Corporate Office Properties Trust 10-K 25 Feb 16 2015 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Feb. 16, 2016	Jun. 30, 2015
Entity Information [Line Items]
Entity Registrant Name	CORPORATE OFFICE PROPERTIES TRUST
Entity Central Index Key	860,546
Document Type	10-K
Document Period End Date	Dec. 31, 2015
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$ 2,200
Entity Common Stock, Shares Outstanding		94,528,118
Document Fiscal Year Focus	2,015
Document Fiscal Period Focus	FY
Corporate Office Properties, L.P. [Member]
Entity Information [Line Items]
Entity Registrant Name	CORPORATE OFFICE PROPERTIES, L.P.
Entity Central Index Key	1,577,966
Document Type	10-K
Document Period End Date	Dec. 31, 2015
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Public Float			$ 79.4
Document Fiscal Year Focus	2,015
Document Fiscal Period Focus	FY

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Properties, net:
Operating properties, net	$ 2,920,529	$ 2,751,488
Projects in development or held for future development	429,219	545,426
Total properties, net	3,349,748	3,296,914
Assets held for sale, net	96,782	14,339
Cash and cash equivalents	60,310	6,077
Restricted cash and marketable securities	7,716	9,069
Accounts receivable (net of allowance for doubtful accounts of $1,525 and $717, respectively)	29,167	30,698
Deferred rent receivable (net of allowance of $1,962 and $1,418, respectively)	105,484	95,910
Intangible assets on real estate acquisitions, net	98,338	43,854
Deferred leasing costs (net of accumulated amortization of $66,364 and $69,998, respectively)	53,868	53,927
Investing receivables	47,875	52,147
Prepaid expenses and other assets, net	60,024	61,301
Total assets	3,909,312	3,664,236
Liabilities:
Debt, net	2,077,752	1,914,036
Accounts payable and accrued expenses	91,755	123,035
Rents received in advance and security deposits	37,148	31,011
Dividends and distributions payable	30,178	29,862
Deferred revenue associated with operating leases	19,758	13,031
Interest rate derivatives	3,160	1,855
Other liabilities	13,779	12,105
Total liabilities	$ 2,273,530	$ 2,124,935
Commitments and contingencies
Redeemable noncontrolling interest	$ 19,218	$ 18,417
Corporate Office Properties Trust’s shareholders’ equity:
Preferred Shares of beneficial interest at liquidation preference	199,083	199,083
Common Shares of beneficial interest ($0.01 par value; 125,000,000 shares authorized, shares issued and outstanding of 94,531,512 at December 31, 2015 and 93,255,284 at December 31, 2014)	945	933
Additional paid-in capital	2,004,507	1,969,968
Cumulative distributions in excess of net income	(657,172)	(717,264)
Accumulated other comprehensive loss	(2,838)	(1,297)
Total Corporate Office Properties Trust’s shareholders’ equity	1,544,525	1,451,423
Noncontrolling interests in subsidiaries:
Common units in COPLP	52,359	51,534
Preferred units in COPLP	8,800	8,800
Other consolidated entities	10,880	9,127
Noncontrolling interests in subsidiaries	72,039	69,461
Total equity	1,616,564	1,520,884
Total liabilities, redeemable noncontrolling interest and equity	3,909,312	3,664,236
Corporate Office Properties, L.P. [Member]
Properties, net:
Operating properties, net	2,920,529	2,751,488
Projects in development or held for future development	429,219	545,426
Total properties, net	3,349,748	3,296,914
Assets held for sale, net	96,782	14,339
Cash and cash equivalents	60,310	6,077
Restricted cash and marketable securities	1,953	3,187
Accounts receivable (net of allowance for doubtful accounts of $1,525 and $717, respectively)	29,167	30,698
Deferred rent receivable (net of allowance of $1,962 and $1,418, respectively)	105,484	95,910
Intangible assets on real estate acquisitions, net	98,338	43,854
Deferred leasing costs (net of accumulated amortization of $66,364 and $69,998, respectively)	53,868	53,927
Investing receivables	47,875	52,147
Prepaid expenses and other assets, net	60,024	61,301
Total assets	3,903,549	3,658,354
Liabilities:
Debt, net	2,077,752	1,914,036
Accounts payable and accrued expenses	91,755	123,035
Rents received in advance and security deposits	37,148	31,011
Dividends and distributions payable	30,178	29,862
Deferred revenue associated with operating leases	19,758	13,031
Interest rate derivatives	3,160	1,855
Other liabilities	8,016	6,223
Total liabilities	$ 2,267,767	$ 2,119,053
Commitments and contingencies
Redeemable noncontrolling interest	$ 19,218	$ 18,417
Corporate Office Properties Trust’s shareholders’ equity:
Preferred Shares of beneficial interest at liquidation preference	199,083
Common Shares of beneficial interest ($0.01 par value; 125,000,000 shares authorized, shares issued and outstanding of 94,531,512 at December 31, 2015 and 93,255,284 at December 31, 2014)	1,400,745	1,305,219
Accumulated other comprehensive loss	(2,985)	(1,381)
Total Corporate Office Properties Trust’s shareholders’ equity	1,605,643	1,511,721
Noncontrolling interests in subsidiaries:
Noncontrolling interests in subsidiaries	10,921	9,163
Total equity	1,616,564	1,520,884
Total liabilities, redeemable noncontrolling interest and equity	3,903,549	3,658,354
Corporate Office Properties, L.P. [Member] | General Partner [Member]
Corporate Office Properties Trust’s shareholders’ equity:
Preferred Units, Preferred Partners' Capital Accounts	199,083	199,083
Corporate Office Properties, L.P. [Member] | Limited Partner [Member]
Corporate Office Properties Trust’s shareholders’ equity:
Preferred Units, Preferred Partners' Capital Accounts	$ 8,800	$ 8,800

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Allowance for doubtful accounts- AR	$ 1,525	$ 717
Allowance for deferred rent receivable	1,962	1,418
Accumulated amortization of deferred leasing costs	$ 66,364	$ 69,998
Common Shares of beneficial interest, par value (in dollars per share)	$ 0.01	$ 0.01
Common Shares of beneficial interest, shares authorized	125,000,000	125,000,000
Common Shares of beneficial interest, shares issued	94,531,512	93,255,284
Common Shares of beneficial interest, shares outstanding	94,531,512	93,255,284
Corporate Office Properties, L.P. [Member]
Allowance for doubtful accounts- AR	$ 1,525	$ 717
Allowance for deferred rent receivable	1,962	1,418
Accumulated amortization of deferred leasing costs	$ 66,364	$ 69,998
General Partner [Member] | Corporate Office Properties, L.P. [Member]
Preferred Units, Outstanding	7,431,667	7,431,667
Common Shares of beneficial interest, shares outstanding	94,531,512	93,255,284
Limited Partner [Member] | Corporate Office Properties, L.P. [Member]
Preferred Units, Outstanding	352,000	352,000
Common Shares of beneficial interest, shares outstanding	3,677,391	3,837,551

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenues
Rental revenue		$ 420,340	$ 386,396	$ 377,611
Tenant recoveries and other real estate operations revenue		98,724	93,329	83,386
Construction contract and other service revenues		106,402	106,748	62,363
Total revenues		625,466	586,473	523,360
Expenses
Property operating expenses		194,494	179,934	167,199
Depreciation and amortization associated with real estate operations		140,025	136,086	113,214
Construction contract and other service expenses		102,696	100,058	58,875
Impairment losses		23,289	1,416	5,857
General, administrative and leasing expenses		31,361	31,794	30,869
Business development expenses and land carry costs		13,507	5,573	5,436
Total operating expenses		505,372	454,861	381,450
Operating income		120,094	131,612	141,910
Interest expense		(89,074)	(92,393)	(82,010)
Interest and other income		4,517	4,923	3,834
Gain (loss) on early extinguishment of debt		85,275	(9,552)	(27,030)
Income from continuing operations before equity in income of unconsolidated entities and income taxes		120,812	34,590	36,704
Equity in income of unconsolidated entities		62	229	2,110
Income tax expense		(199)	(310)	(1,978)
Income from continuing operations		120,675	34,509	36,836
Discontinued operations		156	26	55,692
Income before gain on sales of real estate		120,831	34,535	92,528
Gain on sales of real estate, net of income taxes		68,047	10,671	9,016
Net income		188,878	45,206	101,544
Net income attributable to noncontrolling interests:
Common units in COPLP		(6,403)	(1,006)	(3,283)
Preferred units in COPLP		(660)	(660)	(660)
Other consolidated entities		(3,515)	(3,285)	(3,894)
Net income		178,300	40,255	93,707
Preferred share/ unit dividends/ distributions		(14,210)	(15,939)	(19,971)
Issuance costs associated with redeemed preferred shares		0	(1,769)	(2,904)
Net income attributable to COPT common shareholders		164,090	22,547	70,832
Net income attributable to COPT:
Income from continuing operations		178,147	40,225	41,366
Discontinued operations, net		153	30	52,341
Net income		$ 178,300	$ 40,255	$ 93,707
Basic earnings per common share
Income from continuing operations (in dollars per share/unit)	[1]	$ 1.74	$ 0.25	$ 0.21
Discontinued operations (in dollars per share/unit)	[1]	0	0	0.62
Net income attributable to COPT common shareholders (in dollars per share/unit)	[1]	1.74	0.25	0.83
Diluted earnings per common share
Income from continuing operations (in dollars per share/unit)	[1]	1.74	0.25	0.21
Discontinued operations (in dollars per share/unit)	[1]	0	0	0.62
Net income attributable to COPT common shareholders (in dollars per share/unit)	[1]	$ 1.74	$ 0.25	$ 0.83
Corporate Office Properties, L.P. [Member]
Revenues
Rental revenue		$ 420,340	$ 386,396	$ 377,611
Tenant recoveries and other real estate operations revenue		98,724	93,329	83,386
Construction contract and other service revenues		106,402	106,748	62,363
Total revenues		625,466	586,473	523,360
Expenses
Property operating expenses		194,494	179,934	167,199
Depreciation and amortization associated with real estate operations		140,025	136,086	113,214
Construction contract and other service expenses		102,696	100,058	58,875
Impairment losses		23,289	1,416	5,857
General, administrative and leasing expenses		31,361	31,794	30,869
Business development expenses and land carry costs		13,507	5,573	5,436
Total operating expenses		505,372	454,861	381,450
Operating income		120,094	131,612	141,910
Interest expense		(89,074)	(92,393)	(82,010)
Interest and other income		4,517	4,923	3,834
Gain (loss) on early extinguishment of debt		85,275	(9,552)	(27,030)
Income from continuing operations before equity in income of unconsolidated entities and income taxes		120,812	34,590	36,704
Equity in income of unconsolidated entities		62	229	2,110
Income tax expense		(199)	(310)	(1,978)
Income from continuing operations		120,675	34,509	36,836
Discontinued operations		156	26	55,692
Income before gain on sales of real estate		120,831	34,535	92,528
Gain on sales of real estate, net of income taxes		68,047	10,671	9,016
Net income		188,878	45,206	101,544
Net income attributable to noncontrolling interests in consolidated entities		(3,520)	(3,276)	(3,907)
Net income attributable to noncontrolling interests:
Net income		185,358	41,930	97,637
Preferred share/ unit dividends/ distributions		(14,870)	(16,599)	(20,631)
Issuance costs associated with redeemed preferred shares		0	(1,769)	(2,904)
Net income attributable to COPT common shareholders		170,488	23,562	74,102
Net income attributable to COPT:
Income from continuing operations		185,199	41,899	42,875
Discontinued operations, net		159	31	54,762
Net income		$ 185,358	$ 41,930	$ 97,637
Basic earnings per common share
Income from continuing operations (in dollars per share/unit)	[2]	$ 1.74	$ 0.25	$ 0.21
Discontinued operations (in dollars per share/unit)	[2]	0	0	0.62
Net income attributable to COPT common shareholders (in dollars per share/unit)	[2]	1.74	0.25	0.83
Diluted earnings per common share
Income from continuing operations (in dollars per share/unit)	[2]	1.74	0.25	0.21
Discontinued operations (in dollars per share/unit)	[2]	0	0	0.62
Net income attributable to COPT common shareholders (in dollars per share/unit)	[2]	$ 1.74	$ 0.25	$ 0.83
[1]	Basic and diluted earnings per common share are calculated based on amounts attributable to common shareholders of Corporate Office Properties Trust.
[2]	Basic and diluted earnings per common unit are calculated based on amounts attributable to common unitholders of Corporate Office Properties, L.P.

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income/Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Net income	$ 188,878	$ 45,206	$ 101,544
Other comprehensive (loss) income
Amount of (losses) gains recognized in accumulated other comprehensive income (loss) (“AOCI”) (effective portion)	(4,739)	(7,799)	6,791
Unrealized equity in other comprehensive (loss) income of equity method investee	(264)	0	1,070
Realized equity in other comprehensive income of equity method investee	0	0	(1,070)
Other comprehensive (loss) income	(1,404)	(4,771)	9,531
Comprehensive income	187,474	40,435	111,075
Comprehensive income attributable to noncontrolling interests	(10,715)	(4,957)	(8,453)
Comprehensive income attributable to COPT	176,759	35,478	102,622
Interest expense [Member]
Other comprehensive (loss) income
Losses on interest rate derivatives	3,599	2,990	2,740
Loss on early extinguishment of debt [Member]
Other comprehensive (loss) income
Losses on interest rate derivatives	0	38	0
Corporate Office Properties, L.P. [Member]
Net income	188,878	45,206	101,544
Other comprehensive (loss) income
Amount of (losses) gains recognized in accumulated other comprehensive income (loss) (“AOCI”) (effective portion)	(4,739)	(7,799)	6,791
Unrealized equity in other comprehensive (loss) income of equity method investee	(264)	0	1,070
Realized equity in other comprehensive income of equity method investee	0	0	(1,070)
Other comprehensive (loss) income	(1,404)	(4,771)	9,531
Comprehensive income	187,474	40,435	111,075
Comprehensive income attributable to noncontrolling interests	(3,720)	(3,492)	(4,125)
Comprehensive income attributable to COPT	183,754	36,943	106,950
Corporate Office Properties, L.P. [Member] | Interest expense [Member]
Other comprehensive (loss) income
Losses on interest rate derivatives	3,599	2,990	2,740
Corporate Office Properties, L.P. [Member] | Loss on early extinguishment of debt [Member]
Other comprehensive (loss) income
Losses on interest rate derivatives	$ 0	$ 38	$ 0

Consolidated Statements of Equi

Consolidated Statements of Equity - USD ($) $ in Thousands	Total	Common Shares [Member]	Preferred Shares [Member]	Common Shares [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member]Common Shares [Member]	Cumulative Distributions in Excess of Net Income [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Noncontrolling Interests in Subsidiaries [Member]	Corporate Office Properties, L.P. [Member]	Corporate Office Properties, L.P. [Member]Common Shares [Member]	Corporate Office Properties, L.P. [Member]Accumulated Other Comprehensive Income (Loss) [Member]	Corporate Office Properties, L.P. [Member]Noncontrolling Interests in Subsidiaries [Member]	Corporate Office Properties, L.P. [Member]Limited Partner [Member]	Corporate Office Properties, L.P. [Member]Limited Partner [Member]Preferred Shares [Member]	Corporate Office Properties, L.P. [Member]General Partner [Member]	Corporate Office Properties, L.P. [Member]General Partner [Member]Preferred Shares [Member]
Balance at Dec. 31, 2012	$ 1,436,499		$ 333,833	$ 809	$ 1,653,672		$ (617,455)	$ (5,435)	$ 71,075	$ 1,436,499	$ 1,089,391	$ (5,708)	$ 10,183		$ 8,800		$ 333,833
Balance (in units/ shares) at Dec. 31, 2012	80,952,986										85,020,528
Balance (preferred units) at Dec. 31, 2012															352,000		12,821,667
Increase (Decrease) in Shareholders' Equity
Redemption of preferred shares/units			(3,390,000)														(3,390,000)
Redemption of preferred shares/units (3,390,000 and 2,000,000 shares in 2013 and 2014, respectively)	$ (84,750)		$ (84,750)		2,904		(2,904)			(84,750)							$ (84,750)
Conversion of common units to common shares (311,343, 140,149 and 160,160 shares in 2013, 2014 and 2015 respectively)	0			3	3,994				(3,997)
Shares issued to the public (4,485,000 and 5,520,000 common shares in 2013 and 2014, respectively)(in dollars)	117,961			$ 45	117,916					117,961	$ 117,961
Shares issued to the public (in units/shares)				4,485,000							4,485,000
Issuance of common units resulting from common shares issued under at-the-market program (in units)											1,500,000
Issuance of common units resulting from common shares issued under at-the-market program (1,500,000 shares in 2013 and 890,241 shares in 2015) (in dollars)		$ 38,447		$ 15		$ 38,432				38,447	$ 38,447
Acquisition of property and noncontrolling interest in other consolidated entity for COPLP common units	$ 1,369				(1,296)				2,665
Acquisition of property and noncontrolling interest in other consolidated entity for COPLP common units (in units)											221,501
Acquisition of property and noncontrolling interest in other consolidated entity for COPLP common units (in dollars)										1,369	$ 3,899		(2,530)
Exercise of share options (in units/shares)	39,331										39,331
Exercise of share options (39,331, 62,888 and 76,474 shares in 2013, 2014 and 2015 respectively)(in dollars)	$ 779				779					779	$ 779
Share-based compensation (105,852, 137,735 and 149,353 shares issued in 2013, 2014 and 2015)	$ 7,605			2	7,603					7,605	$ 7,605
Share-based compensation (in shares/units)	105,852										105,852
Redemptions of vested equity awards	$ (2,002)				(2,002)					(2,002)	$ (2,002)
Contributions from noncontrolling interests	0				(744)				744
Comprehensive income	109,699						93,707	8,915	7,077	109,699	77,006	9,313	2,749		$ 660		19,971
Dividends/Distributions	(115,216)						(115,216)			(120,156)	(99,525)				$ (660)		$ (19,971)
Distributions to owners of common and preferred units in COPLP	(4,940)								(4,940)
Contributions from noncontrolling interests in other consolidates entities	86								86	86			86
Distributions to noncontrolling interests	(1,045)								(1,045)	(1,045)			(1,045)
Adjustment to arrive at fair value of redeemable noncontrolling interest	(7,121)				(7,121)					(7,121)	(7,121)
Tax loss from share-based compensation	$ (122)				(122)					(122)	$ (122)
Balance (preferred units) at Dec. 31, 2013															352,000		9,431,667
Balance (in units/ shares) at Dec. 31, 2013	87,394,512										91,372,212
Balance 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
Increase (Decrease) in Shareholders' Equity
Redemption of preferred shares/units			(2,000,000)														(2,000,000)
Redemption of preferred shares/units (3,390,000 and 2,000,000 shares in 2013 and 2014, respectively)	(50,000)		$ (50,000)		1,769		(1,769)			(50,000)							$ (50,000)
Conversion of common units to common shares (311,343, 140,149 and 160,160 shares in 2013, 2014 and 2015 respectively)	0			2	1,841				(1,843)
Shares issued to the public (4,485,000 and 5,520,000 common shares in 2013 and 2014, respectively)(in dollars)	$ 148,666			$ 55	148,611					148,666	$ 148,666
Shares issued to the public (in units/shares)				5,520,000							5,520,000
Exercise of share options (in units/shares)	62,888										62,888
Exercise of share options (39,331, 62,888 and 76,474 shares in 2013, 2014 and 2015 respectively)(in dollars)	$ 1,489			$ 0	1,489					1,489	$ 1,489
Share-based compensation (105,852, 137,735 and 149,353 shares issued in 2013, 2014 and 2015)	$ 7,050			2	7,048					7,050	$ 7,050
Share-based compensation (in shares/units)	137,735										137,735
Redemptions of vested equity awards	$ (1,554)				(1,554)					(1,554)	$ (1,554)
Contributions from noncontrolling interests	0				(3,382)				3,382
Comprehensive income	38,274						40,255	(4,777)	2,796	38,274	25,331	(4,986)	1,330		660		15,939
Dividends/Distributions	(113,882)						(113,882)			(118,811)	(102,212)				$ (660)		$ (15,939)
Distributions to owners of common and preferred units in COPLP	(4,929)								(4,929)
Contributions from noncontrolling interests in other consolidates entities	3				0				3	3			3
Distributions to noncontrolling interests	(1,613)								(1,613)	(1,613)			(1,613)
Adjustment to arrive at fair value of redeemable noncontrolling interest	134				134					134	134
Tax loss from share-based compensation	$ (3)				(3)					(3)	$ (3)
Balance (preferred units) at Dec. 31, 2014														352,000	352,000	7,431,667	7,431,667
Balance (in units/ shares) at Dec. 31, 2014	93,255,284										97,092,835			3,837,551		93,255,284
Balance 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
Increase (Decrease) in Shareholders' Equity
Conversion of common units to common shares (311,343, 140,149 and 160,160 shares in 2013, 2014 and 2015 respectively)	0			2	2,149				(2,151)
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 (1,500,000 shares in 2013 and 890,241 shares in 2015) (in dollars)	$ 26,535			9	26,526					26,535	$ 26,535
Exercise of share options (in units/shares)	76,474										76,474
Exercise of share options (39,331, 62,888 and 76,474 shares in 2013, 2014 and 2015 respectively)(in dollars)	$ 2,008				2,008					2,008	$ 2,008
Share-based compensation (105,852, 137,735 and 149,353 shares issued in 2013, 2014 and 2015)	$ 7,398			1	7,397					7,398	$ 7,398
Share-based compensation (in shares/units)	149,353										149,353
Redemptions of vested equity awards	$ (2,462)				(2,462)					(2,462)	$ (2,462)
Contributions from noncontrolling interests	0				(682)				682
Comprehensive income	185,247						178,300	(1,541)	8,488	185,247	170,488	(1,604)	1,493		660		14,210
Dividends/Distributions	(118,208)						(118,208)			(122,914)	(108,044)				$ (660)		$ (14,210)
Distributions to owners of common and preferred units in COPLP	(4,706)								(4,706)
Contributions from noncontrolling interests in other consolidates entities	300								300	300			300
Distributions to noncontrolling interests	(35)								(35)	(35)			(35)
Adjustment to arrive at fair value of redeemable noncontrolling interest	116				116					116	116
Tax loss from share-based compensation	$ (513)				(513)					(513)	$ (513)
Balance (preferred units) at Dec. 31, 2015														352,000	352,000	7,431,667	7,431,667
Balance (in units/ shares) at Dec. 31, 2015	94,531,512										98,208,903			3,677,391		94,531,512
Balance at Dec. 31, 2015	$ 1,616,564		$ 199,083	$ 945	$ 2,004,507		$ (657,172)	$ (2,838)	$ 72,039	$ 1,616,564	$ 1,400,745	$ (2,985)	$ 10,921		$ 8,800		$ 199,083

Consolidated Statements of Equ7

Consolidated Statements of Equity (Parenthetical) - shares	Jul. 15, 2013	Nov. 30, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Balance (in units/ shares)			94,531,512	93,255,284	87,394,512	80,952,986
Conversion of common units to common shares (in shares/units)			160,160	140,149	311,343
Shares issued to the public (in units/shares)		5,520,000
Exercise of share options (in units/shares)			76,474	62,888	39,331
Restricted common units/shares redemptions			149,353	137,735	105,852
Preferred Shares [Member]
Stock redeemed or called during period (in units/ shares)				(2,000,000)	(3,390,000)
Common Shares [Member]
Shares issued to the public (in units/shares)				5,520,000	4,485,000
Common Stock Issued to Public Under At-the-Market Program [Member]
Shares issued to the public (in units/shares)	1,500,000		890,241		1,500,000

Consolidated Statements of Cash

Consolidated Statements of Cash Flows $ in Thousands	12 Months Ended
	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)
Cash flows from operating activities
Revenues from real estate operations received	$ 501,779	$ 479,605	$ 482,763
Construction contract and other service revenues received	117,107	89,180	63,647
Property operating expenses paid	(190,281)	(178,803)	(176,243)
Construction contract and other service expenses paid	(124,481)	(79,271)	(63,853)
General, administrative, leasing, business development and land carry costs paid	(38,113)	(29,521)	(28,022)
Interest expense paid	(65,816)	(79,095)	(81,575)
Previously accreted interest expense paid	0	0	(11,116)
Exit costs on property dispositions	0	0	(979)
Payments in connection with early extinguishment of debt	(373)	(9,017)	(27,909)
Interest and other income received	4,194	607	2,260
Income taxes refunded (paid)	(8)	200	6
Net cash provided by operating activities	204,008	193,885	158,979
Cash flows from investing activities
Acquisitions of operating properties and related intangible assets	(202,866)	0	0
Construction, development and redevelopment	(234,346)	(200,385)	(201,808)
Tenant improvements on operating properties	(29,413)	(27,037)	(21,950)
Other capital improvements on operating properties	(23,147)	(28,720)	(23,940)
Proceeds from dispositions of properties	193,735	57,782	148,569
Investing receivables funded	(22)	(3,731)	(14,077)
Investing receivables payments received	5,114	10,279	144
Leasing costs paid	(13,710)	(16,234)	(14,429)
Decrease in restricted cash associated with investing activities	1,455	1,137	8,178
Other	(4,332)	(2,780)	(477)
Net cash used in investing activities	(307,532)	(209,689)	(119,790)
Proceeds from debt
Revolving Credit Facility	522,000	232,000	504,000
Unsecured senior notes	296,580	297,342	592,413
Other debt proceeds	164,000	11,569	94,049
Repayments of debt
Revolving Credit Facility	(561,500)	(149,000)	(504,000)
Scheduled principal amortization	(6,728)	(6,517)	(9,481)
Other debt repayments	(155,307)	(394,653)	(612,093)
Deferred financing costs paid	(7,522)	(708)	(9,361)
Net proceeds from issuance of common shares	28,567	150,174	157,444
Redemption of preferred shares	0	(50,000)	(84,750)
Common share/unit dividends/distributions paid	(103,638)	(96,330)	(93,474)
Preferred share/unit dividends/distributions paid	(14,210)	(16,731)	(21,335)
Distributions paid to noncontrolling interests in COPLP	(4,752)	(5,008)	(4,958)
Redemption of vested equity awards	(2,462)	(1,554)	(2,002)
Other	2,729	(3,076)	(1,862)
Net cash provided by (used in) financing activities	157,757	(32,492)	4,590
Net increase (decrease) in cash and cash equivalents	54,233	(48,296)	43,779
Cash and cash equivalents
Beginning of period	6,077	54,373	10,594
End of period	60,310	6,077	54,373
Reconciliation of net income to net cash provided by operating activities:
Net income	188,878	45,206	101,544
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and other amortization	142,231	138,490	119,773
Impairment losses	23,523	1,419	31,068
Settlement of previously accreted interest expense	0	0	(11,116)
Amortization of deferred financing costs	4,466	4,666	5,451
Increase in deferred rent receivable	(14,969)	(3,520)	(5,196)
Amortization of net debt discounts	1,122	921	1,159
Gain on sales of real estate	(68,047)	(10,695)	(11,687)
Share-based compensation	6,574	6,164	6,530
(Gain) loss on early extinguishment of debt	(86,028)	651	(68,689)
Other	528	(3,242)	(3,093)
Changes in operating assets and liabilities:
Decrease (increase) in accounts receivable	1,331	(2,011)	(11,698)
Decrease in restricted cash and marketable securities	(1,241)	1,352	576
Decrease (increase) in prepaid expenses and other assets, net	2,853	(10,126)	(3,764)
(Decrease) increase in accounts payable, accrued expenses and other liabilities	(3,620)	25,091	3,960
Increase (decrease) in rents received in advance and security deposits	6,407	(481)	4,161
Net cash provided by operating activities	204,008	193,885	158,979
Supplemental schedule of non-cash investing and financing activities:
(Decrease) increase in accrued capital improvements, leasing and other investing activity costs	(14,797)	(3,779)	2,947
Debt assumed on acquisition of operating property	55,490	0	0
Other liabilities assumed on acquisition of operating properties	5,179	0	0
Decrease in property in connection with transfer of property in settlement of debt	(82,738)	0	(73,780)
Decrease in debt in connection with transfer of property in settlement of debt	(150,000)	0	(146,500)
Increase in property and redeemable noncontrolling interests in connection with property contribution by a redeemable noncontrolling interest in a joint venture	1,415	0	0
(Decrease) increase in fair value of derivatives applied to accumulated other comprehensive income (loss) and noncontrolling interests	(1,140)	(4,866)	9,470
Equity in other comprehensive loss of an equity method investee	(264)	0	0
Dividends/distribution payable	30,178	29,862	29,080
COPLP common units issued to acquire property and noncontrolling interest in other consolidated entity	0	0	5,194
Decrease in noncontrolling interests and increase in shareholders’ equity in connection with the conversion of common units into common shares	2,151	1,843	3,997
Adjustments to noncontrolling interests resulting from changes in COPLP ownership	682	3,382	744
(Decrease) increase in redeemable noncontrolling interest and (increase) decrease in equity to carry redeemable noncontrolling interests at fair value	(116)	(134)	7,121
Corporate Office Properties, L.P. [Member]
Cash flows from operating activities
Revenues from real estate operations received	501,779	479,605	482,763
Construction contract and other service revenues received	117,107	89,180	63,647
Property operating expenses paid	(190,281)	(178,803)	(176,243)
Construction contract and other service expenses paid	(124,481)	(79,271)	(63,853)
General, administrative, leasing, business development and land carry costs paid	(38,113)	(29,521)	(28,022)
Interest expense paid	(65,816)	(79,095)	(81,575)
Previously accreted interest expense paid	0	0	(11,116)
Exit costs on property dispositions	0	0	(979)
Payments in connection with early extinguishment of debt	(373)	(9,017)	(27,909)
Interest and other income received	4,194	607	2,260
Income taxes refunded (paid)	(8)	200	6
Net cash provided by operating activities	204,008	193,885	158,979
Cash flows from investing activities
Acquisitions of operating properties and related intangible assets	(202,866)	0	0
Construction, development and redevelopment	(234,346)	(200,385)	(201,808)
Tenant improvements on operating properties	(29,413)	(27,037)	(21,950)
Other capital improvements on operating properties	(23,147)	(28,720)	(23,940)
Proceeds from dispositions of properties	193,735	57,782	148,569
Investing receivables funded	(22)	(3,731)	(14,077)
Investing receivables payments received	5,114	10,279	144
Leasing costs paid	(13,710)	(16,234)	(14,429)
Decrease in restricted cash associated with investing activities	1,455	1,137	8,178
Other	(4,332)	(2,780)	(477)
Net cash used in investing activities	(307,532)	(209,689)	(119,790)
Proceeds from debt
Revolving Credit Facility	522,000	232,000	504,000
Unsecured senior notes	296,580	297,342	592,413
Other debt proceeds	164,000	11,569	94,049
Repayments of debt
Revolving Credit Facility	(561,500)	(149,000)	(504,000)
Scheduled principal amortization	(6,728)	(6,517)	(9,481)
Other debt repayments	(155,307)	(394,653)	(612,093)
Deferred financing costs paid	(7,522)	(708)	(9,361)
Net proceeds from issuance of common shares	28,567	150,174	157,444
Redemption of preferred shares	0	(50,000)	(84,750)
Common share/unit dividends/distributions paid	(107,730)	(100,678)	(97,772)
Preferred share/unit dividends/distributions paid	(14,870)	(17,391)	(21,995)
Redemption of vested equity awards	(2,462)	(1,554)	(2,002)
Other	2,729	(3,076)	(1,862)
Net cash provided by (used in) financing activities	157,757	(32,492)	4,590
Net increase (decrease) in cash and cash equivalents	54,233	(48,296)	43,779
Cash and cash equivalents
Beginning of period	6,077	54,373	10,594
End of period	60,310	6,077	54,373
Reconciliation of net income to net cash provided by operating activities:
Net income	188,878	45,206	101,544
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and other amortization	142,231	138,490	119,773
Impairment losses	23,523	1,419	31,068
Settlement of previously accreted interest expense	0	0	(11,116)
Amortization of deferred financing costs	4,466	4,666	5,451
Increase in deferred rent receivable	(14,969)	(3,520)	(5,196)
Amortization of net debt discounts	1,122	921	1,159
Gain on sales of real estate	(68,047)	(10,695)	(11,687)
Share-based compensation	6,574	6,164	6,530
(Gain) loss on early extinguishment of debt	(86,028)	651	(68,689)
Other	528	(3,242)	(3,093)
Changes in operating assets and liabilities:
Decrease (increase) in accounts receivable	1,331	(2,011)	(11,698)
Decrease in restricted cash and marketable securities	(1,360)	(234)	1,267
Decrease (increase) in prepaid expenses and other assets, net	2,853	(10,126)	(3,764)
(Decrease) increase in accounts payable, accrued expenses and other liabilities	(3,501)	26,677	3,269
Increase (decrease) in rents received in advance and security deposits	6,407	(481)	4,161
Net cash provided by operating activities	204,008	193,885	158,979
Supplemental schedule of non-cash investing and financing activities:
(Decrease) increase in accrued capital improvements, leasing and other investing activity costs	(14,797)	(3,779)	2,947
Debt assumed on acquisition of operating property	55,490	0	0
Other liabilities assumed on acquisition of operating properties	5,179	0	0
Decrease in property in connection with transfer of property in settlement of debt	(82,738)	0	(73,780)
Decrease in debt in connection with transfer of property in settlement of debt	(150,000)	0	(146,500)
Increase in property and redeemable noncontrolling interests in connection with property contribution by a redeemable noncontrolling interest in a joint venture	1,415	0	0
(Decrease) increase in fair value of derivatives applied to accumulated other comprehensive income (loss) and noncontrolling interests	(1,140)	(4,866)	9,470
Equity in other comprehensive loss of an equity method investee	(264)	0	0
Dividends/distribution payable	30,178	29,862	29,080
COPLP common units issued to acquire property and noncontrolling interest in other consolidated entity	0	0	5,194
(Decrease) increase in redeemable noncontrolling interest and (increase) decrease in equity to carry redeemable noncontrolling interests at fair value	$ (116)	$ (134)	$ 7,121

Organization

Organization	12 Months Ended
	Dec. 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 own, manage, lease, develop and selectively acquire office and data center properties. The majority of our portfolio is in locations that support United States Government agencies and their contractors, most of whom are engaged in national security, defense and information technology (“IT”) related activities servicing what we believe are growing, durable priority missions (“Defense/IT Locations”). We also own a complementary portfolio of traditional office properties located in select urban/urban-like submarkets within our regional footprint with durable Class-A office fundamentals and characteristics, as well as other properties supporting general commercial office tenants (“Regional Office”). As of December 31, 2015 , our properties included the following (all references to number of properties, square footage, acres and megawatts are unaudited): • 177 operating office properties totaling 18.1 million square feet, including nine triple-net leased, single-tenant data center properties; • 13 office properties under, or contractually committed for, construction or redevelopment that we estimate will total approximately 1.5 million square feet upon completion, including one partially operational property included above; • 1,439 acres of land we control that we believe are potentially developable into approximately 17.6 million square feet; and • a wholesale data center with 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 December 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 Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 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 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. We use the cost method of accounting when we own an interest in an entity and cannot exert significant influence over its operations. Reclassification We reclassified certain amounts from prior periods to conform to the current period presentation of our consolidated financial statements, including amounts related to our adoption of guidance regarding the presentation of deferred debt issuance costs (discussed below) and our change in reportable segments (discussed in Note 17), with no effect on previously reported net income or equity. Use of Estimates in the Preparation of Financial Statements We make estimates and assumptions when preparing financial statements under generally accepted accounting principles (“GAAP”). These estimates and assumptions affect various matters, including: • the reported amounts of assets and liabilities in our consolidated balance sheets at the dates of the financial statements; • the disclosure of contingent assets and liabilities at the dates of the financial statements; and • the reported amounts of revenues and expenses in our consolidated statements of operations during the reporting periods. Significant estimates are inherent in the presentation of our financial statements in a number of areas, including the evaluation of the collectability of accounts and deferred rent receivable, the allocation of property acquisition costs, the determination of estimated useful lives of assets, the determination of lease terms, the evaluation of impairment of long-lived assets, the amount of impairment losses recognized, the amount of revenue recognized relating to tenant improvements, the level of expense recognized in connection with share-based compensation and the determination of accounting method for investments. Actual results could differ from these and other estimates. Acquisitions of Properties Upon completion of property acquisitions, we allocate the purchase price to tangible and intangible assets and liabilities associated with such acquisitions based on our estimates of their fair values. We determine these fair values by using market data and independent appraisals available to us and making numerous estimates and assumptions. We allocate property acquisitions to the following components: • properties based on a valuation performed under the assumption that the property is vacant upon acquisition (the “if-vacant value”). The if-vacant value is allocated between land and buildings or, in the case of properties under development, construction in progress. We also allocate additional amounts to properties for in-place tenant improvements based on our estimate of improvements per square foot provided under market leases that would be attributable to the remaining non-cancelable terms of the respective leases; • above- and below-market lease intangible assets or liabilities based on the present value (using an interest rate which reflects the risks associated with the leases acquired) of the difference between: (1) the contractual amounts to be received pursuant to the in-place leases; and (2) our estimate of fair market lease rates for the corresponding space, measured over a period equal to the remaining non-cancelable term of the lease. The capitalized above- and below-market lease values are amortized as adjustments to rental revenue over the remaining lease terms of the respective leases, and to renewal periods in the case of below-market leases; • in-place lease value based on our estimates of: (1) the present value of additional income to be realized as a result of leases being in place on the acquired properties; and (2) costs to execute similar leases. Our estimate of additional income to be realized includes carrying costs, such as real estate taxes, insurance and other operating expenses, and revenues during the expected lease-up periods considering current market conditions. Our estimate of costs to execute similar leases includes leasing commissions, legal and other related costs; • tenant relationship value based on our evaluation of the specific characteristics of each tenant's lease and our overall relationship with that respective tenant. Characteristics we consider in determining these values include the nature and extent of our existing business relationships with the tenant, growth prospects for developing new business with the tenant, the tenant's credit quality and expectations of lease renewals, among other factors; and • above- and below- market cost arrangements (such as real estate tax treaties or above- or below- market ground leases) based on the present value of the expected benefit from any such arrangements in place on the property at the time of acquisition. Intangible Assets and Deferred Revenue on Real Estate Acquisitions We amortize the intangible assets and deferred revenue on real estate acquisitions discussed above as follows: Asset Type Amortization Period Above- and below-market leases Related lease terms In-place lease value Related lease terms Tenant relationship value Estimated period of time that tenant will lease space in property Above- and below-market cost arrangements Term of arrangements We recognize the amortization of acquired above-market and below-market leases as adjustments to rental revenue. We recognize the amortization of above- and below- market cost arrangements as adjustments to property operating expenses. We recognize the amortization of other intangible assets on property acquisitions as amortization expense. Properties We report properties to be developed or held and used in operations at our depreciated cost, reduced for impairment losses. The preconstruction stage of the development or redevelopment of an operating property includes efforts and related costs to secure land control and zoning, evaluate feasibility and complete other initial tasks which are essential to development. We capitalize interest expense, real estate taxes and direct and indirect project costs (including related compensation and other indirect costs) associated with properties, or portions thereof, undergoing construction, development and redevelopment activities. In capitalizing interest expense, if there is a specific borrowing for the property undergoing construction, development and redevelopment activities, we apply the interest rate of that borrowing to the average accumulated expenditures that do not exceed such borrowing; for the portion of expenditures exceeding any such specific borrowing, we apply our weighted average interest rate on other borrowings to the expenditures. We continue to capitalize costs while construction, development or redevelopment activities are underway until a property becomes “operational,” which occurs when lease terms commence (generally when the tenant has control of the leased space and we have delivered the premises to the tenant as required under the terms of such lease), but no later than one year after the cessation of major construction activities. When leases commence on portions of a newly-constructed or redeveloped property in the period prior to one year from the cessation of major construction activities, we consider that property to be “partially operational.” When a property is partially operational, we allocate the costs associated with the property between the portion that is operational and the portion under construction. We start depreciating newly-constructed and redeveloped properties as they become operational. Most of our leases involve some form of improvements to leased space. When we are required to provide improvements under the terms of a lease, we determine whether the improvements constitute landlord assets or tenant assets. If the improvements are landlord assets, we capitalize the cost of the improvements and recognize depreciation expense associated with such improvements over the shorter of the useful life of the assets or the term of the lease and recognize any payments from the tenant as rental revenue over the term of the lease. If the improvements are tenant assets, we defer the cost of improvements funded by us as a lease incentive asset and amortize it as a reduction of rental revenue over the term of the lease. In determining whether improvements constitute landlord or tenant assets, we consider numerous factors, including: whether the improvements are unique to the tenant or reusable by other tenants; whether the tenant is permitted to alter or remove the improvements without our consent or without compensating us for any lost fair value; whether the ownership of the improvements remains with us or remains with the tenant at the end of the lease term; and whether the economic substance of the lease terms is properly reflected. We depreciate our fixed assets using the straight-line method over their estimated useful lives as follows: Estimated Useful Lives Buildings and building improvements 10-40 years Land improvements 10-20 years Tenant improvements on operating properties Related lease term Equipment and personal property 3-10 years We assess each of our operating properties for impairment quarterly using cash flow projections and estimated fair values that we derive for each of the properties. We update the leasing and other assumptions used in these projections regularly, paying particular attention to properties that have experienced chronic vacancy or face significant market challenges. We review our plans and intentions for our development projects and land parcels quarterly. If our analyses indicate that the carrying values of operating properties, properties in development or land held for future development may be impaired, we perform a recovery analysis for such properties. For long-lived assets to be held and used, we analyze recoverability based on the estimated undiscounted future cash flows expected to be generated from the operations and eventual disposition of the assets over, in most cases, a ten -year holding period. If we believe there is a significant possibility that we might dispose of the assets earlier, we analyze recoverability using a probability weighted analysis of the estimated undiscounted future cash flows expected to be generated from the operations and eventual disposition of the assets over the various possible holding periods. If the recovery analysis indicates that the carrying value of a tested property is not recoverable from estimated future cash flows, it is written down to its estimated fair value and an impairment loss is recognized. If and when our plans change, we revise our recoverability analyses to use the cash flows expected from the operations and eventual disposition of each asset using holding periods that are consistent with our revised plans. Changes in holding periods may require us to recognize significant impairment losses. Fair values are estimated based on contract prices, indicative bids, discounted cash flow analyses, yield analyses or sales comparison approach. Estimated cash flows used in such analyses are based on our plans for the property and our views of market and economic conditions. The estimates consider factors such as current and future rental rates, occupancies for the tested property and comparable properties, estimated operating and capital expenditures and recent sales data for comparable properties; most of these factors are influenced by market data obtained from real estate leasing and brokerage firms and our direct experience with the properties and their markets. When we determine that a property is held for sale, we stop depreciating the property and estimate the property’s fair value, net of selling costs; if we then determine that the estimated fair value, net of selling costs, is less than the net book value of the property, we recognize an impairment loss equal to the difference and reduce the net book value of the property. For periods in which a property is classified as held for sale, we classify the assets of the property as held for sale on our consolidated balance sheet for such periods. For dispositions of operating properties occurring prior to June 30, 2014 in which we had no significant continuing involvement, or for operating properties held for sale prior to June 30, 2014, we classify the results of operations for such properties as discontinued operations; interest expense that is specifically identifiable to properties included in discontinued operations is used in the computation of interest expense attributable to discontinued operations. We adopted guidance issued by the Financial Accounting Standards Board (“FASB”) related to the reporting of discontinued operations and disclosures of disposals of components of an entity effective for the quarterly period ended June 30, 2014. This guidance defines a discontinued operation as a component or group of components disposed or classified as held for sale that represents a strategic shift that has (or will have) a major effect on an entity’s operations and financial results; the guidance states that a strategic shift could include a disposal of a major geographical area of operations, a major line of business, a major equity method investment or other major parts of an entity. We have had no properties newly classified as discontinued operations subsequent to our adoption of this guidance. Sales of Interests in Real Estate We recognize gains from sales of interests in real estate using the full accrual method, provided that various criteria relating to the terms of sale and any subsequent involvement by us with the real estate sold are met. Cash and Cash Equivalents Cash and cash equivalents include all cash and liquid investments that mature three months or less from when they are purchased. Cash equivalents are reported at cost, which approximates fair value. We maintain our cash in bank accounts in amounts that may exceed Federally insured limits at times. We have not experienced any losses in these accounts in the past and believe that we are not exposed to significant credit risk because our accounts are deposited with major financial institutions. Investments in Marketable Securities We classify marketable securities as trading securities when we have the intent to sell such securities in the near term, and classify other marketable securities as available-for-sale securities. We determine the appropriate classification of investments in marketable securities at the acquisition date and re-evaluate the classification at each balance sheet date. We report investments in marketable securities classified as trading securities at fair value, with unrealized gains and losses recognized through earnings; on our consolidated statements of cash flows, we classify cash flows from these securities as operating activities. Accounts and Deferred Rents Receivable and Investing Receivables We maintain allowances for estimated losses resulting from the failure of our customers or borrowers to satisfy their payment obligations. We use judgment in estimating these allowances based primarily upon the payment history and credit status of the entities associated with the individual receivables. We write off these receivables when we believe the facts and circumstances indicate that continued pursuit of collection is no longer warranted. When cash is received in connection with receivables for which we have established allowances, we reduce the amount of losses previously recognized. We evaluate the collectibility of both interest and principal of loans whenever events or changes in circumstances indicate such amounts may not be recoverable. A loan is impaired when it is probable that we will be unable to collect all amounts due according to the existing contractual terms. When a loan is impaired, the amount of the loss accrual is calculated by comparing the carrying amount of the investment to the present value of expected future cash flows discounted at the loan’s effective interest rate and the value of any collateral under such loan. Interest on impaired loans is recognized when received in cash. Deferred Leasing and Financing Costs We defer costs incurred to obtain new tenant leases or extend existing tenant leases, including related compensation costs. We amortize these costs evenly over the lease terms. When tenant leases are terminated early, we expense any unamortized deferred leasing costs associated with those leases over the shortened term of the lease. We defer costs of financing arrangements and recognize these costs as interest expense over the related loan terms on a straight-line basis, which approximates the amortization that would occur under the effective interest method of amortization. We expense any unamortized loan costs when loans are retired early. In April 2015, the FASB issued guidance that changes the presentation of deferred 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. This guidance was further updated in August 2015 with respect to debt issuance costs of line-of-credit arrangements to note that it will be permissible for an entity to defer and present debt issuance costs as an asset and subsequently amortize the deferred debt issuance costs ratably over the term of a line-of-credit arrangement, regardless of whether there are any outstanding borrowings on the line-of-credit arrangement. We adopted this guidance effective for the quarterly period ended December 31, 2015 retrospectively to each prior period presented. The application of this guidance changed the balance sheet classification of most of our deferred financing costs but did not otherwise affect our consolidated financial statements. For costs of such arrangements attributable to line-of-credit arrangements and interest rate derivatives, we present such costs in the balance sheet in the line entitled prepaid and other assets. Noncontrolling Interests COPT’s consolidated noncontrolling interests are comprised of interests in COPLP not owned by COPT (discussed further in Note 14) and consolidated real estate joint ventures (discussed further in Note 6). COPLP’s consolidated noncontrolling interests are comprised primarily of interests in our consolidated real estate joint ventures. Also included in COPLP’s consolidated noncontrolling interests are interests in several real estate entities owned directly by COPT, or a wholly owned subsidiary of COPT, that generally do not exceed 1% of interests in such entities. We evaluate whether noncontrolling interests are subject to redemption features outside of our control. For noncontrolling interests that are currently redeemable for cash at the option of the holders of such interests or deemed probable to eventually become redeemable, we classify such interests as redeemable noncontrolling interests in the mezzanine section of our consolidated balance sheets; we adjust these interests each period to the greater of their fair value or carrying amount (initial amount as adjusted for allocations of income and losses and contributions and distributions), with a corresponding offset to additional paid-in capital on COPT’s consolidated balance sheets or common units on COPLP’s balance sheet, and only recognize reductions in such interests to the extent of their carrying amount. Our other noncontrolling interests are reported in the equity section of our consolidated balance sheets. The amounts reported for noncontrolling interests on our consolidated statements of operations represent the portion of these entities’ income or losses not attributable to us. Revenue Recognition We recognize minimum rents, net of abatements, on a straight-line basis over the term of tenant leases. A lease term generally commences when: (1) the tenant has control of the leased space (legal right to use the property); and (2) we have delivered the premises to the tenant as required under the terms of such lease. The term of a lease generally includes periods when a tenant: (1) may not terminate its lease obligation early; (2) may terminate its lease obligation early in exchange for a fee or penalty that we consider material enough such that termination would not be probable; (3) possesses renewal rights and the tenant’s failure to exercise such rights imposes a penalty on the tenant material enough such that renewal appears reasonably assured; or (4) possesses bargain renewal options for such periods. We report the amount by which our minimum rental revenue recognized on a straight-line basis under leases exceeds the contractual rent billings associated with such leases as deferred rent receivable on our consolidated balance sheets. Amounts by which our minimum rental revenue recognized on a straight-line basis under leases are less than the contractual rent billings associated with such leases are included in deferred revenue associated with operating leases on our consolidated balance sheets. In connection with a tenant’s entry into, or modification of, a lease, if we make cash payments to, or on behalf of, the tenant for purposes other than funding the construction of landlord assets, we defer the amount of such payments as lease incentives. As discussed above, when we are required to provide improvements under the terms of a lease, we determine whether the improvements constitute landlord assets or tenant assets; if the improvements are tenant assets, we defer the cost of improvements funded by us as a lease incentive asset. We amortize lease incentives as a reduction of rental revenue over the term of the lease. We recognize tenant recovery revenue in the same periods in which we incur the related expenses. Tenant recovery revenue includes payments from tenants as reimbursement for property taxes, utilities and other property operating expenses. We recognize fees received for lease terminations as revenue and write off against such revenue any (1) deferred rents receivable, and (2) deferred revenue, lease incentives and intangible assets that are amortizable into rental revenue associated with the leases; the resulting net amount is the net revenue from the early termination of the leases. When a tenant's lease for space in a property is terminated early but the tenant continues to lease such space under a new or modified lease in the property, the net revenue from the early termination of the lease is recognized evenly over the remaining life of the new or modified lease in place on that property. We recognize fees for services provided by us once services are rendered, fees are determinable and collectability is assured. We recognize revenue under construction contracts using the percentage of completion method when the revenue and costs for such contracts can be estimated with reasonable accuracy; when these criteria do not apply to a contract, we recognize revenue on that contract using the completed contract method. Under the percentage of completion method, we recognize a percentage of the total estimated revenue on a contract based on the cost of services provided on the contract as of a point in time relative to the total estimated costs on the contract. Interest Rate Derivatives Our primary objectives in using interest rate derivatives are to add stability to interest expense and to manage exposure to interest rate movements. To accomplish this objective, we primarily use interest rate swaps as part of our interest rate risk management strategy. Interest rate swaps designated as cash flow hedges involve the receipt of variable-rate amounts from a counterparty in exchange for our making fixed-rate payments over the life of the agreements without exchange of the underlying notional amount. Derivatives are used to hedge the cash flows associated with interest rates on existing debt as well as future debt. We recognize all derivatives as assets or liabilities on our consolidated balance sheet at fair value. We defer the effective portion of changes in fair value of the designated cash flow hedges to AOCI and reclassify such deferrals to interest expense as interest expense is recognized on the hedged forecasted transactions. We recognize the ineffective portion of the change in fair value of interest rate derivatives directly in interest expense. When an interest rate swap designated as a cash flow hedge no longer qualifies for hedge accounting, we recognize changes in fair value of the hedge previously deferred to AOCI, along with any changes in fair value occurring thereafter, through earnings. We do not use interest rate derivatives for trading or speculative purposes. We manage counter-party risk by only entering into contracts with major financial institutions based upon their credit ratings and other risk factors. We use standard market conventions and techniques such as discounted cash flow analysis, option pricing models, replacement cost and termination cost in computing the fair value of derivatives at each balance sheet date. We made an accounting policy election to use an exception provided for in the applicable accounting guidance with respect to measuring counterparty credit risk for derivative instruments; this election enables us to measure the fair value of groups of assets and liabilities associated with derivative instruments consistently with how market participants would price the net risk exposure as of the measurement date. Please refer to Note 11 for additional information pertaining to interest rate derivatives. Expense Classification We classify as property operations expense costs incurred for property taxes, ground rents, utilities, property management, insurance, repairs, exterior and interior maintenance and tenant revenue collection losses, as well as associated labor and indirect costs attributable to these costs. We classify as general and administrative and leasing expenses costs incurred for corporate-level management, public company administration, asset management, leasing, investor relations, marketing and corporate-level insurance (including general business, director and officers and key man life) and leasing prospects, as well as associated labor and indirect costs attributable to these costs. Share-Based Compensation We issue three forms of share-based compensation: restricted COPT common shares (“restricted shares”), deferred share awards and performance share units (“PSUs”). We also issued options to purchase COPT common shares (“options”) in prior years. We account for share-based compensation in accordance with authoritative guidance provided by the FASB that establishes standards for the accounting for transactions in which an entity exchanges its equity instruments for goods or services, focusing primarily on accounting for transactions in which an entity obtains employee services in share-based payment transactions. The guidance requires us to measure the cost of employee services received in exchange for an award of equity instruments based generally on the fair value of the award on the grant date; such cost is then recognized over the period during which the employee is required to provide service in exchange for the award. No compensation cost is recognized for equity instruments for which employees do not render the requisite service. The guidance also requires that share-based compensation be computed based on awards that are ultimately expected to vest; as a result, future forfeitures of awards are estimated at the time of grant and revised, if necessary, in subsequent periods if actual forfeitures differ from those estimates. If an award is voluntarily cancelled by an employee, we recognize the previously unrecognized cost associated with the original award on the date of such cancellation. We capitalize costs associated with share-based compensation attributable to employees engaged in construction and development activities. When we adopted the authoritative guidance on accounting for share-based compensation, we elected to adopt the alternative transition method for calculating the tax effects of share-based compensation. This method enabled us to use a simplified method to establishing the beginning balance of the additional paid-in capital pool related to the tax effects of employee share-based compensation that was available to absorb tax deficiencies recognized subsequent to the adoption of this guidance. We compute the fair value of restricted shares and deferred share awards based on the fair value of COPT common shares on the grant date. We compute the fair value of options using the Black-Scholes option-pricing model. Under that model, the risk-free interest rate is based on the U.S. Treasury yield curve in effect at the time of grant. The expected option life is based on our historical experience of employee exercise behavior. Expected volatility is based on historical volatility of COPT common shares. Expected dividend yield is based on the average historical dividend yield on our common shares over a period of time ending on the grant date of the options. We compute the fair value of PSUs using a Monte Carlo model. Under that model, the baseline common share value is based on the market value on the grant date. The risk-free interest rate is based on the U.S. Treasury yield curve in effect at the time of grant. Expected volatility is based on historical volatility of COPT’s common shares. Income Taxes COPT elected to be treated as a REIT under Sections 856 through 860 of the Internal Revenue Code. To qualify as a REIT, COPT must meet a number of organizational and operational requirements, including a requirement that it distribute at least 90% of the Company’s adjusted taxable income to its shareholders. As a REIT, COPT generally will not be subject to Federal income tax on taxable income that it distributes to its shareholders. If COPT fails to qualify as a REIT in any tax year, it will be subject to Federal income tax on its taxable income at regular corporate rates and may not be able to qualify as a REIT for four subsequent tax years. COPLP is a limited partnership and is not subject to federal income tax. Its partners are required to report their respective share of the Operating Partnership’s taxable income on their respective tax returns. COPT’s share of the Operating Partnership’s taxable income is reported on COPT’s income tax return. For Federal income tax purposes, dividends to shareholders may be characterized as ordinary income, capital gains or return of capital. The characterization of dividends paid on COPT’s common and preferred shares during each of the last three years was as follows: Common Shares Preferred Shares For the Years Ended December 31, For the Years Ended December 31, 2015 2014 2013 2015 2014 2013 Ordinary income 38.3 % 64.5 % 71.8 % 38.3 % 90.9 % 76.2 % Long-term capital gain 61.7 % 6.5 % 22.4 % 61.7 % 9.1 % 23.8 % Return of capital 0.0 % 29.0 % 5.8 % 0.0 % 0.0 % 0.0 % In addition, dividends paid on January 15, 2016 (with a record date of December 31, 2015) on COPT’s common and preferred shares were allocated to 2015 for Federal income tax purposes and characterized based on the percentages set forth above for 2015. We distributed all of COPT’s REIT taxable income in 2015, 2014 and 2013 and, as a result, did not incur Federal income tax in those years. The net basis of our consolidated assets and liabilities for tax reporting purposes was approximately $224 million lower than the amount reported on our consolidated balance sheet at December 31, 2015, which was primaril

