
Income from continuing operations (in dollars per share/unit)	$0.10	[2]	$0	[2]
Discontinued operations (in dollars per share/unit)	$0	[2]	$0	[2]
Net income attributable to COPT common shareholders (in dollars per share/unit)	$0.10	[2]	$0	[2]
Diluted EPS:
Income from continuing operations (in dollars per share/unit)	$0.10	[2]	$0	[2]
Discontinued operations (in dollars per share/unit)	$0	[2]	$0	[2]
Net income attributable to COPT common shareholders (in dollars per share/unit)	$0.10	[2]	$0	[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_and_Ear3

Earnings Per Share ("EPS") and Earnings Per Unit ("EPU") (Details 2)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Conversion of common units
Antidilutive securities
Antidilutive securities excluded from computation of diluted earnings per share (in shares)	3,732	3,958
Conversion of Series I Preferred Units
Antidilutive securities
Antidilutive securities excluded from computation of diluted earnings per share (in shares)	176	176
Conversion of Series K Preferred Shares
Antidilutive securities
Antidilutive securities excluded from computation of diluted earnings per share (in shares)	434	434
Weighted average restricted shares
Antidilutive securities
Antidilutive securities excluded from computation of diluted earnings per share (in shares)	400	390
Weighted average options
Antidilutive securities
Antidilutive securities excluded from computation of diluted earnings per share (in shares)	474	588
Corporate Office Properties, L.P. | Weighted average restricted shares
Antidilutive securities
Antidilutive securities excluded from computation of diluted earnings per share (in shares)	400	390
Corporate Office Properties, L.P. | Weighted average options
Antidilutive securities
Antidilutive securities excluded from computation of diluted earnings per share (in shares)	474	588
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
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

Commitments_and_Contingencies_

Commitments and Contingencies (Details) (USD $)	1 Months Ended	0 Months Ended
	Aug. 31, 2010	Apr. 15, 2015	Mar. 31, 2015
			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			1,400,000
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
Subsequent Event | 2600 Park Tower Drive | Virginia
Other Commitments [Line Items]
Square footage of real estate properties (in square feet)		240,000
Acquired property percentage leased		100.00%
Total acquisition cost		$83,000,000