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2015
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements Accounting standards define fair value as the exit price, or the amount that would be received upon sale of an asset or paid to transfer a liability in an orderly transaction between market participants as of the measurement date. The standards also establish a hierarchy for inputs used in measuring fair value that maximizes the use of observable inputs and minimizes the use of unobservable inputs by requiring that the most observable inputs be used when available. Observable inputs are inputs market participants would use in valuing the asset or liability developed based on market data obtained from sources independent of us. Unobservable inputs are inputs that reflect our assumptions about the factors market participants would use in valuing the asset or liability developed based upon the best information available in the circumstances. The hierarchy of these inputs is broken down into three levels: Level 1 inputs are quoted prices (unadjusted) in active markets for identical assets or liabilities; Level 2 inputs include (1) quoted prices for similar assets or liabilities in active markets, (2) quoted prices for identical or similar assets or liabilities in inactive markets and (3) inputs (other than quoted prices) that are observable for the asset or liability, either directly or indirectly; and Level 3 inputs are unobservable inputs for the asset or liability. Categorization within the valuation hierarchy is based upon the lowest level of input that is most significant to the fair value measurement. Recurring Fair Value Measurements COPT has a non-qualified elective deferred compensation plan for certain members of our management team that permits participants to defer up to 100% of their compensation on a pre-tax basis and receive a tax-deferred return on such deferrals. The assets held in the plan (comprised primarily of mutual funds and equity securities) and the corresponding liability to the participants are measured at fair value on a recurring basis on COPT’s consolidated balance sheet using quoted market prices, as are other marketable securities that we hold. The balance of the plan, which was fully funded, totaled $5.8 million as of December 31, 2015 and $5.9 million as of December 31, 2014 , and is included in the accompanying COPT consolidated balance sheets in the line entitled restricted cash and marketable securities. The offsetting liability associated with the plan is adjusted to fair value at the end of each accounting period based on the fair value of the plan assets and reported in other liabilities on COPT’s consolidated balance sheets. The assets of the plan and other marketable securities that we hold are classified in Level 1 of the fair value hierarchy. The liability associated with the plan is classified in Level 2 of the fair value hierarchy. The fair values of our interest rate derivatives are determined using widely accepted valuation techniques, including a discounted cash flow analysis on the expected cash flows of each derivative. This analysis reflects the contractual terms of the derivatives, including the period to maturity, and uses observable market-based inputs, including interest rate market data and implied volatilities in such interest rates. While we determined that the majority of the inputs used to value our derivatives fall within Level 2 of the fair value hierarchy, the credit valuation adjustments associated with our interest rate derivatives utilize Level 3 inputs, such as estimates of current credit spreads to evaluate the likelihood of default. However, as of December 31, 2015 , we assessed the significance of the impact of the credit valuation adjustments on the overall valuation of our derivatives and determined that these adjustments are not significant. As a result, we determined that our interest rate derivative valuations in their entirety are classified in Level 2 of the fair value hierarchy. As discussed further in Note 6, our partners in two real estate joint ventures, LW Redstone Company, LLC and Stevens Investors, LLC, have the right to require us to acquire their respective interests at fair value; accordingly, we classify the fair value of our partners’ interests as redeemable noncontrolling interests in the mezzanine section of our consolidated balance sheet. We determine the fair value of the interests based on unobservable inputs after considering the assumptions that market participants would make in pricing the interest. We apply a discount rate to the estimated future cash flows allocable to our partners from the properties underlying the respective joint ventures. Estimated cash flows used in such analyses are based on our plans for the properties and our views of market and economic conditions, and consider items such as current and future rental rates, occupancies for the properties and comparable properties and estimated operating and capital expenditures. 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 8, 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 10, 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 8 for investing receivables, Note 10 for debt and Note 11 for interest rate derivatives. COPT and Subsidiaries The tables below set forth financial assets and liabilities of COPT and its subsidiaries that are accounted for at fair value on a recurring basis as of December 31, 2015 and 2014 and the hierarchy level of inputs used in measuring their respective fair values under applicable accounting standards (in thousands): Description Quoted Prices in Significant Other Significant Total December 31, 2015: Assets: Marketable securities in deferred compensation plan (1) Mutual funds $ 5,658 $ — $ — $ 5,658 Other 105 — — 105 Interest rate derivatives (2) — 53 — 53 Total assets $ 5,763 $ 53 $ — $ 5,816 Liabilities: Deferred compensation plan liability (3) $ — $ 5,763 $ — $ 5,763 Interest rate derivatives — 3,160 — 3,160 Total liabilities $ — $ 8,923 $ — $ 8,923 December 31, 2014: Assets: Marketable securities in deferred compensation plan (1) Mutual funds $ 5,756 $ — $ — $ 5,756 Other 126 — — 126 Interest rate derivatives (2) — 274 — 274 Total assets $ 5,882 $ 274 $ — $ 6,156 Liabilities: Deferred compensation plan liability (3) $ — $ 5,882 $ — $ 5,882 Interest rate derivatives — 1,855 — 1,855 Total liabilities $ — $ 7,737 $ — $ 7,737 (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 tables below set forth financial assets and liabilities of COPLP and its subsidiaries that are accounted for at fair value on a recurring basis as of December 31, 2015 and 2014 and the hierarchy level of inputs used in measuring their respective fair values under applicable accounting standards (in thousands): Description Quoted Prices in Significant Other Significant Total December 31, 2015: Assets: Interest rate derivatives (1) $ — $ 53 $ — $ 53 Total Assets $ — $ 53 $ — $ 53 Liabilities: Interest rate derivatives $ — $ 3,160 $ — $ 3,160 December 31, 2014: Assets: Interest rate derivatives (1) $ — $ 274 $ — $ 274 Total Assets $ — $ 274 $ — $ 274 Liabilities: Interest rate derivatives $ — $ 1,855 $ — $ 1,855 (1) Included in the line entitled “prepaid expenses and other assets” on COPLP ’ s consolidated balance sheet. Nonrecurring Fair Value Measurements In 2015, we recognized the following impairment losses resulting from nonrecurring fair value measurements: • $12.8 million on land in Colorado Springs, Colorado (“Colorado Springs”). We classified some of this land as held for sale in the fourth quarter of 2015, at which time we adjusted the land to its estimated fair value less costs to sell. Due to the impairment loss on the land held for sale, we updated our estimates of fair value for other land owned in Colorado Springs and determined that the carrying value of some of this land exceeded such land’s estimated fair value, which resulted in recognition of an additional impairment loss; • $6.6 million on land in Aberdeen, Maryland. After concluding during the fourth quarter that we no longer expected to develop operating properties on the land, we determined that the carrying amount of the land would not likely be recovered from the sale of this property over the likely remaining holding period. Accordingly, we adjusted the land to its estimated fair value; • $2.6 million on operating properties in White Marsh, Maryland (included in our Regional Office segment) that we decided to sell and whose carrying amounts exceeded their estimated fair values less costs to sell. These properties were reclassified as held for sale during the year; and • $1.3 million on an operating property in Northern Virginia (included in our Regional Office segment) that we sold on July 27, 2015 following receipt of an unsolicited offer. This property’s carrying value exceeded its fair value less costs to sell. The table below sets forth the fair value hierarchy of the valuation technique we used to determine nonrecurring fair measurements of assets as of December 31, 2015 (dollars in thousands): Fair Values as of December 31, 2015 Quoted Prices in Significant Active Markets for Significant Other Unobservable Identical Assets Observable Inputs Inputs Description (Level 1) (Level 2) (Level 3) Total Assets: Total properties, net (1) $ — $ — $ 13,850 $ 13,850 Assets held for sale, net (2) — — 21,423 21,423 (1) Represents estimated fair values. (2) Represents estimated fair values less costs to sell. The table below sets forth quantitative information about significant unobservable inputs used for the Level 3 fair value measurements reported above as of December 31, 2015 (dollars in thousands): Valuation Technique Fair Values on Measurement Date Unobservable Input Range (Weighted Average) (1) Bids for property indicative of value $ 24,423 Indicative bids N/A Comparable sales analysis 10,850 Comparable sales prices N/A (1) Only one value applied for these unobservable inputs. In 2014, we recognized impairment losses totaling $1.4 million primarily in connection with certain of our operating properties in White Marsh, Maryland (included in our Regional Office segment) that we decided to sell and whose carrying amounts exceeded the cash flows from the operations and sales of the properties over the shortened period. These properties were subsequently disposed during the year. In 2013, we recognized the following impairment losses: • for certain of our operating properties that served as collateral for a nonrecourse loan, we expected that the cash flows to be generated by the properties would be insufficient to fund debt service requirements on the loan. While we sought to negotiate various alternatives with the lender, on December 23, 2013, we conveyed the properties to the lender to extinguish the loan. We recognized non-cash impairment losses of $11.0 million (all classified as discontinued operations and including $560,000 in exit costs) on these properties in 2013 resulting primarily from the carrying amount of certain of these properties located in Colorado Springs (included in our Regional Office and Colorado Defense/IT Locations segments) exceeding their fair value; • $15.2 million (all classified as discontinued operations and including $419,000 in exit costs) in connection with properties and land no longer aligned with our strategy that we sold, mostly in Colorado Springs (operating properties included primarily in our Regional Office and Colorado Defense/IT Locations segments); and • $5.9 million on two properties in White Marsh, Maryland (included in our Regional Office segment) that we concluded no longer met our investment criteria during the year and whose carrying amounts exceeded the cash flows from the operations and sales of the properties over the shortened period.

Concentration of Revenue

Concentration of Revenue	12 Months Ended
	Dec. 31, 2015
Risks and Uncertainties [Abstract]
Concentration of Revenue	Concentration of Revenue A large concentration of our revenue from real estate operations was earned from our largest tenant, the United States Government, including 20% of our rental revenue in 2015, 18% in 2014 and 18% in 2013 (continuing and discontinued operations, and excluding tenant recoveries and other real estate operations revenue). Our rental revenue from the United States Government was earned primarily from properties in the Fort Meade/Baltimore Washington Corridor, Lackland Air Force Base, Navy Support Locations and Redstone Arsenal business subsegments (see Note 17). No other individual tenants accounted for 10% or more of our revenue from real estate operations. In addition, we also derived in excess of 90% of our construction contract revenue from the United States Government in each of the years set forth on the consolidated statements of operations. In addition, we derived large concentrations of our revenue from real estate operations from certain business segments as set forth in Note 17.

Properties, net

Properties, net	12 Months Ended
	Dec. 31, 2015
Real Estate [Abstract]
Properties, net	Properties, net Operating properties, net consisted of the following (in thousands): December 31, 2015 2014 Land $ 463,305 $ 439,355 Buildings and improvements 3,157,587 3,015,216 Less: accumulated depreciation (700,363 ) (703,083 ) Operating properties, net $ 2,920,529 $ 2,751,488 In 2014, we recognized $12.9 million in additional depreciation expense resulting from our shortening of the useful life of a property in Greater Philadelphia, Pennsylvania (included in our Regional Office segment) that was removed from service for redevelopment. Projects we had in development or held for future development consisted of the following (in thousands): December 31, 2015 2014 Land $ 207,774 $ 214,977 Development in progress, excluding land 221,445 330,449 Projects in development or held for future development $ 429,219 $ 545,426 As of December 31, 2015 , we had 13 operating properties in White Marsh, Maryland (included in our Regional Office segment), two operating properties in San Antonio (included in our Other segment) and land in Northern Virginia and Colorado Springs classified as held for sale. The table below sets forth the components of assets held for sale on our consolidated balance sheet for these properties (in thousands): December 31, 2015 Properties, net $ 90,188 Deferred rent receivable 2,891 Intangible assets on real estate acquisitions, net 1,591 Deferred leasing costs, net 1,391 Lease incentives, net 721 Assets held for sale, net $ 96,782 As of December 31, 2014, we had two land parcels in White Marsh, Maryland classified as held for sale with aggregate carrying amounts of $14.3 million that were sold in 2015. 2015 Acquisitions In 2015, we acquired the following operating properties: • 250 W. Pratt Street, a 367,000 square foot office property in Baltimore, Maryland that was 96.2% leased, for $61.8 million on March 19, 2015; • 2600 Park Tower Drive, a 237,000 square foot office property in Vienna, Virginia that was 100% leased, for $80.5 million on April 15, 2015; and • 100 Light Street, a 558,000 square foot office property in Baltimore, Maryland that was 93.5% leased, and its structured parking garage, 30 Light Street, for $121.2 million on August 7, 2015. In connection with that acquisition, we assumed a $55.0 million mortgage loan with a fair value at assumption of $55.5 million . The table below sets forth the allocation of the aggregate purchase price of these properties to the value of the acquired assets and liabilities (in thousands): Land, operating properties $ 55,076 Building and improvements 139,540 Intangible assets on real estate acquisitions 75,729 Total assets 270,345 Below-market leases (6,808 ) Total acquisition cost $ 263,537 Intangible assets recorded in connection with these acquisitions included the following (dollars in thousands): Weighted Average Amortization Period (in Years) Tenant relationship value $ 31,183 12 In-place lease value 35,139 7 Above-market leases 6,720 4 Below-market cost arrangements 2,687 40 $ 75,729 10 These properties contributed revenues of $20.2 million and net income from continuing operations of $1.2 million in 2015. We expensed operating property acquisition costs of $4.1 million in 2015 that are included in business development expenses and land carry costs on our consolidated statements of operations. We accounted for these acquisitions as business combinations. We included the results of operations for the acquisitions in our consolidated statements of operations from their respective purchase dates through December 31, 2015 . The following table presents pro forma information for COPT and subsidiaries as if these acquisitions had occurred on January 1, 2014. This pro forma information also includes adjustments to reclassify the operating property acquisition costs disclosed above from the 2015 periods in which they were incurred to the year ended December 31, 2014. The pro forma financial information was prepared for comparative purposes only and is not necessarily indicative of what would have occurred had these acquisitions been made at that time or of results which may occur in the future (in thousands, except per shares amounts). For the Year Ended December 31, 2015 2014 (Unaudited) Pro forma total revenues $ 641,982 $ 623,013 Pro forma net income attributable to COPT common shareholders $ 167,079 $ 20,796 Pro forma EPS: Basic $ 1.77 $ 0.23 Diluted $ 1.77 $ 0.23 2015 Dispositions In 2015, we completed dispositions of the following operating properties (dollars in thousands): Project Name City, State Segment Date of Disposition Number of Buildings Total Rentable Square Feet Transaction Value (1) Gain on Disposition 1550 Westbranch Drive McLean, VA Regional Office 7/27/2015 1 160,000 $ 27,800 $ — 15000 and 15010 Conference Center Drive Chantilly, VA Northern VA Defense/IT 8/28/2015 2 665,000 167,335 — 13200 Woodland Park Road Herndon, VA Regional Office 10/27/2015 1 397,000 84,000 42,515 9900, 9910 and 9920 Franklin Square Drive White Marsh, MD Regional Office 11/9/2015 3 135,000 24,150 6,468 9690 Deereco Road and 375 W. Padonia Road Timonium, MD Regional Office 12/17/2015 2 240,000 44,500 15,050 9 1,597,000 $ 347,785 $ 64,033 (1) Each of these properties were sold except for 15000 and 15010 Conference Center Drive, the disposition of which was completed in connection with a debt extinguishment, as discussed further below. We also sold land in 2015 for $18.1 million and recognized gains of $4.0 million on the sales. On August 28, 2015, ownership in 15000 and 15010 Conference Center Drive was transferred to the mortgage lender on a $150.0 million nonrecourse mortgage loan that was secured by the properties and we removed the debt obligation and accrued interest from our balance sheet. Upon completion of this transfer, we recognized a gain on early extinguishment of debt of $84.8 million , representing the difference between the mortgage loan and accrued interest payable extinguished over the carrying value of the properties transferred as of the transfer date and related closing costs. 2015 Construction Activities In 2015, we placed into service an aggregate of 897,000 square feet in seven newly constructed office properties and 170,000 square feet in two redeveloped properties. As of December 31, 2015 , we had nine office properties under construction, or for which we were contractually committed to construct, that we estimate will total 1.3 million square feet upon completion and four office properties under redevelopment that we estimate will total 156,000 square feet upon completion. 2014 Dispositions In 2014, we completed dispositions of the following operating properties (dollars in thousands): Project Name City, State Segment Date of Sale Number of Buildings Total Rentable Square Feet Transaction Value Gain on Disposition 4969 and 4979 Mercantile Road White Marsh, MD Regional Office 7/14/2014 2 96,721 $ 5,960 $ 2,124 9930 and 9940 Franklin Square White Marsh, MD Regional Office 7/30/2014 2 71,992 10,475 2,303 5020, 5022, 5024 and 5026 Campbell Boulevard White Marsh, MD Regional Office 8/4/2014 4 134,245 12,400 666 8 302,958 $ 28,835 $ 5,093 We also sold land in 2014 for $28.3 million and recognized gains of $5.6 million on the sales. 2014 Construction Activities In 2014, we placed into service an aggregate of 692,000 square feet in five newly constructed office properties. 2013 Dispositions In 2013, we completed dispositions of the following operating properties (dollars in thousands): Project Name City, State Segment Date of Sale Number of Buildings Total Rentable Square Feet Transaction Value (1) Gain on Disposition 920 Elkridge Landing Road Linthicum, MD Fort Meade/BW Corridor 6/25/2013 1 103,000 $ 6,900 $ — 4230 Forbes Boulevard Lanham, MD Regional Office 12/11/2013 1 56,000 5,600 1,507 December 2013 Colorado Springs Portfolio Disposition Colorado Springs, CO Colorado Defense/IT and Regional Office 12/12/2013 15 1,165,000 133,925 1,164 December 2013 Portfolio Conveyance Colorado Springs, CO and Linthicum, MD Various (2) 12/23/2013 14 1,021,000 146,876 — 31 2,345,000 $ 293,301 $ 2,671 (1) Each of these properties were sold except for the December 2013 Portfolio Conveyance, the disposition of which was completed in connection with a debt extinguishment, as described further below. (2) Included properties in our Fort Meade/BW Corridor, Colorado Defense/IT and Regional Office segments. We also disposed of a non-operating property in 2013 for an aggregate transaction value of $3.5 million . In addition to the gains on dispositions reflected above, we recognized impairment losses on certain of these assets that are disclosed in Note 3. On December 23, 2013, the mortgage lender on a $146.5 million nonrecourse mortgage loan that was secured by the December 2013 Portfolio Conveyance accepted a deed in lieu of foreclosure on the properties. As a result, we transferred title to the properties to the mortgage lender and we were relieved of the debt obligation plus accrued interest. Upon completion of this transfer, we recognized a gain on extinguishment of debt of $67.8 million , representing the excess of the mortgage loan and interest payable extinguished over the carrying values of the properties transferred as of the transfer date (which included the effect of previous impairment losses) and related closing costs. 2013 Construction Activities In 2013, we placed into service an aggregate of 812,000 square feet in eight newly constructed office properties.

Real Estate Joint Ventures

Real Estate Joint Ventures	12 Months Ended
	Dec. 31, 2015
Equity Method Investments and Joint Ventures [Abstract]
Real Estate Joint Ventures	Real Estate Joint Ventures Consolidated Real Estate Joint Ventures The table below sets forth information pertaining to our investments in consolidated real estate joint ventures as of December 31, 2015 (dollars in thousands): Nominal Ownership December 31, 2015 (1) Date % as of Total Encumbered Total Acquired 12/31/2015 Nature of Activity Assets Assets Liabilities LW Redstone Company, LLC 3/23/2010 85% Development and operation of real estate (2) $ 147,612 $ 82,721 $ 54,022 M Square Associates, LLC 6/26/2007 50% Development and operation of real estate (3) 56,455 48,245 37,568 Stevens Investors, LLC 8/11/2015 95% Development of real estate (4) 5,284 — 393 $ 209,351 $ 130,966 $ 91,983 (1) Excludes amounts eliminated in consolidation. (2) This joint venture’s property is in Huntsville, Alabama. (3) This joint venture’s properties are in College Park, Maryland. (4) This joint venture’s property is in Washington, DC. With regard to our consolidated joint ventures: • For LW Redstone, LLC, we anticipate funding certain infrastructure costs (up to a maximum of $76.0 million excluding accrued interest thereon) due to be reimbursed by the City of Huntsville as discussed further in Note 8; as of December 31, 2015 , we had advanced $37.2 million to the City to fund such costs (excluding accrued interest; the aggregate amount outstanding under these notes, including accrued interest, was $44.9 million as of December 31, 2015 and is included in investing receivables on our consolidated balance sheets). We also expect to fund additional development and construction costs through equity contributions to the extent that third party financing is not obtained. Our partner was credited with a $9.0 million capital account upon formation and is not required to make any future equity contributions. While net cash flow distributions to the partners vary depending on the source of the funds distributed, cash flows are generally distributed as follows: • cumulative preferred returns on capital invested to fund the project’s infrastructure costs on a pro rata basis to us and our partner; • cumulative preferred returns on our capital invested to fund the project’s vertical construction; • return of our invested capital; • return of our partner’s capital; • any remaining residual 85% to us and 15% to our partner. Our partner has the right to require us to acquire its interest for fair value beginning in March 2020; accordingly, we classify the fair value of our partner’s interest as redeemable noncontrolling interests in the mezzanine section of our consolidated balance sheet. We have the right to purchase our partner’s interest at fair value upon the earlier of five years following the project’s achievement of a construction commencement threshold of 4.4 million square feet or March 2040; the project had achieved 594,000 square feet of construction commencement through December 31, 2015 ; • For M Square Associates, LLC, net cash flows of this entity will be distributed to the partners as follows: (1) member loans and accrued interest; (2) our preferred return and capital contributions used to fund infrastructure costs; (3) the partners’ preferred returns and capital contributions used to fund all other costs, including the base land value credit, in proportion to the accrued returns and capital accounts; and (4) residual amounts distributed 50% to each member. • For Stevens Investors, LLC, net cash flows of this entity will be distributed to the partners as follows: (1) member loans and accrued interest; (2) pro rata return of the partners’ capital; (3) pro rata return of the partners’ respective unpaid preferred returns; and (4) varying splits of 85% to 60% to us and the balance to our partners as we reach specified return hurdles. Our partners have the right to require us to acquire some or all of their interests for fair value for a defined period of time following the construction completion and stabilization (as defined in the operating agreement) of the joint venture’s office property; accordingly, we classify the fair value of our partners’ interest as redeemable noncontrolling interests in the mezzanine section of our consolidated balance sheet. Our partners have the right to receive some or all of the consideration for the acquisition of their interests in the form of common units in COPLP. We disclose the activity of our redeemable noncontrolling interests in Note 12. The ventures discussed above include only ones in which parties other than COPLP and COPT own interests. During the periods included herein, we also owned investments in the following consolidated real estate joint ventures: • Arundel Preserve #5, LLC, a joint venture owning property in Hanover, Maryland (in the Baltimore/Washington Corridor) and in which we had a 50% nominal ownership interest. On September 17, 2013, we acquired our partner’s noncontrolling interest, along with incremental additional land value in the venture, in exchange for 221,501 common units in COPLP valued at $5.2 million ; • MOR Forbes 2 LLC, a joint venture owning property in Lanham, Maryland (in the Baltimore/Washington Corridor) and in which we had a 50% nominal ownership interest. On December 11, 2013, the joint venture sold the property, after which the proceeds were distributed to the partners and there was substantially no remaining business operations or property; and • COPT-FD Indian Head, LLC, a joint venture owning property in Charles County, Maryland (in our “Other” region). On August 7, 2014, the joint venture’s property was repurchased by Charles County under the terms of a development agreement for $6.4 million , after which the proceeds were distributed to the partners and there was no remaining business operations or assets. We consolidate the real estate joint ventures described above because we have: (1) the power to direct the matters that most significantly impact the activities of the joint ventures, including development, leasing and management of the properties constructed by the VIEs; and (2) the right to receive returns on our fundings and, in many cases, the obligation to fund the activities of the ventures to the extent that third-party financing is not obtained, both of which could be potentially significant to the VIEs. Our commitments and contingencies pertaining to our real estate joint ventures are disclosed in Note 20. Unconsolidated Real Estate Joint Venture We had a 20% ownership interest in an unconsolidated real estate joint venture that operated 16 operating properties, and in which we had a negative investment balance of $6.4 million as of December 31, 2012. We accounted for the investment in the joint venture using the equity method of accounting. On December 6, 2013, the holder of mortgage debt encumbering all of the joint venture’s properties foreclosed on the properties. As a result, title to the properties was transferred to the mortgage lender and the joint venture was relieved of the debt obligation. We recognized a gain on the substantive disposition of our investment in the joint venture in 2013 of $6.3 million , which is included in the line entitled “gain on sales of real estate, net of income taxes” on our consolidated statements of operations.

Intangible Assets on Real Estat

Intangible Assets on Real Estate Acquisitions	12 Months Ended
	Dec. 31, 2015
Intangible Assets on Real Estate Acquisitions
Intangible Assets on Real Estate Acquisitions	Intangible Assets on Real Estate Acquisitions Intangible assets on real estate acquisitions consisted of the following (in thousands): December 31, 2015 December 31, 2014 Gross Carrying Amount Accumulated Amortization Net Carrying Amount Gross Carrying Amount Accumulated Amortization Net Carrying Amount In-place lease value $ 134,664 $ 89,540 $ 45,124 $ 123,759 $ 101,040 $ 22,719 Tenant relationship value 62,172 23,468 38,704 42,301 28,492 13,809 Below-market cost arrangements 15,102 6,692 8,410 12,415 5,984 6,431 Above-market leases 14,210 8,491 5,719 8,659 8,159 500 Other 1,333 952 381 1,333 938 395 $ 227,481 $ 129,143 $ 98,338 $ 188,467 $ 144,613 $ 43,854 Amortization of the intangible asset categories set forth above totaled $ 18.5 million in 2015, $ 15.2 million in 2014 and $16.2 million in 2013. The approximate weighted average amortization periods of the categories set forth above follow: in-place lease value: six years; tenant relationship value: 11 years; below-market cost arrangements: 35 years; above-market leases: four years; and other: 27 years. The approximate weighted average amortization period for all of the categories combined is eleven years. The estimated amortization (to amortization associated with real estate operations, rental revenue and property operating expenses) associated with the intangible asset categories set forth above for the next five years is: $18.8 million for 2016; $16.3 million for 2017; $11.6 million for 2018; $8.8 million for 2019; and $7.0 million for 2020.

Investing Receivables

Investing Receivables	12 Months Ended
	Dec. 31, 2015
Receivables [Abstract]
Investing Receivables	Investing Receivables Investing receivables, including accrued interest thereon, consisted of the following (in thousands): December 31, 2015 2014 Notes receivable from City of Huntsville $ 44,875 $ 49,147 Other investing loans receivable 3,000 3,000 $ 47,875 $ 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 6) and carry an interest rate of 9.95% . These notes and the accrued and unpaid interest thereon, which is compounded annually on March 1st, will be repaid using the real estate taxes generated by the properties constructed by the joint venture. When these tax revenues are sufficient to cover the debt service on a certain increment of municipal bonds, the City of Huntsville will be required to issue bonds to repay the notes receivable and the accrued and unpaid interest thereon. Each note has a maturity date of the earlier of 30 years from the date issued or the expiration of the tax increment district comprising the constructed properties in 2045. We did not have an allowance for credit losses in connection with our investing receivables as of December 31, 2015 or December 31, 2014 . The fair value of these receivables approximated their carrying amounts as of December 31, 2015 and December 31, 2014 .

Prepaid Expenses and Other Asse

Prepaid Expenses and Other Assets, Net	12 Months Ended
	Dec. 31, 2015
Prepaid Expense and Other Assets [Abstract]
Prepaid Expenses and Other Assets, Net	Prepaid Expenses and Other Assets, Net Prepaid expenses and other assets, net consisted of the following (in thousands): December 31, 2015 2014 Prepaid expenses $ 23,009 $ 20,570 Lease incentives 11,133 13,344 Furniture, fixtures and equipment, net 6,004 6,637 Deferred financing costs, net (1) 5,867 4,849 Deferred tax asset, net 3,467 4,002 Construction contract costs incurred in excess of billings 3,261 6,656 Equity method investments 1,636 2,368 Other assets 5,647 2,875 Prepaid expenses and other assets, net $ 60,024 $ 61,301 (1) Represents deferred costs, net of accumulated amortization, attributable to our Revolving Credit Facility and interest rate derivatives. Deferred tax asset, net reported above includes the following tax effects of temporary differences and carry forwards of our TRS (in thousands): December 31, 2015 2014 Operating loss carry forward $ 5,065 $ 5,012 Share-based compensation 363 976 Accrued payroll 133 195 Property (32 ) (119 ) Valuation allowance (2,062 ) (2,062 ) Deferred tax asset, net $ 3,467 $ 4,002 We recognize a valuation allowance on our deferred tax asset if we believe all or some portion of the asset may not be realized. An increase or decrease in the valuation allowance resulting from a change in circumstances that causes a change in our judgment about the realizability of our deferred tax asset is included in income. The deferred tax asset valuation allowance is due to a decrease in future projected operating income in our TRS resulting primarily from our dispositions of certain properties to which the TRS provided amenity services and our planned reduction in amenity services provided by the TRS at certain other properties. We believe it is more likely than not that the results of future operations in our TRS will generate sufficient taxable income to realize our December 31, 2015 net deferred tax asset.

Debt

Debt	12 Months Ended
	Dec. 31, 2015
Debt Disclosure [Abstract]
Debt	Debt Debt Summary Our debt consisted of the following (dollars in thousands): Carrying Value (1) as of December 31, December 31, Stated Interest Rates as of Scheduled Maturity as of December 31, 2015 December 31, 2015 Mortgage and Other Secured Loans: Fixed rate mortgage loans (2) $ 281,208 $ 385,769 3.96% - 7.87% (3) 2016-2024 Variable rate secured loans 49,792 36,698 LIBOR + 1.85% - 2.00% (4) 2016-2020 Total mortgage and other secured loans 331,000 422,467 Revolving Credit Facility (5) 43,500 83,000 LIBOR + 0.875% to 1.60% May 2019 Term Loan Facilities (6) 515,902 517,846 LIBOR + 0.90% to 2.60% (7) 2019-2022 Unsecured Senior Notes (5) 3.600%, $350,000 aggregate principal 346,714 346,311 3.60% (8) May 2023 5.250%, $250,000 aggregate principal 245,731 245,306 5.25% (9) February 2024 3.700%, $300,000 aggregate principal 297,378 296,927 3.70% (10) June 2021 5.000%, $300,000 aggregate principal 296,019 — 5.00% (10) July 2025 Unsecured notes payable 1,508 1,607 0% (11) 2026 4.25% Exchangeable Senior Notes (12) — 572 N/A N/A Total debt $ 2,077,752 $ 1,914,036 (1) The carrying values of our loans other than the Revolving Credit Facility reflect net deferred financing costs of $8.0 million as of December 31, 2015 and $6.0 million as of December 31, 2014. (2) Several of the fixed rate mortgages carry interest rates that were above or below market rates upon assumption and therefore were recorded at their fair value based on applicable effective interest rates. The carrying values of these loans reflect net unamortized premiums totaling $514,000 as of December 31, 2015 and $42,000 as of December 31, 2014 . (3) The weighted average interest rate on our fixed rate mortgage loans was 6.08% as of December 31, 2015 . (4) The weighted average interest rate on our variable rate secured loans was 2.2% as of December 31, 2015 . (5) Refer to the paragraphs below for further disclosure. (6) As discussed below, an additional $150.0 million in borrowings is available to be drawn under a term loan entered into in the current year. In addition, we have the ability to borrow an additional $430.0 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. (7) The weighted average interest rate on these loans was 1.92% as of December 31, 2015 . (8) The carrying value of these notes reflects an unamortized discount totaling $2.2 million as of December 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% . (9) The carrying value of these notes reflects an unamortized discount totaling $3.8 million as of December 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% . (10) Refer to the paragraphs below for further disclosure. (11) These notes carry interest rates that were below market rates upon assumption and therefore were recorded at their fair value based on applicable effective interest rates. The carrying value of these notes reflects an unamortized discount totaling $554,000 as of December 31, 2015 and $654,000 as of December 31, 2014 . (12) On April 20, 2015, we redeemed the remaining $575,000 principal amount of these notes at 100% of their principal amount. The carrying value of these notes as of December 31, 2014 included an unamortized discount totaling $3,000 . 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 and Unsecured Senior Notes. Certain of our debt instruments require that we comply with a number of restrictive financial covenants, including maximum leverage ratio, unencumbered leverage ratio, minimum net worth, minimum fixed charge coverage, minimum unencumbered interest coverage ratio, minimum debt service and maximum secured indebtedness ratio. In addition, the terms of some of COPLP’s debt may limit its ability to make certain types of payments and other distributions to COPT in the event of default or when such payments or distributions may prompt failure of debt covenants. As of December 31, 2015 , we were within the compliance requirements of these financial covenants. Our debt matures on the following schedule (in thousands): 2016 $ 208,109 2017 3,252 2018 3,400 2019 167,014 (1) 2020 315,252 Thereafter 1,400,203 Total $ 2,097,230 (2) (1) Includes $43.5 million in 2019 that may be extended to 2020 at our option, subject to certain conditions. (2) Represents scheduled principal amortization and maturities only and therefore excludes net discounts and deferred financing costs of $19.5 million . We capitalized interest costs of $7.2 million in 2015, $6.1 million in 2014 and $8.8 million in 2013. The following table sets forth information pertaining to the fair value of our debt (in thousands): December 31, 2015 December 31, 2014 Carrying Estimated Carrying Estimated Amount Fair Value Amount Fair Value Fixed-rate debt Unsecured Senior Notes $ 1,185,842 $ 1,211,658 $ 888,544 $ 901,599 Other fixed-rate debt 282,716 291,991 387,948 356,377 Variable-rate debt 609,194 610,987 637,544 642,091 $ 2,077,752 $ 2,114,636 $ 1,914,036 $ 1,900,067 Revolving Credit Facility On May 6, 2015, we entered into a credit agreement with a group of lenders for which KeyBanc Capital Markets and J.P. Morgan Securities LLC acted as joint lead arrangers and joint book runners, KeyBank National Association acted as administrative agent and JPMorgan Chase Bank, N.A. acted as syndication agent (the “Consolidated Credit Agreement”) to amend, restate and consolidate the terms of our existing unsecured revolving credit facility (the “Revolving Credit Facility”) and one of our term loan facilities discussed below. The lenders’ aggregate commitment under the Revolving Credit Facility is $800.0 million , with the ability for us to increase the lenders’ aggregate commitment to $1.3 billion , provided that there is no default under the facility and subject to the approval of the lenders. Amounts available under the facility are computed based on 60% of our unencumbered asset value, as defined in the agreement. The facility matures on May 6, 2019, with the ability for us to further extend such maturity by two six -month periods at our option, provided that there is no default under the facility and we pay an extension fee of 0.075% of the total availability under the facility for each extension period. The interest rate on the facility is based on LIBOR (customarily the 30 -day rate) plus 0.875% to 1.600% , as determined by the credit ratings assigned to COPLP by Standard & Poor’s Rating Services, Moody’s Investor Services, Inc. or Fitch Ratings Ltd. (collectively, the “Ratings Agencies”). The facility also carries a quarterly fee that is based on the lenders’ aggregate commitment under the facility multiplied by a per annum rate of 0.125% to 0.300% , as determined by the credit ratings assigned to COPLP by the Ratings Agencies. As of December 31, 2015 , the maximum borrowing capacity under this facility totaled $800.0 million , of which $741.7 million was available. Weighted average borrowings under our Revolving Credit Facility totaled $125.0 million in 2015 and $15.9 million in 2014. The weighted average interest rate on our Revolving Credit Facility was 1.40% in 2015 and 1.47% in 2014. Term Loan Facilities Effective February 14, 2012, we entered into an unsecured term loan agreement under which we borrowed $250.0 million . In connection with our entry into the Consolidated Credit Agreement on May 6, 2015 discussed above, we increased the loan amount to $300.0 million , with a right for us to borrow up to an additional $200.0 million during the term for an aggregate maximum loan of $500.0 million , subject to certain conditions. The term loan matures on May 6, 2020. The variable interest rate on the loan is based on the LIBOR rate (customarily the 30-day rate) plus 0.90% to 1.85% , as determined by the credit ratings assigned to COPLP by the Ratings Agencies. Effective August 3, 2012, we entered into an unsecured term loan agreement under which we borrowed $120.0 million , with the ability for us to borrow an additional $80.0 million , provided that there is no default under the loan and subject to the approval of the lenders. The term loan matures on August 2, 2019. The variable interest rate on the loan is based on the LIBOR rate (customarily the 30-day rate) plus 2.10% to 2.60% , as determined by our leverage levels. Effective December 17, 2015, we entered into an unsecured term loan agreement with an initial commitment of $250.0 million , of which we borrowed $100.0 million . We have until September 2016 to draw the remaining initial commitment. We also have the ability to borrow $150.0 million above the initial commitment, provided that there is no default under the loan and subject to the approval of the lenders. The term loan matures on December 17, 2022. The variable interest rate on the loan is based on the LIBOR rate (customarily the 30-day rate) plus 1.40% to 2.35% , as determined by our leverage levels. We also had an unsecured term loan agreement under which we borrowed $400.0 million in 2011. In 2013, we amended this term loan and repaid $150.0 million of the loan balance. We repaid the loan balance by an additional $100.0 million in 2014 and repaid the remaining balance in 2015. Unsecured Senior Notes We issued the following senior notes in 2014 and 2015: • $300.0 million of 3.700% Senior Notes at an initial offering price of 99.739% of their face value on May 14, 2014, resulting in proceeds, after deducting underwriting discounts, but before other offering expenses, of $297.3 million . The carrying value of these notes reflects an unamortized discount totaling $2.1 million as of December 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% ; and • $300.0 million of 5.000% Senior Notes at an initial offering price of 99.510% of their face value on June 29, 2015, resulting in proceeds, after deducting underwriting discounts, but before other offering expenses of $296.6 million . The carrying value of these notes reflects an unamortized discount totaling $3.3 million at December 31, 2015 . The effective interest rate under the notes, including amortization of the issuance costs, was 5.15% . We may redeem our unsecured senior notes, in whole at any time or in part from time to time, at our option, at a redemption price equal to the greater of (1) the aggregate principal amount of the notes being redeemed or (2) the sum of the present values of the remaining scheduled payments of principal and interest thereon (not including any portion of such payments of interest accrued as of the date of redemption) discounted to its present value, on a semi-annual basis at an adjusted treasury rate plus a spread ( 30 basis points for the 3.600% Senior Notes, 40 basis points for the 5.250% Senior Notes, 25 basis points for the 3.700% Senior Notes and 45 basis points for the 5.000% Senior Notes), plus, in each case, accrued and unpaid interest thereon to the date of redemption. However, in each case, if this redemption occurs on or after three months prior to the maturity date, the redemption price will be equal to 100% of the principal amount of the notes being redeemed, plus accrued and unpaid interest thereon to, but not including, the applicable redemption date. These notes are unconditionally guaranteed by COPT. Gains (Losses) on Early Extinguishment of Debt Our gains (losses) on early extinguishment of debt in 2013, 2014 and 2015 included the following: • a gain of $67.8 million on December 23, 2013 resulting from our transfer of title to properties to the lender on a $146.5 million nonrecourse mortgage loan and being relieved of the debt obligation plus accrued interest as discussed further in Note 5; • a loss of $25.9 million in 2013 resulting from early repayment of $239.4 million principal amount of 4.25% Exchangeable Senior Notes; • a loss of $9.1 million in December 2014, when we completed the defeasance of, and full satisfaction of our obligations with respect to, (1) $103.0 million principal amount of secured nonrecourse mortgage loan due to mature on November 6, 2015 and bearing an interest rate of 5.53% and (2) $108.5 million principal amount of secured nonrecourse mortgage loan due to mature on January 1, 2016 and bearing an interest rate of 5.56% , as well as costs related to the defeasance and satisfaction; and • a gain of $84.8 million on August 28, 2015 pertaining to the removal of a $150.0 million nonrecourse mortgage loan from our balance sheet as discussed further in Note 5.

Interest Rate Derivatives

Interest Rate Derivatives	12 Months Ended
	Dec. 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 Effective Expiration December 31, Amount Fixed Rate Floating Rate Index Date Date 2015 2014 $ 100,000 0.8055 % One-Month LIBOR 9/2/2014 9/1/2016 $ (148 ) $ (317 ) 100,000 0.8100 % One-Month LIBOR 9/2/2014 9/1/2016 (151 ) (324 ) 100,000 1.6730 % One-Month LIBOR 9/1/2015 8/1/2019 (1,217 ) 239 100,000 1.7300 % One-Month LIBOR 9/1/2015 8/1/2019 (1,429 ) 35 13,941 (1) 1.3900 % One-Month LIBOR 10/13/2015 10/1/2020 53 — 100,000 1.9013 % One-Month LIBOR 9/1/2016 12/1/2022 (138 ) — 100,000 1.9050 % One-Month LIBOR 9/1/2016 12/1/2022 (45 ) — 50,000 1.9079 % One-Month LIBOR 9/1/2016 12/1/2022 (32 ) — 36,200 -1 3.8300 % One-Month LIBOR + 2.25% 11/2/2010 11/2/2015 — (400 ) 100,000 0.8320 % One-Month LIBOR 1/3/2012 9/1/2015 — (407 ) 100,000 0.8320 % One-Month LIBOR 1/3/2012 9/1/2015 — (407 ) $ (3,107 ) $ (1,581 ) (1) The notional amount of this instrument is scheduled to amortize to $12.1 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): Fair Value at December 31, Derivatives Balance Sheet Location 2015 2014 Interest rate swaps designated as cash flow hedges Prepaid expenses and other assets $ 53 $ 274 Interest rate swaps designated as cash flow hedges Interest rate derivatives (3,160 ) (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 Years Ended December 31, 2015 2014 2013 Amount of (losses) gains recognized in accumulated other comprehensive income (loss) (“AOCI”) (effective portion) $ (4,739 ) $ (7,799 ) $ 6,791 Amount of losses reclassified from AOCI into interest expense (effective portion) 3,599 2,990 2,740 Amount of loss recognized in interest expense (ineffective portion) 386 — — Amount of loss reclassified from AOCI into loss on early extinguishment of debt — 38 — Over the next 12 months, we estimate that approximately $3.3 million of losses will be reclassified from AOCI as an increase to interest expense. We have agreements with each of our interest rate derivative counterparties that contain provisions under which, if we default or are capable of being declared in default on defined levels of our indebtedness, we could also be declared in default on our derivative obligations. These agreements also incorporate the loan covenant provisions of our indebtedness with a lender affiliate of the derivative counterparties. Failure to comply with the loan covenant provisions could result in our being declared in default on any derivative instrument obligations covered by the agreements. As of December 31, 2015 , the fair value of interest rate derivatives in a liability position related to these agreements was $3.2 million , excluding the effects of accrued interest and credit valuation adjustments. As of December 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 $3.6 million .

Redeemable Noncontrolling Inter

Redeemable Noncontrolling Interest	12 Months Ended
	Dec. 31, 2015
Noncontrolling Interest [Abstract]
Redeemable Noncontrolling Interest	Redeemable Noncontrolling Interests The table below sets forth the activity in redeemable noncontrolling interests in our LW Redstone, LLC and Stevens Investors, LLC joint ventures described in Note 6 (in thousands): For the Years Ended December 31, 2015 2014 2013 Beginning balance $ 18,417 $ 17,758 $ 10,298 Contributions from noncontrolling interests 1,654 — — Distributions to noncontrolling interests (2,964 ) (1,369 ) (1,037 ) Net income attributable to noncontrolling interests 2,227 2,162 1,376 Adjustment to arrive at fair value of interests (116 ) (134 ) 7,121 Ending balance $ 19,218 $ 18,417 $ 17,758

Equity - COPT and Subsidiaries

Equity - COPT and Subsidiaries	12 Months Ended
	Dec. 31, 2015
Stockholders' Equity Note [Abstract]
Equity - COPT and Subsidiaries	Equity - COPT and Subsidiaries Preferred Shares As of December 31, 2015 , COPT had 25.0 million preferred shares authorized at $0.01 par value per share. The table below sets forth additional information pertaining to COPT’s outstanding preferred shares (dollars in thousands, except per share data): Series # of Shares Issued Aggregate Liquidation Preference Month of Issuance Annual Dividend Yield Annual Dividend Per Share Earliest Redemption Date Series K 531,667 $ 26,583 January 2007 5.600 % $ 2.80000 1/9/2017 Series L 6,900,000 172,500 June 2012 7.375 % $ 1.84375 6/27/2017 7,431,667 $ 199,083 Each series of preferred shares is nonvoting and redeemable for cash in the amount of its liquidation preference at COPT’s option on or after the earliest redemption date. The Series K Cumulative Redeemable Preferred Shares are also convertible, subject to certain conditions, into common shares on the basis of 0.8163 common shares for each preferred share. Holders of all preferred shares are entitled to cumulative dividends, payable quarterly (as and if declared by the Board of Trustees). In the case of each series of preferred shares, there is a series of COPLP preferred units owned by COPT that carries substantially the same terms. COPT redeemed all of the outstanding shares of its 7.500% Series H Preferred Shares on June 16, 2014 at a price of $25.00 per share, or $50.0 million in the aggregate, plus accrued and unpaid dividends thereon through the date of redemption, and recognized a $1.8 million decrease to net income available to common shareholders pertaining to the shares’ original issuance costs incurred at the time of the redemption. Common Shares COPT completed the public offering of 5.52 million common shares in November 2014 at an offering price of $27.30 per share for net proceeds of $148.9 million after underwriter discounts but before offering expenses. COPT contributed the net proceeds from this issuance to COPLP in exchange for an equal number of units in COPLP. In October 2012, COPT established an at-the-market (“ATM”) stock offering program under which it may, from time to time, offer and sell common shares in “at the market” stock offerings having an aggregate gross sales price of up to $150.0 million . Through December 31, 2015, COPT issued the following common shares under the program: • 1.5 million shares on July 15, 2013 at a weighted average price of $26.05 per share. Net proceeds from the shares issued totaled $38.5 million , after payment of $0.6 million in commissions to sales agents; and • 890,241 common shares issued in 2015 at a weighted average price of $30.29 per share. Net proceeds from the shares issued totaled $26.6 million , after payment of $0.4 million in commissions to sales agents. COPT contributed the net proceeds from these issuances to COPLP in exchange for an equal number of units in COPLP. COPT’s remaining capacity under the ATM Plan is an aggregate gross sales price of $84.0 million in common share sales. Holders of COPLP common units converted their units into COPT common shares on the basis of one common share for each common unit in the amount of 160,160 in 2015, 140,149 in 2014 and 311,343 in 2013. COPT declared dividends per common share of $1.10 in 2015, 2014 and 2013. See Note 15 for disclosure of common share activity pertaining to our share-based compensation plans.

Equity - COPLP and Subsidiaries

Equity - COPLP and Subsidiaries	12 Months Ended
	Dec. 31, 2015
Class of Stock [Line Items]
Equity - COPLP and Subsidiaries	Equity - COPT and Subsidiaries Preferred Shares As of December 31, 2015 , COPT had 25.0 million preferred shares authorized at $0.01 par value per share. The table below sets forth additional information pertaining to COPT’s outstanding preferred shares (dollars in thousands, except per share data): Series # of Shares Issued Aggregate Liquidation Preference Month of Issuance Annual Dividend Yield Annual Dividend Per Share Earliest Redemption Date Series K 531,667 $ 26,583 January 2007 5.600 % $ 2.80000 1/9/2017 Series L 6,900,000 172,500 June 2012 7.375 % $ 1.84375 6/27/2017 7,431,667 $ 199,083 Each series of preferred shares is nonvoting and redeemable for cash in the amount of its liquidation preference at COPT’s option on or after the earliest redemption date. The Series K Cumulative Redeemable Preferred Shares are also convertible, subject to certain conditions, into common shares on the basis of 0.8163 common shares for each preferred share. Holders of all preferred shares are entitled to cumulative dividends, payable quarterly (as and if declared by the Board of Trustees). In the case of each series of preferred shares, there is a series of COPLP preferred units owned by COPT that carries substantially the same terms. COPT redeemed all of the outstanding shares of its 7.500% Series H Preferred Shares on June 16, 2014 at a price of $25.00 per share, or $50.0 million in the aggregate, plus accrued and unpaid dividends thereon through the date of redemption, and recognized a $1.8 million decrease to net income available to common shareholders pertaining to the shares’ original issuance costs incurred at the time of the redemption. Common Shares COPT completed the public offering of 5.52 million common shares in November 2014 at an offering price of $27.30 per share for net proceeds of $148.9 million after underwriter discounts but before offering expenses. COPT contributed the net proceeds from this issuance to COPLP in exchange for an equal number of units in COPLP. In October 2012, COPT established an at-the-market (“ATM”) stock offering program under which it may, from time to time, offer and sell common shares in “at the market” stock offerings having an aggregate gross sales price of up to $150.0 million . Through December 31, 2015, COPT issued the following common shares under the program: • 1.5 million shares on July 15, 2013 at a weighted average price of $26.05 per share. Net proceeds from the shares issued totaled $38.5 million , after payment of $0.6 million in commissions to sales agents; and • 890,241 common shares issued in 2015 at a weighted average price of $30.29 per share. Net proceeds from the shares issued totaled $26.6 million , after payment of $0.4 million in commissions to sales agents. COPT contributed the net proceeds from these issuances to COPLP in exchange for an equal number of units in COPLP. COPT’s remaining capacity under the ATM Plan is an aggregate gross sales price of $84.0 million in common share sales. Holders of COPLP common units converted their units into COPT common shares on the basis of one common share for each common unit in the amount of 160,160 in 2015, 140,149 in 2014 and 311,343 in 2013. COPT declared dividends per common share of $1.10 in 2015, 2014 and 2013. See Note 15 for disclosure of common share activity pertaining to our share-based compensation plans.
Corporate Office Properties, L.P. [Member]
Class of Stock [Line Items]
Equity - COPLP and Subsidiaries	Equity - COPLP and Subsidiaries General Partner Preferred Units The table below sets forth information pertaining to preferred units in COPLP held by COPT at December 31, 2015 (dollars in thousands, except per unit data): Series # of Units Issued Aggregate Liquidation Preference Month of Issuance Annual Distribution Yield Annual Distribution Per Unit Earliest Redemption Date Series K 531,667 $ 26,583 January 2007 5.600 % $ 2.80000 1/9/2017 Series L 6,900,000 172,500 June 2012 7.375 % $ 1.84375 6/27/2017 7,431,667 $ 199,083 In the case of each series of preferred units, COPT had preferred shares that carry substantially the same terms. Each series of preferred units are redeemable for cash in the amount of its liquidation preference at our option on or after the earliest redemption date. The Series K Preferred Units are also convertible, subject to certain conditions, into common units on the basis of 0.8163 common units for each preferred unit. COPT, as holder of these preferred units, is entitled to cumulative distributions, payable quarterly (as and if declared by the Board of Trustees). COPLP redeemed all of the outstanding units of its 7.500% Series H Preferred Units held by COPT on June 16, 2014 at a price of $25.00 per unit, or $50.0 million in the aggregate, plus accrued and unpaid distributions thereon through the date of redemption, and recognized a $1.8 million decrease to net income available to common unitholders pertaining to the units’ original issuance costs at the time of the redemption. Limited Partner Preferred Units COPLP has 352,000 Series I Preferred Units issued to an unrelated party that have an aggregate liquidation preference of $8.8 million ( $25.00 per unit), plus any accrued and unpaid distributions of return thereon (as described below), and may be redeemed for cash by COPLP at COPLP’s option any time after September 22, 2019. The owner of these units is entitled to a priority annual cumulative return equal to 7.5% of their liquidation preference through September 22, 2019; the annual cumulative preferred return increases for each subsequent five -year period, subject to certain maximum limits. These units are convertible into common units on the basis of 0.5 common units for each Series I Preferred Unit; the resulting common units would then be exchangeable for COPT common shares in accordance with the terms of COPLP’s agreement of limited partnership. Common Units COPT owned 96.3% of COPLP’s common units as of December 31, 2015 and 96.0% as of December 31, 2014. COPT acquired additional common units through the public offering of 5.52 million common shares in November 2014 at an offering price of $27.30 per share for net proceeds of $148.9 million (after underwriter discounts but before offering expenses) that were contributed to COPLP in exchange for an equal number of common units in COPLP. Through December 31, 2015, COPT issued the following common shares under the ATM program: • 1.5 million common shares on July 15, 2013 at a weighted average price of $26.05 per share. Net proceeds from the shares issued totaled $38.5 million , after payment of $0.6 million in commissions to sales agents; and • 890,241 common shares in 2015 at a weighted average price of $30.29 per share; net proceeds from the shares issued totaled $26.6 million , after payment of $0.4 million in commissions to sales agents. COPT contributed the net proceeds from these issuances to COPLP in exchange for an equal number of units in COPLP. Limited partners in COPLP holding common units have the right to require COPLP to redeem all or a portion of their common units. COPLP (or COPT as the general partner) has the right, in its sole discretion, to deliver to such redeeming limited partners for each partnership unit either one COPT common share (subject to anti-dilution adjustment) or a cash payment equal to the then fair market value of such share (so adjusted) (based on the formula for determining such value set forth in the partnership agreement). Limited partners holding common units redeemed their units into common shares on the basis of one common share for each common unit in the amount of 160,160 in 2015, 140,149 in 2014 and 311,343 in 2013. We declared distributions per common unit of $1.10 in 2015, 2014 and 2013.

Share-Based Compensation and Ot

Share-Based Compensation and Other Compensation Matters	12 Months Ended
	Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-Based Compensation and Other Compensation Matters	Share-Based Compensation and Other Compensation Matters Share-Based Compensation Plans In May 2010, COPT adopted the Amended and Restated 2008 Omnibus Equity and Incentive Plan. COPT may issue equity-based awards under this plan to officers, employees, non-employee trustees and any other key persons of us and our subsidiaries, as defined in the plan. The plan provides for a maximum of 5.9 million common shares in COPT to be issued in the form of options, share appreciation rights, deferred share awards, restricted share awards, unrestricted share awards, performance shares, dividend equivalent rights and other equity-based awards and for the granting of cash-based awards. The plan expires on May 13, 2020. In March 1998, COPT adopted a long-term incentive plan for our Trustees and employees. This plan, which expired in March 2008, provided for the award of options, restricted shares and dividend equivalents. Awards under these plans to nonemployee Trustees generally vest on the first anniversary of the grant date provided that the Trustee remains in his or her position. Awards 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. Options expire ten years after the date of grant. Shares for each of the share-based compensation plans are issued under registration statements on Form S-8 that became effective upon filing with the Securities and Exchange Commission. In connection with awards of common shares granted by COPT under such share-based compensation plans, COPLP issues to COPT an equal number of equity instruments with identical terms. The following table summarizes restricted shares under the share-based compensation plans for 2013, 2014 and 2015: Shares Weighted Average Grant Date Fair Value Unvested at December 31, 2012 434,643 $ 29.67 Granted 193,833 25.91 Forfeited (9,541 ) 27.59 Vested (241,487 ) 30.97 Unvested at December 31, 2013 377,448 26.96 Granted 216,607 26.73 Forfeited (21,335 ) 25.10 Vested (182,213 ) 28.56 Unvested at December 31, 2014 390,507 26.19 Granted 201,024 28.69 Forfeited (10,550 ) 26.05 Vested (202,781 ) 26.07 Unvested at December 31, 2015 378,200 $ 27.58 Unvested shares as of December 31, 2015 that are expected to vest 364,666 $ 27.61 The aggregate intrinsic value of restricted shares that vested was $4.9 million in 2015, $4.9 million in 2014 and $6.3 million in 2013. We made the following grants of PSUs to executives from 2011 through 2015 (dollars in thousands): Grant Date Number of PSUs Granted Performance Period Commencement Date Performance Period End Date Grant Date Fair Value Number of PSUs Outstanding as of December 31, 2015 3/3/2011 56,883 3/3/2011 3/2/2014 $ 2,796 — 3/1/2012 54,070 1/1/2012 12/31/2014 1,772 — 3/1/2013 69.579 1/1/2013 12/31/2015 1,867 51,556 3/6/2014 49,103 1/1/2014 12/31/2016 1,723 37,843 3/5/2015 45,656 1/1/2015 12/31/2017 1,678 45,656 The PSUs each have three year performance periods concluding on the earlier of the respective performance period end dates set forth above or the date of: (1) termination by us without cause, death or disability of the executive or constructive discharge of the executive (collectively, “qualified termination”); or (2) a sale event. The number of PSUs earned (“earned PSUs”) at the end of the performance period will be determined based on the percentile rank of COPT’s total shareholder return relative to a peer group of companies, as set forth in the following schedule: Percentile Rank Earned PSUs Payout % 75th or greater 200% of PSUs granted 50th or greater 100% of PSUs granted 25th 50% of PSUs granted Below 25th 0% of PSUs granted If the percentile rank exceeds the 25th percentile and is between two of the percentile ranks set forth in the table above, then the percentage of the earned PSUs will be interpolated between the ranges set forth in the table above to reflect any performance between the listed percentiles. At the end of the performance period, we, in settlement of the award, will issue a number of fully-vested COPT common shares equal to the sum of: • the number of earned PSUs in settlement of the award plan; plus • the aggregate dividends that would have been paid with respect to the common shares issued in settlement of the earned PSUs through the date of settlement had such shares been issued on the grant date, divided by the share price on such settlement date, as defined under the terms of the agreement. If a performance period ends due to a sale event or qualified termination, the number of earned PSUs is prorated based on the portion of the three -year performance period that has elapsed. If employment is terminated by the employee or by us for cause, all PSUs are forfeited. PSUs do not carry voting rights. Based on COPT’s total shareholder return relative to its peer group of companies: • for the 2011 PSUs that vested in 2014, there was no payout value in connection with the vesting; • for the 2012 PSUs that vested in 2014, we issued 40,309 common shares in settlement of the PSUs on March 5, 2015; • for 2013 and 2014 PSUs issued to Mr. Stephen E. Riffee, our former Chief Financial Officer who departed on February 3, 2015, we issued 15,289 common shares on March 5, 2015 in settlement of such PSUs; and • for the 2013 PSUs that vested on December 31, 2015, there was no payout value in connection with the vesting. We computed grant date fair values for PSUs using Monte Carlo models and are recognizing these values over three -year periods that commenced on the respective grant dates. The grant date fair value and certain of the assumptions used in the Monte Carlo models for the PSUs granted in 2013, 2014 and 2015 are set forth below: For the Years Ended December 31, 2015 2014 2013 Grant date fair value $ 36.76 $ 35.09 $ 26.84 Baseline common share value $ 29.28 $ 26.52 $ 25.85 Expected volatility of common shares 19.9 % 28.6 % 29.5 % Risk-free interest rate 0.99 % 0.66 % 0.33 % In 2015, nonemployee members of our Board of Trustees were granted a total of 24,056 deferred share awards with an aggregate grant date fair value of $642,000 ( $26.70 per share). Deferred share awards vest on the first anniversary of the grant date, provided that the Trustee remains in his or her position. We settle deferred share awards by issuing an equivalent number of common shares upon vesting of the awards or a later date elected by the Trustee (generally upon cessation of being a Trustee). In 2015, we issued 15,485 common shares in settlement of deferred share awards granted in 2014; these shares had a grant date fair value of $26.77 per share, and the aggregate intrinsic value of the shares on the settlement date was $413,000 . We have not issued options since 2009, and all of our options were vested and fully expensed as of December 31, 2015 . The table below sets forth information regarding our outstanding options (dollars in thousands, except per share data): Options Outstanding and Exercisable Weighted Average Exercise Price Per Share Weighted Average Remaining Contractual Term (in Years) Aggregate Intrinsic Value December 31, 2012 798,210 $37.62 3 $ 325 December 31, 2013 640,927 $38.11 2 $ 68 December 31, 2014 559,736 $39.60 2 $ 167 December 31, 2015 425,347 $42.75 1 $ — The aggregate intrinsic value of options exercised was $300,000 in 2015, $225,000 in 2014 and $258,000 in 2013. We own a taxable REIT subsidiary that is subject to Federal and state income taxes. We realized a windfall tax loss of $513,000 in 2015, $3,000 in 2014 and $122,000 in 2013 on options exercised and vesting restricted shares in connection with employees of that subsidiary. The table below sets forth our reporting for share based compensation cost (in thousands): For the Years Ended December 31, 2015 2014 2013 General, administrative and leasing expenses $ 5,574 $ 5,307 $ 5,412 Property operating expenses 1,000 857 1,118 Capitalized to development activities 824 886 1,075 Share-based compensation cost $ 7,398 $ 7,050 $ 7,605 The amounts included in our consolidated statements of operations for share-based compensation reflected an estimate of pre-vesting forfeitures of 0% for PSUs and deferred share awards and 0% to 5% for restricted shares. As of December 31, 2015 , unrecognized compensation costs related to unvested awards included: • $6.3 million on restricted shares expected to be recognized over a weighted average period of approximately two years; • $1.7 million on PSUs expected to be recognized over a weighted average performance period of approximately two years and • $223,000 on deferred share awards expected to be recognized through May 2016.

Operating Leases

Operating Leases	12 Months Ended
	Dec. 31, 2015
Leases, Operating [Abstract]
Operating Leases	Operating Leases We lease our properties to tenants under operating leases with various expiration dates extending to the year 2030. Gross minimum future rentals on noncancelable leases in our properties as of December 31, 2015 were as follows (in thousands): Year Ending December 31, 2016 $ 378,780 2017 349,192 2018 278,714 2019 218,977 2020 154,207 Thereafter 433,892 $ 1,813,762

Information by Business Segment

Information by Business Segment	12 Months Ended
	Dec. 31, 2015
Segment Reporting [Abstract]
Information by Business Segment	Information by Business Segment Effective in the quarter ended December 31, 2015, we changed the reportable segments that we use to review operating results and make decisions regarding segment performance and resource allocation. Given the changes in our portfolio resulting from development, acquisition and disposition activities, we made this change to better align our segments with our business strategy by contemplating the market characteristics of our properties. Our new segment reporting includes the following segments: Defense/IT Locations; Regional Office; our operating wholesale data center; and other. Our segment reporting also includes reporting for Defense/IT Locations sub-segments, which include the following: Fort George G. Meade and the Baltimore/Washington Corridor (referred to herein as “Fort Meade/BW Corridor”); Northern Virginia Defense/IT Locations; Lackland Air Force Base (in San Antonio); locations serving the U.S. Navy ( “Navy Support Locations”), which included properties proximate to the Washington Navy Yard, the Naval Air Station Patuxent River in Maryland and the Naval Surface Warfare Center Dahlgren Division in Virginia); Redstone Arsenal (in Huntsville); Colorado Defense/IT Locations; and data center shells (properties leased to tenants to be operated as data centers in which the tenants generally fund the costs for the power, fiber connectivity and data center infrastructure). 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 Defense/Information Technology Locations Fort Meade/BW Corridor Northern Virginia Defense/IT Lackland Air Force Base Navy Support Locations Redstone Arsenal Colorado Defense/IT Data Center Shells Total Defense/IT Locations Regional Office Operating Wholesale Data Center Other Total Year Ended December 31, 2015 Revenues from real estate operations $ 244,274 $ 49,199 $ 39,659 $ 28,177 $ 11,228 $ — $ 21,746 $ 394,283 $ 98,165 $ 19,032 $ 7,588 $ 519,068 Property operating expenses 83,309 20,107 22,004 13,229 3,497 — 2,298 144,444 36,165 10,402 3,477 194,488 NOI from real estate operations $ 160,965 $ 29,092 $ 17,655 $ 14,948 $ 7,731 $ — $ 19,448 $ 249,839 $ 62,000 $ 8,630 $ 4,111 $ 324,580 Additions to long-lived assets $ 31,883 $ 90,248 $ — $ 7,656 $ 883 $ — $ — $ 130,670 $ 204,139 $ 132 $ 328 $ 335,269 Transfers from non-operating properties $ 45,560 $ 50,690 $ 32,307 $ 1,408 $ 13,190 $ — $ 51,492 $ 194,647 $ 22,313 $ 89,745 $ 415 $ 307,120 Segment assets at December 31, 2015 $ 1,290,028 $ 411,196 $ 134,381 $ 196,090 $ 108,038 $ — $ 203,013 $ 2,342,746 $ 608,471 $ 243,338 $ 70,914 $ 3,265,469 Year Ended December 31, 2014 Revenues from real estate operations $ 233,764 $ 48,313 $ 34,463 $ 31,335 $ 10,446 $ — $ 18,421 $ 376,742 $ 85,025 $ 10,430 $ 7,514 $ 479,711 Property operating expenses 80,824 19,071 19,677 12,576 3,066 — 2,272 137,486 31,427 7,286 3,600 179,799 NOI from real estate operations $ 152,940 $ 29,242 $ 14,786 $ 18,759 $ 7,380 $ — $ 16,149 $ 239,256 $ 53,598 $ 3,144 $ 3,914 $ 299,912 Additions to long-lived assets $ 24,173 $ 7,119 $ — $ 10,010 $ 4,077 $ — $ — $ 45,379 $ 19,290 $ 22 $ (90 ) $ 64,601 Transfers from non-operating properties $ 56,699 $ 360 $ — $ — $ 21,014 $ — $ 43,154 $ 121,227 $ 17,942 $ 1,108 $ 1,622 $ 141,899 Segment assets at December 31, 2014 $ 1,264,353 $ 372,041 $ 102,232 $ 195,972 $ 97,209 $ — $ 156,214 $ 2,188,021 $ 526,988 $ 163,177 $ 73,309 $ 2,951,495 Year Ended December 31, 2013 Revenues from real estate operations $ 234,857 $ 56,524 $ 31,252 $ 33,431 $ 5,050 $ 12,229 $ 12,520 $ 385,863 $ 98,487 $ 7,271 $ 7,012 $ 498,633 Property operating expenses 80,149 19,718 16,784 12,872 1,282 3,797 1,304 135,906 35,488 6,360 2,950 180,704 NOI from real estate operations $ 154,708 $ 36,806 $ 14,468 $ 20,559 $ 3,768 $ 8,432 $ 11,216 $ 249,957 $ 62,999 $ 911 $ 4,062 $ 317,929 Additions to long-lived assets $ 28,009 $ 5,522 $ 5 $ 4,945 $ 3,563 $ 200 $ — $ 42,244 $ 13,244 $ 598 $ 791 $ 56,877 Transfers from non-operating properties $ 50,067 $ 37,234 $ — $ 14 $ 48,799 $ 987 $ 24,200 $ 161,301 $ 32,692 $ 70,106 $ 191 $ 264,290 Segment assets at December 31, 2013 $ 1,229,192 $ 385,061 $ 103,650 $ 195,105 $ 77,773 $ — $ 117,012 $ 2,107,793 $ 556,718 $ 166,790 $ 74,329 $ 2,905,630 The following table reconciles our segment revenues to total revenues as reported on our consolidated statements of operations (in thousands): For the Years Ended December 31, 2015 2014 2013 Segment revenues from real estate operations $ 519,068 $ 479,711 $ 498,633 Construction contract and other service revenues 106,402 106,748 62,363 Less: Revenues from discontinued operations (Note 18) (4 ) 14 (37,636 ) Total revenues $ 625,466 $ 586,473 $ 523,360 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 Years Ended December 31, 2015 2014 2013 Segment property operating expenses $ 194,488 $ 179,799 $ 180,704 Less: Property operating expenses from discontinued operations (Note 18) 6 135 (13,505 ) Total property operating expenses $ 194,494 $ 179,934 $ 167,199 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 Years Ended December 31, 2015 2014 2013 Construction contract and other service revenues $ 106,402 $ 106,748 $ 62,363 Construction contract and other service expenses (102,696 ) (100,058 ) (58,875 ) NOI from service operations $ 3,706 $ 6,690 $ 3,488 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 Years Ended December 31, 2015 2014 2013 NOI from real estate operations $ 324,580 $ 299,912 $ 317,929 NOI from service operations 3,706 6,690 3,488 Interest and other income 4,517 4,923 3,834 Equity in income of unconsolidated entities 62 229 2,110 Income tax expense (199 ) (310 ) (1,978 ) Other adjustments: — Depreciation and other amortization associated with real estate operations (140,025 ) (136,086 ) (113,214 ) Impairment losses (23,289 ) (1,416 ) (5,857 ) General, administrative and leasing expenses (31,361 ) (31,794 ) (30,869 ) Business development expenses and land carry costs (13,507 ) (5,573 ) (5,436 ) Interest expense on continuing operations (89,074 ) (92,393 ) (82,010 ) NOI from discontinued operations (10 ) (121 ) (24,131 ) Gain (loss) on early extinguishment of debt 85,275 (9,552 ) (27,030 ) COPT consolidated income from continuing operations $ 120,675 $ 34,509 $ 36,836 The following table reconciles our segment assets to the consolidated total assets of COPT and subsidiaries (in thousands): As of December 31, 2015 2014 Segment assets $ 3,265,469 $ 2,951,495 Non-operating property assets 450,679 567,586 Other assets 193,164 145,155 Total COPT consolidated assets $ 3,909,312 $ 3,664,236 The accounting policies of the segments are the same as those used to prepare our consolidated financial statements, except that discontinued operations are not presented separately for segment purposes. In the segment reporting presented above, we did not allocate interest expense, depreciation and amortization, impairment losses, loss on early extinguishment of debt and gain on sales of real estate to our real estate segments since they are not included in the measure of segment profit reviewed by management. We also did not allocate general and administrative expenses, business development expenses and land carry costs, interest and other income, equity in income of unconsolidated entities, income taxes and noncontrolling interests because these items represent general corporate or non-operating property items not attributable to segments.

Discontinued Operations

Discontinued Operations	12 Months Ended
	Dec. 31, 2015
Discontinued Operations and Disposal Groups [Abstract]
Discontinued Operations	Discontinued Operations Income from discontinued operations primarily includes revenues and expenses associated with the following: • 920 Elkridge Landing Road in the Baltimore/Washington Corridor that was sold on June 25, 2013; • 4230 Forbes Boulevard in the Baltimore/Washington Corridor that was sold on December 11, 2013; • 15 operating properties in Colorado Springs that were sold on December 12, 2013; and • nine operating properties in the Baltimore/Washington Corridor and five operating properties in Colorado Springs for which the title to the properties was transferred to the mortgage lender on December 23, 2013. The table below sets forth the components of discontinued operations reported on our consolidated statements of operations (in thousands): For the Years Ended December 31, 2015 2014 2013 Revenue from real estate operations $ 4 $ (14 ) $ 37,636 Property operating expenses 6 135 (13,505 ) Depreciation and amortization — — (4,505 ) Impairment losses (234 ) (3 ) (26,190 ) General, administrative and leasing expenses — — (4 ) Interest expense — — (8,221 ) Gain on sales of real estate — 24 2,671 Gain (loss) on early extinguishment of debt 380 (116 ) 67,810 Discontinued operations $ 156 $ 26 $ 55,692

Earnings Per Share ("EPS") and

Earnings Per Share ("EPS") and Earnings Per Unit (“EPU”)	12 Months Ended
	Dec. 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 Years Ended December 31, 2015 2014 2013 Numerator: Income from continuing operations $ 120,675 $ 34,509 $ 36,836 Gain on sales of real estate, net 68,047 10,671 9,016 Preferred share dividends (14,210 ) (15,939 ) (19,971 ) Issuance costs associated with redeemed preferred shares — (1,769 ) (2,904 ) Income from continuing operations attributable to noncontrolling interests (10,575 ) (4,955 ) (4,486 ) Income from continuing operations attributable to restricted shares (706 ) (432 ) (414 ) Numerator for basic EPS from continuing operations attributable to COPT common shareholders $ 163,231 $ 22,085 $ 18,077 Dilutive effect of common units in the Operating Partnership on diluted EPS from continuing operations 6,397 — — Numerator for diluted EPS from continuing operations attributable to COPT common shareholders $ 169,628 $ 22,085 $ 18,077 Numerator for basic EPS from continuing operations attributable to COPT common shareholders $ 163,231 $ 22,085 $ 18,077 Discontinued operations 156 26 55,692 Discontinued operations attributable to noncontrolling interests (3 ) 4 (3,351 ) Numerator for basic EPS on net income attributable to COPT common shareholders $ 163,384 $ 22,115 $ 70,418 Dilutive effect of common units in COPLP 6,403 — — Numerator for diluted EPS on net income attributable to COPT common shareholders $ 169,787 $ 22,115 $ 70,418 Denominator (all weighted averages): Denominator for basic EPS (common shares) 93,914 88,092 85,167 Dilutive effect of common units 3,692 — — Dilutive effect of share-based compensation awards 61 171 57 Denominator for diluted EPS (common shares) 97,667 88,263 85,224 Basic EPS: Income from continuing operations attributable to COPT common shareholders $ 1.74 $ 0.25 $ 0.21 Discontinued operations attributable to COPT common shareholders — — 0.62 Net income attributable to COPT common shareholders $ 1.74 $ 0.25 $ 0.83 Diluted EPS: Income from continuing operations attributable to COPT common shareholders $ 1.74 $ 0.25 $ 0.21 Discontinued operations attributable to COPT common shareholders — — 0.62 Net income attributable to COPT common shareholders $ 1.74 $ 0.25 $ 0.83 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 Years Ended December 31, 2015 2014 2013 Conversion of common units — 3,897 3,869 Conversion of Series I preferred units 176 176 176 Conversion of Series K preferred shares 434 434 434 The following share-based compensation securities were excluded from the computation of diluted EPS because their effect was antidilutive: • weighted average restricted shares and deferred share awards of 410,000 for 2015, 401,000 for 2014 and 385,000 for 2013; and • weighted average options of 469,000 for 2015, 492,000 for 2014 and 636,000 for 2013, respectively. As discussed in Note 10, 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 Years Ended December 31, 2015 2014 2013 Numerator: Income from continuing operations $ 120,675 $ 34,509 $ 36,836 Gain on sales of real estate, net 68,047 10,671 9,016 Preferred unit distributions (14,870 ) (16,599 ) (20,631 ) Issuance costs associated with redeemed preferred units — (1,769 ) (2,904 ) Income from continuing operations attributable to noncontrolling interests (3,523 ) (3,281 ) (2,977 ) Income from continuing operations attributable to restricted units (706 ) (432 ) (414 ) Numerator for basic and diluted EPU from continuing operations attributable to COPLP common unitholders $ 169,623 $ 23,099 $ 18,926 Discontinued operations 156 26 55,692 Discontinued operations attributable to noncontrolling interests 3 5 (930 ) Numerator for basic and diluted EPU on net income attributable to COPLP common unitholders $ 169,782 $ 23,130 $ 73,688 Denominator (all weighted averages): Denominator for basic EPU (common units) 97,606 91,989 89,036 Dilutive effect of share-based compensation awards 61 171 57 Denominator for diluted EPU (common units) 97,667 92,160 89,093 Basic EPU: Income from continuing operations attributable to COPLP common unitholders $ 1.74 $ 0.25 $ 0.21 Discontinued operations attributable to COPLP common unitholders — — 0.62 Net income attributable to COPLP common unitholders $ 1.74 $ 0.25 $ 0.83 Diluted EPU: Income from continuing operations attributable to COPLP common unitholders $ 1.74 $ 0.25 $ 0.21 Discontinued operations attributable to COPLP common unitholders — — 0.62 Net income attributable to COPLP common unitholders $ 1.74 $ 0.25 $ 0.83 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 Years Ended December 31, 2015 2014 2013 Conversion of Series I preferred units 176 176 176 Conversion of Series K preferred units 434 434 434 The following share-based compensation securities were excluded from the computation of diluted EPU because their effect was antidilutive: • weighted average restricted units and deferred share awards of 410,000 for 2015, 401,000 for 2014 and 385,000 for 2013; and • weighted average options of 469,000 for 2015, 492,000 for 2014 and 636,000 for 2013. As discussed in Note 10, 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	12 Months Ended
	Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Litigation In the normal course of business, we are involved in legal actions arising from our ownership and administration of properties. We establish reserves for specific legal proceedings when we determine that the likelihood of an unfavorable outcome is probable and the amount of loss can be reasonably estimated. Management does not anticipate that any liabilities that may result from such proceedings will have a materially adverse effect on our financial position, operations or liquidity. Our assessment of the potential outcomes of these matters involves significant judgment and is subject to change based on future developments. Environmental We are subject to various Federal, state and local environmental regulations related to our property ownership and operation. We have performed environmental assessments of our properties, the results of which have not revealed any environmental liability that we believe would have a materially adverse effect on our financial position, operations or liquidity. 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 $1.9 million liability through December 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. Operating Leases We are obligated as lessee under operating leases (mostly ground leases) with various expiration dates extending to the year 2100. Future minimum rental payments due under the terms of these operating leases as of December 31, 2015 follow (in thousands): Year Ending December 31, 2016 $ 1,189 2017 1,117 2018 1,071 2019 1,049 2020 1,067 Thereafter 86,798 $ 92,291 Contractual Obligations We had amounts remaining to be incurred under various contractual obligations as of December 31, 2015 that included the following: • new development and redevelopment obligations of $87.8 million (including acquisitions of land); • capital expenditures for operating properties of $47.9 million ; • third party construction and development of $10.7 million ; and • other purchase obligations of $3.6 million . Environmental Indemnity Agreement In connection with a lease and subsequent sale in 2008 and 2010 of three properties in Dayton, New Jersey, we agreed to provide certain environmental indemnifications. 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.

Subsequent Event

Subsequent Event	12 Months Ended
	Dec. 31, 2015
Subsequent Events [Abstract]
Subsequent Event	Subsequent Event On February 10, 2016, our Board of Trustees appointed Stephen E. Budorick, our Executive Vice President and Chief Operating Officer since September 2011, to become our President and Chief Executive Officer effective May 12, 2016, the date of the Company’s 2016 Annual Meeting of Shareholders. On that date, Roger A. Waesche, Jr., our current President and Chief Executive Officer, will leave the Company to pursue other interests and he will not be nominated for reelection as a Trustee. The Board expects to appoint Mr. Budorick to our Board of Trustees after the 2016 Annual Meeting of Shareholders. In connection with the departure of Mr. Waesche, we will recognize executive transition costs of approximately $2.9 million in 2016.

Quarterly Data (Unaudited)

Quarterly Data (Unaudited)	12 Months Ended
	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Quarterly Data (Unaudited)	Quarterly Data (Unaudited) The tables below set forth selected quarterly information for the years ended December 31, 2015 and 2014 (in thousands, except per share data). Certain of the amounts below have been reclassified to conform to the current period presentation of our consolidated financial statements. For the Year Ended December 31, 2015 For the Year Ended December 31, 2014 First Quarter Second Quarter Third Quarter Fourth Quarter First Quarter Second Quarter Third Quarter Fourth Quarter COPT and Subsidiaries Revenues $ 161,034 $ 170,363 $ 150,744 $ 143,325 $ 146,667 $ 139,820 $ 153,015 $ 146,971 Operating income $ 30,575 $ 37,471 $ 31,993 $ 20,055 $ 25,206 $ 31,836 $ 37,422 $ 37,148 Income from continuing operations $ 10,987 $ 16,839 $ 94,279 $ (1,430 ) $ 5,660 $ 9,248 $ 13,727 $ 5,874 Discontinued operations $ (238 ) $ 394 $ — $ — $ 11 $ (198 ) $ 191 $ 22 Net income $ 14,735 $ 17,232 $ 94,294 $ 62,617 $ 5,671 $ 9,050 $ 24,548 $ 5,937 Net income attributable to noncontrolling interests (1,380 ) (1,451 ) (4,494 ) (3,253 ) (930 ) (1,160 ) (1,828 ) (1,033 ) Net income attributable to COPT 13,355 15,781 89,800 59,364 4,741 7,890 22,720 4,904 Preferred share dividends (3,552 ) (3,553 ) (3,552 ) (3,553 ) (4,490 ) (4,344 ) (3,553 ) (3,552 ) Issuance costs associated with redeemed preferred shares — — — — — (1,769 ) — — Net income attributable to COPT common shareholders $ 9,803 $ 12,228 $ 86,248 $ 55,811 $ 251 $ 1,777 $ 19,167 $ 1,352 Basic earnings per common share $ 0.10 $ 0.13 $ 0.91 $ 0.59 $ 0.00 $ 0.02 $ 0.22 $ 0.01 Diluted earnings per common share $ 0.10 $ 0.13 $ 0.91 $ 0.59 $ 0.00 $ 0.02 $ 0.22 $ 0.01 COPLP and Subsidiaries Revenues $ 161,034 $ 170,363 $ 150,744 $ 143,325 $ 146,667 $ 139,820 $ 153,015 $ 146,971 Operating income $ 30,575 $ 37,471 $ 31,993 $ 20,055 $ 25,206 $ 31,836 $ 37,422 $ 37,148 Income from continuing operations $ 10,987 $ 16,839 $ 94,279 $ (1,430 ) $ 5,660 $ 9,248 $ 13,727 $ 5,874 Discontinued operations $ (238 ) $ 394 $ — $ — $ 11 $ (198 ) $ 191 $ 22 Net income $ 14,735 $ 17,232 $ 94,294 $ 62,617 $ 5,671 $ 9,050 $ 24,548 $ 5,937 Net income attributable to noncontrolling interests (818 ) (812 ) (972 ) (918 ) (737 ) (837 ) (897 ) (805 ) Net income attributable to COPLP 13,917 16,420 93,322 61,699 4,934 8,213 23,651 5,132 Preferred unit distributions (3,717 ) (3,718 ) (3,717 ) (3,718 ) (4,655 ) (4,509 ) (3,718 ) (3,717 ) Issuance costs associated with redeemed preferred units — — — — — (1,769 ) — — Net income attributable to COPLP common unitholders $ 10,200 $ 12,702 $ 89,605 $ 57,981 $ 279 $ 1,935 $ 19,933 $ 1,415 Basic earnings per common unit $ 0.10 $ 0.13 $ 0.91 $ 0.59 $ 0.00 $ 0.02 $ 0.22 $ 0.01 Diluted earnings per common unit $ 0.10 $ 0.13 $ 0.91 $ 0.59 $ 0.00 $ 0.02 $ 0.22 $ 0.01

Schedule II - Valuation and Qua

Schedule II - Valuation and Qualifying Accounts (Notes)	12 Months Ended
	Dec. 31, 2015
Valuation and Qualifying Accounts [Abstract]
Schedule II—Valuation and Qualifying Accounts	Corporate Office Properties Trust and Subsidiaries and Corporate Office Properties, L.P. and Subsidiaries Schedule II—Valuation and Qualifying Accounts Years Ended December 31, 2015, 2014 and 2013 (Dollars in thousands) Balance at Charged to Charged to Other Accounts (2) Deductions (3) Balance at End of Year Accounts Receivables-Allowance for doubtful accounts Year ended December 31, 2015 $ 717 $ 1,125 $ 98 $ (415 ) $ 1,525 Year ended December 31, 2014 $ 2,976 $ 278 $ — $ (2,537 ) $ 717 Year ended December 31, 2013 $ 4,694 $ (65 ) $ — $ (1,653 ) $ 2,976 Allowance for Deferred Rent Receivable Year ended December 31, 2015 $ 1,418 $ — $ 544 $ — $ 1,962 Year ended December 31, 2014 $ 2,126 $ — $ (708 ) $ — $ 1,418 Year ended December 31, 2013 $ 913 $ — $ 1,213 $ — $ 2,126 Allowance for Deferred Tax Asset Year ended December 31, 2015 $ 2,062 $ — $ — $ — $ 2,062 Year ended December 31, 2014 $ 2,062 $ — $ — $ — $ 2,062 Year ended December 31, 2013 $ 207 $ 1,855 $ — $ — $ 2,062 (1) Amounts charged to costs and expenses are net of recoveries. (2) Allowances for certain accounts receivables were charged to service company revenue. Deferred rent receivable allowances were charged to rental revenue. (3) Deductions reflect adjustments to reserves due to actual write-offs of accounts.

Schedule III - Real Estate and

Schedule III - Real Estate and Accumulated Depreciation	12 Months Ended
	Dec. 31, 2015
SEC Schedule III, Real Estate and Accumulated Depreciation Disclosure [Abstract]
Schedule III-Real Estate and Accumulated Depreciation	Corporate Office Properties Trust and Subsidiaries and Corporate Office Properties, L.P. and Subsidiaries Schedule III—Real Estate and Accumulated Depreciation December 31, 2015 (Dollars in thousands) Initial Cost Gross Amounts Carried At Close of Period Property (Type) (1) Location Encumbrances (2) Land Building and Land Improvements Costs Capitalized Subsequent to Acquisition Land Building and Land Improvements Total (3)(4) Accumulated Depreciation (5) Year Built or Renovated Date Acquired (6) 100 Light Street (O) Baltimore, MD $ 50,732 $ 26,715 $ 58,343 $ 55 $ 26,715 $ 58,398 $ 85,113 $ (1,588 ) 1973 8/7/2015 1000 Redstone Gateway (O) Huntsville, AL 11,370 — 20,533 — — 20,533 20,533 (1,438 ) 2013 3/23/2010 1100 Redstone Gateway (O) Huntsville, AL 11,791 — 19,593 — — 19,593 19,593 (965 ) 2014 3/23/2010 114 National Business Parkway (O) Annapolis Junction, MD — 364 3,109 60 364 3,169 3,533 (1,125 ) 2002 6/30/2000 11751 Meadowville Lane (O) Richmond, VA — 1,305 52,098 112 1,305 52,210 53,515 (11,194 ) 2007 9/15/2006 1200 Redstone Gateway (O) Huntsville, AL 13,639 — 22,389 — — 22,389 22,389 (1,145 ) 2013 3/23/2010 1201 M Street (O) Washington, DC — — 49,785 4,225 — 54,010 54,010 (8,947 ) 2001 9/28/2010 1201 Winterson Road (O) Linthicum, MD — 1,288 12,175 460 1,288 12,635 13,923 (3,824 ) 1985 (7) 4/30/1998 1220 12th Street, SE (O) Washington, DC — — 42,464 2,431 — 44,895 44,895 (8,523 ) 2003 9/28/2010 1243 Winterson Road (L) Linthicum, MD — 630 — — 630 — 630 — (8) 12/19/2001 1302 Concourse Drive (O) Linthicum, MD — 2,078 8,313 6,559 2,078 14,872 16,950 (6,188 ) 1996 11/18/1999 1304 Concourse Drive (O) Linthicum, MD — 1,999 12,934 2,884 1,999 15,818 17,817 (6,360 ) 2002 11/18/1999 1306 Concourse Drive (O) Linthicum, MD — 2,796 11,186 8,152 2,796 19,338 22,134 (6,922 ) 1990 11/18/1999 131 National Business Parkway (O) Annapolis Junction, MD — 1,906 7,623 3,524 1,906 11,147 13,053 (5,596 ) 1990 9/28/1998 132 National Business Parkway (O) Annapolis Junction, MD — 2,917 12,259 3,005 2,917 15,264 18,181 (7,700 ) 2000 5/28/1999 133 National Business Parkway (O) Annapolis Junction, MD — 2,517 10,068 5,204 2,517 15,272 17,789 (8,230 ) 1997 9/28/1998 1331 Ashton Road (O) Hanover, MD — 587 2,347 1,575 587 3,922 4,509 (1,337 ) 1989 4/28/1999 1334 Ashton Road (O) Hanover, MD — 736 1,488 2,639 736 4,127 4,863 (2,467 ) 1989 4/28/1999 134 National Business Parkway (O) Annapolis Junction, MD — 3,684 7,517 3,496 3,684 11,013 14,697 (4,062 ) 1999 11/13/1998 1340 Ashton Road (O) Hanover, MD — 905 3,620 1,469 905 5,089 5,994 (2,621 ) 1989 4/28/1999 1341 Ashton Road (O) Hanover, MD — 306 1,223 604 306 1,827 2,133 (932 ) 1989 4/28/1999 1343 Ashton Road (O) Hanover, MD — 193 774 405 193 1,179 1,372 (640 ) 1989 4/28/1999 13450 Sunrise Valley Road (O) Herndon, VA — 1,386 5,576 3,335 1,386 8,911 10,297 (3,902 ) 1998 7/25/2003 13454 Sunrise Valley Road (O) Herndon, VA — 2,899 11,986 6,750 2,899 18,736 21,635 (7,177 ) 1998 7/25/2003 135 National Business Parkway (O) Annapolis Junction, MD 11,125 2,484 9,750 5,700 2,484 15,450 17,934 (6,467 ) 1998 12/30/1998 1362 Mellon Road (O) Hanover, MD — 1,706 8,670 182 1,706 8,852 10,558 (1,517 ) 2006 2/10/2006 13857 McLearen Road (O) Herndon, VA — 3,507 30,177 1,565 3,507 31,742 35,249 (6,961 ) 2007 7/11/2012 140 National Business Parkway (O) Annapolis Junction, MD — 3,407 24,167 688 3,407 24,855 28,262 (7,647 ) 2003 12/31/2003 141 National Business Parkway (O) Annapolis Junction, MD 11,566 2,398 9,590 3,194 2,398 12,784 15,182 (5,963 ) 1990 9/28/1998 14280 Park Meadow Drive (O) Chantilly, VA — 3,731 15,953 1,764 3,731 17,717 21,448 (6,185 ) 1999 9/29/2004 1460 Dorsey Road (L) Hanover, MD — 1,800 — — 1,800 — 1,800 — (8) 2/28/2006 14840 Conference Center Drive (O) Chantilly, VA — 1,572 8,175 2,860 1,572 11,035 12,607 (4,320 ) 2000 7/25/2003 14850 Conference Center Drive (O) Chantilly, VA — 1,615 8,358 2,947 1,615 11,305 12,920 (4,646 ) 2000 7/25/2003 14900 Conference Center Drive (O) Chantilly, VA — 3,436 14,402 5,239 3,436 19,641 23,077 (8,221 ) 1999 7/25/2003 Initial Cost Gross Amounts Carried At Close of Period Property (Type) (1) Location Encumbrances (2) Land Building and Land Improvements Costs Capitalized Subsequent to Acquisition Land Building and Land Improvements Total (3)(4) Accumulated Depreciation (5) Year Built or Renovated Date Acquired (6) 1501 South Clinton Street (O) Baltimore, MD — 27,964 52,146 8,165 27,964 60,311 88,275 (14,472 ) 2006 10/27/2009 15049 Conference Center Drive (O) Chantilly, VA — 4,415 20,365 3,619 4,415 23,984 28,399 (9,236 ) 1997 8/14/2002 15059 Conference Center Drive (O) Chantilly, VA — 5,753 13,615 2,065 5,753 15,680 21,433 (6,697 ) 2000 8/14/2002 15395 John Marshall Highway (O) Haymarket, VA — 2,465 24,768 — 2,465 24,768 27,233 (1,195 ) 2014 5/7/2013 1550 West Nursery Road (O) Linthicum, MD — 14,071 16,930 — 14,071 16,930 31,001 (3,626 ) 2009 10/28/2009 1560 West Nursery Road (O) Linthicum, MD — 1,441 113 — 1,441 113 1,554 (5 ) 2014 10/28/2009 1560A Cable Ranch Road (O) (10) San Antonio, TX — 1,097 3,770 394 1,097 4,164 5,261 (1,298 ) 1985/2007 6/19/2008 1560B Cable Ranch Road (O) (10) San Antonio, TX — 2,299 6,545 159 2,299 6,704 9,003 (1,738 ) 1985/2006 6/19/2008 16442 Commerce Drive (O) Dahlgren, VA — 613 2,582 883 613 3,465 4,078 (1,332 ) 2002 12/21/2004 16480 Commerce Drive (O) Dahlgren, VA — 1,856 7,425 1,283 1,856 8,708 10,564 (2,316 ) 2000 12/28/2004 16501 Commerce Drive (O) Dahlgren, VA — 522 2,090 482 522 2,572 3,094 (797 ) 2002 12/21/2004 16539 Commerce Drive (O) Dahlgren, VA — 688 2,860 1,524 688 4,384 5,072 (1,934 ) 1990 12/21/2004 16541 Commerce Drive (O) Dahlgren, VA — 773 3,094 1,368 773 4,462 5,235 (1,727 ) 1996 12/21/2004 16543 Commerce Drive (O) Dahlgren, VA — 436 1,742 502 436 2,244 2,680 (573 ) 2002 12/21/2004 1751 Pinnacle Drive (O) McLean, VA — 10,486 42,339 24,059 10,486 66,398 76,884 (23,750 ) 1989/1995 9/23/2004 1753 Pinnacle Drive (O) McLean, VA — 8,275 34,353 10,894 8,275 45,247 53,522 (15,849 ) 1976/2004 9/23/2004 201 Technology Drive (O) Lebanon, VA — 726 31,091 59 726 31,150 31,876 (6,358 ) 2007 10/5/2007 206 Research Boulevard (O) Aberdeen, MD — 1,813 17,485 — 1,813 17,485 19,298 (1,416 ) 2012 9/14/2007 209 Research Boulevard (O) Aberdeen, MD — 1,045 16,087 79 1,045 16,166 17,211 (2,069 ) 2010 9/14/2007 210 Research Boulevard (O) Aberdeen, MD — 1,065 15,102 75 1,065 15,177 16,242 (1,569 ) 2010 9/14/2007 2100 L Street (L) Washington, DC — 1,273 1,685 — 1,273 1,685 2,958 — (8) 8/11/2015 2100 Rideout Road (O) Huntsville, AL — — 2,586 — — 2,586 2,586 — (7) 3/23/2010 21267 Smith Switch Road (O) Ashburn, VA — 4,040 10,135 — 4,040 10,135 14,175 (365 ) 2014 12/27/2012 21271 Smith Switch Road (O) Ashburn, VA — 7,346 18,267 — 7,346 18,267 25,613 (881 ) 2013 12/27/2012 22289 Exploration Drive (O) Lexington Park, MD — 1,422 5,719 1,660 1,422 7,379 8,801 (2,826 ) 2000 3/24/2004 22299 Exploration Drive (O) Lexington Park, MD — 1,362 5,791 1,218 1,362 7,009 8,371 (2,641 ) 1998 3/24/2004 22300 Exploration Drive (O) Lexington Park, MD — 1,094 5,038 1,315 1,094 6,353 7,447 (2,024 ) 1997 11/9/2004 22309 Exploration Drive (O) Lexington Park, MD — 2,243 10,419 7,787 2,243 18,206 20,449 (4,683 ) 1984/1997 3/24/2004 23535 Cottonwood Parkway (O) California, MD — 692 3,051 248 692 3,299 3,991 (1,206 ) 1984 3/24/2004 250 W Pratt St (O) Baltimore, MD — 8,057 34,654 4,765 8,057 39,419 47,476 (1,652 ) 1985 3/19/2015 2500 Riva Road (O) Annapolis, MD — 2,791 12,145 1 2,791 12,146 14,937 (4,417 ) 2000 3/4/2003 2600 Park Tower Drive (O) Vienna, VA — 20,304 34,443 346 20,304 34,789 55,093 (1,051 ) 1999 4/15/2015 2691 Technology Drive (O) Annapolis Junction, MD — 2,098 17,334 5,305 2,098 22,639 24,737 (7,827 ) 2005 5/26/2000 2701 Technology Drive (O) Annapolis Junction, MD 22,053 1,737 15,266 2,821 1,737 18,087 19,824 (7,567 ) 2001 5/26/2000 2711 Technology Drive (O) Annapolis Junction, MD 17,153 2,251 21,611 1,468 2,251 23,079 25,330 (10,158 ) 2002 11/13/2000 2720 Technology Drive (O) Annapolis Junction, MD — 3,863 29,272 146 3,863 29,418 33,281 (8,316 ) 2004 1/31/2002 2721 Technology Drive (O) Annapolis Junction, MD — 4,611 14,597 1,263 4,611 15,860 20,471 (6,400 ) 2000 10/21/1999 2730 Hercules Road (O) Annapolis Junction, MD 34,256 8,737 31,612 7,213 8,737 38,825 47,562 (15,674 ) 1990 9/28/1998 2900 Towerview Road (O) Herndon, VA — 3,207 16,379 6,509 3,207 22,888 26,095 (7,181 ) 1982/2008 12/20/2005 Initial Cost Gross Amounts Carried At Close of Period Property (Type) (1) Location Encumbrances (2) Land Building and Land Improvements Costs Capitalized Subsequent to Acquisition Land Building and Land Improvements Total (3)(4) Accumulated Depreciation (5) Year Built or Renovated Date Acquired (6) 30 Light Street (O) Baltimore, MD 4,268 — 12,101 37 — 12,138 12,138 (128 ) 2009 8/7/2015 300 Sentinel Drive (O) Annapolis Junction, MD — 1,517 59,016 219 1,517 59,235 60,752 (8,458 ) 2009 11/14/2003 302 Sentinel Drive (O) Annapolis Junction, MD — 2,648 29,687 445 2,648 30,132 32,780 (6,000 ) 2007 11/14/2003 304 Sentinel Drive (O) Annapolis Junction, MD — 3,411 24,917 193 3,411 25,110 28,521 (6,304 ) 2005 11/14/2003 306 Sentinel Drive (O) Annapolis Junction, MD 15,878 3,260 22,592 611 3,260 23,203 26,463 (5,269 ) 2006 11/14/2003 308 Sentinel Drive (O) Annapolis Junction, MD — 1,422 26,208 37 1,422 26,245 27,667 (3,050 ) 2010 11/14/2003 310 Sentinel Way (O) Annapolis Junction, MD — 2,372 36,468 — 2,372 36,468 38,840 (191 ) (7) 11/14/2003 310 The Bridge Street (O) Huntsville, AL — 261 26,531 1,695 261 28,226 28,487 (5,136 ) 2009 8/9/2011 312 Sentinel Way (O) Annapolis Junction, MD — 3,138 27,789 — 3,138 27,789 30,927 (913 ) 2014 11/14/2003 3120 Fairview Park Drive (O) Falls Church, VA — 6,863 35,606 8,773 6,863 44,379 51,242 (7,229 ) 2008 11/23/2010 314 Sentinel Way (O) Annapolis Junction, MD — 1,254 1,439 — 1,254 1,439 2,693 (249 ) 2008 11/14/2003 316 Sentinel Way (O) Annapolis Junction, MD — 2,748 38,156 138 2,748 38,294 41,042 (3,570 ) 2011 11/14/2003 318 Sentinel Way (O) Annapolis Junction, MD — 2,185 28,426 — 2,185 28,426 30,611 (6,981 ) 2005 11/14/2003 320 Sentinel Way (O) Annapolis Junction, MD — 2,067 21,623 — 2,067 21,623 23,690 (4,310 ) 2007 11/14/2003 322 Sentinel Way (O) Annapolis Junction, MD 21,122 2,605 22,827 — 2,605 22,827 25,432 (5,143 ) 2006 11/14/2003 324 Sentinel Way (O) Annapolis Junction, MD — 1,656 23,018 — 1,656 23,018 24,674 (3,078 ) 2010 6/29/2006 410 National Business Parkway (O) Annapolis Junction, MD — 1,831 23,257 112 1,831 23,369 25,200 (1,719 ) 2012 6/29/2006 420 National Business Parkway (O) Annapolis Junction, MD — 2,370 27,490 106 2,370 27,596 29,966 (1,230 ) 2013 6/29/2006 430 National Business Parkway (O) Annapolis Junction, MD — 1,852 21,153 119 1,852 21,272 23,124 (2,034 ) 2011 6/29/2006 44408 Pecan Court (O) California, MD — 817 1,583 581 817 2,164 2,981 (560 ) 1986 3/24/2004 44414 Pecan Court (O) California, MD — 405 1,619 337 405 1,956 2,361 (725 ) 1986 3/24/2004 44417 Pecan Court (O) California, MD — 434 3,348 88 434 3,436 3,870 (1,127 ) 1989/2015 3/24/2004 44420 Pecan Court (O) California, MD — 344 890 148 344 1,038 1,382 (255 ) 1989 11/9/2004 44425 Pecan Court (O) California, MD — 1,309 3,506 1,590 1,309 5,096 6,405 (1,784 ) 1997 5/5/2004 45310 Abell House Lane (O) California, MD — 2,272 13,808 — 2,272 13,808 16,080 (1,403 ) 2011 8/30/2010 46579 Expedition Drive (O) Lexington Park, MD — 1,406 5,796 1,335 1,406 7,131 8,537 (3,060 ) 2002 3/24/2004 46591 Expedition Drive (O) Lexington Park, MD — 1,200 7,199 1,134 1,200 8,333 9,533 (1,933 ) 2005 3/24/2004 4851 Stonecroft Boulevard (O) Chantilly, VA — 1,878 11,558 21 1,878 11,579 13,457 (3,248 ) 2004 8/14/2002 4940 Campbell Drive (O) (10) White Marsh, MD — 1,379 2,728 1,464 1,379 4,192 5,571 (445 ) 1990 1/9/2007 525 Babcock Road (O) Colorado Springs, CO — 355 397 79 355 476 831 (173 ) 1967 7/12/2007 5325 Nottingham Drive (O) White Marsh, MD — 816 3,976 485 816 4,461 5,277 (1,233 ) 2002 1/9/2007 5355 Nottingham Drive (O) White Marsh, MD — 761 3,562 1,758 761 5,320 6,081 (2,132 ) 2005 1/9/2007 540 National Business Parkway (O) Annapolis Junction, MD — 2,035 9,496 — 2,035 9,496 11,531 — (7) 6/29/2006 5520 Research Park Drive (O) Catonsville, MD — — 20,072 201 — 20,273 20,273 (3,195 ) 2009 4/4/2006 5522 Research Park Drive (O) Catonsville, MD — — 4,550 — — 4,550 4,550 (955 ) 2007 3/8/2006 5825 University Research Court (O) College Park, MD 15,255 — 22,740 87 — 22,827 22,827 (3,817 ) 2008 1/29/2008 5850 University Research Court (O) College Park, MD 20,771 — 31,906 406 — 32,312 32,312 (4,580 ) 2008 1/29/2008 6700 Alexander Bell Drive (O) Columbia, MD — 1,755 7,019 6,226 1,755 13,245 15,000 (5,913 ) 1988 5/14/2001 6708 Alexander Bell Drive (O) Columbia, MD — 897 8,844 1,591 897 10,435 11,332 (3,414 ) 1988 (7) 5/14/2001 Initial Cost Gross Amounts Carried At Close of Period Property (Type) (1) Location Encumbrances (2) Land Building and Land Improvements Costs Capitalized Subsequent to Acquisition Land Building and Land Improvements Total (3)(4) Accumulated Depreciation (5) Year Built or Renovated Date Acquired (6) 6711 Columbia Gateway Drive (O) Columbia, MD — 2,683 23,239 619 2,683 23,858 26,541 (5,407 ) 2006-2007 9/28/2000 6716 Alexander Bell Drive (O) Columbia, MD — 1,242 4,969 3,352 1,242 8,321 9,563 (4,359 ) 1990 12/31/1998 6721 Columbia Gateway Drive (O) Columbia, MD 16,663 1,753 34,090 76 1,753 34,166 35,919 (5,798 ) 2009 9/28/2000 6724 Alexander Bell Drive (O) Columbia, MD — 449 5,039 708 449 5,747 6,196 (2,266 ) 2001 5/14/2001 6731 Columbia Gateway Drive (O) Columbia, MD — 2,807 19,098 2,916 2,807 22,014 24,821 (8,494 ) 2002 3/29/2000 6740 Alexander Bell Drive (O) Columbia, MD — 1,424 5,696 3,340 1,424 9,036 10,460 (5,058 ) 1992 12/31/1998 6741 Columbia Gateway Drive (O) Columbia, MD — 675 1,711 124 675 1,835 2,510 (357 ) 2008 9/28/2000 6750 Alexander Bell Drive (O) Columbia, MD — 1,263 12,461 3,457 1,263 15,918 17,181 (7,576 ) 2001 12/31/1998 6760 Alexander Bell Drive (O) Columbia, MD — 890 3,561 2,974 890 6,535 7,425 (3,248 ) 1991 12/31/1998 6940 Columbia Gateway Drive (O) Columbia, MD — 3,545 9,916 5,537 3,545 15,453 18,998 (7,000 ) 1999 11/13/1998 6950 Columbia Gateway Drive (O) Columbia, MD — 3,596 14,269 3,087 3,596 17,356 20,952 (7,506 ) 1998 10/22/1998 7000 Columbia Gateway Drive (O) Columbia, MD — 3,131 12,103 2,138 3,131 14,241 17,372 (4,482 ) 1999 5/31/2002 7005 Columbia Gateway Drive (O) Columbia, MD — 3,036 189 — 3,036 189 3,225 — (8) 6/26/2014 7015 Albert Einstein Drive (O) Columbia, MD 1,570 2,058 6,093 1,527 2,058 7,620 9,678 (2,915 ) 1999 12/1/2005 7061 Columbia Gateway Drive (O) Columbia, MD — 729 3,094 1,407 729 4,501 5,230 (1,802 ) 2000 8/30/2001 7063 Columbia Gateway Drive (O) Columbia, MD — 902 3,684 2,512 902 6,196 7,098 (2,452 ) 2000 8/30/2001 7065 Columbia Gateway Drive (O) Columbia, MD — 919 3,763 3,095 919 6,858 7,777 (2,773 ) 2000 8/30/2001 7067 Columbia Gateway Drive (O) Columbia, MD — 1,829 11,823 2,838 1,829 14,661 16,490 (6,154 ) 2001 8/30/2001 7125 Columbia Gateway Drive (L) Columbia, MD — 3,361 1,677 279 3,361 1,956 5,317 — 1973/1999 (8) 6/29/2006 7125 Columbia Gateway Drive (O) Columbia, MD — 17,126 46,994 9,813 17,126 56,807 73,933 (16,632 ) 1973/1999 6/29/2006 7130 Columbia Gateway Drive (O) Columbia, MD — 1,350 4,359 1,906 1,350 6,265 7,615 (2,692 ) 1989 9/19/2005 7134 Columbia Gateway Drive (O) Columbia, MD — 704 2,139 314 704 2,453 3,157 (1,234 ) 1990 (7) 9/19/2005 7138 Columbia Gateway Drive (O) Columbia, MD — 1,104 3,518 2,118 1,104 5,636 6,740 (3,139 ) 1990 9/19/2005 7142 Columbia Gateway Drive (O) Columbia, MD — 1,342 3,978 2,502 1,342 6,480 7,822 (2,322 ) 1994 9/19/2005 7150 Columbia Gateway Drive (O) Columbia, MD — 1,032 3,429 665 1,032 4,094 5,126 (1,170 ) 1991 9/19/2005 7150 Riverwood Drive (O) Columbia, MD — 1,821 4,388 1,713 1,821 6,101 7,922 (1,933 ) 2000 1/10/2007 7160 Riverwood Drive (O) Columbia, MD — 2,732 7,006 2,275 2,732 9,281 12,013 (3,265 ) 2000 1/10/2007 7170 Riverwood Drive (O) Columbia, MD — 1,283 3,096 692 1,283 3,788 5,071 (1,386 ) 2000 1/10/2007 7175 Riverwood Drive (O) Columbia, MD — 1,788 7,269 — 1,788 7,269 9,057 (389 ) 1996/2013 7/27/2005 7200 Redstone Gateway (O) Huntsville, AL 6,649 — 8,347 — — 8,347 8,347 (328 ) 2013 3/23/2010 7200 Riverwood Road (O) Columbia, MD — 4,089 22,544 3,994 4,089 26,538 30,627 (8,761 ) 1986 10/13/1998 7205 Riverwood Drive (O) Columbia, MD — 1,367 21,484 — 1,367 21,484 22,851 (1,310 ) 2013 7/27/2005 7272 Park Circle Drive (O) Hanover, MD 4,747 1,479 6,300 3,883 1,479 10,183 11,662 (2,956 ) 1991/1996 1/10/2007 7318 Parkway Drive (O) Hanover, MD — 972 3,888 1,002 972 4,890 5,862 (2,084 ) 1984 4/16/1999 7320 Parkway Drive (O) Hanover, MD — 905 3,570 5,048 905 8,618 9,523 (2,883 ) 1983 4/4/2002 7400 Redstone Gateway (O) Huntsville, AL 7,292 — 9,012 — — 9,012 9,012 (121 ) 2015 3/23/2010 7467 Ridge Road (O) Hanover, MD — 1,629 6,517 3,645 1,629 10,162 11,791 (4,439 ) 1990 4/28/1999 7740 Milestone Parkway (O) Hanover, MD 18,987 3,825 34,176 404 3,825 34,580 38,405 (4,692 ) 2009 7/2/2007 7770 Backlick Road (O) Springfield, VA — 6,387 74,325 141 6,387 74,466 80,853 (5,292 ) 2012 3/10/2010 Initial Cost Gross Amounts Carried At Close of Period Property (Type) (1) Location Encumbrances (2) Land Building and Land Improvements Costs Capitalized Subsequent to Acquisition Land Building and Land Improvements Total (3)(4) Accumulated Depreciation (5) Year Built or Renovated Date Acquired (6) 7880 Milestone Parkway (O) Hanover, MD — 4,857 23,647 — 4,857 23,647 28,504 (215 ) 2015 (7) 9/17/2013 8003 Corporate Drive (O) (10) White Marsh, MD — 611 1,126 685 611 1,811 2,422 (99 ) 1999 1/9/2007 8007 Corporate Drive (O) (10) White Marsh, MD — 1,434 350 1,218 1,434 1,568 3,002 (278 ) 1995 1/9/2007 8010 Corporate Drive (O) (10) White Marsh, MD — 1,349 3,262 1,744 1,349 5,006 6,355 (1,448 ) 1998 1/9/2007 8013 Corporate Drive (O) (10) White Marsh, MD — 642 1,072 1,808 642 2,880 3,522 (519 ) 1990 1/9/2007 8015 Corporate Drive (O) (10) White Marsh, MD — 446 1,116 466 446 1,582 2,028 (468 ) 1990 1/9/2007 8019 Corporate Drive (O) (10) White Marsh, MD — 680 1,898 825 680 2,723 3,403 (827 ) 1990 1/9/2007 8020 Corporate Drive (O) (10) White Marsh, MD — 2,184 3,767 2,205 2,184 5,972 8,156 (1,703 ) 1997 1/9/2007 8023 Corporate Drive (O) (10) White Marsh, MD — 651 1,164 5 651 1,169 1,820 (47 ) 1990 1/9/2007 8094 Sandpiper Circle (O) (10) White Marsh, MD — 1,960 3,716 1,129 1,960 4,845 6,805 (1,267 ) 1998 1/9/2007 8098 Sandpiper Circle (O) (10) White Marsh, MD — 1,797 3,651 639 1,797 4,290 6,087 (929 ) 1998 1/9/2007 8110 Corporate Drive (O) (10) White Marsh, MD — 2,285 10,117 989 2,285 11,106 13,391 (3,133 ) 2001 1/9/2007 8140 Corporate Drive (O) (10) White Marsh, MD — 2,158 8,457 3,163 2,158 11,620 13,778 (4,117 ) 2003 1/9/2007 8621 Robert Fulton Drive (O) Columbia, MD — 2,317 12,642 344 2,317 12,986 15,303 (3,378 ) 2005-2006 6/10/2005 8661 Robert Fulton Drive (O) Columbia, MD — 1,510 3,764 2,420 1,510 6,184 7,694 (2,117 ) 2002 12/30/2003 8671 Robert Fulton Drive (O) Columbia, MD — 1,718 4,280 3,931 1,718 8,211 9,929 (3,060 ) 2002 12/30/2003 870 Elkridge Landing Road (O) Linthicum, MD — 2,003 9,442 7,616 2,003 17,058 19,061 (8,411 ) 1981 8/3/2001 891 Elkridge Landing Road (O) Linthicum, MD — 1,165 4,772 2,928 1,165 7,700 8,865 (3,488 ) 1984 7/2/2001 900 Elkridge Landing Road (O) Linthicum, MD — 1,993 7,972 3,603 1,993 11,575 13,568 (6,009 ) 1982 4/30/1998 901 Elkridge Landing Road (O) Linthicum, MD — 1,156 4,437 2,390 1,156 6,827 7,983 (3,015 ) 1984 7/2/2001 911 Elkridge Landing Road (O) Linthicum, MD — 1,215 4,861 2,024 1,215 6,885 8,100 (3,462 ) 1985 4/30/1998 921 Elkridge Landing Road (O) Linthicum, MD — 1,044 4,727 748 1,044 5,475 6,519 (4,924 ) 1983 (7) 4/30/1998 938 Elkridge Landing Road (O) Linthicum, MD — 922 4,748 1,263 922 6,011 6,933 (2,116 ) 1984 7/2/2001 939 Elkridge Landing Road (O) Linthicum, MD — 939 3,756 4,245 939 8,001 8,940 (3,337 ) 1983 4/30/1998 940 Elkridge Landing Road (L) Linthicum, MD — 842 4 — 842 4 846 — (8) 7/2/2001 9651 Hornbaker Road (D) Manassas, VA — 6,050 249,706 985 6,050 250,691 256,741 (18,119 ) 2010 9/14/2010 Aerotech Commerce (L) Colorado Springs, CO — 900 — — 900 — 900 — (8) 5/19/2006 Arborcrest (O) Blue Bell, PA — 21,968 122,759 1,301 21,968 124,060 146,028 (25,901 ) 1991-96/2015 10/14/1997 Arundel Preserve (L) Hanover, MD — 13,401 9,331 — 13,401 9,331 22,732 — (8) 7/2/2007 Ashburn Crossing - DC10 (O) Ashburn, VA — 4,408 10,689 — 4,408 10,689 15,097 (225 ) 2015 12/27/2012 Canton Crossing Land (L) Baltimore, MD — 16,085 2,134 — 16,085 2,134 18,219 — (8) 10/27/2009 Canton Crossing Util Distr Ctr (O) Baltimore, MD — 7,300 15,556 942 7,300 16,498 23,798 (3,326 ) 2006 10/27/2009 Columbia Gateway - Southridge (L) Columbia, MD — 6,387 3,539 — 6,387 3,539 9,926 — (8) 9/20/2004 Dahlgren Technology Center (L) Dahlgren, VA — 978 178 — 978 178 1,156 — (8) 3/16/2005 Expedition VII (L) Lexington Park, MD — 705 728 — 705 728 1,433 — (8) 3/24/2004 InterQuest (L) (10) Colorado Springs, CO — 8,201 — — 8,201 — 8,201 — (8) 9/28/2005 M Square Research Park (L) College Park, MD — — 2,986 — — 2,986 2,986 — (8) 1/29/2008 National Business Park North (L) Annapolis Junction, MD — 28,066 41,936 — 28,066 41,936 70,002 — (8) 6/29/2006 North Gate Business Park (L) Aberdeen, MD — 6,309 4,540 — 6,309 4,540 10,849 — (8) 9/14/2007 Initial Cost Gross Amounts Carried At Close of Period Property (Type) (1) Location Encumbrances (2) Land Building and Land Improvements Costs Capitalized Subsequent to Acquisition Land Building and Land Improvements Total (3)(4) Accumulated Depreciation (5) Year Built or Renovated Date Acquired (6) Northwest Crossroads (L) San Antonio, TX — 7,430 847 — 7,430 847 8,277 — (8) 1/20/2006 NOVA Office A (O) (9) Chantilly, VA — 2,096 47,364 — 2,096 47,364 49,460 (1,046 ) 2015 7/31/2002 NOVA Office B (O) (9) Chantilly, VA — 739 26,277 — 739 26,277 27,016 — (7) 7/31/2002 NOVA Office D (O) Chantilly, VA — 6,587 103 — 6,587 103 6,690 — (7) 7/31/2002 Old Annapolis Road (O) Columbia, MD — 1,637 5,500 3,457 1,637 8,957 10,594 (2,983 ) 1974/1985 12/14/2000 Patriot Park (L) Colorado Springs, CO — 2,328 1 — 2,328 1 2,329 — (8) 7/8/2005 Patriot Point - DC15 (O) Ashburn, VA — 12,156 7,112 — 12,156 7,112 19,268 — (7) 10/15/2015 Patriot Point - DC16 (O) Ashburn, VA — 12,156 4,110 — 12,156 4,110 16,266 — (7) 10/15/2015 Patriot Point - DC17 (O) Ashburn, VA — 6,078 472 — 6,078 472 6,550 — (7) 10/15/2015 Patriot Ridge (L) Springfield, VA — 18,517 14,423 — 18,517 14,423 32,940 — (8) 3/10/2010 Redstone Gateway (L) Huntsville, AL — — 14,250 — — 14,250 14,250 — (8) 3/23/2010 Route 15/Biggs Ford Road (L) Frederick, MD — 8,703 596 — 8,703 596 9,299 — (8) 8/28/2008 Sentry Gateway (L) San Antonio, TX — 8,275 3,645 — 8,275 3,645 11,920 — (8) 3/30/2005 Sentry Gateway - T (O) San Antonio, TX — 14,020 38,804 13 14,020 38,817 52,837 (8,618 ) 1982/1985 3/30/2005 Sentry Gateway - V (O) San Antonio, TX — — 1,066 — — 1,066 1,066 (188 ) 2007 3/30/2005 Sentry Gateway - W (O) San Antonio, TX — — 1,884 — — 1,884 1,884 (296 ) 2009 3/30/2005 Sentry Gateway - X (O) San Antonio, TX — 1,964 21,178 — 1,964 21,178 23,142 (2,728 ) 2010 1/20/2006 Sentry Gateway - Y (O) San Antonio, TX — 1,964 21,298 — 1,964 21,298 23,262 (2,745 ) 2010 1/20/2006 Sentry Gateway - Z (O) San Antonio, TX — 1,964 30,335 — 1,964 30,335 32,299 (616 ) 2015 6/14/2005 Southpoint Manassas - DC12 (O) Manassas, VA — 4,518 12,701 — 4,518 12,701 17,219 (127 ) 2015 2/6/2015 Southpoint Manassas - DC14 (O) Manassas, VA — 4,518 13,009 — 4,518 13,009 17,527 (101 ) 2015 2/6/2015 Westfields - Park Center (L) Chantilly, VA — 16,418 7,488 — 16,418 7,488 23,906 — (8) 7/18/2002 Westfields Corporate Center (L) Chantilly, VA — 7,141 1,423 — 7,141 1,423 8,564 — (8) 7/31/2002 White Marsh (L) White Marsh, MD — 7,852 2,570 — 7,852 2,570 10,422 — (8) 1/9/2007 Woodland Park (L) (10) Herndon, VA — 9,614 85 — 9,614 85 9,699 — (8) 4/29/2004 Other Developments, including intercompany eliminations (V) Various — 94 34 431 94 465 559 (39 ) Various Various $ 316,887 $ 709,953 $ 3,106,271 $ 342,392 $ 709,953 $ 3,448,663 $ 4,158,616 $ (718,680 ) (1) A legend for the Property Type follows: (O) = Office Property; (L) = Land held or pre-construction; (D) = Data Center; and (V) = Various. (2) Excludes our Revolving Credit Facility of $43.5 million , term loan facilities of $515.9 million , unsecured senior notes of $1.2 billion , unsecured notes payable of $ 1.5 million , a letter of credit on a mortgage loan of $14.8 million , and deferred financing costs, net of premiums, on the remaining loans of $668,000 . (3) The aggregate cost of these assets for Federal income tax purposes was approximately $3.5 billion at December 31, 2015 . (4) As discussed in Note 3 to our Consolidated Financial Statements, we recognized impairment losses of $23.5 million primarily in connection with certain of our land and operating properties, including $22.0 million related to land and operating properties still owned as of December 31, 2015. (5) The estimated lives over which depreciation is recognized follow: Building and land improvements: 10 - 40 years; and tenant improvements: related lease terms. (6) The acquisition date of multi-parcel properties reflects the date of the earliest parcel acquisition. (7) Under construction or redevelopment at December 31, 2015 . (8) Held or under pre-construction at December 31, 2015 . (9) The carrying amounts of these properties under construction exclude allocated costs of the garage being constructed to support the properties. (10) Classified as held for sale as of December 31, 2015. The following table summarizes our changes in cost of properties for the years ended December 31, 2015, 2014 and 2013 (in thousands): 2015 2014 2013 Beginning balance $ 4,014,336 $ 3,811,950 $ 3,859,960 Acquisitions of operating properties 194,616 — — Improvements and other additions 273,761 254,868 249,639 Sales (172,628 ) (48,466 ) (141,045 ) Impairments (29,548 ) (3,042 ) (45,931 ) Other dispositions (121,921 ) (974 ) (110,673 ) Ending balance $ 4,158,616 $ 4,014,336 $ 3,811,950 The following table summarizes our changes in accumulated depreciation for the same time periods (in thousands): 2015 2014 2013 Beginning balance $ 703,083 $ 597,649 $ 568,176 Depreciation expense 112,695 111,326 92,677 Sales (49,614 ) (3,129 ) (9,542 ) Impairments (6,092 ) (1,671 ) (14,863 ) Other dispositions (41,392 ) (1,092 ) (38,799 ) Ending balance $ 718,680 $ 703,083 $ 597,649

Summary of Significant Accoun33

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 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 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. We use the cost method of accounting when we own an interest in an entity and cannot exert significant influence over its operations.
Reclassification	Reclassification We reclassified certain amounts from prior periods to conform to the current period presentation of our consolidated financial statements, including amounts related to our adoption of guidance regarding the presentation of deferred debt issuance costs (discussed below) and our change in reportable segments (discussed in Note 17), with no effect on previously reported net income or equity.
Use of Estimates in the Preparation of Financial Statements	Use of Estimates in the Preparation of Financial Statements We make estimates and assumptions when preparing financial statements under generally accepted accounting principles (“GAAP”). These estimates and assumptions affect various matters, including: • the reported amounts of assets and liabilities in our consolidated balance sheets at the dates of the financial statements; • the disclosure of contingent assets and liabilities at the dates of the financial statements; and • the reported amounts of revenues and expenses in our consolidated statements of operations during the reporting periods. Significant estimates are inherent in the presentation of our financial statements in a number of areas, including the evaluation of the collectability of accounts and deferred rent receivable, the allocation of property acquisition costs, the determination of estimated useful lives of assets, the determination of lease terms, the evaluation of impairment of long-lived assets, the amount of impairment losses recognized, the amount of revenue recognized relating to tenant improvements, the level of expense recognized in connection with share-based compensation and the determination of accounting method for investments. Actual results could differ from these and other estimates.
Acquisitions of Properties	Acquisitions of Properties Upon completion of property acquisitions, we allocate the purchase price to tangible and intangible assets and liabilities associated with such acquisitions based on our estimates of their fair values. We determine these fair values by using market data and independent appraisals available to us and making numerous estimates and assumptions. We allocate property acquisitions to the following components: • properties based on a valuation performed under the assumption that the property is vacant upon acquisition (the “if-vacant value”). The if-vacant value is allocated between land and buildings or, in the case of properties under development, construction in progress. We also allocate additional amounts to properties for in-place tenant improvements based on our estimate of improvements per square foot provided under market leases that would be attributable to the remaining non-cancelable terms of the respective leases; • above- and below-market lease intangible assets or liabilities based on the present value (using an interest rate which reflects the risks associated with the leases acquired) of the difference between: (1) the contractual amounts to be received pursuant to the in-place leases; and (2) our estimate of fair market lease rates for the corresponding space, measured over a period equal to the remaining non-cancelable term of the lease. The capitalized above- and below-market lease values are amortized as adjustments to rental revenue over the remaining lease terms of the respective leases, and to renewal periods in the case of below-market leases; • in-place lease value based on our estimates of: (1) the present value of additional income to be realized as a result of leases being in place on the acquired properties; and (2) costs to execute similar leases. Our estimate of additional income to be realized includes carrying costs, such as real estate taxes, insurance and other operating expenses, and revenues during the expected lease-up periods considering current market conditions. Our estimate of costs to execute similar leases includes leasing commissions, legal and other related costs; • tenant relationship value based on our evaluation of the specific characteristics of each tenant's lease and our overall relationship with that respective tenant. Characteristics we consider in determining these values include the nature and extent of our existing business relationships with the tenant, growth prospects for developing new business with the tenant, the tenant's credit quality and expectations of lease renewals, among other factors; and • above- and below- market cost arrangements (such as real estate tax treaties or above- or below- market ground leases) based on the present value of the expected benefit from any such arrangements in place on the property at the time of acquisition.
Intangible Assets and Deferred Revenue on Real Estate Acquisitions	Intangible Assets and Deferred Revenue on Real Estate Acquisitions We amortize the intangible assets and deferred revenue on real estate acquisitions discussed above as follows: Asset Type Amortization Period Above- and below-market leases Related lease terms In-place lease value Related lease terms Tenant relationship value Estimated period of time that tenant will lease space in property Above- and below-market cost arrangements Term of arrangements We recognize the amortization of acquired above-market and below-market leases as adjustments to rental revenue. We recognize the amortization of above- and below- market cost arrangements as adjustments to property operating expenses. We recognize the amortization of other intangible assets on property acquisitions as amortization expense.
Properties	Properties We report properties to be developed or held and used in operations at our depreciated cost, reduced for impairment losses. The preconstruction stage of the development or redevelopment of an operating property includes efforts and related costs to secure land control and zoning, evaluate feasibility and complete other initial tasks which are essential to development. We capitalize interest expense, real estate taxes and direct and indirect project costs (including related compensation and other indirect costs) associated with properties, or portions thereof, undergoing construction, development and redevelopment activities. In capitalizing interest expense, if there is a specific borrowing for the property undergoing construction, development and redevelopment activities, we apply the interest rate of that borrowing to the average accumulated expenditures that do not exceed such borrowing; for the portion of expenditures exceeding any such specific borrowing, we apply our weighted average interest rate on other borrowings to the expenditures. We continue to capitalize costs while construction, development or redevelopment activities are underway until a property becomes “operational,” which occurs when lease terms commence (generally when the tenant has control of the leased space and we have delivered the premises to the tenant as required under the terms of such lease), but no later than one year after the cessation of major construction activities. When leases commence on portions of a newly-constructed or redeveloped property in the period prior to one year from the cessation of major construction activities, we consider that property to be “partially operational.” When a property is partially operational, we allocate the costs associated with the property between the portion that is operational and the portion under construction. We start depreciating newly-constructed and redeveloped properties as they become operational. Most of our leases involve some form of improvements to leased space. When we are required to provide improvements under the terms of a lease, we determine whether the improvements constitute landlord assets or tenant assets. If the improvements are landlord assets, we capitalize the cost of the improvements and recognize depreciation expense associated with such improvements over the shorter of the useful life of the assets or the term of the lease and recognize any payments from the tenant as rental revenue over the term of the lease. If the improvements are tenant assets, we defer the cost of improvements funded by us as a lease incentive asset and amortize it as a reduction of rental revenue over the term of the lease. In determining whether improvements constitute landlord or tenant assets, we consider numerous factors, including: whether the improvements are unique to the tenant or reusable by other tenants; whether the tenant is permitted to alter or remove the improvements without our consent or without compensating us for any lost fair value; whether the ownership of the improvements remains with us or remains with the tenant at the end of the lease term; and whether the economic substance of the lease terms is properly reflected. We depreciate our fixed assets using the straight-line method over their estimated useful lives as follows: Estimated Useful Lives Buildings and building improvements 10-40 years Land improvements 10-20 years Tenant improvements on operating properties Related lease term Equipment and personal property 3-10 years We assess each of our operating properties for impairment quarterly using cash flow projections and estimated fair values that we derive for each of the properties. We update the leasing and other assumptions used in these projections regularly, paying particular attention to properties that have experienced chronic vacancy or face significant market challenges. We review our plans and intentions for our development projects and land parcels quarterly. If our analyses indicate that the carrying values of operating properties, properties in development or land held for future development may be impaired, we perform a recovery analysis for such properties. For long-lived assets to be held and used, we analyze recoverability based on the estimated undiscounted future cash flows expected to be generated from the operations and eventual disposition of the assets over, in most cases, a ten -year holding period. If we believe there is a significant possibility that we might dispose of the assets earlier, we analyze recoverability using a probability weighted analysis of the estimated undiscounted future cash flows expected to be generated from the operations and eventual disposition of the assets over the various possible holding periods. If the recovery analysis indicates that the carrying value of a tested property is not recoverable from estimated future cash flows, it is written down to its estimated fair value and an impairment loss is recognized. If and when our plans change, we revise our recoverability analyses to use the cash flows expected from the operations and eventual disposition of each asset using holding periods that are consistent with our revised plans. Changes in holding periods may require us to recognize significant impairment losses. Fair values are estimated based on contract prices, indicative bids, discounted cash flow analyses, yield analyses or sales comparison approach. Estimated cash flows used in such analyses are based on our plans for the property and our views of market and economic conditions. The estimates consider factors such as current and future rental rates, occupancies for the tested property and comparable properties, estimated operating and capital expenditures and recent sales data for comparable properties; most of these factors are influenced by market data obtained from real estate leasing and brokerage firms and our direct experience with the properties and their markets. When we determine that a property is held for sale, we stop depreciating the property and estimate the property’s fair value, net of selling costs; if we then determine that the estimated fair value, net of selling costs, is less than the net book value of the property, we recognize an impairment loss equal to the difference and reduce the net book value of the property. For periods in which a property is classified as held for sale, we classify the assets of the property as held for sale on our consolidated balance sheet for such periods. For dispositions of operating properties occurring prior to June 30, 2014 in which we had no significant continuing involvement, or for operating properties held for sale prior to June 30, 2014, we classify the results of operations for such properties as discontinued operations; interest expense that is specifically identifiable to properties included in discontinued operations is used in the computation of interest expense attributable to discontinued operations. We adopted guidance issued by the Financial Accounting Standards Board (“FASB”) related to the reporting of discontinued operations and disclosures of disposals of components of an entity effective for the quarterly period ended June 30, 2014. This guidance defines a discontinued operation as a component or group of components disposed or classified as held for sale that represents a strategic shift that has (or will have) a major effect on an entity’s operations and financial results; the guidance states that a strategic shift could include a disposal of a major geographical area of operations, a major line of business, a major equity method investment or other major parts of an entity. We have had no properties newly classified as discontinued operations subsequent to our adoption of this guidance.
Sale of Interests in Real Estate	Sales of Interests in Real Estate We recognize gains from sales of interests in real estate using the full accrual method, provided that various criteria relating to the terms of sale and any subsequent involvement by us with the real estate sold are met.
Cash and Cash Equivalents	Cash and Cash Equivalents Cash and cash equivalents include all cash and liquid investments that mature three months or less from when they are purchased. Cash equivalents are reported at cost, which approximates fair value. We maintain our cash in bank accounts in amounts that may exceed Federally insured limits at times. We have not experienced any losses in these accounts in the past and believe that we are not exposed to significant credit risk because our accounts are deposited with major financial institutions.
Investments in Marketable Securities	Investments in Marketable Securities We classify marketable securities as trading securities when we have the intent to sell such securities in the near term, and classify other marketable securities as available-for-sale securities. We determine the appropriate classification of investments in marketable securities at the acquisition date and re-evaluate the classification at each balance sheet date. We report investments in marketable securities classified as trading securities at fair value, with unrealized gains and losses recognized through earnings; on our consolidated statements of cash flows, we classify cash flows from these securities as operating activities.
Accounts and Deferred Rents Receivable and Investing Receivables	Accounts and Deferred Rents Receivable and Investing Receivables We maintain allowances for estimated losses resulting from the failure of our customers or borrowers to satisfy their payment obligations. We use judgment in estimating these allowances based primarily upon the payment history and credit status of the entities associated with the individual receivables. We write off these receivables when we believe the facts and circumstances indicate that continued pursuit of collection is no longer warranted. When cash is received in connection with receivables for which we have established allowances, we reduce the amount of losses previously recognized. We evaluate the collectibility of both interest and principal of loans whenever events or changes in circumstances indicate such amounts may not be recoverable. A loan is impaired when it is probable that we will be unable to collect all amounts due according to the existing contractual terms. When a loan is impaired, the amount of the loss accrual is calculated by comparing the carrying amount of the investment to the present value of expected future cash flows discounted at the loan’s effective interest rate and the value of any collateral under such loan. Interest on impaired loans is recognized when received in cash.
Deferred Leasing and Financing Costs, Net	Deferred Leasing and Financing Costs We defer costs incurred to obtain new tenant leases or extend existing tenant leases, including related compensation costs. We amortize these costs evenly over the lease terms. When tenant leases are terminated early, we expense any unamortized deferred leasing costs associated with those leases over the shortened term of the lease. We defer costs of financing arrangements and recognize these costs as interest expense over the related loan terms on a straight-line basis, which approximates the amortization that would occur under the effective interest method of amortization. We expense any unamortized loan costs when loans are retired early. In April 2015, the FASB issued guidance that changes the presentation of deferred 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. This guidance was further updated in August 2015 with respect to debt issuance costs of line-of-credit arrangements to note that it will be permissible for an entity to defer and present debt issuance costs as an asset and subsequently amortize the deferred debt issuance costs ratably over the term of a line-of-credit arrangement, regardless of whether there are any outstanding borrowings on the line-of-credit arrangement. We adopted this guidance effective for the quarterly period ended December 31, 2015 retrospectively to each prior period presented. The application of this guidance changed the balance sheet classification of most of our deferred financing costs but did not otherwise affect our consolidated financial statements. For costs of such arrangements attributable to line-of-credit arrangements and interest rate derivatives, we present such costs in the balance sheet in the line entitled prepaid and other assets.
Noncontrolling Interests	Noncontrolling Interests COPT’s consolidated noncontrolling interests are comprised of interests in COPLP not owned by COPT (discussed further in Note 14) and consolidated real estate joint ventures (discussed further in Note 6). COPLP’s consolidated noncontrolling interests are comprised primarily of interests in our consolidated real estate joint ventures. Also included in COPLP’s consolidated noncontrolling interests are interests in several real estate entities owned directly by COPT, or a wholly owned subsidiary of COPT, that generally do not exceed 1% of interests in such entities. We evaluate whether noncontrolling interests are subject to redemption features outside of our control. For noncontrolling interests that are currently redeemable for cash at the option of the holders of such interests or deemed probable to eventually become redeemable, we classify such interests as redeemable noncontrolling interests in the mezzanine section of our consolidated balance sheets; we adjust these interests each period to the greater of their fair value or carrying amount (initial amount as adjusted for allocations of income and losses and contributions and distributions), with a corresponding offset to additional paid-in capital on COPT’s consolidated balance sheets or common units on COPLP’s balance sheet, and only recognize reductions in such interests to the extent of their carrying amount. Our other noncontrolling interests are reported in the equity section of our consolidated balance sheets. The amounts reported for noncontrolling interests on our consolidated statements of operations represent the portion of these entities’ income or losses not attributable to us.
Revenue Recognition	Revenue Recognition We recognize minimum rents, net of abatements, on a straight-line basis over the term of tenant leases. A lease term generally commences when: (1) the tenant has control of the leased space (legal right to use the property); and (2) we have delivered the premises to the tenant as required under the terms of such lease. The term of a lease generally includes periods when a tenant: (1) may not terminate its lease obligation early; (2) may terminate its lease obligation early in exchange for a fee or penalty that we consider material enough such that termination would not be probable; (3) possesses renewal rights and the tenant’s failure to exercise such rights imposes a penalty on the tenant material enough such that renewal appears reasonably assured; or (4) possesses bargain renewal options for such periods. We report the amount by which our minimum rental revenue recognized on a straight-line basis under leases exceeds the contractual rent billings associated with such leases as deferred rent receivable on our consolidated balance sheets. Amounts by which our minimum rental revenue recognized on a straight-line basis under leases are less than the contractual rent billings associated with such leases are included in deferred revenue associated with operating leases on our consolidated balance sheets. In connection with a tenant’s entry into, or modification of, a lease, if we make cash payments to, or on behalf of, the tenant for purposes other than funding the construction of landlord assets, we defer the amount of such payments as lease incentives. As discussed above, when we are required to provide improvements under the terms of a lease, we determine whether the improvements constitute landlord assets or tenant assets; if the improvements are tenant assets, we defer the cost of improvements funded by us as a lease incentive asset. We amortize lease incentives as a reduction of rental revenue over the term of the lease. We recognize tenant recovery revenue in the same periods in which we incur the related expenses. Tenant recovery revenue includes payments from tenants as reimbursement for property taxes, utilities and other property operating expenses. We recognize fees received for lease terminations as revenue and write off against such revenue any (1) deferred rents receivable, and (2) deferred revenue, lease incentives and intangible assets that are amortizable into rental revenue associated with the leases; the resulting net amount is the net revenue from the early termination of the leases. When a tenant's lease for space in a property is terminated early but the tenant continues to lease such space under a new or modified lease in the property, the net revenue from the early termination of the lease is recognized evenly over the remaining life of the new or modified lease in place on that property. We recognize fees for services provided by us once services are rendered, fees are determinable and collectability is assured. We recognize revenue under construction contracts using the percentage of completion method when the revenue and costs for such contracts can be estimated with reasonable accuracy; when these criteria do not apply to a contract, we recognize revenue on that contract using the completed contract method. Under the percentage of completion method, we recognize a percentage of the total estimated revenue on a contract based on the cost of services provided on the contract as of a point in time relative to the total estimated costs on the contract.
Interest Rate Derivatives	Interest Rate Derivatives Our primary objectives in using interest rate derivatives are to add stability to interest expense and to manage exposure to interest rate movements. To accomplish this objective, we primarily use interest rate swaps as part of our interest rate risk management strategy. Interest rate swaps designated as cash flow hedges involve the receipt of variable-rate amounts from a counterparty in exchange for our making fixed-rate payments over the life of the agreements without exchange of the underlying notional amount. Derivatives are used to hedge the cash flows associated with interest rates on existing debt as well as future debt. We recognize all derivatives as assets or liabilities on our consolidated balance sheet at fair value. We defer the effective portion of changes in fair value of the designated cash flow hedges to AOCI and reclassify such deferrals to interest expense as interest expense is recognized on the hedged forecasted transactions. We recognize the ineffective portion of the change in fair value of interest rate derivatives directly in interest expense. When an interest rate swap designated as a cash flow hedge no longer qualifies for hedge accounting, we recognize changes in fair value of the hedge previously deferred to AOCI, along with any changes in fair value occurring thereafter, through earnings. We do not use interest rate derivatives for trading or speculative purposes. We manage counter-party risk by only entering into contracts with major financial institutions based upon their credit ratings and other risk factors. We use standard market conventions and techniques such as discounted cash flow analysis, option pricing models, replacement cost and termination cost in computing the fair value of derivatives at each balance sheet date. We made an accounting policy election to use an exception provided for in the applicable accounting guidance with respect to measuring counterparty credit risk for derivative instruments; this election enables us to measure the fair value of groups of assets and liabilities associated with derivative instruments consistently with how market participants would price the net risk exposure as of the measurement date. Please refer to Note 11 for additional information pertaining to interest rate derivatives.
Expense Classification	Expense Classification We classify as property operations expense costs incurred for property taxes, ground rents, utilities, property management, insurance, repairs, exterior and interior maintenance and tenant revenue collection losses, as well as associated labor and indirect costs attributable to these costs. We classify as general and administrative and leasing expenses costs incurred for corporate-level management, public company administration, asset management, leasing, investor relations, marketing and corporate-level insurance (including general business, director and officers and key man life) and leasing prospects, as well as associated labor and indirect costs attributable to these costs.
Share-Based Compensation	Share-Based Compensation We issue three forms of share-based compensation: restricted COPT common shares (“restricted shares”), deferred share awards and performance share units (“PSUs”). We also issued options to purchase COPT common shares (“options”) in prior years. We account for share-based compensation in accordance with authoritative guidance provided by the FASB that establishes standards for the accounting for transactions in which an entity exchanges its equity instruments for goods or services, focusing primarily on accounting for transactions in which an entity obtains employee services in share-based payment transactions. The guidance requires us to measure the cost of employee services received in exchange for an award of equity instruments based generally on the fair value of the award on the grant date; such cost is then recognized over the period during which the employee is required to provide service in exchange for the award. No compensation cost is recognized for equity instruments for which employees do not render the requisite service. The guidance also requires that share-based compensation be computed based on awards that are ultimately expected to vest; as a result, future forfeitures of awards are estimated at the time of grant and revised, if necessary, in subsequent periods if actual forfeitures differ from those estimates. If an award is voluntarily cancelled by an employee, we recognize the previously unrecognized cost associated with the original award on the date of such cancellation. We capitalize costs associated with share-based compensation attributable to employees engaged in construction and development activities. When we adopted the authoritative guidance on accounting for share-based compensation, we elected to adopt the alternative transition method for calculating the tax effects of share-based compensation. This method enabled us to use a simplified method to establishing the beginning balance of the additional paid-in capital pool related to the tax effects of employee share-based compensation that was available to absorb tax deficiencies recognized subsequent to the adoption of this guidance. We compute the fair value of restricted shares and deferred share awards based on the fair value of COPT common shares on the grant date. We compute the fair value of options using the Black-Scholes option-pricing model. Under that model, the risk-free interest rate is based on the U.S. Treasury yield curve in effect at the time of grant. The expected option life is based on our historical experience of employee exercise behavior. Expected volatility is based on historical volatility of COPT common shares. Expected dividend yield is based on the average historical dividend yield on our common shares over a period of time ending on the grant date of the options. We compute the fair value of PSUs using a Monte Carlo model. Under that model, the baseline common share value is based on the market value on the grant date. The risk-free interest rate is based on the U.S. Treasury yield curve in effect at the time of grant. Expected volatility is based on historical volatility of COPT’s common shares.
Income Taxes	Income Taxes COPT elected to be treated as a REIT under Sections 856 through 860 of the Internal Revenue Code. To qualify as a REIT, COPT must meet a number of organizational and operational requirements, including a requirement that it distribute at least 90% of the Company’s adjusted taxable income to its shareholders. As a REIT, COPT generally will not be subject to Federal income tax on taxable income that it distributes to its shareholders. If COPT fails to qualify as a REIT in any tax year, it will be subject to Federal income tax on its taxable income at regular corporate rates and may not be able to qualify as a REIT for four subsequent tax years. COPLP is a limited partnership and is not subject to federal income tax. Its partners are required to report their respective share of the Operating Partnership’s taxable income on their respective tax returns. COPT’s share of the Operating Partnership’s taxable income is reported on COPT’s income tax return. For Federal income tax purposes, dividends to shareholders may be characterized as ordinary income, capital gains or return of capital. The characterization of dividends paid on COPT’s common and preferred shares during each of the last three years was as follows: Common Shares Preferred Shares For the Years Ended December 31, For the Years Ended December 31, 2015 2014 2013 2015 2014 2013 Ordinary income 38.3 % 64.5 % 71.8 % 38.3 % 90.9 % 76.2 % Long-term capital gain 61.7 % 6.5 % 22.4 % 61.7 % 9.1 % 23.8 % Return of capital 0.0 % 29.0 % 5.8 % 0.0 % 0.0 % 0.0 % In addition, dividends paid on January 15, 2016 (with a record date of December 31, 2015) on COPT’s common and preferred shares were allocated to 2015 for Federal income tax purposes and characterized based on the percentages set forth above for 2015. We distributed all of COPT’s REIT taxable income in 2015, 2014 and 2013 and, as a result, did not incur Federal income tax in those years. The net basis of our consolidated assets and liabilities for tax reporting purposes was approximately $224 million lower than the amount reported on our consolidated balance sheet at December 31, 2015, which was primarily related to differences in basis for net properties, intangible assets on property acquisitions and deferred rent receivable. We are subject to certain state and local income and franchise taxes. The expense associated with these state and local taxes is included in general and administrative expense and property operating expenses on our consolidated statements of operations. We did not separately state these amounts on our consolidated statements of operations because they are insignificant.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In May 2014, the FASB issued guidance regarding the recognition of revenue from contracts with customers. Under this guidance, an entity will recognize revenue to depict the transfer of goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. Additionally, this guidance requires improved disclosures regarding the nature, amount, timing and uncertainty of revenue and cash flows arising from contracts with customers. We are required to adopt this guidance for our annual and interim periods beginning January 1, 2018 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. As discussed further above, we adopted guidance issued by the FASB effective for the quarterly period ended December 31, 2015 retrospectively to each prior period presented that requires an entity to present deferred debt issuance costs (other than costs of line-of-credit arrangements) in the balance sheet as a direct deduction from the related debt liability rather than as an asset. In September 2015, the FASB issued guidance that eliminates the requirement to restate prior period financial statements for measurement period adjustments following a business combination. The guidance requires that the cumulative impact of a measurement period adjustment (including the impact on prior periods) be recognized in the reporting period in which the adjustment is identified. The prior period impact of the adjustment should be either presented separately on the face of the statement of operations or disclosed in the notes. This guidance is effective for annual reporting periods beginning after December 15, 2015. This guidance will be applied prospectively for measurement period adjustments that occur after the effective date. We expect that the application of this guidance will not materially affect our consolidated financial statements.

Summary of Significant Accoun34

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2015
Accounting Policies [Abstract]
Schedule of amortization of intangible assets and deferred revenue	We amortize the intangible assets and deferred revenue on real estate acquisitions discussed above as follows: Asset Type Amortization Period Above- and below-market leases Related lease terms In-place lease value Related lease terms Tenant relationship value Estimated period of time that tenant will lease space in property Above- and below-market cost arrangements Term of arrangements
Schedule of the estimated useful lives of fixed assets	We depreciate our fixed assets using the straight-line method over their estimated useful lives as follows: Estimated Useful Lives Buildings and building improvements 10-40 years Land improvements 10-20 years Tenant improvements on operating properties Related lease term Equipment and personal property 3-10 years
Schedule of characterization of dividends declared on common and preferred shares	The characterization of dividends paid on COPT’s common and preferred shares during each of the last three years was as follows: Common Shares Preferred Shares For the Years Ended December 31, For the Years Ended December 31, 2015 2014 2013 2015 2014 2013 Ordinary income 38.3 % 64.5 % 71.8 % 38.3 % 90.9 % 76.2 % Long-term capital gain 61.7 % 6.5 % 22.4 % 61.7 % 9.1 % 23.8 % Return of capital 0.0 % 29.0 % 5.8 % 0.0 % 0.0 % 0.0 %

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2015
Fair Value Disclosures [Abstract]
Schedule of fair value assets and liabilities measured on recurring basis	The tables below set forth financial assets and liabilities of COPT and its subsidiaries that are accounted for at fair value on a recurring basis as of December 31, 2015 and 2014 and the hierarchy level of inputs used in measuring their respective fair values under applicable accounting standards (in thousands): Description Quoted Prices in Significant Other Significant Total December 31, 2015: Assets: Marketable securities in deferred compensation plan (1) Mutual funds $ 5,658 $ — $ — $ 5,658 Other 105 — — 105 Interest rate derivatives (2) — 53 — 53 Total assets $ 5,763 $ 53 $ — $ 5,816 Liabilities: Deferred compensation plan liability (3) $ — $ 5,763 $ — $ 5,763 Interest rate derivatives — 3,160 — 3,160 Total liabilities $ — $ 8,923 $ — $ 8,923 December 31, 2014: Assets: Marketable securities in deferred compensation plan (1) Mutual funds $ 5,756 $ — $ — $ 5,756 Other 126 — — 126 Interest rate derivatives (2) — 274 — 274 Total assets $ 5,882 $ 274 $ — $ 6,156 Liabilities: Deferred compensation plan liability (3) $ — $ 5,882 $ — $ 5,882 Interest rate derivatives — 1,855 — 1,855 Total liabilities $ — $ 7,737 $ — $ 7,737 (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 tables below set forth financial assets and liabilities of COPLP and its subsidiaries that are accounted for at fair value on a recurring basis as of December 31, 2015 and 2014 and the hierarchy level of inputs used in measuring their respective fair values under applicable accounting standards (in thousands): Description Quoted Prices in Significant Other Significant Total December 31, 2015: Assets: Interest rate derivatives (1) $ — $ 53 $ — $ 53 Total Assets $ — $ 53 $ — $ 53 Liabilities: Interest rate derivatives $ — $ 3,160 $ — $ 3,160 December 31, 2014: Assets: Interest rate derivatives (1) $ — $ 274 $ — $ 274 Total Assets $ — $ 274 $ — $ 274 Liabilities: Interest rate derivatives $ — $ 1,855 $ — $ 1,855 (1) Included in the line entitled “prepaid expenses and other assets” on COPLP ’ s consolidated balance sheet.
Schedule of fair value hierarchy of impaired properties and other assets associated with such properties	The table below sets forth the fair value hierarchy of the valuation technique we used to determine nonrecurring fair measurements of assets as of December 31, 2015 (dollars in thousands): Fair Values as of December 31, 2015 Quoted Prices in Significant Active Markets for Significant Other Unobservable Identical Assets Observable Inputs Inputs Description (Level 1) (Level 2) (Level 3) Total Assets: Total properties, net (1) $ — $ — $ 13,850 $ 13,850 Assets held for sale, net (2) — — 21,423 21,423 (1) Represents estimated fair values. (2) Represents estimated fair values less costs to sell.
Schedule of quantitative information about significant unobservable inputs used for Level 3 fair value measurements	The table below sets forth quantitative information about significant unobservable inputs used for the Level 3 fair value measurements reported above as of December 31, 2015 (dollars in thousands): Valuation Technique Fair Values on Measurement Date Unobservable Input Range (Weighted Average) (1) Bids for property indicative of value $ 24,423 Indicative bids N/A Comparable sales analysis 10,850 Comparable sales prices N/A (1) Only one value applied for these unobservable inputs.

Properties, net (Tables)

Properties, net (Tables)	12 Months Ended
	Dec. 31, 2015
Real Estate [Abstract]
Schedule of operating properties, net	Operating properties, net consisted of the following (in thousands): December 31, 2015 2014 Land $ 463,305 $ 439,355 Buildings and improvements 3,157,587 3,015,216 Less: accumulated depreciation (700,363 ) (703,083 ) Operating properties, net $ 2,920,529 $ 2,751,488
Schedule of projects in development or held for future development	Projects we had in development or held for future development consisted of the following (in thousands): December 31, 2015 2014 Land $ 207,774 $ 214,977 Development in progress, excluding land 221,445 330,449 Projects in development or held for future development $ 429,219 $ 545,426
Components of assets held for sale	The table below sets forth the components of assets held for sale on our consolidated balance sheet for these properties (in thousands): December 31, 2015 Properties, net $ 90,188 Deferred rent receivable 2,891 Intangible assets on real estate acquisitions, net 1,591 Deferred leasing costs, net 1,391 Lease incentives, net 721 Assets held for sale, net $ 96,782
Schedule of allocation of acquisition costs	The table below sets forth the allocation of the aggregate purchase price of these properties to the value of the acquired assets and liabilities (in thousands): Land, operating properties $ 55,076 Building and improvements 139,540 Intangible assets on real estate acquisitions 75,729 Total assets 270,345 Below-market leases (6,808 ) Total acquisition cost $ 263,537
Schedule of intangible assets recorded in connection with acquisition	Intangible assets recorded in connection with these acquisitions included the following (dollars in thousands): Weighted Average Amortization Period (in Years) Tenant relationship value $ 31,183 12 In-place lease value 35,139 7 Above-market leases 6,720 4 Below-market cost arrangements 2,687 40 $ 75,729 10
Pro Forma information	The pro forma financial information was prepared for comparative purposes only and is not necessarily indicative of what would have occurred had these acquisitions been made at that time or of results which may occur in the future (in thousands, except per shares amounts). For the Year Ended December 31, 2015 2014 (Unaudited) Pro forma total revenues $ 641,982 $ 623,013 Pro forma net income attributable to COPT common shareholders $ 167,079 $ 20,796 Pro forma EPS: Basic $ 1.77 $ 0.23 Diluted $ 1.77 $ 0.23
Schedule of operating property dispositions	In 2014, we completed dispositions of the following operating properties (dollars in thousands): Project Name City, State Segment Date of Sale Number of Buildings Total Rentable Square Feet Transaction Value Gain on Disposition 4969 and 4979 Mercantile Road White Marsh, MD Regional Office 7/14/2014 2 96,721 $ 5,960 $ 2,124 9930 and 9940 Franklin Square White Marsh, MD Regional Office 7/30/2014 2 71,992 10,475 2,303 5020, 5022, 5024 and 5026 Campbell Boulevard White Marsh, MD Regional Office 8/4/2014 4 134,245 12,400 666 8 302,958 $ 28,835 $ 5,093 In 2013, we completed dispositions of the following operating properties (dollars in thousands): Project Name City, State Segment Date of Sale Number of Buildings Total Rentable Square Feet Transaction Value (1) Gain on Disposition 920 Elkridge Landing Road Linthicum, MD Fort Meade/BW Corridor 6/25/2013 1 103,000 $ 6,900 $ — 4230 Forbes Boulevard Lanham, MD Regional Office 12/11/2013 1 56,000 5,600 1,507 December 2013 Colorado Springs Portfolio Disposition Colorado Springs, CO Colorado Defense/IT and Regional Office 12/12/2013 15 1,165,000 133,925 1,164 December 2013 Portfolio Conveyance Colorado Springs, CO and Linthicum, MD Various (2) 12/23/2013 14 1,021,000 146,876 — 31 2,345,000 $ 293,301 $ 2,671 (1) Each of these properties were sold except for the December 2013 Portfolio Conveyance, the disposition of which was completed in connection with a debt extinguishment, as described further below. In 2015, we completed dispositions of the following operating properties (dollars in thousands): Project Name City, State Segment Date of Disposition Number of Buildings Total Rentable Square Feet Transaction Value (1) Gain on Disposition 1550 Westbranch Drive McLean, VA Regional Office 7/27/2015 1 160,000 $ 27,800 $ — 15000 and 15010 Conference Center Drive Chantilly, VA Northern VA Defense/IT 8/28/2015 2 665,000 167,335 — 13200 Woodland Park Road Herndon, VA Regional Office 10/27/2015 1 397,000 84,000 42,515 9900, 9910 and 9920 Franklin Square Drive White Marsh, MD Regional Office 11/9/2015 3 135,000 24,150 6,468 9690 Deereco Road and 375 W. Padonia Road Timonium, MD Regional Office 12/17/2015 2 240,000 44,500 15,050 9 1,597,000 $ 347,785 $ 64,033 (1) Each of these properties were sold except for 15000 and 15010 Conference Center Drive, the disposition of which was completed in connection with a debt extinguishment, as discussed further below.

Real Estate Joint Ventures (Tab

Real Estate Joint Ventures (Tables)	12 Months Ended
	Dec. 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 December 31, 2015 (dollars in thousands): Nominal Ownership December 31, 2015 (1) Date % as of Total Encumbered Total Acquired 12/31/2015 Nature of Activity Assets Assets Liabilities LW Redstone Company, LLC 3/23/2010 85% Development and operation of real estate (2) $ 147,612 $ 82,721 $ 54,022 M Square Associates, LLC 6/26/2007 50% Development and operation of real estate (3) 56,455 48,245 37,568 Stevens Investors, LLC 8/11/2015 95% Development of real estate (4) 5,284 — 393 $ 209,351 $ 130,966 $ 91,983 (1) Excludes amounts eliminated in consolidation. (2) This joint venture’s property is in Huntsville, Alabama. (3) This joint venture’s properties are in College Park, Maryland. (4) This joint venture’s property is in Washington, DC.

Intangible Assets on Real Est38

Intangible Assets on Real Estate Acquisitions (Tables)	12 Months Ended
	Dec. 31, 2015
Intangible Assets on Real Estate Acquisitions
Schedule of intangible assets on real estate acquisitions	Intangible assets on real estate acquisitions consisted of the following (in thousands): December 31, 2015 December 31, 2014 Gross Carrying Amount Accumulated Amortization Net Carrying Amount Gross Carrying Amount Accumulated Amortization Net Carrying Amount In-place lease value $ 134,664 $ 89,540 $ 45,124 $ 123,759 $ 101,040 $ 22,719 Tenant relationship value 62,172 23,468 38,704 42,301 28,492 13,809 Below-market cost arrangements 15,102 6,692 8,410 12,415 5,984 6,431 Above-market leases 14,210 8,491 5,719 8,659 8,159 500 Other 1,333 952 381 1,333 938 395 $ 227,481 $ 129,143 $ 98,338 $ 188,467 $ 144,613 $ 43,854

Investing Receivables (Tables)

Investing Receivables (Tables)	12 Months Ended
	Dec. 31, 2015
Receivables [Abstract]
Schedule of investing receivables	Investing receivables, including accrued interest thereon, consisted of the following (in thousands): December 31, 2015 2014 Notes receivable from City of Huntsville $ 44,875 $ 49,147 Other investing loans receivable 3,000 3,000 $ 47,875 $ 52,147

Prepaid Expenses and Other As40

Prepaid Expenses and Other Assets, Net (Tables)	12 Months Ended
	Dec. 31, 2015
Prepaid Expense and Other Assets [Abstract]
Schedule of prepaid expenses and other assets	Prepaid expenses and other assets, net consisted of the following (in thousands): December 31, 2015 2014 Prepaid expenses $ 23,009 $ 20,570 Lease incentives 11,133 13,344 Furniture, fixtures and equipment, net 6,004 6,637 Deferred financing costs, net (1) 5,867 4,849 Deferred tax asset, net 3,467 4,002 Construction contract costs incurred in excess of billings 3,261 6,656 Equity method investments 1,636 2,368 Other assets 5,647 2,875 Prepaid expenses and other assets, net $ 60,024 $ 61,301 (1) Represents deferred costs, net of accumulated amortization, attributable to our Revolving Credit Facility and interest rate derivatives.
Schedule of tax effects of temporary differences and carry forwards in net deferred tax assets	Deferred tax asset, net reported above includes the following tax effects of temporary differences and carry forwards of our TRS (in thousands): December 31, 2015 2014 Operating loss carry forward $ 5,065 $ 5,012 Share-based compensation 363 976 Accrued payroll 133 195 Property (32 ) (119 ) Valuation allowance (2,062 ) (2,062 ) Deferred tax asset, net $ 3,467 $ 4,002

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2015
Debt Disclosure [Abstract]
Schedule of debt	Our debt consisted of the following (dollars in thousands): Carrying Value (1) as of December 31, December 31, Stated Interest Rates as of Scheduled Maturity as of December 31, 2015 December 31, 2015 Mortgage and Other Secured Loans: Fixed rate mortgage loans (2) $ 281,208 $ 385,769 3.96% - 7.87% (3) 2016-2024 Variable rate secured loans 49,792 36,698 LIBOR + 1.85% - 2.00% (4) 2016-2020 Total mortgage and other secured loans 331,000 422,467 Revolving Credit Facility (5) 43,500 83,000 LIBOR + 0.875% to 1.60% May 2019 Term Loan Facilities (6) 515,902 517,846 LIBOR + 0.90% to 2.60% (7) 2019-2022 Unsecured Senior Notes (5) 3.600%, $350,000 aggregate principal 346,714 346,311 3.60% (8) May 2023 5.250%, $250,000 aggregate principal 245,731 245,306 5.25% (9) February 2024 3.700%, $300,000 aggregate principal 297,378 296,927 3.70% (10) June 2021 5.000%, $300,000 aggregate principal 296,019 — 5.00% (10) July 2025 Unsecured notes payable 1,508 1,607 0% (11) 2026 4.25% Exchangeable Senior Notes (12) — 572 N/A N/A Total debt $ 2,077,752 $ 1,914,036 (1) The carrying values of our loans other than the Revolving Credit Facility reflect net deferred financing costs of $8.0 million as of December 31, 2015 and $6.0 million as of December 31, 2014. (2) Several of the fixed rate mortgages carry interest rates that were above or below market rates upon assumption and therefore were recorded at their fair value based on applicable effective interest rates. The carrying values of these loans reflect net unamortized premiums totaling $514,000 as of December 31, 2015 and $42,000 as of December 31, 2014 . (3) The weighted average interest rate on our fixed rate mortgage loans was 6.08% as of December 31, 2015 . (4) The weighted average interest rate on our variable rate secured loans was 2.2% as of December 31, 2015 . (5) Refer to the paragraphs below for further disclosure. (6) As discussed below, an additional $150.0 million in borrowings is available to be drawn under a term loan entered into in the current year. In addition, we have the ability to borrow an additional $430.0 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. (7) The weighted average interest rate on these loans was 1.92% as of December 31, 2015 . (8) The carrying value of these notes reflects an unamortized discount totaling $2.2 million as of December 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% . (9) The carrying value of these notes reflects an unamortized discount totaling $3.8 million as of December 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% . (10) Refer to the paragraphs below for further disclosure. (11) These notes carry interest rates that were below market rates upon assumption and therefore were recorded at their fair value based on applicable effective interest rates. The carrying value of these notes reflects an unamortized discount totaling $554,000 as of December 31, 2015 and $654,000 as of December 31, 2014 . (12) On April 20, 2015, we redeemed the remaining $575,000 principal amount of these notes at 100% of their principal amount. The carrying value of these notes as of December 31, 2014 included an unamortized discount totaling $3,000 .
Schedule of debt maturities	Our debt matures on the following schedule (in thousands): 2016 $ 208,109 2017 3,252 2018 3,400 2019 167,014 (1) 2020 315,252 Thereafter 1,400,203 Total $ 2,097,230 (2) (1) Includes $43.5 million in 2019 that may be extended to 2020 at our option, subject to certain conditions. (2) Represents scheduled principal amortization and maturities only and therefore excludes net discounts and deferred financing costs of $19.5 million .
Schedule of the fair value of debt	The following table sets forth information pertaining to the fair value of our debt (in thousands): December 31, 2015 December 31, 2014 Carrying Estimated Carrying Estimated Amount Fair Value Amount Fair Value Fixed-rate debt Unsecured Senior Notes $ 1,185,842 $ 1,211,658 $ 888,544 $ 901,599 Other fixed-rate debt 282,716 291,991 387,948 356,377 Variable-rate debt 609,194 610,987 637,544 642,091 $ 2,077,752 $ 2,114,636 $ 1,914,036 $ 1,900,067

Interest Rate Derivatives (Tabl

Interest Rate Derivatives (Tables)	12 Months Ended
	Dec. 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 Effective Expiration December 31, Amount Fixed Rate Floating Rate Index Date Date 2015 2014 $ 100,000 0.8055 % One-Month LIBOR 9/2/2014 9/1/2016 $ (148 ) $ (317 ) 100,000 0.8100 % One-Month LIBOR 9/2/2014 9/1/2016 (151 ) (324 ) 100,000 1.6730 % One-Month LIBOR 9/1/2015 8/1/2019 (1,217 ) 239 100,000 1.7300 % One-Month LIBOR 9/1/2015 8/1/2019 (1,429 ) 35 13,941 (1) 1.3900 % One-Month LIBOR 10/13/2015 10/1/2020 53 — 100,000 1.9013 % One-Month LIBOR 9/1/2016 12/1/2022 (138 ) — 100,000 1.9050 % One-Month LIBOR 9/1/2016 12/1/2022 (45 ) — 50,000 1.9079 % One-Month LIBOR 9/1/2016 12/1/2022 (32 ) — 36,200 -1 3.8300 % One-Month LIBOR + 2.25% 11/2/2010 11/2/2015 — (400 ) 100,000 0.8320 % One-Month LIBOR 1/3/2012 9/1/2015 — (407 ) 100,000 0.8320 % One-Month LIBOR 1/3/2012 9/1/2015 — (407 ) $ (3,107 ) $ (1,581 ) (1) The notional amount of this instrument is scheduled to amortize to $12.1 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): Fair Value at December 31, Derivatives Balance Sheet Location 2015 2014 Interest rate swaps designated as cash flow hedges Prepaid expenses and other assets $ 53 $ 274 Interest rate swaps designated as cash flow hedges Interest rate derivatives (3,160 ) (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 Years Ended December 31, 2015 2014 2013 Amount of (losses) gains recognized in accumulated other comprehensive income (loss) (“AOCI”) (effective portion) $ (4,739 ) $ (7,799 ) $ 6,791 Amount of losses reclassified from AOCI into interest expense (effective portion) 3,599 2,990 2,740 Amount of loss recognized in interest expense (ineffective portion) 386 — — Amount of loss reclassified from AOCI into loss on early extinguishment of debt — 38 —

Redeemable Noncontrolling Int43

Redeemable Noncontrolling Interest (Tables)	12 Months Ended
	Dec. 31, 2015
Noncontrolling Interest [Abstract]
Schedule of activity for redeemable noncontrolling interest	The table below sets forth the activity in redeemable noncontrolling interests in our LW Redstone, LLC and Stevens Investors, LLC joint ventures described in Note 6 (in thousands): For the Years Ended December 31, 2015 2014 2013 Beginning balance $ 18,417 $ 17,758 $ 10,298 Contributions from noncontrolling interests 1,654 — — Distributions to noncontrolling interests (2,964 ) (1,369 ) (1,037 ) Net income attributable to noncontrolling interests 2,227 2,162 1,376 Adjustment to arrive at fair value of interests (116 ) (134 ) 7,121 Ending balance $ 19,218 $ 18,417 $ 17,758

Equity - COPT and Subsidiaries

Equity - COPT and Subsidiaries (Tables)	12 Months Ended
	Dec. 31, 2015
Stockholders' Equity Note [Abstract]
Schedule of additional information pertaining to preferred shares	The table below sets forth additional information pertaining to COPT’s outstanding preferred shares (dollars in thousands, except per share data): Series # of Shares Issued Aggregate Liquidation Preference Month of Issuance Annual Dividend Yield Annual Dividend Per Share Earliest Redemption Date Series K 531,667 $ 26,583 January 2007 5.600 % $ 2.80000 1/9/2017 Series L 6,900,000 172,500 June 2012 7.375 % $ 1.84375 6/27/2017 7,431,667 $ 199,083

Equity - COPLP and Subsidiari45

Equity - COPLP and Subsidiaries (Tables)	12 Months Ended
	Dec. 31, 2015
Class of Stock [Line Items]
Schedule of additional information pertaining to preferred shares	The table below sets forth additional information pertaining to COPT’s outstanding preferred shares (dollars in thousands, except per share data): Series # of Shares Issued Aggregate Liquidation Preference Month of Issuance Annual Dividend Yield Annual Dividend Per Share Earliest Redemption Date Series K 531,667 $ 26,583 January 2007 5.600 % $ 2.80000 1/9/2017 Series L 6,900,000 172,500 June 2012 7.375 % $ 1.84375 6/27/2017 7,431,667 $ 199,083
Corporate Office Properties, L.P. [Member]
Class of Stock [Line Items]
Schedule of additional information pertaining to preferred shares	The table below sets forth information pertaining to preferred units in COPLP held by COPT at December 31, 2015 (dollars in thousands, except per unit data): Series # of Units Issued Aggregate Liquidation Preference Month of Issuance Annual Distribution Yield Annual Distribution Per Unit Earliest Redemption Date Series K 531,667 $ 26,583 January 2007 5.600 % $ 2.80000 1/9/2017 Series L 6,900,000 172,500 June 2012 7.375 % $ 1.84375 6/27/2017 7,431,667 $ 199,083

Share-Based Compensation and 46

Share-Based Compensation and Other Compensation Matters (Tables)	12 Months Ended
	Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary of restricted share transactions under the entity's share-based compensation plans	The following table summarizes restricted shares under the share-based compensation plans for 2013, 2014 and 2015: Shares Weighted Average Grant Date Fair Value Unvested at December 31, 2012 434,643 $ 29.67 Granted 193,833 25.91 Forfeited (9,541 ) 27.59 Vested (241,487 ) 30.97 Unvested at December 31, 2013 377,448 26.96 Granted 216,607 26.73 Forfeited (21,335 ) 25.10 Vested (182,213 ) 28.56 Unvested at December 31, 2014 390,507 26.19 Granted 201,024 28.69 Forfeited (10,550 ) 26.05 Vested (202,781 ) 26.07 Unvested at December 31, 2015 378,200 $ 27.58 Unvested shares as of December 31, 2015 that are expected to vest 364,666 $ 27.61
Schedule of PSU's Granted	We made the following grants of PSUs to executives from 2011 through 2015 (dollars in thousands): Grant Date Number of PSUs Granted Performance Period Commencement Date Performance Period End Date Grant Date Fair Value Number of PSUs Outstanding as of December 31, 2015 3/3/2011 56,883 3/3/2011 3/2/2014 $ 2,796 — 3/1/2012 54,070 1/1/2012 12/31/2014 1,772 — 3/1/2013 69.579 1/1/2013 12/31/2015 1,867 51,556 3/6/2014 49,103 1/1/2014 12/31/2016 1,723 37,843 3/5/2015 45,656 1/1/2015 12/31/2017 1,678 45,656
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
Schedule of assumptions used in Monte Carlo models for PSUs	The grant date fair value and certain of the assumptions used in the Monte Carlo models for the PSUs granted in 2013, 2014 and 2015 are set forth below: For the Years Ended December 31, 2015 2014 2013 Grant date fair value $ 36.76 $ 35.09 $ 26.84 Baseline common share value $ 29.28 $ 26.52 $ 25.85 Expected volatility of common shares 19.9 % 28.6 % 29.5 % Risk-free interest rate 0.99 % 0.66 % 0.33 %
Summary of stock option transactions under the entity's share-based compensation plans	The table below sets forth information regarding our outstanding options (dollars in thousands, except per share data): Options Outstanding and Exercisable Weighted Average Exercise Price Per Share Weighted Average Remaining Contractual Term (in Years) Aggregate Intrinsic Value December 31, 2012 798,210 $37.62 3 $ 325 December 31, 2013 640,927 $38.11 2 $ 68 December 31, 2014 559,736 $39.60 2 $ 167 December 31, 2015 425,347 $42.75 1 $ —
Reporting for share-based compensation expense	The table below sets forth our reporting for share based compensation cost (in thousands): For the Years Ended December 31, 2015 2014 2013 General, administrative and leasing expenses $ 5,574 $ 5,307 $ 5,412 Property operating expenses 1,000 857 1,118 Capitalized to development activities 824 886 1,075 Share-based compensation cost $ 7,398 $ 7,050 $ 7,605

Operating Leases (Tables)

Operating Leases (Tables)	12 Months Ended
	Dec. 31, 2015
Leases, Operating [Abstract]
Schedule of gross minimum future rentals on noncancelable leases in the entity's properties	Gross minimum future rentals on noncancelable leases in our properties as of December 31, 2015 were as follows (in thousands): Year Ending December 31, 2016 $ 378,780 2017 349,192 2018 278,714 2019 218,977 2020 154,207 Thereafter 433,892 $ 1,813,762

Information by Business Segme48

Information by Business Segment (Tables)	12 Months Ended
	Dec. 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 Defense/Information Technology Locations Fort Meade/BW Corridor Northern Virginia Defense/IT Lackland Air Force Base Navy Support Locations Redstone Arsenal Colorado Defense/IT Data Center Shells Total Defense/IT Locations Regional Office Operating Wholesale Data Center Other Total Year Ended December 31, 2015 Revenues from real estate operations $ 244,274 $ 49,199 $ 39,659 $ 28,177 $ 11,228 $ — $ 21,746 $ 394,283 $ 98,165 $ 19,032 $ 7,588 $ 519,068 Property operating expenses 83,309 20,107 22,004 13,229 3,497 — 2,298 144,444 36,165 10,402 3,477 194,488 NOI from real estate operations $ 160,965 $ 29,092 $ 17,655 $ 14,948 $ 7,731 $ — $ 19,448 $ 249,839 $ 62,000 $ 8,630 $ 4,111 $ 324,580 Additions to long-lived assets $ 31,883 $ 90,248 $ — $ 7,656 $ 883 $ — $ — $ 130,670 $ 204,139 $ 132 $ 328 $ 335,269 Transfers from non-operating properties $ 45,560 $ 50,690 $ 32,307 $ 1,408 $ 13,190 $ — $ 51,492 $ 194,647 $ 22,313 $ 89,745 $ 415 $ 307,120 Segment assets at December 31, 2015 $ 1,290,028 $ 411,196 $ 134,381 $ 196,090 $ 108,038 $ — $ 203,013 $ 2,342,746 $ 608,471 $ 243,338 $ 70,914 $ 3,265,469 Year Ended December 31, 2014 Revenues from real estate operations $ 233,764 $ 48,313 $ 34,463 $ 31,335 $ 10,446 $ — $ 18,421 $ 376,742 $ 85,025 $ 10,430 $ 7,514 $ 479,711 Property operating expenses 80,824 19,071 19,677 12,576 3,066 — 2,272 137,486 31,427 7,286 3,600 179,799 NOI from real estate operations $ 152,940 $ 29,242 $ 14,786 $ 18,759 $ 7,380 $ — $ 16,149 $ 239,256 $ 53,598 $ 3,144 $ 3,914 $ 299,912 Additions to long-lived assets $ 24,173 $ 7,119 $ — $ 10,010 $ 4,077 $ — $ — $ 45,379 $ 19,290 $ 22 $ (90 ) $ 64,601 Transfers from non-operating properties $ 56,699 $ 360 $ — $ — $ 21,014 $ — $ 43,154 $ 121,227 $ 17,942 $ 1,108 $ 1,622 $ 141,899 Segment assets at December 31, 2014 $ 1,264,353 $ 372,041 $ 102,232 $ 195,972 $ 97,209 $ — $ 156,214 $ 2,188,021 $ 526,988 $ 163,177 $ 73,309 $ 2,951,495 Year Ended December 31, 2013 Revenues from real estate operations $ 234,857 $ 56,524 $ 31,252 $ 33,431 $ 5,050 $ 12,229 $ 12,520 $ 385,863 $ 98,487 $ 7,271 $ 7,012 $ 498,633 Property operating expenses 80,149 19,718 16,784 12,872 1,282 3,797 1,304 135,906 35,488 6,360 2,950 180,704 NOI from real estate operations $ 154,708 $ 36,806 $ 14,468 $ 20,559 $ 3,768 $ 8,432 $ 11,216 $ 249,957 $ 62,999 $ 911 $ 4,062 $ 317,929 Additions to long-lived assets $ 28,009 $ 5,522 $ 5 $ 4,945 $ 3,563 $ 200 $ — $ 42,244 $ 13,244 $ 598 $ 791 $ 56,877 Transfers from non-operating properties $ 50,067 $ 37,234 $ — $ 14 $ 48,799 $ 987 $ 24,200 $ 161,301 $ 32,692 $ 70,106 $ 191 $ 264,290 Segment assets at December 31, 2013 $ 1,229,192 $ 385,061 $ 103,650 $ 195,105 $ 77,773 $ — $ 117,012 $ 2,107,793 $ 556,718 $ 166,790 $ 74,329 $ 2,905,630
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 Years Ended December 31, 2015 2014 2013 Segment revenues from real estate operations $ 519,068 $ 479,711 $ 498,633 Construction contract and other service revenues 106,402 106,748 62,363 Less: Revenues from discontinued operations (Note 18) (4 ) 14 (37,636 ) Total revenues $ 625,466 $ 586,473 $ 523,360
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 Years Ended December 31, 2015 2014 2013 Segment property operating expenses $ 194,488 $ 179,799 $ 180,704 Less: Property operating expenses from discontinued operations (Note 18) 6 135 (13,505 ) Total property operating expenses $ 194,494 $ 179,934 $ 167,199
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 Years Ended December 31, 2015 2014 2013 Construction contract and other service revenues $ 106,402 $ 106,748 $ 62,363 Construction contract and other service expenses (102,696 ) (100,058 ) (58,875 ) NOI from service operations $ 3,706 $ 6,690 $ 3,488
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 Years Ended December 31, 2015 2014 2013 NOI from real estate operations $ 324,580 $ 299,912 $ 317,929 NOI from service operations 3,706 6,690 3,488 Interest and other income 4,517 4,923 3,834 Equity in income of unconsolidated entities 62 229 2,110 Income tax expense (199 ) (310 ) (1,978 ) Other adjustments: — Depreciation and other amortization associated with real estate operations (140,025 ) (136,086 ) (113,214 ) Impairment losses (23,289 ) (1,416 ) (5,857 ) General, administrative and leasing expenses (31,361 ) (31,794 ) (30,869 ) Business development expenses and land carry costs (13,507 ) (5,573 ) (5,436 ) Interest expense on continuing operations (89,074 ) (92,393 ) (82,010 ) NOI from discontinued operations (10 ) (121 ) (24,131 ) Gain (loss) on early extinguishment of debt 85,275 (9,552 ) (27,030 ) COPT consolidated income from continuing operations $ 120,675 $ 34,509 $ 36,836
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): As of December 31, 2015 2014 Segment assets $ 3,265,469 $ 2,951,495 Non-operating property assets 450,679 567,586 Other assets 193,164 145,155 Total COPT consolidated assets $ 3,909,312 $ 3,664,236

Discontinued Operations (Tables

Discontinued Operations (Tables)	12 Months Ended
	Dec. 31, 2015
Discontinued Operations and Disposal Groups [Abstract]
Components of discontinued operations	The table below sets forth the components of discontinued operations reported on our consolidated statements of operations (in thousands): For the Years Ended December 31, 2015 2014 2013 Revenue from real estate operations $ 4 $ (14 ) $ 37,636 Property operating expenses 6 135 (13,505 ) Depreciation and amortization — — (4,505 ) Impairment losses (234 ) (3 ) (26,190 ) General, administrative and leasing expenses — — (4 ) Interest expense — — (8,221 ) Gain on sales of real estate — 24 2,671 Gain (loss) on early extinguishment of debt 380 (116 ) 67,810 Discontinued operations $ 156 $ 26 $ 55,692

Earnings Per Share ("EPS") an50

Earnings Per Share ("EPS") and Earnings Per Unit (“EPU”) (Tables)	12 Months Ended
	Dec. 31, 2015
Earnings Per Share [Line Items]
Summary of calculation of numerator and denominator in basic and diluted earning 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 Years Ended December 31, 2015 2014 2013 Numerator: Income from continuing operations $ 120,675 $ 34,509 $ 36,836 Gain on sales of real estate, net 68,047 10,671 9,016 Preferred share dividends (14,210 ) (15,939 ) (19,971 ) Issuance costs associated with redeemed preferred shares — (1,769 ) (2,904 ) Income from continuing operations attributable to noncontrolling interests (10,575 ) (4,955 ) (4,486 ) Income from continuing operations attributable to restricted shares (706 ) (432 ) (414 ) Numerator for basic EPS from continuing operations attributable to COPT common shareholders $ 163,231 $ 22,085 $ 18,077 Dilutive effect of common units in the Operating Partnership on diluted EPS from continuing operations 6,397 — — Numerator for diluted EPS from continuing operations attributable to COPT common shareholders $ 169,628 $ 22,085 $ 18,077 Numerator for basic EPS from continuing operations attributable to COPT common shareholders $ 163,231 $ 22,085 $ 18,077 Discontinued operations 156 26 55,692 Discontinued operations attributable to noncontrolling interests (3 ) 4 (3,351 ) Numerator for basic EPS on net income attributable to COPT common shareholders $ 163,384 $ 22,115 $ 70,418 Dilutive effect of common units in COPLP 6,403 — — Numerator for diluted EPS on net income attributable to COPT common shareholders $ 169,787 $ 22,115 $ 70,418 Denominator (all weighted averages): Denominator for basic EPS (common shares) 93,914 88,092 85,167 Dilutive effect of common units 3,692 — — Dilutive effect of share-based compensation awards 61 171 57 Denominator for diluted EPS (common shares) 97,667 88,263 85,224 Basic EPS: Income from continuing operations attributable to COPT common shareholders $ 1.74 $ 0.25 $ 0.21 Discontinued operations attributable to COPT common shareholders — — 0.62 Net income attributable to COPT common shareholders $ 1.74 $ 0.25 $ 0.83 Diluted EPS: Income from continuing operations attributable to COPT common shareholders $ 1.74 $ 0.25 $ 0.21 Discontinued operations attributable to COPT common shareholders — — 0.62 Net income attributable to COPT common shareholders $ 1.74 $ 0.25 $ 0.83
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 Years Ended December 31, 2015 2014 2013 Conversion of common units — 3,897 3,869 Conversion of Series I preferred units 176 176 176 Conversion of Series K preferred shares 434 434 434
Corporate Office Properties, L.P. [Member]
Earnings Per Share [Line Items]
Summary of calculation of numerator and denominator in basic and diluted earning 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 Years Ended December 31, 2015 2014 2013 Numerator: Income from continuing operations $ 120,675 $ 34,509 $ 36,836 Gain on sales of real estate, net 68,047 10,671 9,016 Preferred unit distributions (14,870 ) (16,599 ) (20,631 ) Issuance costs associated with redeemed preferred units — (1,769 ) (2,904 ) Income from continuing operations attributable to noncontrolling interests (3,523 ) (3,281 ) (2,977 ) Income from continuing operations attributable to restricted units (706 ) (432 ) (414 ) Numerator for basic and diluted EPU from continuing operations attributable to COPLP common unitholders $ 169,623 $ 23,099 $ 18,926 Discontinued operations 156 26 55,692 Discontinued operations attributable to noncontrolling interests 3 5 (930 ) Numerator for basic and diluted EPU on net income attributable to COPLP common unitholders $ 169,782 $ 23,130 $ 73,688 Denominator (all weighted averages): Denominator for basic EPU (common units) 97,606 91,989 89,036 Dilutive effect of share-based compensation awards 61 171 57 Denominator for diluted EPU (common units) 97,667 92,160 89,093 Basic EPU: Income from continuing operations attributable to COPLP common unitholders $ 1.74 $ 0.25 $ 0.21 Discontinued operations attributable to COPLP common unitholders — — 0.62 Net income attributable to COPLP common unitholders $ 1.74 $ 0.25 $ 0.83 Diluted EPU: Income from continuing operations attributable to COPLP common unitholders $ 1.74 $ 0.25 $ 0.21 Discontinued operations attributable to COPLP common unitholders — — 0.62 Net income attributable to COPLP common unitholders $ 1.74 $ 0.25 $ 0.83
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 Years Ended December 31, 2015 2014 2013 Conversion of Series I preferred units 176 176 176 Conversion of Series K preferred units 434 434 434

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Schedule of future minimum rental payments	Future minimum rental payments due under the terms of these operating leases as of December 31, 2015 follow (in thousands): Year Ending December 31, 2016 $ 1,189 2017 1,117 2018 1,071 2019 1,049 2020 1,067 Thereafter 86,798 $ 92,291

Quarterly Data (Unaudited) (Tab

Quarterly Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2015
Quarterly Data [Line Items]
Schedule of selected quarterly information	The tables below set forth selected quarterly information for the years ended December 31, 2015 and 2014 (in thousands, except per share data). Certain of the amounts below have been reclassified to conform to the current period presentation of our consolidated financial statements. For the Year Ended December 31, 2015 For the Year Ended December 31, 2014 First Quarter Second Quarter Third Quarter Fourth Quarter First Quarter Second Quarter Third Quarter Fourth Quarter COPT and Subsidiaries Revenues $ 161,034 $ 170,363 $ 150,744 $ 143,325 $ 146,667 $ 139,820 $ 153,015 $ 146,971 Operating income $ 30,575 $ 37,471 $ 31,993 $ 20,055 $ 25,206 $ 31,836 $ 37,422 $ 37,148 Income from continuing operations $ 10,987 $ 16,839 $ 94,279 $ (1,430 ) $ 5,660 $ 9,248 $ 13,727 $ 5,874 Discontinued operations $ (238 ) $ 394 $ — $ — $ 11 $ (198 ) $ 191 $ 22 Net income $ 14,735 $ 17,232 $ 94,294 $ 62,617 $ 5,671 $ 9,050 $ 24,548 $ 5,937 Net income attributable to noncontrolling interests (1,380 ) (1,451 ) (4,494 ) (3,253 ) (930 ) (1,160 ) (1,828 ) (1,033 ) Net income attributable to COPT 13,355 15,781 89,800 59,364 4,741 7,890 22,720 4,904 Preferred share dividends (3,552 ) (3,553 ) (3,552 ) (3,553 ) (4,490 ) (4,344 ) (3,553 ) (3,552 ) Issuance costs associated with redeemed preferred shares — — — — — (1,769 ) — — Net income attributable to COPT common shareholders $ 9,803 $ 12,228 $ 86,248 $ 55,811 $ 251 $ 1,777 $ 19,167 $ 1,352 Basic earnings per common share $ 0.10 $ 0.13 $ 0.91 $ 0.59 $ 0.00 $ 0.02 $ 0.22 $ 0.01 Diluted earnings per common share $ 0.10 $ 0.13 $ 0.91 $ 0.59 $ 0.00 $ 0.02 $ 0.22 $ 0.01
Corporate Office Properties, L.P. [Member]
Quarterly Data [Line Items]
Schedule of selected quarterly information	COPLP and Subsidiaries Revenues $ 161,034 $ 170,363 $ 150,744 $ 143,325 $ 146,667 $ 139,820 $ 153,015 $ 146,971 Operating income $ 30,575 $ 37,471 $ 31,993 $ 20,055 $ 25,206 $ 31,836 $ 37,422 $ 37,148 Income from continuing operations $ 10,987 $ 16,839 $ 94,279 $ (1,430 ) $ 5,660 $ 9,248 $ 13,727 $ 5,874 Discontinued operations $ (238 ) $ 394 $ — $ — $ 11 $ (198 ) $ 191 $ 22 Net income $ 14,735 $ 17,232 $ 94,294 $ 62,617 $ 5,671 $ 9,050 $ 24,548 $ 5,937 Net income attributable to noncontrolling interests (818 ) (812 ) (972 ) (918 ) (737 ) (837 ) (897 ) (805 ) Net income attributable to COPLP 13,917 16,420 93,322 61,699 4,934 8,213 23,651 5,132 Preferred unit distributions (3,717 ) (3,718 ) (3,717 ) (3,718 ) (4,655 ) (4,509 ) (3,718 ) (3,717 ) Issuance costs associated with redeemed preferred units — — — — — (1,769 ) — — Net income attributable to COPLP common unitholders $ 10,200 $ 12,702 $ 89,605 $ 57,981 $ 279 $ 1,935 $ 19,933 $ 1,415 Basic earnings per common unit $ 0.10 $ 0.13 $ 0.91 $ 0.59 $ 0.00 $ 0.02 $ 0.22 $ 0.01 Diluted earnings per common unit $ 0.10 $ 0.13 $ 0.91 $ 0.59 $ 0.00 $ 0.02 $ 0.22 $ 0.01

Organization (Details)

Organization (Details) ft² in Thousands	Dec. 31, 2015ft²aPropertyMW
Properties under redevelopment [Member]
Investments in real estate
Number of real estate properties	4
Square footage of real estate properties (in square feet) | ft²	156
Operating office properties [Member]
Investments in real estate
Number of real estate properties	177
Square footage of real estate properties (in square feet) | ft²	18,100
Operating office properties [Member] | Single-tenant data centers [Member]
Investments in real estate
Number of real estate properties	9
Office properties under, or contractually committed for, construction or approved for redevelopment [Member]
Investments in real estate
Number of real estate properties	13
Square footage of real estate properties (in square feet) | ft²	1,500
Partially operational properties [Member] | Properties under redevelopment [Member]
Investments in real estate
Number of real estate properties	1
Land controlled for future development [Member]
Investments in real estate
Square footage of real estate properties (in square feet) | a	1,439
Developable square footage of real estate properties (in square feet) | ft²	17,600
Operating Wholesale Data Center [Member]
Investments in real estate
Critical load (in MW's) | MW	19.25

Organization (Details 2)

Organization (Details 2) - Corporate Office Properties, L.P. [Member]	12 Months Ended
	Dec. 31, 2015
Common Units [Member]
Forms of ownership in Operating Partnership and ownership percentage by the entity
Percentage ownership in operating partnership	96.30%
Preferred Units [Member]
Forms of ownership in Operating Partnership and ownership percentage by the entity
Percentage ownership in operating partnership	95.50%

Summary of Significant Accoun55

Summary of Significant Accounting Policies (Details)	12 Months Ended
	Dec. 31, 2015
Properties
Period after the cessation of major construction activities for considering property as operational if leases have not commenced earlier (in years)	1 year
Period after the cessation of major construction activities for considering property as partially operational if leases have commenced earlier (in years)	1 year
Property, Plant and Equipment [Line Items]
Period used for recovery analysis of long-lived assets to be held and used that may be impaired (in years)	10 years
Minimum [Member] | Buildings and building improvements [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	10 years
Minimum [Member] | Land improvements [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	10 years
Minimum [Member] | Equipment and personal property [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	3 years
Maximum [Member] | Buildings and building improvements [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	40 years
Maximum [Member] | Land improvements [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	20 years
Maximum [Member] | Equipment and personal property [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	10 years

Summary of Significant Accoun56

Summary of Significant Accounting Policies (Details 1)	12 Months Ended
	Dec. 31, 2015Form
Cash and Cash Equivalents
Maximum term until original maturity to classify cash and liquid investments as cash and cash equivalent (in months)	3 months
Share-Based Compensation
Number of forms of share based compensation plans	3
Corporate Office Properties, L.P. [Member]
Noncontrolling Interest [Abstract]
Interests in several real estate entities owned directly by COPT	1.00%

Summary of Significant Accoun57

Summary of Significant Accounting Policies (Details 2) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accounting Policies [Abstract]
Minimum adjusted taxable income to be distributed to shareholders to qualify as a REIT (as a percent)	90.00%
Subsequent period during which entity would be disqualified as a REIT if during any tax year entity fails to qualify as a REIT (in years)	4 years
Dividends Payable [Line Items]
Approximate amount by which basis of assets and liabilities for tax reporting purposes is lower than amount reported on consolidated balance sheet	$ 224
Common Shares [Member]
Dividends Payable [Line Items]
Ordinary income (as a percent)	38.30%	64.50%	71.80%
Long-term capital gain (as a percent)	61.70%	6.50%	22.40%
Return of capital (as a percent)	0.00%	29.00%	5.80%
Preferred Shares [Member]
Dividends Payable [Line Items]
Ordinary income (as a percent)	38.30%	90.90%	76.20%
Long-term capital gain (as a percent)	61.70%	9.10%	23.80%
Return of capital (as a percent)	0.00%	0.00%	0.00%

Fair Value Measurements (Detail

Fair Value Measurements (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015USD ($)joint_ventnure	Dec. 31, 2014USD ($)
Schedule of Trading Security [Line Items]
Number of joint ventures | joint_ventnure	2
Deferred Compensation, Excluding Share-based Payments and Retirement Benefits [Member] | Management [Member]
Schedule of Trading Security [Line Items]
Maximum percentage of participants' compensation which is deferrable (as a percent)	100.00%
Balance of the plan which was fully funded | $	$ 5.8	$ 5.9

Fair Value Measurements (Deta59

Fair Value Measurements (Details 2) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Liabilities:
Interest rate derivatives	$ 3,160	$ 1,855
Fair value measurement on a recurring basis [Member]
Assets:
Interest rate derivatives	53	274
Assets	5,816	6,156
Liabilities:
Deferred compensation plan liability	5,763	5,882
Interest rate derivatives	3,160	1,855
Liabilities	8,923	7,737
Fair value measurement on a recurring basis [Member] | Mutual funds [Member]
Assets:
Marketable securities in deferred compensation plan	5,658	5,756
Fair value measurement on a recurring basis [Member] | Other [Member]
Assets:
Marketable securities in deferred compensation plan	105	126
Fair value measurement on a recurring basis [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets:
Assets	5,763	5,882
Liabilities:
Liabilities	0	0
Fair value measurement on a recurring basis [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Mutual funds [Member]
Assets:
Marketable securities in deferred compensation plan	5,658	5,756
Fair value measurement on a recurring basis [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Other [Member]
Assets:
Marketable securities in deferred compensation plan	105	126
Fair value measurement on a recurring basis [Member] | Significant Other Observable Inputs (Level 2) [Member]
Assets:
Interest rate derivatives	53	274
Assets	53	274
Liabilities:
Deferred compensation plan liability	5,763	5,882
Interest rate derivatives	3,160	1,855
Liabilities	8,923	7,737
Corporate Office Properties, L.P. [Member]
Liabilities:
Interest rate derivatives	3,160	1,855
Corporate Office Properties, L.P. [Member] | Fair value measurement on a recurring basis [Member]
Assets:
Interest rate derivatives	53	274
Assets	53	274
Liabilities:
Interest rate derivatives	3,160	1,855
Corporate Office Properties, L.P. [Member] | Fair value measurement on a recurring basis [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets:
Assets	0	0
Corporate Office Properties, L.P. [Member] | Fair value measurement on a recurring basis [Member] | Significant Other Observable Inputs (Level 2) [Member]
Assets:
Interest rate derivatives	53	274
Assets	53	274
Liabilities:
Interest rate derivatives	$ 3,160	$ 1,855

Fair Value Measurements (Deta60

Fair Value Measurements (Details 3) $ in Thousands	12 Months Ended
	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)Property
Fair value of impaired properties and other assets
Impairment losses	$ 23,289	$ 1,416	$ 5,857
Exit costs included in impairment losses in connection with dispositions	0	0	979
Fair value measurement on a recurring basis [Member]
Fair value of impaired properties and other assets
Assets	5,816	6,156
Fair value measurement on a nonrecurring basis [Member]
Fair value of impaired properties and other assets
Exit costs included in impairment losses in connection with dispositions			419
Fair value measurement on a nonrecurring basis [Member] | Discontinued Operations [Member]
Fair value of impaired properties and other assets
Impairment losses			15,200
Real Estate Investment Properties, Net [Member] | Fair value measurement on a nonrecurring basis [Member]
Fair value of impaired properties and other assets
Impairment losses	23,500	$ 1,400
Assets	13,850
Real Estate Investment Properties, Net [Member] | Fair value measurement on a nonrecurring basis [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair value of impaired properties and other assets
Assets	13,850
Real Estate Investment Properties, Net [Member] | Fair value measurement on a nonrecurring basis [Member] | Indicative Bids [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair value of impaired properties and other assets
Assets	24,423
Real Estate Investment Properties, Net [Member] | Fair value measurement on a nonrecurring basis [Member] | Comparable Sales Price [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair value of impaired properties and other assets
Assets	10,850
Real Estate Investment Properties, Net [Member] | Fair value measurement on a nonrecurring basis [Member] | Colorado Springs [Member]
Fair value of impaired properties and other assets
Impairment losses	12,800
Real Estate Investment Properties, Net [Member] | Fair value measurement on a nonrecurring basis [Member] | Aberdeen, Maryland [Member]
Fair value of impaired properties and other assets
Impairment losses	6,600
Real Estate Investment Properties, Net [Member] | Fair value measurement on a nonrecurring basis [Member] | White Marsh, Maryland [Member]
Fair value of impaired properties and other assets
Impairment losses	2,600
Real Estate Investment Properties, Net [Member] | Fair value measurement on a nonrecurring basis [Member] | Land controlled for future development [Member] | Northern Virginia [Member]
Fair value of impaired properties and other assets
Impairment losses	1,300
Assets Held-for-sale, Net [Member] | Fair value measurement on a nonrecurring basis [Member]
Fair value of impaired properties and other assets
Assets	21,423
Assets Held-for-sale, Net [Member] | Fair value measurement on a nonrecurring basis [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair value of impaired properties and other assets
Assets	$ 21,423
Property Conveyances [Member] | Fair value measurement on a nonrecurring basis [Member]
Fair value of impaired properties and other assets
Impairment losses			11,000
Exit costs included in impairment losses in connection with dispositions			560
Other Properties with Shortened Holding Periods [Member] | Fair value measurement on a nonrecurring basis [Member] | White Marsh, Maryland [Member]
Fair value of impaired properties and other assets
Impairment losses			$ 5,900
Number of properties, shortened holding period | Property			2

Concentration of Revenue (Detai

Concentration of Revenue (Details) - Tenants [Member] - United States Government [Member]	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Rental revenue [Member]
Concentration Risk
Percentage of revenue	20.00%	18.00%	18.00%
Construction contract revenue [Member]
Concentration Risk
Percentage of revenue	90.00%	90.00%	90.00%

Properties, net (Details)

Properties, net (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Properties
Gross	$ 4,158,616	$ 4,014,336	$ 3,811,950	$ 3,859,960
Operating properties, net	2,920,529	2,751,488
Operating properties, net [Member]
Properties
Less: accumulated depreciation	(700,363)	(703,083)
Operating properties, net [Member] | Land [Member]
Properties
Gross	463,305	439,355
Operating properties, net [Member] | Buildings and improvements [Member]
Properties
Gross	$ 3,157,587	$ 3,015,216

Properties, net (Details 2)

Properties, net (Details 2) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Properties
Projects in development or held for future development	$ 429,219	$ 545,426
Projects in development or held for future development [Member] | Land in development or held for future development [Member]
Properties
Projects in development or held for future development	207,774	214,977
Projects in development or held for future development [Member] | Development in progress, excluding land [Member]
Properties
Projects in development or held for future development	$ 221,445	$ 330,449

Properties, net (Details 3)

Properties, net (Details 3) $ in Thousands	Dec. 31, 2015USD ($)Property	Dec. 31, 2014USD ($)Property
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets held for sale, net	$ 96,782	$ 14,339
2015 Assets Held For Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Properties, net	90,188
Deferred rent receivable	2,891
Intangible assets on real estate acquisitions, net	1,591
Deferred leasing costs, net	1,391
Lease incentives, net	721
Assets held for sale, net	$ 96,782
2015 Assets Held For Sale [Member] | White Marsh, Maryland [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of land parcels held-for-sale | Property	13
2015 Assets Held For Sale [Member] | San Antonio [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of land parcels held-for-sale | Property	2
Held-for-sale [Member] | White Marsh, Maryland [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of land parcels held-for-sale | Property		2
Assets held for sale, net		$ 14,300

Properties, net (Details 4)

Properties, net (Details 4) $ / shares in Units, ft² in Thousands, $ in Thousands	Aug. 07, 2015USD ($)ft²	Apr. 15, 2015USD ($)ft²	Mar. 19, 2015USD ($)ft²	Dec. 31, 2015USD ($)$ / shares	Dec. 31, 2014USD ($)$ / shares
Business Combination
Weighted average amortization period of intangible assets				11 years
Pro Forma Information
Pro forma total revenues				$ 641,982	$ 623,013
Pro forma net income attributable to COPT common shareholders				$ 167,079	$ 20,796
Pro Forma EPS Basic (in dollars per share) | $ / shares				$ 1.77	$ 0.23
Pro Forma EPS Diluted (in dollars per share) | $ / shares				$ 1.77	$ 0.23
Tenant relationship value [Member]
Business Combination
Weighted average amortization period of intangible assets				11 years
Below-market cost arrangements [Member]
Business Combination
Weighted average amortization period of intangible assets				35 years
250 W Pratt St [Member] | Baltimore, Maryland [Member]
Business Acquisition [Line Items]
Square footage of real estate properties (in square feet) | ft²			367
Acquired property percentage leased			96.20%
Payments to acquire real estate			$ 61,800
2600 Park Tower Drive [Member] | Vienna, Virginia [Member]
Business Acquisition [Line Items]
Square footage of real estate properties (in square feet) | ft²		237
Acquired property percentage leased		100.00%
Payments to acquire real estate		$ 80,500
100 and 30 Light Street [Member] | Baltimore, Maryland [Member]
Business Acquisition [Line Items]
Square footage of real estate properties (in square feet) | ft²	558
Acquired property percentage leased	93.50%
Payments to acquire real estate	$ 121,200
Liabilities incurred	55,000
Fair value assumption of debt	$ 55,500
2015 Property Acquisitions [Member]
Business Combination
Land, operating properties				$ 55,076
Building and improvements				139,540
Intangible assets on real estate acquisitions				75,729
Total assets				270,345
Below-market leases				(6,808)
Total acquisition cost				263,537
Finite-lived Intangible Assets Acquired				$ 75,729
Weighted average amortization period of intangible assets				10 years
Contributed revenues				$ 20,200
Net income from continuing operations since acquisition date				1,200
Acquisition related costs				4,100
2015 Property Acquisitions [Member] | Tenant relationship value [Member]
Business Combination
Finite-lived Intangible Assets Acquired				$ 31,183
Weighted average amortization period of intangible assets				12 years
2015 Property Acquisitions [Member] | Leases, Acquired-in-Place [Member]
Business Combination
Finite-lived Intangible Assets Acquired				$ 35,139
Weighted average amortization period of intangible assets				7 years
2015 Property Acquisitions [Member] | Above Market Leases [Member]
Business Combination
Finite-lived Intangible Assets Acquired				$ 6,720
Weighted average amortization period of intangible assets				4 years
2015 Property Acquisitions [Member] | Below-market cost arrangements [Member]
Business Combination
Finite-lived Intangible Assets Acquired				$ 2,687
Weighted average amortization period of intangible assets				40 years

Properties, net (Details 5)

Properties, net (Details 5) $ in Thousands	Aug. 28, 2015USD ($)	Dec. 23, 2013USD ($)	Dec. 31, 2015USD ($)ft²Propertybuillding	Dec. 31, 2014USD ($)ft²Propertybuillding	Dec. 31, 2013USD ($)ft²Propertybuillding
Dispositions
Gain (loss) on early extinguishment of debt			$ 85,275	$ (9,552)	$ (27,030)
Non-operating Properties [Member]
Dispositions
Gain on Disposition			4,000	5,600
Sale price of non-operating properties			$ 18,100	$ 28,300	$ 3,500
Operating properties, net [Member]
Dispositions
Number of Buildings Disposed | buillding			9	8	31
Total Rentable Square Feet | ft²			1,597,000	302,958	2,345,000
Transaction value of operating property dispositions			$ 347,785	$ 28,835	$ 293,301
Gain on Disposition			$ 64,033	$ 5,093	$ 2,671
Operating properties, net [Member] | 1550 Westbranch Drive [Member]
Dispositions
Number of Buildings Disposed | buillding			1
Total Rentable Square Feet | ft²			160,000
Transaction value of operating property dispositions			$ 27,800
Gain on Disposition			$ 0
Operating properties, net [Member] | 15000 and 15010 Conference Center Drive [Member]
Dispositions
Number of Buildings Disposed | buillding			2
Total Rentable Square Feet | ft²			665,000
Transaction value of operating property dispositions			$ 167,335
Gain on Disposition			$ 0
Operating properties, net [Member] | 13200 Woodland Park Road [Member]
Dispositions
Number of Buildings Disposed | buillding			1
Total Rentable Square Feet | ft²			397,000
Transaction value of operating property dispositions			$ 84,000
Gain on Disposition			$ 42,515
Operating properties, net [Member] | 9900, 9910, 9920 Franklin Square Drive [Member]
Dispositions
Number of Buildings Disposed | buillding			3
Total Rentable Square Feet | ft²			135,000
Transaction value of operating property dispositions			$ 24,150
Gain on Disposition			$ 6,468
Operating properties, net [Member] | 9690 Deereco Road and 375 W Padonia Road [Member]
Dispositions
Number of Buildings Disposed | buillding			2
Total Rentable Square Feet | ft²			240,000
Transaction value of operating property dispositions			$ 44,500
Gain on Disposition			$ 15,050
Operating properties, net [Member] | 4969 and 4979 Mercantile Rd [Member]
Dispositions
Number of Buildings Disposed | buillding				2
Total Rentable Square Feet | ft²				96,721
Transaction value of operating property dispositions				$ 5,960
Gain on Disposition				$ 2,124
Operating properties, net [Member] | 9930 and 9940 Franklin Square [Member]
Dispositions
Number of Buildings Disposed | buillding				2
Total Rentable Square Feet | ft²				71,992
Transaction value of operating property dispositions				$ 10,475
Gain on Disposition				$ 2,303
Operating properties, net [Member] | 5020, 5022, 5024 and 5026 Campbell Boulevard [Member]
Dispositions
Number of Buildings Disposed | buillding				4
Total Rentable Square Feet | ft²				134,245
Transaction value of operating property dispositions				$ 12,400
Gain on Disposition				666
Operating properties, net [Member] | 920 Elkridge Landing Road [Member]
Dispositions
Number of Buildings Disposed | buillding					1
Total Rentable Square Feet | ft²					103,000
Transaction value of operating property dispositions					$ 6,900
Gain on Disposition					$ 0
Operating properties, net [Member] | 4230 Forbes Boulevard [Member]
Dispositions
Number of Buildings Disposed | buillding					1
Total Rentable Square Feet | ft²					56,000
Transaction value of operating property dispositions					$ 5,600
Gain on Disposition					$ 1,507
Operating properties, net [Member] | December 2013 Colorado Springs Portfolio Disposition [Member]
Dispositions
Number of Buildings Disposed | buillding					15
Total Rentable Square Feet | ft²					1,165,000
Transaction value of operating property dispositions					$ 133,925
Gain on Disposition					$ 1,164
Operating properties, net [Member] | December 2013 Portfolio Conveyance [Member]
Dispositions
Number of Buildings Disposed | buillding					14
Total Rentable Square Feet | ft²					1,021,000
Transaction value of operating property dispositions					$ 146,876
Gain on Disposition					$ 0
Greater Philadelphia [Member]
Dispositions
Depreciation Expense Resulting From Useful Life Change				$ 12,900
15000 and 15010 Conference Center Drive [Member] | Northern Virginia [Member] | Operating properties, net [Member]
Dispositions
Debt settled through the surrender of real estate assets	$ 150,000
Gain (loss) on early extinguishment of debt	$ 84,800
December 2013 Portfolio Conveyance [Member] | Operating properties, net [Member]
Dispositions
Debt settled through the surrender of real estate assets		$ 146,500
Gain (loss) on early extinguishment of debt		$ 67,800
Newly-constructed properties placed in service [Member]
Dispositions
Square Feet of Properties Placed in Service | ft²			897,000	692,000	812,000
Number of real estate properties | Property			7	5	8
Newly Redeveloped Properties Placed In Service [Member] | Greater Philadelphia and St. Mary's County, Maryland [Member]
Dispositions
Square Feet of Properties Placed in Service | ft²			170,000
Number of real estate properties | Property			2
Properties under Construction or Contractually Commited for Construction [Member]
Dispositions
Total Rentable Square Feet | ft²			1,300,000
Number of real estate properties | Property			9
Properties under redevelopment [Member]
Dispositions
Total Rentable Square Feet | ft²			156,000
Number of real estate properties | Property			4

Real Estate Joint Ventures (Det

Real Estate Joint Ventures (Details) $ in Thousands	Aug. 07, 2014USD ($)	Dec. 11, 2013	Sep. 17, 2013USD ($)shares	Dec. 31, 2015USD ($)ft²	Dec. 31, 2013USD ($)Property	Dec. 31, 2014USD ($)	Dec. 31, 2012USD ($)
Investments in consolidated real estate joint ventures
Investing receivables				$ 47,875		$ 52,147
Notes receivable from City of Huntsville [Member]
Investments in consolidated real estate joint ventures
Amount of advanced				37,200
Investing receivables				$ 44,875		$ 49,147
Stevens Investors, LLC [Member] | Minimum [Member]
Investments in consolidated real estate joint ventures
Percentage of residual distributable cash flows in excess of unpaid cumulative preferred returns and return of invested capital entitled to the company				60.00%
LW Redstone Company, LLC [Member]
Investments in consolidated real estate joint ventures
Partner's capital account upon formation				$ 9,000
Percentage of residual distributable cash flows in excess of unpaid cumulative preferred returns and return of invested capital entitled to the company				85.00%
Percentage of residual distributable cash flows in excess of unpaid cumulative preferred returns and return of invested capital entitled to the entity's partners				15.00%
Number of years following construction commencement threshold achievement before partner's interest can be purchased at fair value				5 years
Construction commencement threshold (in square feet) | ft²				4,400,000
Construction commencement completed (in sqft) | ft²				594,000
LW Redstone Company, LLC [Member] | Maximum [Member]
Investments in consolidated real estate joint ventures
Infrastructure costs anticipated to be funded by entity for reimbursement by the City of Huntsville (in dollars)				$ 76,000
Percentage of residual distributable cash flows in excess of unpaid cumulative preferred returns and return of invested capital entitled to the company				85.00%
M Square Associates, LLC [Member]
Investments in consolidated real estate joint ventures
Percentage of residual amounts distributed to each member				50.00%
Consolidated real estate joint ventures [Member]
Investments in consolidated real estate joint ventures
Total Assets				$ 209,351
Encumbered Assets				130,966
Total Liabilities				$ 91,983
Arundel Preserve 5, LLC [Member]
Investments in consolidated real estate joint ventures
Ownership (as a percent)			50.00%
Arundel Preserve 5, LLC [Member] | Common Shares [Member]
Investments in consolidated real estate joint ventures
Acquisition of property and noncontrolling interest in other consolidated entity for COPLP common units (in units) | shares			221,501
Acquisition of property and noncontrolling interest in other consolidated entity for COPLP common units (in dollars)			$ 5,200
MOR Forbes 2 LLC [Member]
Investments in consolidated real estate joint ventures
Ownership (as a percent)		50.00%
Unconsolidated real estate joint ventures [Member]
Investments in consolidated real estate joint ventures
Ownership (as a percent)					20.00%
Number of buildings operated by the joint venture | Property					16
Investment balance							$ (6,400)
Gain on disposal					$ 6,300
LW Redstone Company, LLC [Member] | Primary Beneficiary [Member]
Investments in consolidated real estate joint ventures
Ownership (as a percent)				85.00%
Total Assets				$ 147,612
Encumbered Assets				82,721
Total Liabilities				$ 54,022
M Square Associates, LLC [Member] | Primary Beneficiary [Member]
Investments in consolidated real estate joint ventures
Ownership (as a percent)				50.00%
Total Assets				$ 56,455
Encumbered Assets				48,245
Total Liabilities				$ 37,568
Stevens Investors, LLC [Member] | Primary Beneficiary [Member]
Investments in consolidated real estate joint ventures
Ownership (as a percent)				95.00%
Total Assets				$ 5,284
Encumbered Assets				0
Total Liabilities				$ 393
COPT-FD Indian Head LLC [Member] | Primary Beneficiary [Member]
Investments in consolidated real estate joint ventures
Proceeds from real estate and real estate joint ventures	$ 6,400

Intangible Assets on Real Est68

Intangible Assets on Real Estate Acquisitions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Intangible assets on real estate acquisitions
Gross Carrying Amount	$ 227,481	$ 188,467
Accumulated Amortization	129,143	144,613
Net Carrying Amount	98,338	43,854
Amortization of the intangible assets	$ 18,500	15,200	$ 16,200
Weighted average amortization period of intangible assets	11 years
Estimated future amortization expense associated with the intangible asset categories for the next five years
2,016	$ 18,800
2,017	16,300
2,018	11,600
2,019	8,800
2,020	7,000
In-place lease value [Member]
Intangible assets on real estate acquisitions
Gross Carrying Amount	134,664	123,759
Accumulated Amortization	89,540	101,040
Net Carrying Amount	$ 45,124	22,719
Weighted average amortization period of intangible assets	6 years
Tenant relationship value [Member]
Intangible assets on real estate acquisitions
Gross Carrying Amount	$ 62,172	42,301
Accumulated Amortization	23,468	28,492
Net Carrying Amount	$ 38,704	13,809
Weighted average amortization period of intangible assets	11 years
Below-market cost arrangements [Member]
Intangible assets on real estate acquisitions
Gross Carrying Amount	$ 15,102	12,415
Accumulated Amortization	6,692	5,984
Net Carrying Amount	$ 8,410	6,431
Weighted average amortization period of intangible assets	35 years
Above-market leases [Member]
Intangible assets on real estate acquisitions
Gross Carrying Amount	$ 14,210	8,659
Accumulated Amortization	8,491	8,159
Net Carrying Amount	$ 5,719	500
Weighted average amortization period of intangible assets	4 years
Other [Member]
Intangible assets on real estate acquisitions
Gross Carrying Amount	$ 1,333	1,333
Accumulated Amortization	952	938
Net Carrying Amount	$ 381	$ 395
Weighted average amortization period of intangible assets	27 years

Investing Receivables (Details)

Investing Receivables (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Investing receivables	$ 47,875	$ 52,147
Notes receivable from City of Huntsville [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Investing receivables	$ 44,875	49,147
Notes receivable from City of Huntsville [Member] | LW Redstone Company, LLC [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Stated interest rate (as a percent)	9.95%
Debt instrument, term	30 years
Other investing loans receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Investing receivables	$ 3,000	$ 3,000

Prepaid Expenses and Other As70

Prepaid Expenses and Other Assets, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Prepaid Expense and Other Assets [Abstract]
Prepaid expenses	$ 23,009	$ 20,570
Lease incentives	11,133	13,344
Furniture, fixtures and equipment, net	6,004	6,637
Deferred financing costs, net	5,867	4,849
Deferred tax asset, net	3,467	4,002
Construction contract costs incurred in excess of billings	3,261	6,656
Equity method investments	1,636	2,368
Other assets	5,647	2,875
Prepaid expenses and other assets, net	$ 60,024	$ 61,301

Prepaid Expenses and Other As71

Prepaid Expenses and Other Assets, Net (Details 2) - TRS [Member] - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Tax Credit Carryforward [Line Items]
Operating loss carry forward	$ 5,065	$ 5,012
Share-based compensation	363	976
Accrued payroll	133	195
Property	(32)	(119)
Valuation allowance	(2,062)	(2,062)
Deferred tax asset, net	$ 3,467	$ 4,002

Debt (Details)

Debt (Details) - USD ($)	May. 06, 2015	Apr. 20, 2015	Dec. 31, 2015	Jun. 29, 2015	Dec. 31, 2014	May. 14, 2014	Dec. 31, 2013
Debt
Debt, net			$ 2,077,752,000		$ 1,914,036,000
Deferred financing costs, net			5,867,000		4,849,000
Long-term debt gross			2,097,230,000
Mortgage and Other Secured Loans [Member]
Debt
Debt, net			331,000,000		422,467,000
Fixed rate mortgage loans [Member]
Debt
Debt, net			$ 281,208,000		385,769,000
Stated Interest Rates, low end of range (as a percent)			3.96%
Stated Interest Rates, high end of range (as a percent)			7.87%
Unamortized premium included in carrying value			$ 514,000		42,000
Weighted average interest rate excluding incremental rate			6.08%
Variable rate secured loans [Member]
Debt
Debt, net			$ 49,792,000		36,698,000
Stated interest rate (as a percent)			2.20%
Revolving Credit Facility [Member]
Debt
Debt, net			$ 43,500,000		83,000,000
Term Loan Facilities [Member]
Debt
Debt, net			$ 515,900,000		517,846,000
Weighted average interest rate (as a percent)			1.92%
Additional borrowing capacity available provided there is no default under the agreement			$ 150,000,000
Aggregate additional borrowing capacity			430,000,000
Unsecured notes payable [Member]
Debt
Debt, net			1,500,000		1,607,000
Unamortized discount included in carrying value			554,000		654,000
4.25% Exchangeable Senior Notes [Member]
Debt
Debt, net			$ 0		572,000
Stated interest rate (as a percent)							4.25%
Debt Instrument, Redemption, Actual Redemption Price As Percentage Of Principal		100.00%
Unamortized discount included in carrying value					3,000
Long-term debt gross					$ 575,000
London Interbank Offered Rate (LIBOR) [Member] | Variable rate secured loans [Member] | Minimum [Member]
Debt
Variable rate, spread (as a percent)			1.85%
London Interbank Offered Rate (LIBOR) [Member] | Variable rate secured loans [Member] | Maximum [Member]
Debt
Variable rate, spread (as a percent)			2.00%
London Interbank Offered Rate (LIBOR) [Member] | Revolving Credit Facility [Member] | Minimum [Member]
Debt
Variable rate, spread (as a percent)			0.875%
London Interbank Offered Rate (LIBOR) [Member] | Revolving Credit Facility [Member] | Maximum [Member]
Debt
Variable rate, spread (as a percent)			1.60%
London Interbank Offered Rate (LIBOR) [Member] | Term Loan Facilities [Member] | Minimum [Member]
Debt
Variable rate, spread (as a percent)			0.90%
London Interbank Offered Rate (LIBOR) [Member] | Term Loan Facilities [Member] | Maximum [Member]
Debt
Variable rate, spread (as a percent)			2.60%
Revolving Credit Facility [Member] | Revolving Credit Facility [Member]
Debt
Weighted average interest rate (as a percent)			1.40%		1.47%
Loans Payable [Member]
Debt
Deferred financing costs, net			$ 8,000,000		$ 6,000,000
Line of Credit [Member] | Revolving Credit Facility [Member] | London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member]
Debt
Variable rate, spread (as a percent)	0.875%
Line of Credit [Member] | Revolving Credit Facility [Member] | London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member]
Debt
Variable rate, spread (as a percent)	1.60%
Senior Notes [Member] | 3.60% Senior Notes [Member]
Debt
Debt, net			$ 346,714,000		346,311,000
Stated interest rate (as a percent)			3.60%
Interest rate on debt (as a percent)			3.70%
Unamortized discount included in carrying value			$ 2,200,000		2,500,000
Senior Notes [Member] | 5.250% Senior Notes [Member]
Debt
Debt, net			$ 245,731,000		245,306,000
Stated interest rate (as a percent)			5.25%
Interest rate on debt (as a percent)			5.49%
Unamortized discount included in carrying value			$ 3,800,000		4,200,000
Senior Notes [Member] | 3.70% Senior Notes [Member]
Debt
Debt, net			$ 297,378,000		296,927,000
Stated interest rate (as a percent)			3.70%			3.70%
Interest rate on debt (as a percent)			3.85%
Unamortized discount included in carrying value			$ 2,100,000		2,400,000
Senior Notes [Member] | 5.0% Senior Notes [Member]
Debt
Debt, net			$ 296,019,000		$ 0
Stated interest rate (as a percent)			5.00%	5.00%
Interest rate on debt (as a percent)			5.15%
Unamortized discount included in carrying value			$ 3,300,000
Senior Notes [Member] | 4.25% Exchangeable Senior Notes [Member]
Debt
Stated interest rate (as a percent)					4.25%

Debt (Details 2)

Debt (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule on basis of which debt matures
2,016	$ 208,109
2,017	3,252
2,018	3,400
2,019	167,014
2,020	315,252
Thereafter	1,400,203
Total	2,097,230
Debt maturing in 2019 that may be extended for a one year period	43,500
Net discounts and deferred financing costs	19,500
Capitalized interest costs	$ 7,200	$ 6,100	$ 8,800

Debt (Details 3)

Debt (Details 3) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Debt, net	$ 2,077,752	$ 1,914,036
Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Debt, net	1,200,000
Carrying Amount [Member]
Debt Instrument [Line Items]
Variable-rate debt	609,194	637,544
Debt, net	2,077,752	1,914,036
Carrying Amount [Member] | Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Fixed-rate debt	1,185,842	888,544
Carrying Amount [Member] | Other Fixed-Rate Debt [Member]
Debt Instrument [Line Items]
Fixed-rate debt	282,716	387,948
Estimated Fair Value [Member]
Debt Instrument [Line Items]
Variable-rate debt	610,987	642,091
Debt, net	2,114,636	1,900,067
Estimated Fair Value [Member] | Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Fixed-rate debt	1,211,658	901,599
Estimated Fair Value [Member] | Other Fixed-Rate Debt [Member]
Debt Instrument [Line Items]
Fixed-rate debt	$ 291,991	$ 356,377

Debt (Details 4)

Debt (Details 4)

Dec. 17, 2015USD ($)

Aug. 28, 2015USD ($)

Jun. 29, 2015USD ($)

May. 06, 2015USD ($)extensions

May. 14, 2014USD ($)

Dec. 23, 2013USD ($)

Aug. 03, 2012USD ($)

Feb. 14, 2012USD ($)

Sep. 01, 2011USD ($)

Dec. 31, 2014USD ($)

Dec. 31, 2015USD ($)

Dec. 31, 2014USD ($)

Dec. 31, 2013USD ($)

Debt Instrument [Line Items]

Gain (loss) on early extinguishment of debt

$ 85,275,000

$ (9,552,000)

$ (27,030,000)

Term Credit Facility Effective February 2012 [Member]

Debt Instrument [Line Items]

Initial amount borrowed

$ 250,000,000

Term Credit Facility Effective August 2012 [Member]

Debt Instrument [Line Items]

Initial amount borrowed

$ 120,000,000

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

$ 80,000,000

Term Credit Facility Effective September 2011 [Member]

Debt Instrument [Line Items]

Initial amount borrowed

$ 400,000,000

Repayments of debt

100,000,000

$ 150,000,000

4.25% Exchangeable Senior Notes [Member]

Debt Instrument [Line Items]

Stated interest rate (as a percent)

4.25%

Unamortized discount included in carrying value

$ 3,000

$ 3,000

Gain (loss) on early extinguishment of debt

$ (25,900,000)

Amount of loans repaid upon entry into Revolving Credit Facility and Term Loan Agreement

$ 239,400,000

Fixed rate mortgage loans [Member]

Debt Instrument [Line Items]

Debt settled through the surrender of real estate assets

$ 150,000,000

Minimum [Member] | Revolving Credit Facility [Member] | London Interbank Offered Rate (LIBOR) [Member]

Debt Instrument [Line Items]

Variable rate, spread (as a percent)

0.875%

Minimum [Member] | Term Credit Facility Effective February 2012 [Member] | London Interbank Offered Rate (LIBOR) [Member]

Debt Instrument [Line Items]

Variable rate, spread (as a percent)

0.90%

Minimum [Member] | Term Credit Facility Effective August 2012 [Member] | London Interbank Offered Rate (LIBOR) [Member]

Debt Instrument [Line Items]

Variable rate, spread (as a percent)

2.10%

Maximum [Member] | Revolving Credit Facility [Member] | London Interbank Offered Rate (LIBOR) [Member]

Debt Instrument [Line Items]

Variable rate, spread (as a percent)

1.60%

Maximum [Member] | Term Credit Facility Effective February 2012 [Member] | London Interbank Offered Rate (LIBOR) [Member]

Debt Instrument [Line Items]

Variable rate, spread (as a percent)

1.85%

Maximum [Member] | Term Credit Facility Effective August 2012 [Member] | London Interbank Offered Rate (LIBOR) [Member]

Debt Instrument [Line Items]

Variable rate, spread (as a percent)

2.60%

Secured Loans Due November 2015 And January 2016 [Member]

Debt Instrument [Line Items]

Gain (loss) on early extinguishment of debt

$ (9,100,000)

Secured Loan Due November 2015 [Member]

Debt Instrument [Line Items]

Stated interest rate (as a percent)

5.53%

5.53%

Amount of loans repaid upon entry into Revolving Credit Facility and Term Loan Agreement

$ 103,000,000

Secured Loan Due January 2016 [Member]

Debt Instrument [Line Items]

Stated interest rate (as a percent)

5.56%

5.56%

Amount of loans repaid upon entry into Revolving Credit Facility and Term Loan Agreement

$ 108,500,000

Revolving Credit Facility [Member] | Revolving Credit Facility [Member]

Debt Instrument [Line Items]

Maximum availability

$ 800,000,000

Expansion right, maximum borrowing capacity

$ 1,300,000,000

Amount available as a percentage of unencumbered Asset value

60.00%

Line of credit facility, extension fee percentage

0.075%

Remaining borrowing capacity

$ 741,700,000

Weighted average borrowings

$ 125,000,000

$ 15,900,000

Weighted average interest rate (as a percent)

1.47%

1.40%

1.47%

Line of Credit [Member] | Revolving Credit Facility [Member]

Debt Instrument [Line Items]

Number of extensions | extensions

2

Extension option period (in years)

6 months

Line of Credit [Member] | Revolving Credit Facility [Member] | Minimum [Member]

Debt Instrument [Line Items]

Commitment fee percentage

0.125%

Line of Credit [Member] | Revolving Credit Facility [Member] | Minimum [Member] | London Interbank Offered Rate (LIBOR) [Member]

Debt Instrument [Line Items]

Variable rate, spread (as a percent)

0.875%

Line of Credit [Member] | Revolving Credit Facility [Member] | Maximum [Member]

Debt Instrument [Line Items]

Commitment fee percentage

0.30%

Line of Credit [Member] | Revolving Credit Facility [Member] | Maximum [Member] | London Interbank Offered Rate (LIBOR) [Member]

Debt Instrument [Line Items]

Variable rate, spread (as a percent)

1.60%

Line of Credit [Member] | Term Loan Facilities [Member]

Debt Instrument [Line Items]

Maximum availability

$ 300,000,000

Increase to maximum borrowing capacity

200,000,000

Aggregate maximum borrowing capacity

$ 500,000,000

Unsecured Debt [Member] | Term Credit Facility Effective December 2015 [Member]

Debt Instrument [Line Items]

Maximum availability

$ 250,000,000

Initial amount borrowed

100,000,000

Increase to maximum borrowing capacity

$ 150,000,000

Unsecured Debt [Member] | Minimum [Member] | Term Credit Facility Effective December 2015 [Member] | London Interbank Offered Rate (LIBOR) [Member]

Debt Instrument [Line Items]

Variable rate, spread (as a percent)

1.40%

Unsecured Debt [Member] | Maximum [Member] | Term Credit Facility Effective December 2015 [Member] | London Interbank Offered Rate (LIBOR) [Member]

Debt Instrument [Line Items]

Variable rate, spread (as a percent)

2.35%

Senior Notes [Member]

Debt Instrument [Line Items]

Redemption price (percent)

100.00%

Senior Notes [Member] | 3.70% Senior Notes [Member]

Debt Instrument [Line Items]

Initial amount borrowed

$ 297,300,000

Loan amount

$ 300,000,000

Stated interest rate (as a percent)

3.70%

3.70%

Debt issuance as a percentage of principal amount

99.739%

Unamortized discount included in carrying value

$ 2,400,000

$ 2,100,000

$ 2,400,000

Interest rate on debt (as a percent)

3.85%

Senior Notes [Member] | 3.70% Senior Notes [Member] | Adjusted Treasury [Member]

Debt Instrument [Line Items]

Basis points used in determining redemption price prior to maturity

250.00%

Senior Notes [Member] | 5.0% Senior Notes [Member]

Debt Instrument [Line Items]

Loan amount

$ 300,000,000

Stated interest rate (as a percent)

5.00%

5.00%

Debt issuance as a percentage of principal amount

99.51%

Unamortized discount included in carrying value

$ 3,300,000

Interest rate on debt (as a percent)

5.15%

Proceeds from debt, net

$ 296,600,000

Senior Notes [Member] | 5.0% Senior Notes [Member] | Adjusted Treasury [Member]

Debt Instrument [Line Items]

Basis points used in determining redemption price prior to maturity

450.00%

Senior Notes [Member] | 3.60% Senior Notes [Member]

Debt Instrument [Line Items]

Stated interest rate (as a percent)

3.60%

Unamortized discount included in carrying value

2,500,000

$ 2,200,000

2,500,000

Interest rate on debt (as a percent)

3.70%

Senior Notes [Member] | 3.60% Senior Notes [Member] | Adjusted Treasury [Member]

Debt Instrument [Line Items]

Basis points used in determining redemption price prior to maturity

300.00%

Senior Notes [Member] | 5.250% Senior Notes [Member]

Debt Instrument [Line Items]

Stated interest rate (as a percent)

5.25%

Unamortized discount included in carrying value

$ 4,200,000

$ 3,800,000

$ 4,200,000

Interest rate on debt (as a percent)

5.49%

Senior Notes [Member] | 5.250% Senior Notes [Member] | Adjusted Treasury [Member]

Debt Instrument [Line Items]

Basis points used in determining redemption price prior to maturity

400.00%

Senior Notes [Member] | 4.25% Exchangeable Senior Notes [Member]

Debt Instrument [Line Items]

Stated interest rate (as a percent)

4.25%

4.25%

Operating properties, net [Member] | December 2013 Portfolio Conveyance [Member]

Debt Instrument [Line Items]

Gain (loss) on early extinguishment of debt

$ 67,800,000

Debt settled through the surrender of real estate assets

$ 146,500,000

Operating properties, net [Member] | 15000 and 15010 Conference Center Drive [Member] | Northern Virginia [Member]

Debt Instrument [Line Items]

Gain (loss) on early extinguishment of debt

84,800,000

Debt settled through the surrender of real estate assets

$ 150,000,000

Interest Rate Derivatives (Deta

Interest Rate Derivatives (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Effect of interest rate derivatives on consolidated statements of operations and comprehensive income
Amount of (losses) gains recognized in accumulated other comprehensive income (loss) (“AOCI”) (effective portion)	$ (4,739)	$ (7,799)	$ 6,791
Interest expense [Member]
Effect of interest rate derivatives on consolidated statements of operations and comprehensive income
Amount of losses reclassified from AOCI into interest rate derivatives	3,599	2,990	2,740
Loss on early extinguishment of debt [Member]
Effect of interest rate derivatives on consolidated statements of operations and comprehensive income
Amount of losses reclassified from AOCI into interest rate derivatives	0	38	0
Interest rate swaps
Effect of interest rate derivatives on consolidated statements of operations and comprehensive income
Amount of (losses) gains recognized in accumulated other comprehensive income (loss) (“AOCI”) (effective portion)	(4,739)	(7,799)	6,791
Approximate amount of losses to be reclassified from AOCL to interest expense over the next 12 months	3,300
Interest rate derivatives in liability position, fair value	3,200
Termination value to settle obligations under interest rate derivative agreements	3,600
Interest rate swaps | Interest expense [Member]
Effect of interest rate derivatives on consolidated statements of operations and comprehensive income
Amount of losses reclassified from AOCI into interest rate derivatives	3,599	2,990	2,740
Amount of loss recognized in interest expense (ineffective portion)	386	0	0
Interest rate swaps | Loss on early extinguishment of debt [Member]
Effect of interest rate derivatives on consolidated statements of operations and comprehensive income
Amount of losses reclassified from AOCI into interest rate derivatives	0	38	$ 0
Interest rate swaps | Prepaid expenses and other assets [Member]
Fair value of interest rate derivatives and balance sheet classification
Interest rate derivatives	53	274
Interest rate swaps | Interest rate derivative [Member]
Fair value of interest rate derivatives and balance sheet classification
Fair value of Interest rate swaps classified as interest rate derivatives	(3,160)	(1,855)
Designated [Member]
Fair values of interest rate swap derivatives
Fair value of interest rate swaps	(3,107)	(1,581)
Designated [Member] | Interest rate swap, effective date September 2, 2014, swap one [Member]
Fair values of interest rate swap derivatives
Notional Amount	$ 100,000
Fixed Rate (as a percent)	0.8055%
Fair value of interest rate swaps	$ (148)	(317)
Designated [Member] | Interest rate swap, effective date September 2, 2014, swap two [Member]
Fair values of interest rate swap derivatives
Notional Amount	$ 100,000
Fixed Rate (as a percent)	0.81%
Fair value of interest rate swaps	$ (151)	(324)
Designated [Member] | Interest rate swap, effective date September 1, 2015, swap one [Member]
Fair values of interest rate swap derivatives
Notional Amount	$ 100,000
Fixed Rate (as a percent)	1.673%
Fair value of interest rate swaps	$ (1,217)	239
Designated [Member] | Interest rate swap, effective date September 1, 2015, swap two [Member]
Fair values of interest rate swap derivatives
Notional Amount	$ 100,000
Fixed Rate (as a percent)	1.73%
Fair value of interest rate swaps	$ (1,429)	35
Designated [Member] | Interest rate swap, effective October 1, 2015 [Member]
Fair values of interest rate swap derivatives
Notional Amount	$ 13,941
Fixed Rate (as a percent)	1.39%
Fair value of interest rate swaps	$ 53	0
Notional amount after scheduled amortization	12,100
Designated [Member] | Interest rate swap, effective September 1, 2016, swap one [Member]
Fair values of interest rate swap derivatives
Notional Amount	$ 100,000
Fixed Rate (as a percent)	1.9013%
Fair value of interest rate swaps	$ (138)	0
Designated [Member] | Interest rate swap, effective September 1, 2016, swap two [Member]
Fair values of interest rate swap derivatives
Notional Amount	$ 100,000
Fixed Rate (as a percent)	1.905%
Fair value of interest rate swaps	$ (45)	0
Designated [Member] | Interest rate swap, effective September 1, 2016, swap three [Member]
Fair values of interest rate swap derivatives
Notional Amount	$ 50,000
Fixed Rate (as a percent)	1.9079%
Fair value of interest rate swaps	$ (32)	0
Designated [Member] | Interest rate swap, effective date November 2, 2010 [Member]
Fair values of interest rate swap derivatives
Notional Amount	$ 36,200
Fixed Rate (as a percent)	3.83%
Fair value of interest rate swaps	$ 0	(400)
Designated [Member] | Interest rate swap, effective date November 2, 2010 [Member] | London Interbank Offered Rate (LIBOR) [Member]
Fair values of interest rate swap derivatives
Floating Rate Index (percent)	2.25%
Designated [Member] | Interest rate swap, effective date January 3, 2012, swap three [Member]
Fair values of interest rate swap derivatives
Notional Amount	$ 100,000
Fixed Rate (as a percent)	0.832%
Fair value of interest rate swaps	$ 0	(407)
Designated [Member] | Interest rate swap, effective date January 3, 2012, swap four [Member]
Fair values of interest rate swap derivatives
Notional Amount	$ 100,000
Fixed Rate (as a percent)	0.832%
Fair value of interest rate swaps	$ 0	$ (407)

Redeemable Noncontrolling Int77

Redeemable Noncontrolling Interest (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Redeemable Noncontrolling Interest [Roll Forward]
Beginning balance	$ 18,417	$ 17,758	$ 10,298
Contributions from noncontrolling interests	1,654	0	0
Distributions to noncontrolling interests	(2,964)	(1,369)	(1,037)
Net income attributable to noncontrolling interests	2,227	2,162	1,376
Adjustment to arrive at fair value of interests	(116)	(134)	7,121
Ending balance	$ 19,218	$ 18,417	$ 17,758

Equity - COPT and Subsidiarie78

Equity - COPT and Subsidiaries (Details) - USD ($)	Jun. 16, 2014	Jul. 15, 2013	Nov. 30, 2014	Oct. 31, 2012	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Preferred Stock
Number of shares issued					7,431,667
Aggregate liquidation preference					$ 199,083,000	$ 199,083,000
Stock redeemed or called during period value						50,000,000	$ 84,750,000
Common Shares
Shares issued to the public			5,520,000
Per share value of common and preferred shares issued to the public (in dollars per share)			$ 27.30
Net proceeds from issue of shares after underwriter discounts but before offering expenses			$ 148,900,000			$ 148,666,000	$ 117,961,000
Number of common shares for each converted common unit (in shares)					1
Number of operating partnerships units converted into common shares (in units)					160,160	140,149	311,343
Dividends declared per common share					$ 1.1000	$ 1.10	$ 1.10
Common Stock Issued to Public Under At-the-Market Program [Member]
Common Shares
Shares issued to the public		1,500,000			890,241		1,500,000
At-market-stock, offering program established, aggregate value				$ 150,000,000
Issuance of stock, weighted average price per share					$ 30.29
Payments of stock issuance costs		$ 600,000			$ 400,000
At-market stock, offering program established, remaining capacity					84,000,000
Share Price		$ 26.05
Proceeds from Issuance of Common Stock, Net of Issuance Costs		$ 38,500,000			$ 26,600,000
Preferred Shares [Member]
Preferred Stock
Number of preferred shares authorized					25,000,000
Number of preferred shares of beneficial interest authorized (in dollars per share)					$ 0.01
Series H [Member]
Preferred Stock
Annual dividend yield	7.50%
Preferred stock, redemption price per share	$ 25
Stock redeemed or called during period value	$ 50,000,000
Issuance costs associated with redeemed preferred shares	$ 1,800,000
Series K [Member]
Preferred Stock
Number of shares issued					531,667
Aggregate liquidation preference					$ 26,583,000
Annual dividend yield					5.60%
Annual dividend per share (in dollars per share)					$ 2.80000
Number of common shares to be issued on conversion of each preferred share					0.8163
Series L [Member]
Preferred Stock
Number of shares issued					6,900,000
Aggregate liquidation preference					$ 172,500,000
Annual dividend yield					7.375%
Annual dividend per share (in dollars per share)					$ 1.84375

Equity - COPLP and Subsidiari79

Equity - COPLP and Subsidiaries (Details) - USD ($) $ / shares in Units, $ in Thousands	Jun. 16, 2014	Jul. 16, 2013	Jul. 15, 2013	Nov. 30, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Preferred Stock
Number of shares issued					7,431,667
Aggregate liquidation preference					$ 199,083	$ 199,083
Stock redeemed or called during period value						50,000	$ 84,750
Common Shares
Shares issued to the public				5,520,000
Per share value of common and preferred shares issued to the public (in dollars per share)				$ 27.30
Net proceeds from issue of shares after underwriter discounts but before offering expenses				$ 148,900		$ 148,666	$ 117,961
Number of common shares for each converted common unit (in shares)					1
Number of operating partnerships units converted into common shares (in units)					160,160	140,149	311,343
Dividends declared per common share					$ 1.1000	$ 1.10	$ 1.10
Preferred units in COPLP					$ 8,800	$ 8,800
Common Stock Issued to Public Under At-the-Market Program [Member]
Common Shares
Shares issued to the public			1,500,000		890,241		1,500,000
Issuance of stock, weighted average price per share					$ 30.29
Payments of stock issuance costs			$ 600		$ 400
Series H [Member]
Preferred Stock
Annual dividend yield	7.50%
Preferred stock, redemption price per share	$ 25
Stock redeemed or called during period value	$ 50,000
Issuance costs associated with redeemed preferred shares	$ 1,800
Series K [Member]
Preferred Stock
Number of shares issued					531,667
Aggregate liquidation preference					$ 26,583
Annual dividend yield					5.60%
Annual dividend per share (in dollars per share)					$ 2.80000
Number of common shares to be issued on conversion of each preferred share					0.8163
Series L [Member]
Preferred Stock
Number of shares issued					6,900,000
Aggregate liquidation preference					$ 172,500
Annual dividend yield					7.375%
Annual dividend per share (in dollars per share)					$ 1.84375
Corporate Office Properties, L.P. [Member]
Preferred Stock
Number of shares issued					7,431,667
Aggregate liquidation preference					$ 199,083
Stock redeemed or called during period value						50,000	$ 84,750
Common Shares
Net proceeds from issue of shares after underwriter discounts but before offering expenses						$ 148,666	$ 117,961
Number of common shares for each converted common unit (in shares)					1
Number of operating partnerships units converted into common shares (in units)					160,160	140,149	311,343
Dividends declared per common share					$ 1.1	$ 1.10	$ 1.10
Corporate Office Properties, L.P. [Member] | Common Stock Issued to Public Under At-the-Market Program [Member]
Common Shares
Shares issued to the public		1,500,000			890,241
Net proceeds from issue of shares after underwriter discounts but before offering expenses		$ 38,500			$ 26,600
Issuance of stock, weighted average price per share		$ 26.05			$ 30.29
Payments of stock issuance costs		$ 600			$ 400
Corporate Office Properties, L.P. [Member] | Common Units [Member]
Common Shares
Percentage ownership in operating partnership					96.30%	96.00%
Corporate Office Properties, L.P. [Member] | Conversion of Series I preferred units [Member]
Preferred Stock
Annual dividend yield					7.50%
Annual cumulative preferred return increment frequency					5 years
Common Shares
Limited partners' capital account, units issued					352,000
Preferred units in COPLP					$ 8,800
Preferred stock, liquidation preference per share					$ 25
Common units conversion basis units issuable					0.5
Corporate Office Properties, L.P. [Member] | Series H [Member]
Preferred Stock
Annual dividend yield	7.50%
Preferred stock, redemption price per share	$ 25
Stock redeemed or called during period value	$ 50,000
Issuance costs associated with redeemed preferred shares	$ 1,800
Corporate Office Properties, L.P. [Member] | Series K [Member]
Preferred Stock
Number of shares issued					531,667
Aggregate liquidation preference					$ 26,583
Annual dividend yield					5.60%
Annual dividend per share (in dollars per share)					$ 2.80000
Number of common shares to be issued on conversion of each preferred share					0.8163
Corporate Office Properties, L.P. [Member] | Series L [Member]
Preferred Stock
Number of shares issued					6,900,000
Aggregate liquidation preference					$ 172,500
Annual dividend yield					7.375%
Annual dividend per share (in dollars per share)					$ 1.84375
Corporate Office Properties, L.P. [Member] | Common Shares [Member]
Common Shares
Shares issued to the public				5,520,000
Per share value of common and preferred shares issued to the public (in dollars per share)				$ 27.30
Net proceeds from issue of shares after underwriter discounts but before offering expenses				$ 148,900

Share-Based Compensation and 80

Share-Based Compensation and Other Compensation Matters (Details 1) - Restricted shares [Member] - $ / shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Shares
Unvested at the beginning of the period (in shares)	390,507	377,448	434,643
Stock awards granted (in shares or units)	201,024	216,607	193,833
Forfeited (in shares)	(10,550)	(21,335)	(9,541)
Vested (in shares)	(202,781)	(182,213)	(241,487)
Unvested at the end of the period (in shares)	378,200	390,507	377,448
Weighted Average Grant Date Fair Value
Unvested at the beginning of the period (in dollars per share)	$ 26.19	$ 26.96	$ 29.67
Grant date fair value (in dollars per share)	28.69	26.73	25.91
Forfeited (in dollars per share)	26.05	25.10	27.59
Vested (in dollars per share)	26.07	28.56	30.97
Unvested at the end of the period (in dollars per share)	$ 27.58	$ 26.19	$ 26.96
Restricted shares expected to vest (in shares)	364,666
Restricted shares expected to vest (in dollars per share)	$ 27.61

Share-Based Compensation and 81

Share-Based Compensation and Other Compensation Matters (Details 2) - Performance share units [Member] - USD ($) $ in Thousands	Mar. 05, 2015	Mar. 06, 2014	Mar. 01, 2013	Mar. 02, 2012	Mar. 03, 2011	Dec. 31, 2015
2011 PSU Grants [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock awards granted (in shares or units)					56,883
Aggregate grant date fair value					$ 2,796
Unvested at the end of the period (in shares)						0
2012 PSU Grants [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock awards granted (in shares or units)				54,070
Aggregate grant date fair value				$ 1,772
Unvested at the end of the period (in shares)						0
2013 PSU Grants [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock awards granted (in shares or units)			69,579
Aggregate grant date fair value			$ 1,867
Unvested at the end of the period (in shares)						51,556
2014 PSU Grants [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock awards granted (in shares or units)		49,103
Aggregate grant date fair value		$ 1,723
Unvested at the end of the period (in shares)						37,843
2015 PSU Grants [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock awards granted (in shares or units)	45,656
Aggregate grant date fair value	$ 1,678
Unvested at the end of the period (in shares)						45,656

Share-Based Compensation and 82

Share-Based Compensation and Other Compensation Matters (Details 3) - Performance share units [Member] - $ / shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
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%
Assumptions used to value stock awards
Grant date fair value (in dollars per share)	$ 36.76	$ 35.09	$ 26.84
Baseline value per common share (in dollars per share)	$ 29.28	$ 26.52	$ 25.85
Expected volatility of common shares (as a percent)	19.90%	28.60%	29.50%
Risk-free interest rate (as a percent)	0.99%	0.66%	0.33%

Share-Based Compensation and 83

Share-Based Compensation and Other Compensation Matters (Details 4) - Options [Member] - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options Outstanding and Exercisable (in shares)	425,347	559,736	640,927	798,210
Weighted Average Exercise Price Per Share (in dollars per share)	$ 42.75	$ 39.60	$ 38.11	$ 37.62
Weighted Average Remaining Contractual Term (years)	1 year	2 years	2 years	3 years
Aggregate Intrinsic Value		$ 167	$ 68	$ 325

Share-Based Compensation and 84

Share-Based Compensation and Other Compensation Matters (Details 5) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-Based Compensation
Share-based compensation cost	$ 7,398	$ 7,050	$ 7,605
General, adminstrative and leasing expenses [Member]
Share-Based Compensation
Share-based compensation cost	5,574	5,307	5,412
Property operating expenses [Member]
Share-Based Compensation
Share-based compensation cost	1,000	857	1,118
Capitalized to development activities [Member]
Share-Based Compensation
Share-based compensation cost	$ 824	$ 886	$ 1,075

Share-Based Compensation and 85

Share-Based Compensation and Other Compensation Matters (Details 6) $ / shares in Units, $ in Thousands	Mar. 05, 2015shares	Mar. 06, 2014USD ($)shares	Mar. 01, 2013USD ($)shares	Mar. 02, 2012USD ($)shares	Dec. 31, 2015USD ($)Percentile_Rank$ / sharesshares	Dec. 31, 2014USD ($)$ / sharesshares	Dec. 31, 2013USD ($)$ / sharesshares	May. 31, 2010shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Decrease in tax benefit from share-based compensation					$ 513	$ 3	$ 122
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period					10 years
Restricted shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate intrinsic value of vested shares					$ 4,900	$ 4,900	$ 6,300
Stock awards granted (in shares or units) | shares					201,024	216,607	193,833
Grant date fair value (in dollars per share) | $ / shares					$ 28.69	$ 26.73	$ 25.91
Vested (in shares) | shares					(202,781)	(182,213)	(241,487)
Vested (in dollars per share) | $ / shares					$ 26.07	$ 28.56	$ 30.97
Unrecognized compensation cost					$ 6,300
Expected weighted average period during which unrecognized compensation cost will be recognized					2 years
Restricted shares [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Estimated pre-vesting forfeitures (as a percent)					0.00%
Restricted shares [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Estimated pre-vesting forfeitures (as a percent)					5.00%
Performance share units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance period of the award					3 years
The number of percentile ranks to fall between to earn interpolated PSUs between such percentile ranks, conditioned on the percentile rank exceeding 25% | Percentile_Rank					2
Grant date fair value (in dollars per share) | $ / shares					$ 36.76	$ 35.09	$ 26.84
Estimated pre-vesting forfeitures (as a percent)					0.00%
Unrecognized compensation cost					$ 1,700
Expected weighted average period during which unrecognized compensation cost will be recognized					2 years
Phantom Share Units (PSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate intrinsic value of vested shares					$ 413
Stock awards granted (in shares or units) | shares					24,056
Aggregate grant date fair value					$ 642
Grant date fair value (in dollars per share) | $ / shares					$ 26.70
Vested (in shares) | shares					(15,485)
Vested (in dollars per share) | $ / shares					$ 26.77
Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate intrinsic value of options exercised					$ 300	$ 225	$ 258
Deferred Share Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Estimated pre-vesting forfeitures (as a percent)					0.00%
Unrecognized compensation cost					$ 223
Amended and Restated 2008 Omnibus Equity and Incentive Plan [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common shares of beneficial interest authorized to be issued | shares								5,900,000
2014 PSU Grants [Member] | Performance share units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock awards granted (in shares or units) | shares		49,103
Aggregate grant date fair value		$ 1,723
2012 PSU Grants [Member] | Performance share units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued for PSU Awards Vested in Period (in shares) | shares	40,309
Stock awards granted (in shares or units) | shares				54,070
Aggregate grant date fair value				$ 1,772
2013 PSU Grants [Member] | Performance share units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock awards granted (in shares or units) | shares			69,579
Aggregate grant date fair value			$ 1,867
2013 and 2014 PSU Grants [Member] | Performance share units [Member] | Former Chief Financial Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued for PSU Awards Vested in Period (in shares) | shares	15,289

Operating Leases (Details)

Operating Leases (Details) $ in Thousands	Dec. 31, 2015USD ($)
Gross minimum future rentals
2,016	$ 378,780
2,017	349,192
2,018	278,714
2,019	218,977
2,020	154,207
Thereafter	433,892
Total	$ 1,813,762

Information by Business Segme87

Information by Business Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment financial information for real estate operations
Revenues from real estate operations	$ 519,068	$ 479,711	$ 498,633
Property operating expenses	194,488	179,799	180,704
NOI from real estate operations	324,580	299,912	317,929
Segment assets	3,909,312	3,664,236
Operating Segments [Member]
Segment financial information for real estate operations
Revenues from real estate operations	519,068	479,711	498,633
Property operating expenses	194,488	179,799	180,704
NOI from real estate operations	324,580	299,912	317,929
Additions to long-lived assets	335,269	64,601	56,877
Transfers from non-operating properties	307,120	141,899	264,290
Segment assets	3,265,469	2,951,495	2,905,630
Operating Segments [Member] | Defense/Information Technology Sector [Member]
Segment financial information for real estate operations
Revenues from real estate operations	394,283	376,742	385,863
Property operating expenses	144,444	137,486	135,906
NOI from real estate operations	249,839	239,256	249,957
Additions to long-lived assets	130,670	45,379	42,244
Transfers from non-operating properties	194,647	121,227	161,301
Segment assets	2,342,746	2,188,021	2,107,793
Operating Segments [Member] | Defense/Information Technology Sector [Member] | Fort Meade/BW Corridor [Member]
Segment financial information for real estate operations
Revenues from real estate operations	244,274	233,764	234,857
Property operating expenses	83,309	80,824	80,149
NOI from real estate operations	160,965	152,940	154,708
Additions to long-lived assets	31,883	24,173	28,009
Transfers from non-operating properties	45,560	56,699	50,067
Segment assets	1,290,028	1,264,353	1,229,192
Operating Segments [Member] | Defense/Information Technology Sector [Member] | Northern Virginia Defense/IT [Member]
Segment financial information for real estate operations
Revenues from real estate operations	49,199	48,313	56,524
Property operating expenses	20,107	19,071	19,718
NOI from real estate operations	29,092	29,242	36,806
Additions to long-lived assets	90,248	7,119	5,522
Transfers from non-operating properties	50,690	360	37,234
Segment assets	411,196	372,041	385,061
Operating Segments [Member] | Defense/Information Technology Sector [Member] | Lackland Air Force Base [Member]
Segment financial information for real estate operations
Revenues from real estate operations	39,659	34,463	31,252
Property operating expenses	22,004	19,677	16,784
NOI from real estate operations	17,655	14,786	14,468
Additions to long-lived assets	0	0	5
Transfers from non-operating properties	32,307	0	0
Segment assets	134,381	102,232	103,650
Operating Segments [Member] | Defense/Information Technology Sector [Member] | Navy Support Locations [Member]
Segment financial information for real estate operations
Revenues from real estate operations	28,177	31,335	33,431
Property operating expenses	13,229	12,576	12,872
NOI from real estate operations	14,948	18,759	20,559
Additions to long-lived assets	7,656	10,010	4,945
Transfers from non-operating properties	1,408	0	14
Segment assets	196,090	195,972	195,105
Operating Segments [Member] | Defense/Information Technology Sector [Member] | Redstone Arsenal [Member]
Segment financial information for real estate operations
Revenues from real estate operations	11,228	10,446	5,050
Property operating expenses	3,497	3,066	1,282
NOI from real estate operations	7,731	7,380	3,768
Additions to long-lived assets	883	4,077	3,563
Transfers from non-operating properties	13,190	21,014	48,799
Segment assets	108,038	97,209	77,773
Operating Segments [Member] | Defense/Information Technology Sector [Member] | Colorado Defense / IT [Member]
Segment financial information for real estate operations
Revenues from real estate operations	0	0	12,229
Property operating expenses	0	0	3,797
NOI from real estate operations	0	0	8,432
Additions to long-lived assets	0	0	200
Transfers from non-operating properties	0	0	987
Segment assets	0	0	0
Operating Segments [Member] | Defense/Information Technology Sector [Member] | Data Center Shells [Member]
Segment financial information for real estate operations
Revenues from real estate operations	21,746	18,421	12,520
Property operating expenses	2,298	2,272	1,304
NOI from real estate operations	19,448	16,149	11,216
Additions to long-lived assets	0	0	0
Transfers from non-operating properties	51,492	43,154	24,200
Segment assets	203,013	156,214	117,012
Operating Segments [Member] | Regional Office [Member]
Segment financial information for real estate operations
Revenues from real estate operations	98,165	85,025	98,487
Property operating expenses	36,165	31,427	35,488
NOI from real estate operations	62,000	53,598	62,999
Additions to long-lived assets	204,139	19,290	13,244
Transfers from non-operating properties	22,313	17,942	32,692
Segment assets	608,471	526,988	556,718
Operating Segments [Member] | Operating Wholesale Data Center [Member]
Segment financial information for real estate operations
Revenues from real estate operations	19,032	10,430	7,271
Property operating expenses	10,402	7,286	6,360
NOI from real estate operations	8,630	3,144	911
Additions to long-lived assets	132	22	598
Transfers from non-operating properties	89,745	1,108	70,106
Segment assets	243,338	163,177	166,790
Operating Segments [Member] | Other Segments [Member]
Segment financial information for real estate operations
Revenues from real estate operations	7,588	7,514	7,012
Property operating expenses	3,477	3,600	2,950
NOI from real estate operations	4,111	3,914	4,062
Additions to long-lived assets	328	(90)	791
Transfers from non-operating properties	415	1,622	191
Segment assets	$ 70,914	$ 73,309	$ 74,329

Information by Business Segme88

Information by Business Segment (Details 2) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reconciliation of segment revenues to total revenues
Segment revenues from real estate operations									$ 519,068	$ 479,711	$ 498,633
Construction contract and other service revenues									106,402	106,748	62,363
Less: Revenues from discontinued operations									(4)	14	(37,636)
Total revenues	$ 143,325	$ 150,744	$ 170,363	$ 161,034	$ 146,971	$ 153,015	$ 139,820	$ 146,667	625,466	586,473	523,360
Reconciliation of segment property operating expenses to property operating expenses
Segment property operating expenses									194,488	179,799	180,704
Less: Property operating expenses from discontinued operations									6	135	(13,505)
Total property operating expenses									194,494	179,934	167,199
Computation of net operating income from service operations
Construction contract and other service revenues									106,402	106,748	62,363
Construction contract and other service expenses									(102,696)	(100,058)	(58,875)
NOI from service operations									3,706	6,690	3,488
Reconciliation of NOI from real estate operations and NOI from service operations to (loss) income from continuing operations
NOI from real estate operations									324,580	299,912	317,929
NOI from service operations									3,706	6,690	3,488
Interest and other income									4,517	4,923	3,834
Equity in income of unconsolidated entities									62	229	2,110
Income tax expense									(199)	(310)	(1,978)
Other adjustments:
Depreciation and amortization associated with real estate operations									(140,025)	(136,086)	(113,214)
Impairment losses									(23,289)	(1,416)	(5,857)
General, administrative and leasing expenses									(31,361)	(31,794)	(30,869)
Business development expenses and land carry costs									(13,507)	(5,573)	(5,436)
Interest expense on continuing operations									(89,074)	(92,393)	(82,010)
NOI from discontinued operations									(10)	(121)	(24,131)
Gain (loss) on early extinguishment of debt									85,275	(9,552)	(27,030)
Income from continuing operations	$ (1,430)	$ 94,279	$ 16,839	$ 10,987	$ 5,874	$ 13,727	$ 9,248	$ 5,660	$ 120,675	$ 34,509	$ 36,836

Information by Business Segme89

Information by Business Segment (Details 3) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reconciliation of segment assets to total assets
Total COPT consolidated assets	$ 3,909,312	$ 3,664,236
Segment assets [Member]
Reconciliation of segment assets to total assets
Total COPT consolidated assets	3,265,469	2,951,495	$ 2,905,630
Non-operating property assets [Member]
Reconciliation of segment assets to total assets
Total COPT consolidated assets	450,679	567,586
Other assets [Member]
Reconciliation of segment assets to total assets
Total COPT consolidated assets	$ 193,164	$ 145,155

Discontinued Operations (Detail

Discontinued Operations (Details) $ in Thousands	Dec. 23, 2013Property	Dec. 12, 2013Property	Dec. 31, 2015USD ($)	Sep. 30, 2015USD ($)	Jun. 30, 2015USD ($)	Mar. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Sep. 30, 2014USD ($)	Jun. 30, 2014USD ($)	Mar. 31, 2014USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)
Properties
Revenue from real estate operations											$ 4	$ (14)	$ 37,636
Property operating expenses											6	135	(13,505)
Depreciation and amortization											0	0	(4,505)
Impairment losses											(234)	(3)	(26,190)
General, administrative and leasing expenses											0	0	(4)
Interest expense											0	0	(8,221)
Gain on sales of real estate											0	24	2,671
Gain (loss) on early extinguishment of debt											380	(116)	67,810
Discontinued operations			$ 0	$ 0	$ 394	$ (238)	$ 22	$ 191	$ (198)	$ 11	$ 156	$ 26	$ 55,692
December 2013 Colorado Springs Portfolio Disposition [Member]
Properties
Number of properties sold | Property		15
Baltimore/Washington Corridor [Member]
Properties
Number of properties transferred | Property	9
Colorado Springs [Member]
Properties
Number of properties transferred | Property	5

Earnings Per Share ("EPS") an91

Earnings Per Share ("EPS") and Earnings Per Unit (“EPU”) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2013
Numerator:
Income (loss) from continuing operations		$ (1,430)	$ 94,279	$ 16,839	$ 10,987	$ 5,874	$ 13,727	$ 9,248	$ 5,660	$ 120,675		$ 34,509		$ 36,836
Gain on sales of real estate, net										68,047		10,671		9,016
Preferred share/ unit dividends/ distributions		(3,553)	(3,552)	(3,553)	(3,552)	(3,552)	(3,553)	(4,344)	(4,490)	(14,210)		(15,939)		(19,971)
Issuance costs associated with redeemed preferred shares		0	0	0	0	0	0	(1,769)	0	0		(1,769)		(2,904)
(Income) loss from continuing operations attributable to noncontrolling interests										(10,575)		(4,955)		(4,486)
Income from continuing operations attributable to restricted units										(706)		(432)		(414)
Numerator for basic EPS/EPU from continuing operations attributable to COPT/COPLP common shareholders/unitholders										163,231		22,085		18,077
Dilutive effect of common units in the Operating Partnership on diluted EPS from continuing operations										6,397		0		0
Numerator for diluted EPS/EPU from continuing operations attributable to COPT/COPLP common shareholders/unitholders										169,628		22,085		18,077
Discontinued operations		$ 0	$ 0	$ 394	$ (238)	$ 22	$ 191	$ (198)	$ 11	156		26		55,692
Discontinued operations attributable to noncontrolling interests										(3)		4		(3,351)
Numerator for basic EPS on net income attributable to COPT common shareholders										163,384		22,115		70,418
Dilutive effect of common units in COPLP										6,403		0		0
Numerator for diluted EPS on net income attributable to COPT common shareholders										$ 169,787		$ 22,115		$ 70,418
Denominator (all weighted averages):
Denominator for basic EPS (common shares)										93,914		88,092		85,167
Dilutive effect of common units										3,692		0		0
Dilutive effect of share-based compensation awards										61		171		57
Denominator for basic and diluted EPS (common shares)										97,667		88,263		85,224
Basic EPS:
Income from continuing operations (in dollars per share/unit)	[1]									$ 1.74		$ 0.25		$ 0.21
Discontinued operations (in dollars per share/unit)	[1]									0		0		0.62
Net income attributable to COPT common shareholders (in dollars per share/unit)		$ 0.59	$ 0.91	$ 0.13	$ 0.10	$ 0.01	$ 0.22	$ 0.02	$ 0	1.74	[1]	0.25	[1]	0.83	[1]
Diluted EPS:
Income from continuing operations (in dollars per share/unit)	[1]									1.74		0.25		0.21
Discontinued operations (in dollars per share/unit)	[1]									0		0		0.62
Net income attributable to COPT common shareholders (in dollars per share/unit)		$ 0.59	$ 0.91	$ 0.13	$ 0.10	$ 0.01	$ 0.22	$ 0.02	$ 0	$ 1.74	[1]	$ 0.25	[1]	$ 0.83	[1]
Corporate Office Properties, L.P. [Member]
Numerator:
Income (loss) from continuing operations		$ (1,430)	$ 94,279	$ 16,839	$ 10,987	$ 5,874	$ 13,727	$ 9,248	$ 5,660	$ 120,675		$ 34,509		$ 36,836
Gain on sales of real estate, net										68,047		10,671		9,016
Preferred share/ unit dividends/ distributions		(3,718)	(3,717)	(3,718)	(3,717)	(3,717)	(3,718)	(4,509)	(4,655)	(14,870)		(16,599)		(20,631)
Issuance costs associated with redeemed preferred shares		0	0	0	0	0	0	(1,769)	0	0		(1,769)		(2,904)
(Income) loss from continuing operations attributable to noncontrolling interests										(3,523)		(3,281)		(2,977)
Income from continuing operations attributable to restricted units										(706)		(432)		(414)
Numerator for basic and diluted EPS/EPU from continuing operations attributable to COPT/COPLP common shareholders/unitholders										169,623		23,099		18,926
Discontinued operations		$ 0	$ 0	$ 394	$ (238)	$ 22	$ 191	$ (198)	$ 11	156		26		55,692
Discontinued operations attributable to noncontrolling interests										3		5		(930)
Numerator for basic and diluted EPU on net income attributable to COPLP common unitholders										$ 169,782		$ 23,130		$ 73,688
Denominator (all weighted averages):
Denominator for basic EPS (common shares)										97,606		91,989		89,036
Dilutive effect of share-based compensation awards										61		171		57
Denominator for basic and diluted EPS (common shares)										97,667		92,160		89,093
Basic EPS:
Income from continuing operations (in dollars per share/unit)	[2]									$ 1.74		$ 0.25		$ 0.21
Discontinued operations (in dollars per share/unit)	[2]									0		0		0.62
Net income attributable to COPT common shareholders (in dollars per share/unit)		$ 0.59	$ 0.91	$ 0.13	$ 0.10	$ 0.01	$ 0.22	$ 0.02	$ 0	1.74	[2]	0.25	[2]	0.83	[2]
Diluted EPS:
Income from continuing operations (in dollars per share/unit)	[2]									1.74		0.25		0.21
Discontinued operations (in dollars per share/unit)	[2]									0		0		0.62
Net income attributable to COPT common shareholders (in dollars per share/unit)		$ 0.59	$ 0.91	$ 0.13	$ 0.10	$ 0.01	$ 0.22	$ 0.02	$ 0	$ 1.74	[2]	$ 0.25	[2]	$ 0.83	[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") an92

Earnings Per Share ("EPS") and Earnings Per Unit (“EPU”) (Details 2) - shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Conversion of common units [Member]
Antidilutive securities
Weighted average antidilutive securities excluded from computation of diluted earnings per share (in shares)	0	3,897,000	3,869,000
Conversion of Series I preferred units [Member]
Antidilutive securities
Weighted average antidilutive securities excluded from computation of diluted earnings per share (in shares)	176,000	176,000	176,000
Conversion of Series K preferred shares [Member]
Antidilutive securities
Weighted average antidilutive securities excluded from computation of diluted earnings per share (in shares)	434,000	434,000	434,000
Stock Options [Member]
Antidilutive securities
Weighted average antidilutive securities excluded from computation of diluted earnings per share (in shares)	469,000	492,000	636,000
Restricted Stock and Deferred Shares [Member]
Antidilutive securities
Weighted average antidilutive securities excluded from computation of diluted earnings per share (in shares)	410,000	401,000	385,000
Corporate Office Properties, L.P. [Member] | Stock Options [Member]
Antidilutive securities
Weighted average antidilutive securities excluded from computation of diluted earnings per share (in shares)	469,000	492,000	636,000
Corporate Office Properties, L.P. [Member] | Restricted Stock and Deferred Shares [Member]
Antidilutive securities
Weighted average antidilutive securities excluded from computation of diluted earnings per share (in shares)	410,000	401,000	385,000
Corporate Office Properties, L.P. [Member] | Conversion of Series I preferred units [Member]
Antidilutive securities
Weighted average antidilutive securities excluded from computation of diluted earnings per share (in shares)	176,000	176,000	176,000
Corporate Office Properties, L.P. [Member] | Conversion of Series K preferred units [Member]
Antidilutive securities
Weighted average antidilutive securities excluded from computation of diluted earnings per share (in shares)	434,000	434,000	434,000

Commitments and Contingencies93

Commitments and Contingencies (Details)	Dec. 31, 2015USD ($)Property	Aug. 31, 2010USD ($)
Tax incremental financing obligation
Liability recognized with regard to tax incremental financing obligation at end of current period	$ 1,900,000
Future minimum rental payments due:
2,016	1,189,000
2,017	1,117,000
2,018	1,071,000
2,019	1,049,000
2,020	1,067,000
Thereafter	86,798,000
Total	$ 92,291,000
Environmental Indemnity Agreement
Number of lease properties which were provided environmental indemnifications | Property	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
New Development and Redevelopment Obligations [Member]
Commitments and Contingencies
Contractual obligation	87,800,000
Capital Expenditures For Operating Properties [Member]
Commitments and Contingencies
Contractual obligation	47,900,000
Third Party Construction and Development [Member]
Commitments and Contingencies
Contractual obligation	10,700,000
Other Purchase Obligations [Member]
Commitments and Contingencies
Contractual obligation	$ 3,600,000
Tax Incremental Financing Bond [Member] | Anne Arundel County, Maryland [Member]
Commitments and Contingencies
Loan amount		$ 30,000,000

Subsequent Event (Details)

Subsequent Event (Details) $ in Millions	Feb. 10, 2016USD ($)
Subsequent Event [Member] | Chief Executive Officer [Member]
Subsequent Event [Line Items]
Defined Benefit Plan, Special Termination Benefits	$ 2.9

Quarterly Data (Unaudited) (Det

Quarterly Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2013
Revenues	$ 143,325	$ 150,744	$ 170,363	$ 161,034	$ 146,971	$ 153,015	$ 139,820	$ 146,667	$ 625,466		$ 586,473		$ 523,360
Operating income	20,055	31,993	37,471	30,575	37,148	37,422	31,836	25,206	120,094		131,612		141,910
Income from continuing operations	(1,430)	94,279	16,839	10,987	5,874	13,727	9,248	5,660	120,675		34,509		36,836
Discontinued operations	0	0	394	(238)	22	191	(198)	11	156		26		55,692
Net income	62,617	94,294	17,232	14,735	5,937	24,548	9,050	5,671	188,878		45,206		101,544
Net income attributable to noncontrolling interests	(3,253)	(4,494)	(1,451)	(1,380)	(1,033)	(1,828)	(1,160)	(930)
Net income	59,364	89,800	15,781	13,355	4,904	22,720	7,890	4,741	178,300		40,255		93,707
Preferred share/ unit dividends/ distributions	(3,553)	(3,552)	(3,553)	(3,552)	(3,552)	(3,553)	(4,344)	(4,490)	(14,210)		(15,939)		(19,971)
Issuance costs associated with redeemed preferred shares	0	0	0	0	0	0	(1,769)	0	0		(1,769)		(2,904)
Net income attributable to COPT common shareholders	$ 55,811	$ 86,248	$ 12,228	$ 9,803	$ 1,352	$ 19,167	$ 1,777	$ 251	$ 164,090		$ 22,547		$ 70,832
Basic earnings per common share/unit (in dollars per share/unit)	$ 0.59	$ 0.91	$ 0.13	$ 0.10	$ 0.01	$ 0.22	$ 0.02	$ 0	$ 1.74	[1]	$ 0.25	[1]	$ 0.83	[1]
Diluted earnings per common share/unit (in dollars per share/unit)	$ 0.59	$ 0.91	$ 0.13	$ 0.10	$ 0.01	$ 0.22	$ 0.02	$ 0	$ 1.74	[1]	$ 0.25	[1]	$ 0.83	[1]
Corporate Office Properties, L.P. [Member]
Revenues	$ 143,325	$ 150,744	$ 170,363	$ 161,034	$ 146,971	$ 153,015	$ 139,820	$ 146,667	$ 625,466		$ 586,473		$ 523,360
Operating income	20,055	31,993	37,471	30,575	37,148	37,422	31,836	25,206	120,094		131,612		141,910
Income from continuing operations	(1,430)	94,279	16,839	10,987	5,874	13,727	9,248	5,660	120,675		34,509		36,836
Discontinued operations	0	0	394	(238)	22	191	(198)	11	156		26		55,692
Net income	62,617	94,294	17,232	14,735	5,937	24,548	9,050	5,671	188,878		45,206		101,544
Net income attributable to noncontrolling interests	(918)	(972)	(812)	(818)	(805)	(897)	(837)	(737)	(3,520)		(3,276)		(3,907)
Net income	61,699	93,322	16,420	13,917	5,132	23,651	8,213	4,934	185,358		41,930		97,637
Preferred share/ unit dividends/ distributions	(3,718)	(3,717)	(3,718)	(3,717)	(3,717)	(3,718)	(4,509)	(4,655)	(14,870)		(16,599)		(20,631)
Issuance costs associated with redeemed preferred shares	0	0	0	0	0	0	(1,769)	0	0		(1,769)		(2,904)
Net income attributable to COPT common shareholders	$ 57,981	$ 89,605	$ 12,702	$ 10,200	$ 1,415	$ 19,933	$ 1,935	$ 279	$ 170,488		$ 23,562		$ 74,102
Basic earnings per common share/unit (in dollars per share/unit)	$ 0.59	$ 0.91	$ 0.13	$ 0.10	$ 0.01	$ 0.22	$ 0.02	$ 0	$ 1.74	[2]	$ 0.25	[2]	$ 0.83	[2]
Diluted earnings per common share/unit (in dollars per share/unit)	$ 0.59	$ 0.91	$ 0.13	$ 0.10	$ 0.01	$ 0.22	$ 0.02	$ 0	$ 1.74	[2]	$ 0.25	[2]	$ 0.83	[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.

Schedule II - Valuation and Q96

Schedule II - Valuation and Qualifying Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accounts Receivables-Allowance for doubtful accounts [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	$ 717	$ 2,976	$ 4,694
Charged to Costs and Expenses	1,125	278	(65)
Charged to Other Accounts	98	0	0
Deductions	(415)	(2,537)	(1,653)
Balance at End of Year	1,525	717	2,976
Allowance for Deferred Rent Receivable
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	1,418	2,126	913
Charged to Costs and Expenses	0	0	0
Charged to Other Accounts	544	(708)	1,213
Deductions	0	0	0
Balance at End of Year	1,962	1,418	2,126
Allowance for Deferred Tax Asset [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	2,062	2,062	207
Charged to Costs and Expenses	0	0	1,855
Charged to Other Accounts	0	0	0
Deductions	0	0	0
Balance at End of Year	$ 2,062	$ 2,062	$ 2,062

Schedule III - Real Estate an97

Schedule III - Real Estate and Accumulated Depreciation (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Real Estate and Accumulated Depreciation
Encumbrances	$ 316,887,000
Initial Cost
Land	709,953,000
Building and Land Improvements	3,106,271,000
Costs Capitalized Subsequent to Acquisition	342,392,000
Gross Amounts Carried at Close of Period
Land	709,953,000
Building and Land Improvements	3,448,663,000
Total	4,158,616,000	$ 4,014,336,000	$ 3,811,950,000	$ 3,859,960,000
Accumulated Depreciation	(718,680,000)	(703,083,000)	(597,649,000)	$ (568,176,000)
Additional information
Debt excluded from encumbrances	2,077,752,000	1,914,036,000
Net discounts and deferred financing costs	19,500,000
Aggregate cost of assets for federal income tax purposes	3,500,000,000
Impairment losses	$ 23,289,000	1,416,000	$ 5,857,000
Buildings improvements [Member] | Minimum [Member]
Additional information
Estimated lives over which depreciation is recognized	10 years
Buildings improvements [Member] | Maximum [Member]
Additional information
Estimated lives over which depreciation is recognized	40 years
Revolving Credit Facility [Member]
Additional information
Debt excluded from encumbrances	$ 43,500,000	83,000,000
Term Loan Facilities [Member]
Additional information
Debt excluded from encumbrances	515,900,000	517,846,000
Unsecured Senior Notes [Member]
Additional information
Debt excluded from encumbrances	1,200,000,000
4.25% Exchangeable Senior Notes [Member]
Additional information
Debt excluded from encumbrances	0	572,000
Unsecured notes payable [Member]
Additional information
Debt excluded from encumbrances	1,500,000	1,607,000
Letter of Credit [Member]
Additional information
Debt excluded from encumbrances	14,800,000
Fixed rate mortgage loans [Member]
Additional information
Debt excluded from encumbrances	281,208,000	385,769,000
Net discounts and deferred financing costs	668,000
100 Light Street [Member]
Real Estate and Accumulated Depreciation
Encumbrances	50,732,000
Initial Cost
Land	26,715,000
Building and Land Improvements	58,343,000
Costs Capitalized Subsequent to Acquisition	55,000
Gross Amounts Carried at Close of Period
Land	26,715,000
Building and Land Improvements	58,398,000
Total	85,113,000
Accumulated Depreciation	(1,588,000)
1000 Redstone Gateway [Member]
Real Estate and Accumulated Depreciation
Encumbrances	11,370,000
Initial Cost
Building and Land Improvements	20,533,000
Gross Amounts Carried at Close of Period
Building and Land Improvements	20,533,000
Total	20,533,000
Accumulated Depreciation	(1,438,000)
1100 Redstone Gateway [Member]
Real Estate and Accumulated Depreciation
Encumbrances	11,791,000
Initial Cost
Building and Land Improvements	19,593,000
Gross Amounts Carried at Close of Period
Building and Land Improvements	19,593,000
Total	19,593,000
Accumulated Depreciation	(965,000)
114 National Business Parkway [Member]
Initial Cost
Land	364,000
Building and Land Improvements	3,109,000
Costs Capitalized Subsequent to Acquisition	60,000
Gross Amounts Carried at Close of Period
Land	364,000
Building and Land Improvements	3,169,000
Total	3,533,000
Accumulated Depreciation	(1,125,000)
11751 Meadowville Lane [Member]
Initial Cost
Land	1,305,000
Building and Land Improvements	52,098,000
Costs Capitalized Subsequent to Acquisition	112,000
Gross Amounts Carried at Close of Period
Land	1,305,000
Building and Land Improvements	52,210,000
Total	53,515,000
Accumulated Depreciation	(11,194,000)
1200 Redstone Gateway [Member]
Real Estate and Accumulated Depreciation
Encumbrances	13,639,000
Initial Cost
Building and Land Improvements	22,389,000
Gross Amounts Carried at Close of Period
Building and Land Improvements	22,389,000
Total	22,389,000
Accumulated Depreciation	(1,145,000)
1201 M Street [Member]
Initial Cost
Building and Land Improvements	49,785,000
Costs Capitalized Subsequent to Acquisition	4,225,000
Gross Amounts Carried at Close of Period
Building and Land Improvements	54,010,000
Total	54,010,000
Accumulated Depreciation	(8,947,000)
1201 Winterson Road [Member]
Initial Cost
Land	1,288,000
Building and Land Improvements	12,175,000
Costs Capitalized Subsequent to Acquisition	460,000
Gross Amounts Carried at Close of Period
Land	1,288,000
Building and Land Improvements	12,635,000
Total	13,923,000
Accumulated Depreciation	(3,824,000)
1220 12th Street, SE [Member]
Initial Cost
Building and Land Improvements	42,464,000
Costs Capitalized Subsequent to Acquisition	2,431,000
Gross Amounts Carried at Close of Period
Building and Land Improvements	44,895,000
Total	44,895,000
Accumulated Depreciation	(8,523,000)
1243 Winterson Road [Member]
Initial Cost
Land	630,000
Gross Amounts Carried at Close of Period
Land	630,000
Total	630,000
1302 Concourse Drive [Member]
Initial Cost
Land	2,078,000
Building and Land Improvements	8,313,000
Costs Capitalized Subsequent to Acquisition	6,559,000
Gross Amounts Carried at Close of Period
Land	2,078,000
Building and Land Improvements	14,872,000
Total	16,950,000
Accumulated Depreciation	(6,188,000)
1304 Concourse Drive [Member]
Initial Cost
Land	1,999,000
Building and Land Improvements	12,934,000
Costs Capitalized Subsequent to Acquisition	2,884,000
Gross Amounts Carried at Close of Period
Land	1,999,000
Building and Land Improvements	15,818,000
Total	17,817,000
Accumulated Depreciation	(6,360,000)
1306 Concourse Drive [Member]
Initial Cost
Land	2,796,000
Building and Land Improvements	11,186,000
Costs Capitalized Subsequent to Acquisition	8,152,000
Gross Amounts Carried at Close of Period
Land	2,796,000
Building and Land Improvements	19,338,000
Total	22,134,000
Accumulated Depreciation	(6,922,000)
131 National Business Parkway [Member]
Initial Cost
Land	1,906,000
Building and Land Improvements	7,623,000
Costs Capitalized Subsequent to Acquisition	3,524,000
Gross Amounts Carried at Close of Period
Land	1,906,000
Building and Land Improvements	11,147,000
Total	13,053,000
Accumulated Depreciation	(5,596,000)
132 National Business Parkway [Member]
Initial Cost
Land	2,917,000
Building and Land Improvements	12,259,000
Costs Capitalized Subsequent to Acquisition	3,005,000
Gross Amounts Carried at Close of Period
Land	2,917,000
Building and Land Improvements	15,264,000
Total	18,181,000
Accumulated Depreciation	(7,700,000)
133 National Business Parkway [Member]
Initial Cost
Land	2,517,000
Building and Land Improvements	10,068,000
Costs Capitalized Subsequent to Acquisition	5,204,000
Gross Amounts Carried at Close of Period
Land	2,517,000
Building and Land Improvements	15,272,000
Total	17,789,000
Accumulated Depreciation	(8,230,000)
1331 Ashton Road [Member]
Initial Cost
Land	587,000
Building and Land Improvements	2,347,000
Costs Capitalized Subsequent to Acquisition	1,575,000
Gross Amounts Carried at Close of Period
Land	587,000
Building and Land Improvements	3,922,000
Total	4,509,000
Accumulated Depreciation	(1,337,000)
1334 Ashton Road [Member]
Initial Cost
Land	736,000
Building and Land Improvements	1,488,000
Costs Capitalized Subsequent to Acquisition	2,639,000
Gross Amounts Carried at Close of Period
Land	736,000
Building and Land Improvements	4,127,000
Total	4,863,000
Accumulated Depreciation	(2,467,000)
134 National Business Parkway [Member]
Initial Cost
Land	3,684,000
Building and Land Improvements	7,517,000
Costs Capitalized Subsequent to Acquisition	3,496,000
Gross Amounts Carried at Close of Period
Land	3,684,000
Building and Land Improvements	11,013,000
Total	14,697,000
Accumulated Depreciation	(4,062,000)
1340 Ashton Road [Member]
Initial Cost
Land	905,000
Building and Land Improvements	3,620,000
Costs Capitalized Subsequent to Acquisition	1,469,000
Gross Amounts Carried at Close of Period
Land	905,000
Building and Land Improvements	5,089,000
Total	5,994,000
Accumulated Depreciation	(2,621,000)
1341 Ashton Road [Member]
Initial Cost
Land	306,000
Building and Land Improvements	1,223,000
Costs Capitalized Subsequent to Acquisition	604,000
Gross Amounts Carried at Close of Period
Land	306,000
Building and Land Improvements	1,827,000
Total	2,133,000
Accumulated Depreciation	(932,000)
1343 Ashton Road [Member]
Initial Cost
Land	193,000
Building and Land Improvements	774,000
Costs Capitalized Subsequent to Acquisition	405,000
Gross Amounts Carried at Close of Period
Land	193,000
Building and Land Improvements	1,179,000
Total	1,372,000
Accumulated Depreciation	(640,000)
13450 Sunrise Valley Road [Member]
Initial Cost
Land	1,386,000
Building and Land Improvements	5,576,000
Costs Capitalized Subsequent to Acquisition	3,335,000
Gross Amounts Carried at Close of Period
Land	1,386,000
Building and Land Improvements	8,911,000
Total	10,297,000
Accumulated Depreciation	(3,902,000)
13454 Sunrise Valley Road [Member]
Initial Cost
Land	2,899,000
Building and Land Improvements	11,986,000
Costs Capitalized Subsequent to Acquisition	6,750,000
Gross Amounts Carried at Close of Period
Land	2,899,000
Building and Land Improvements	18,736,000
Total	21,635,000
Accumulated Depreciation	(7,177,000)
135 National Business Parkway [Member]
Real Estate and Accumulated Depreciation
Encumbrances	11,125,000
Initial Cost
Land	2,484,000
Building and Land Improvements	9,750,000
Costs Capitalized Subsequent to Acquisition	5,700,000
Gross Amounts Carried at Close of Period
Land	2,484,000
Building and Land Improvements	15,450,000
Total	17,934,000
Accumulated Depreciation	(6,467,000)
1362 Mellon Road [Member]
Initial Cost
Land	1,706,000
Building and Land Improvements	8,670,000
Costs Capitalized Subsequent to Acquisition	182,000
Gross Amounts Carried at Close of Period
Land	1,706,000
Building and Land Improvements	8,852,000
Total	10,558,000
Accumulated Depreciation	(1,517,000)
13857 McLearen Road [Member]
Initial Cost
Land	3,507,000
Building and Land Improvements	30,177,000
Costs Capitalized Subsequent to Acquisition	1,565,000
Gross Amounts Carried at Close of Period
Land	3,507,000
Building and Land Improvements	31,742,000
Total	35,249,000
Accumulated Depreciation	(6,961,000)
140 National Business Parkway [Member]
Initial Cost
Land	3,407,000
Building and Land Improvements	24,167,000
Costs Capitalized Subsequent to Acquisition	688,000
Gross Amounts Carried at Close of Period
Land	3,407,000
Building and Land Improvements	24,855,000
Total	28,262,000
Accumulated Depreciation	(7,647,000)
141 National Business Parkway [Member]
Real Estate and Accumulated Depreciation
Encumbrances	11,566,000
Initial Cost
Land	2,398,000
Building and Land Improvements	9,590,000
Costs Capitalized Subsequent to Acquisition	3,194,000
Gross Amounts Carried at Close of Period
Land	2,398,000
Building and Land Improvements	12,784,000
Total	15,182,000
Accumulated Depreciation	(5,963,000)
14280 Park Meadow Drive [Member]
Initial Cost
Land	3,731,000
Building and Land Improvements	15,953,000
Costs Capitalized Subsequent to Acquisition	1,764,000
Gross Amounts Carried at Close of Period
Land	3,731,000
Building and Land Improvements	17,717,000
Total	21,448,000
Accumulated Depreciation	(6,185,000)
1460 Dorsey Road [Member]
Initial Cost
Land	1,800,000
Gross Amounts Carried at Close of Period
Land	1,800,000
Total	1,800,000
14840 Conference Center Drive [Member]
Initial Cost
Land	1,572,000
Building and Land Improvements	8,175,000
Costs Capitalized Subsequent to Acquisition	2,860,000
Gross Amounts Carried at Close of Period
Land	1,572,000
Building and Land Improvements	11,035,000
Total	12,607,000
Accumulated Depreciation	(4,320,000)
14850 Conference Center Drive [Member]
Initial Cost
Land	1,615,000
Building and Land Improvements	8,358,000
Costs Capitalized Subsequent to Acquisition	2,947,000
Gross Amounts Carried at Close of Period
Land	1,615,000
Building and Land Improvements	11,305,000
Total	12,920,000
Accumulated Depreciation	(4,646,000)
14900 Conference Center Drive [Member]
Initial Cost
Land	3,436,000
Building and Land Improvements	14,402,000
Costs Capitalized Subsequent to Acquisition	5,239,000
Gross Amounts Carried at Close of Period
Land	3,436,000
Building and Land Improvements	19,641,000
Total	23,077,000
Accumulated Depreciation	(8,221,000)
1501 South Clinton Street [Member]
Initial Cost
Land	27,964,000
Building and Land Improvements	52,146,000
Costs Capitalized Subsequent to Acquisition	8,165,000
Gross Amounts Carried at Close of Period
Land	27,964,000
Building and Land Improvements	60,311,000
Total	88,275,000
Accumulated Depreciation	(14,472,000)
15049 Conference Center Drive [Member]
Initial Cost
Land	4,415,000
Building and Land Improvements	20,365,000
Costs Capitalized Subsequent to Acquisition	3,619,000
Gross Amounts Carried at Close of Period
Land	4,415,000
Building and Land Improvements	23,984,000
Total	28,399,000
Accumulated Depreciation	(9,236,000)
15059 Conference Center Drive [Member]
Initial Cost
Land	5,753,000
Building and Land Improvements	13,615,000
Costs Capitalized Subsequent to Acquisition	2,065,000
Gross Amounts Carried at Close of Period
Land	5,753,000
Building and Land Improvements	15,680,000
Total	21,433,000
Accumulated Depreciation	(6,697,000)
15395 John Marshall Highway [Member]
Initial Cost
Land	2,465,000
Building and Land Improvements	24,768,000
Gross Amounts Carried at Close of Period
Land	2,465,000
Building and Land Improvements	24,768,000
Total	27,233,000
Accumulated Depreciation	(1,195,000)
1550 West Nursery Road [Member]
Initial Cost
Land	14,071,000
Building and Land Improvements	16,930,000
Gross Amounts Carried at Close of Period
Land	14,071,000
Building and Land Improvements	16,930,000
Total	31,001,000
Accumulated Depreciation	(3,626,000)
1560 West Nursery Road [Member]
Initial Cost
Land	1,441,000
Building and Land Improvements	113,000
Costs Capitalized Subsequent to Acquisition	0
Gross Amounts Carried at Close of Period
Land	1,441,000
Building and Land Improvements	113,000
Total	1,554,000
Accumulated Depreciation	(5,000)
1560A Cable Ranch Road [Member]
Initial Cost
Land	1,097,000
Building and Land Improvements	3,770,000
Costs Capitalized Subsequent to Acquisition	394,000
Gross Amounts Carried at Close of Period
Land	1,097,000
Building and Land Improvements	4,164,000
Total	5,261,000
Accumulated Depreciation	(1,298,000)
1560B Cable Ranch Road [Member]
Initial Cost
Land	2,299,000
Building and Land Improvements	6,545,000
Costs Capitalized Subsequent to Acquisition	159,000
Gross Amounts Carried at Close of Period
Land	2,299,000
Building and Land Improvements	6,704,000
Total	9,003,000
Accumulated Depreciation	(1,738,000)
16442 Commerce Drive [Member]
Initial Cost
Land	613,000
Building and Land Improvements	2,582,000
Costs Capitalized Subsequent to Acquisition	883,000
Gross Amounts Carried at Close of Period
Land	613,000
Building and Land Improvements	3,465,000
Total	4,078,000
Accumulated Depreciation	(1,332,000)
16480 Commerce Drive [Member]
Initial Cost
Land	1,856,000
Building and Land Improvements	7,425,000
Costs Capitalized Subsequent to Acquisition	1,283,000
Gross Amounts Carried at Close of Period
Land	1,856,000
Building and Land Improvements	8,708,000
Total	10,564,000
Accumulated Depreciation	(2,316,000)
16501 Commerce Drive [Member]
Initial Cost
Land	522,000
Building and Land Improvements	2,090,000
Costs Capitalized Subsequent to Acquisition	482,000
Gross Amounts Carried at Close of Period
Land	522,000
Building and Land Improvements	2,572,000
Total	3,094,000
Accumulated Depreciation	(797,000)
16539 Commerce Drive [Member]
Initial Cost
Land	688,000
Building and Land Improvements	2,860,000
Costs Capitalized Subsequent to Acquisition	1,524,000
Gross Amounts Carried at Close of Period
Land	688,000
Building and Land Improvements	4,384,000
Total	5,072,000
Accumulated Depreciation	(1,934,000)
16541 Commerce Drive [Member]
Initial Cost
Land	773,000
Building and Land Improvements	3,094,000
Costs Capitalized Subsequent to Acquisition	1,368,000
Gross Amounts Carried at Close of Period
Land	773,000
Building and Land Improvements	4,462,000
Total	5,235,000
Accumulated Depreciation	(1,727,000)
16543 Commerce Drive [Member]
Initial Cost
Land	436,000
Building and Land Improvements	1,742,000
Costs Capitalized Subsequent to Acquisition	502,000
Gross Amounts Carried at Close of Period
Land	436,000
Building and Land Improvements	2,244,000
Total	2,680,000
Accumulated Depreciation	(573,000)
1751 Pinnacle Drive [Member]
Initial Cost
Land	10,486,000
Building and Land Improvements	42,339,000
Costs Capitalized Subsequent to Acquisition	24,059,000
Gross Amounts Carried at Close of Period
Land	10,486,000
Building and Land Improvements	66,398,000
Total	76,884,000
Accumulated Depreciation	(23,750,000)
1753 Pinnacle Drive [Member]
Initial Cost
Land	8,275,000
Building and Land Improvements	34,353,000
Costs Capitalized Subsequent to Acquisition	10,894,000
Gross Amounts Carried at Close of Period
Land	8,275,000
Building and Land Improvements	45,247,000
Total	53,522,000
Accumulated Depreciation	(15,849,000)
201 Technology Drive [Member]
Initial Cost
Land	726,000
Building and Land Improvements	31,091,000
Costs Capitalized Subsequent to Acquisition	59,000
Gross Amounts Carried at Close of Period
Land	726,000
Building and Land Improvements	31,150,000
Total	31,876,000
Accumulated Depreciation	(6,358,000)
206 Research Boulevard [Member]
Initial Cost
Land	1,813,000
Building and Land Improvements	17,485,000
Gross Amounts Carried at Close of Period
Land	1,813,000
Building and Land Improvements	17,485,000
Total	19,298,000
Accumulated Depreciation	(1,416,000)
209 Research Boulevard [Member]
Initial Cost
Land	1,045,000
Building and Land Improvements	16,087,000
Costs Capitalized Subsequent to Acquisition	79,000
Gross Amounts Carried at Close of Period
Land	1,045,000
Building and Land Improvements	16,166,000
Total	17,211,000
Accumulated Depreciation	(2,069,000)
210 Research Boulevard [Member]
Initial Cost
Land	1,065,000
Building and Land Improvements	15,102,000
Costs Capitalized Subsequent to Acquisition	75,000
Gross Amounts Carried at Close of Period
Land	1,065,000
Building and Land Improvements	15,177,000
Total	16,242,000
Accumulated Depreciation	(1,569,000)
2100 Rideout Road [Member]
Initial Cost
Land	0
Building and Land Improvements	2,586,000
Costs Capitalized Subsequent to Acquisition	0
Gross Amounts Carried at Close of Period
Land	0
Building and Land Improvements	2,586,000
Total	2,586,000
2100 L Street [Member]
Initial Cost
Land	1,273,000
Building and Land Improvements	1,685,000
Costs Capitalized Subsequent to Acquisition	0
Gross Amounts Carried at Close of Period
Land	1,273,000
Building and Land Improvements	1,685,000
Total	2,958,000
21267 Smith Switch Road [Member]
Initial Cost
Land	4,040,000
Building and Land Improvements	10,135,000
Gross Amounts Carried at Close of Period
Land	4,040,000
Building and Land Improvements	10,135,000
Total	14,175,000
Accumulated Depreciation	(365,000)
21271 Smith Switch Road [Member]
Initial Cost
Land	7,346,000
Building and Land Improvements	18,267,000
Gross Amounts Carried at Close of Period
Land	7,346,000
Building and Land Improvements	18,267,000
Total	25,613,000
Accumulated Depreciation	(881,000)
22289 Exploration Drive [Member]
Initial Cost
Land	1,422,000
Building and Land Improvements	5,719,000
Costs Capitalized Subsequent to Acquisition	1,660,000
Gross Amounts Carried at Close of Period
Land	1,422,000
Building and Land Improvements	7,379,000
Total	8,801,000
Accumulated Depreciation	(2,826,000)
22299 Exploration Drive [Member]
Initial Cost
Land	1,362,000
Building and Land Improvements	5,791,000
Costs Capitalized Subsequent to Acquisition	1,218,000
Gross Amounts Carried at Close of Period
Land	1,362,000
Building and Land Improvements	7,009,000
Total	8,371,000
Accumulated Depreciation	(2,641,000)
22300 Exploration Drive [Member]
Initial Cost
Land	1,094,000
Building and Land Improvements	5,038,000
Costs Capitalized Subsequent to Acquisition	1,315,000
Gross Amounts Carried at Close of Period
Land	1,094,000
Building and Land Improvements	6,353,000
Total	7,447,000
Accumulated Depreciation	(2,024,000)
22309 Exploration Drive [Member]
Initial Cost
Land	2,243,000
Building and Land Improvements	10,419,000
Costs Capitalized Subsequent to Acquisition	7,787,000
Gross Amounts Carried at Close of Period
Land	2,243,000
Building and Land Improvements	18,206,000
Total	20,449,000
Accumulated Depreciation	(4,683,000)
23535 Cottonwood Parkway [Member]
Initial Cost
Land	692,000
Building and Land Improvements	3,051,000
Costs Capitalized Subsequent to Acquisition	248,000
Gross Amounts Carried at Close of Period
Land	692,000
Building and Land Improvements	3,299,000
Total	3,991,000
Accumulated Depreciation	(1,206,000)
250 W Pratt St [Member]
Initial Cost
Land	8,057,000
Building and Land Improvements	34,654,000
Costs Capitalized Subsequent to Acquisition	4,765,000
Gross Amounts Carried at Close of Period
Land	8,057,000
Building and Land Improvements	39,419,000
Total	47,476,000
Accumulated Depreciation	(1,652,000)
2500 Riva Road [Member]
Initial Cost
Land	2,791,000
Building and Land Improvements	12,145,000
Costs Capitalized Subsequent to Acquisition	1,000
Gross Amounts Carried at Close of Period
Land	2,791,000
Building and Land Improvements	12,146,000
Total	14,937,000
Accumulated Depreciation	(4,417,000)
2600 Park Tower Drive [Member]
Initial Cost
Land	20,304,000
Building and Land Improvements	34,443,000
Costs Capitalized Subsequent to Acquisition	346,000
Gross Amounts Carried at Close of Period
Land	20,304,000
Building and Land Improvements	34,789,000
Total	55,093,000
Accumulated Depreciation	(1,051,000)
2691 Technology Drive [Member]
Initial Cost
Land	2,098,000
Building and Land Improvements	17,334,000
Costs Capitalized Subsequent to Acquisition	5,305,000
Gross Amounts Carried at Close of Period
Land	2,098,000
Building and Land Improvements	22,639,000
Total	24,737,000
Accumulated Depreciation	(7,827,000)
2701 Technology Drive [Member]
Real Estate and Accumulated Depreciation
Encumbrances	22,053,000
Initial Cost
Land	1,737,000
Building and Land Improvements	15,266,000
Costs Capitalized Subsequent to Acquisition	2,821,000
Gross Amounts Carried at Close of Period
Land	1,737,000
Building and Land Improvements	18,087,000
Total	19,824,000
Accumulated Depreciation	(7,567,000)
2711 Technology Drive [Member]
Real Estate and Accumulated Depreciation
Encumbrances	17,153,000
Initial Cost
Land	2,251,000
Building and Land Improvements	21,611,000
Costs Capitalized Subsequent to Acquisition	1,468,000
Gross Amounts Carried at Close of Period
Land	2,251,000
Building and Land Improvements	23,079,000
Total	25,330,000
Accumulated Depreciation	(10,158,000)
2720 Technology Drive [Member]
Initial Cost
Land	3,863,000
Building and Land Improvements	29,272,000
Costs Capitalized Subsequent to Acquisition	146,000
Gross Amounts Carried at Close of Period
Land	3,863,000
Building and Land Improvements	29,418,000
Total	33,281,000
Accumulated Depreciation	(8,316,000)
2721 Technology Drive [Member]
Initial Cost
Land	4,611,000
Building and Land Improvements	14,597,000
Costs Capitalized Subsequent to Acquisition	1,263,000
Gross Amounts Carried at Close of Period
Land	4,611,000
Building and Land Improvements	15,860,000
Total	20,471,000
Accumulated Depreciation	(6,400,000)
2730 Hercules Road [Member]
Real Estate and Accumulated Depreciation
Encumbrances	34,256,000
Initial Cost
Land	8,737,000
Building and Land Improvements	31,612,000
Costs Capitalized Subsequent to Acquisition	7,213,000
Gross Amounts Carried at Close of Period
Land	8,737,000
Building and Land Improvements	38,825,000
Total	47,562,000
Accumulated Depreciation	(15,674,000)
2900 Towerview Road [Member]
Initial Cost
Land	3,207,000
Building and Land Improvements	16,379,000
Costs Capitalized Subsequent to Acquisition	6,509,000
Gross Amounts Carried at Close of Period
Land	3,207,000
Building and Land Improvements	22,888,000
Total	26,095,000
Accumulated Depreciation	(7,181,000)
30 Light Street [Member]
Real Estate and Accumulated Depreciation
Encumbrances	4,268,000
Initial Cost
Land	0
Building and Land Improvements	12,101,000
Costs Capitalized Subsequent to Acquisition	37,000
Gross Amounts Carried at Close of Period
Land	0
Building and Land Improvements	12,138,000
Total	12,138,000
Accumulated Depreciation	(128,000)
300 Sentinel Drive [Member]
Initial Cost
Land	1,517,000
Building and Land Improvements	59,016,000
Costs Capitalized Subsequent to Acquisition	219,000
Gross Amounts Carried at Close of Period
Land	1,517,000
Building and Land Improvements	59,235,000
Total	60,752,000
Accumulated Depreciation	(8,458,000)
302 Sentinel Drive [Member]
Initial Cost
Land	2,648,000
Building and Land Improvements	29,687,000
Costs Capitalized Subsequent to Acquisition	445,000
Gross Amounts Carried at Close of Period
Land	2,648,000
Building and Land Improvements	30,132,000
Total	32,780,000
Accumulated Depreciation	(6,000,000)
304 Sentinel Drive [Member]
Initial Cost
Land	3,411,000
Building and Land Improvements	24,917,000
Costs Capitalized Subsequent to Acquisition	193,000
Gross Amounts Carried at Close of Period
Land	3,411,000
Building and Land Improvements	25,110,000
Total	28,521,000
Accumulated Depreciation	(6,304,000)
306 Sentinel Drive [Member]
Real Estate and Accumulated Depreciation
Encumbrances	15,878,000
Initial Cost
Land	3,260,000
Building and Land Improvements	22,592,000
Costs Capitalized Subsequent to Acquisition	611,000
Gross Amounts Carried at Close of Period
Land	3,260,000
Building and Land Improvements	23,203,000
Total	26,463,000
Accumulated Depreciation	(5,269,000)
308 Sentinel Drive [Member]
Initial Cost
Land	1,422,000
Building and Land Improvements	26,208,000
Costs Capitalized Subsequent to Acquisition	37,000
Gross Amounts Carried at Close of Period
Land	1,422,000
Building and Land Improvements	26,245,000
Total	27,667,000
Accumulated Depreciation	(3,050,000)
310 Sentinel Way [Member]
Initial Cost
Land	2,372,000
Building and Land Improvements	36,468,000
Gross Amounts Carried at Close of Period
Land	2,372,000
Building and Land Improvements	36,468,000
Total	38,840,000
Accumulated Depreciation	(191,000)
310 The Bridge Street [Member]
Initial Cost
Land	261,000
Building and Land Improvements	26,531,000
Costs Capitalized Subsequent to Acquisition	1,695,000
Gross Amounts Carried at Close of Period
Land	261,000
Building and Land Improvements	28,226,000
Total	28,487,000
Accumulated Depreciation	(5,136,000)
312 Sentinel Way [Member]
Initial Cost
Land	3,138,000
Building and Land Improvements	27,789,000
Gross Amounts Carried at Close of Period
Land	3,138,000
Building and Land Improvements	27,789,000
Total	30,927,000
Accumulated Depreciation	(913,000)
3120 Fairview Park Drive [Member]
Initial Cost
Land	6,863,000
Building and Land Improvements	35,606,000
Costs Capitalized Subsequent to Acquisition	8,773,000
Gross Amounts Carried at Close of Period
Land	6,863,000
Building and Land Improvements	44,379,000
Total	51,242,000
Accumulated Depreciation	(7,229,000)
314 Sentinel Way [Member]
Initial Cost
Land	1,254,000
Building and Land Improvements	1,439,000
Gross Amounts Carried at Close of Period
Land	1,254,000
Building and Land Improvements	1,439,000
Total	2,693,000
Accumulated Depreciation	(249,000)
316 Sentinel Way [Member]
Initial Cost
Land	2,748,000
Building and Land Improvements	38,156,000
Costs Capitalized Subsequent to Acquisition	138,000
Gross Amounts Carried at Close of Period
Land	2,748,000
Building and Land Improvements	38,294,000
Total	41,042,000
Accumulated Depreciation	(3,570,000)
318 Sentinel Way [Member]
Initial Cost
Land	2,185,000
Building and Land Improvements	28,426,000
Gross Amounts Carried at Close of Period
Land	2,185,000
Building and Land Improvements	28,426,000
Total	30,611,000
Accumulated Depreciation	(6,981,000)
320 Sentinel Way [Member]
Initial Cost
Land	2,067,000
Building and Land Improvements	21,623,000
Gross Amounts Carried at Close of Period
Land	2,067,000
Building and Land Improvements	21,623,000
Total	23,690,000
Accumulated Depreciation	(4,310,000)
322 Sentinel Way [Member]
Real Estate and Accumulated Depreciation
Encumbrances	21,122,000
Initial Cost
Land	2,605,000
Building and Land Improvements	22,827,000
Gross Amounts Carried at Close of Period
Land	2,605,000
Building and Land Improvements	22,827,000
Total	25,432,000
Accumulated Depreciation	(5,143,000)
324 Sentinel Way [Member]
Initial Cost
Land	1,656,000
Building and Land Improvements	23,018,000
Gross Amounts Carried at Close of Period
Land	1,656,000
Building and Land Improvements	23,018,000
Total	24,674,000
Accumulated Depreciation	(3,078,000)
410 National Business Parkway [Member]
Initial Cost
Land	1,831,000
Building and Land Improvements	23,257,000
Costs Capitalized Subsequent to Acquisition	112,000
Gross Amounts Carried at Close of Period
Land	1,831,000
Building and Land Improvements	23,369,000
Total	25,200,000
Accumulated Depreciation	(1,719,000)
420 National Business Parkway [Member]
Initial Cost
Land	2,370,000
Building and Land Improvements	27,490,000
Costs Capitalized Subsequent to Acquisition	106,000
Gross Amounts Carried at Close of Period
Land	2,370,000
Building and Land Improvements	27,596,000
Total	29,966,000
Accumulated Depreciation	(1,230,000)
430 National Business Parkway [Member]
Initial Cost
Land	1,852,000
Building and Land Improvements	21,153,000
Costs Capitalized Subsequent to Acquisition	119,000
Gross Amounts Carried at Close of Period
Land	1,852,000
Building and Land Improvements	21,272,000
Total	23,124,000
Accumulated Depreciation	(2,034,000)
44408 Pecan Court [Member]
Initial Cost
Land	817,000
Building and Land Improvements	1,583,000
Costs Capitalized Subsequent to Acquisition	581,000
Gross Amounts Carried at Close of Period
Land	817,000
Building and Land Improvements	2,164,000
Total	2,981,000
Accumulated Depreciation	(560,000)
44414 Pecan Court [Member]
Initial Cost
Land	405,000
Building and Land Improvements	1,619,000
Costs Capitalized Subsequent to Acquisition	337,000
Gross Amounts Carried at Close of Period
Land	405,000
Building and Land Improvements	1,956,000
Total	2,361,000
Accumulated Depreciation	(725,000)
44417 Pecan Court [Member]
Initial Cost
Land	434,000
Building and Land Improvements	3,348,000
Costs Capitalized Subsequent to Acquisition	88,000
Gross Amounts Carried at Close of Period
Land	434,000
Building and Land Improvements	3,436,000
Total	3,870,000
Accumulated Depreciation	(1,127,000)
44420 Pecan Court [Member]
Initial Cost
Land	344,000
Building and Land Improvements	890,000
Costs Capitalized Subsequent to Acquisition	148,000
Gross Amounts Carried at Close of Period
Land	344,000
Building and Land Improvements	1,038,000
Total	1,382,000
Accumulated Depreciation	(255,000)
44425 Pecan Court [Member]
Initial Cost
Land	1,309,000
Building and Land Improvements	3,506,000
Costs Capitalized Subsequent to Acquisition	1,590,000
Gross Amounts Carried at Close of Period
Land	1,309,000
Building and Land Improvements	5,096,000
Total	6,405,000
Accumulated Depreciation	(1,784,000)
45310 Abell House Lane [Member]
Initial Cost
Land	2,272,000
Building and Land Improvements	13,808,000
Gross Amounts Carried at Close of Period
Land	2,272,000
Building and Land Improvements	13,808,000
Total	16,080,000
Accumulated Depreciation	(1,403,000)
46579 Expedition Drive [Member]
Initial Cost
Land	1,406,000
Building and Land Improvements	5,796,000
Costs Capitalized Subsequent to Acquisition	1,335,000
Gross Amounts Carried at Close of Period
Land	1,406,000
Building and Land Improvements	7,131,000
Total	8,537,000
Accumulated Depreciation	(3,060,000)
46591 Expedition Drive [Member]
Initial Cost
Land	1,200,000
Building and Land Improvements	7,199,000
Costs Capitalized Subsequent to Acquisition	1,134,000
Gross Amounts Carried at Close of Period
Land	1,200,000
Building and Land Improvements	8,333,000
Total	9,533,000
Accumulated Depreciation	(1,933,000)
4851 Stonecroft Boulevard [Member]
Initial Cost
Land	1,878,000
Building and Land Improvements	11,558,000
Costs Capitalized Subsequent to Acquisition	21,000
Gross Amounts Carried at Close of Period
Land	1,878,000
Building and Land Improvements	11,579,000
Total	13,457,000
Accumulated Depreciation	(3,248,000)
4940 Campbell Drive [Member]
Initial Cost
Land	1,379,000
Building and Land Improvements	2,728,000
Costs Capitalized Subsequent to Acquisition	1,464,000
Gross Amounts Carried at Close of Period
Land	1,379,000
Building and Land Improvements	4,192,000
Total	5,571,000
Accumulated Depreciation	(445,000)
525 Babcock Road [Member]
Initial Cost
Land	355,000
Building and Land Improvements	397,000
Costs Capitalized Subsequent to Acquisition	79,000
Gross Amounts Carried at Close of Period
Land	355,000
Building and Land Improvements	476,000
Total	831,000
Accumulated Depreciation	(173,000)
5325 Nottingham Drive [Member]
Initial Cost
Land	816,000
Building and Land Improvements	3,976,000
Costs Capitalized Subsequent to Acquisition	485,000
Gross Amounts Carried at Close of Period
Land	816,000
Building and Land Improvements	4,461,000
Total	5,277,000
Accumulated Depreciation	(1,233,000)
5355 Nottingham Drive [Member]
Initial Cost
Land	761,000
Building and Land Improvements	3,562,000
Costs Capitalized Subsequent to Acquisition	1,758,000
Gross Amounts Carried at Close of Period
Land	761,000
Building and Land Improvements	5,320,000
Total	6,081,000
Accumulated Depreciation	(2,132,000)
540 National Business Parkway [Member]
Initial Cost
Land	2,035,000
Building and Land Improvements	9,496,000
Costs Capitalized Subsequent to Acquisition	0
Gross Amounts Carried at Close of Period
Land	2,035,000
Building and Land Improvements	9,496,000
Total	11,531,000
Accumulated Depreciation	0
5520 Research Park Drive [Member]
Initial Cost
Building and Land Improvements	20,072,000
Costs Capitalized Subsequent to Acquisition	201,000
Gross Amounts Carried at Close of Period
Building and Land Improvements	20,273,000
Total	20,273,000
Accumulated Depreciation	(3,195,000)
5522 Research Park Drive [Member]
Initial Cost
Building and Land Improvements	4,550,000
Gross Amounts Carried at Close of Period
Building and Land Improvements	4,550,000
Total	4,550,000
Accumulated Depreciation	(955,000)
5825 University Research Court [Member]
Real Estate and Accumulated Depreciation
Encumbrances	15,255,000
Initial Cost
Building and Land Improvements	22,740,000
Costs Capitalized Subsequent to Acquisition	87,000
Gross Amounts Carried at Close of Period
Building and Land Improvements	22,827,000
Total	22,827,000
Accumulated Depreciation	(3,817,000)
5850 University Research Court [Member]
Real Estate and Accumulated Depreciation
Encumbrances	20,771,000
Initial Cost
Building and Land Improvements	31,906,000
Costs Capitalized Subsequent to Acquisition	406,000
Gross Amounts Carried at Close of Period
Building and Land Improvements	32,312,000
Total	32,312,000
Accumulated Depreciation	(4,580,000)
6700 Alexander Bell Drive [Member]
Initial Cost
Land	1,755,000
Building and Land Improvements	7,019,000
Costs Capitalized Subsequent to Acquisition	6,226,000
Gross Amounts Carried at Close of Period
Land	1,755,000
Building and Land Improvements	13,245,000
Total	15,000,000
Accumulated Depreciation	(5,913,000)
6708 Alexander Bell Drive [Member]
Initial Cost
Land	897,000
Building and Land Improvements	8,844,000
Costs Capitalized Subsequent to Acquisition	1,591,000
Gross Amounts Carried at Close of Period
Land	897,000
Building and Land Improvements	10,435,000
Total	11,332,000
Accumulated Depreciation		(3,414,000)
6711 Columbia Gateway Drive [Member]
Initial Cost
Land	2,683,000
Building and Land Improvements	23,239,000
Costs Capitalized Subsequent to Acquisition	619,000
Gross Amounts Carried at Close of Period
Land	2,683,000
Building and Land Improvements	23,858,000
Total	26,541,000
Accumulated Depreciation		(5,407,000)
6716 Alexander Bell Drive [Member]
Initial Cost
Land	1,242,000
Building and Land Improvements	4,969,000
Costs Capitalized Subsequent to Acquisition	3,352,000
Gross Amounts Carried at Close of Period
Land	1,242,000
Building and Land Improvements	8,321,000
Total	9,563,000
Accumulated Depreciation		(4,359,000)
6721 Columbia Gateway Drive [Member]
Real Estate and Accumulated Depreciation
Encumbrances	16,663,000
Initial Cost
Land	1,753,000
Building and Land Improvements	34,090,000
Costs Capitalized Subsequent to Acquisition	76,000
Gross Amounts Carried at Close of Period
Land	1,753,000
Building and Land Improvements	34,166,000
Total	35,919,000
Accumulated Depreciation		(5,798,000)
6724 Alexander Bell Drive [Member]
Initial Cost
Land	449,000
Building and Land Improvements	5,039,000
Costs Capitalized Subsequent to Acquisition	708,000
Gross Amounts Carried at Close of Period
Land	449,000
Building and Land Improvements	5,747,000
Total	6,196,000
Accumulated Depreciation		(2,266,000)
6731 Columbia Gateway Drive [Member]
Initial Cost
Land	2,807,000
Building and Land Improvements	19,098,000
Costs Capitalized Subsequent to Acquisition	2,916,000
Gross Amounts Carried at Close of Period
Land	2,807,000
Building and Land Improvements	22,014,000
Total	24,821,000
Accumulated Depreciation		(8,494,000)
6740 Alexander Bell Drive [Member]
Initial Cost
Land	1,424,000
Building and Land Improvements	5,696,000
Costs Capitalized Subsequent to Acquisition	3,340,000
Gross Amounts Carried at Close of Period
Land	1,424,000
Building and Land Improvements	9,036,000
Total	10,460,000
Accumulated Depreciation		(5,058,000)
6741 Columbia Gateway Drive [Member]
Initial Cost
Land	675,000
Building and Land Improvements	1,711,000
Costs Capitalized Subsequent to Acquisition	124,000
Gross Amounts Carried at Close of Period
Land	675,000
Building and Land Improvements	1,835,000
Total	2,510,000
Accumulated Depreciation		(357,000)
6750 Alexander Bell Drive [Member]
Initial Cost
Land	1,263,000
Building and Land Improvements	12,461,000
Costs Capitalized Subsequent to Acquisition	3,457,000
Gross Amounts Carried at Close of Period
Land	1,263,000
Building and Land Improvements	15,918,000
Total	17,181,000
Accumulated Depreciation		(7,576,000)
6760 Alexander Bell Drive [Member]
Initial Cost
Land	890,000
Building and Land Improvements	3,561,000
Costs Capitalized Subsequent to Acquisition	2,974,000
Gross Amounts Carried at Close of Period
Land	890,000
Building and Land Improvements	6,535,000
Total	7,425,000
Accumulated Depreciation		(3,248,000)
6940 Columbia Gateway Drive [Member]
Initial Cost
Land	3,545,000
Building and Land Improvements	9,916,000
Costs Capitalized Subsequent to Acquisition	5,537,000
Gross Amounts Carried at Close of Period
Land	3,545,000
Building and Land Improvements	15,453,000
Total	18,998,000
Accumulated Depreciation		(7,000,000)
6950 Columbia Gateway Drive [Member]
Initial Cost
Land	3,596,000
Building and Land Improvements	14,269,000
Costs Capitalized Subsequent to Acquisition	3,087,000
Gross Amounts Carried at Close of Period
Land	3,596,000
Building and Land Improvements	17,356,000
Total	20,952,000
Accumulated Depreciation		(7,506,000)
7000 Columbia Gateway Drive [Member]
Initial Cost
Land	3,131,000
Building and Land Improvements	12,103,000
Costs Capitalized Subsequent to Acquisition	2,138,000
Gross Amounts Carried at Close of Period
Land	3,131,000
Building and Land Improvements	14,241,000
Total	17,372,000
Accumulated Depreciation		(4,482,000)
7005 Columbia Gateway Drive [Member]
Initial Cost
Land	3,036,000
Building and Land Improvements	189,000
Costs Capitalized Subsequent to Acquisition	0
Gross Amounts Carried at Close of Period
Land	3,036,000
Building and Land Improvements	189,000
Total	3,225,000
Accumulated Depreciation		0
7015 Albert Einstein Drive [Member]
Real Estate and Accumulated Depreciation
Encumbrances	1,570,000
Initial Cost
Land	2,058,000
Building and Land Improvements	6,093,000
Costs Capitalized Subsequent to Acquisition	1,527,000
Gross Amounts Carried at Close of Period
Land	2,058,000
Building and Land Improvements	7,620,000
Total	9,678,000
Accumulated Depreciation		(2,915,000)
7061 Columbia Gateway Drive [Member]
Initial Cost
Land	729,000
Building and Land Improvements	3,094,000
Costs Capitalized Subsequent to Acquisition	1,407,000
Gross Amounts Carried at Close of Period
Land	729,000
Building and Land Improvements	4,501,000
Total	5,230,000
Accumulated Depreciation		(1,802,000)
7063 Columbia Gateway Drive [Member]
Initial Cost
Land	902,000
Building and Land Improvements	3,684,000
Costs Capitalized Subsequent to Acquisition	2,512,000
Gross Amounts Carried at Close of Period
Land	902,000
Building and Land Improvements	6,196,000
Total	7,098,000
Accumulated Depreciation		(2,452,000)
7065 Columbia Gateway Drive [Member]
Initial Cost
Land	919,000
Building and Land Improvements	3,763,000
Costs Capitalized Subsequent to Acquisition	3,095,000
Gross Amounts Carried at Close of Period
Land	919,000
Building and Land Improvements	6,858,000
Total	7,777,000
Accumulated Depreciation		(2,773,000)
7067 Columbia Gateway Drive [Member]
Initial Cost
Land	1,829,000
Building and Land Improvements	11,823,000
Costs Capitalized Subsequent to Acquisition	2,838,000
Gross Amounts Carried at Close of Period
Land	1,829,000
Building and Land Improvements	14,661,000
Total	16,490,000
Accumulated Depreciation		(6,154,000)
7125 Columbia Gateway Drive [Member]
Initial Cost
Land	3,361,000
Building and Land Improvements	1,677,000
Costs Capitalized Subsequent to Acquisition	279,000
Gross Amounts Carried at Close of Period
Land	3,361,000
Building and Land Improvements	1,956,000
Total	5,317,000
7125 Columbia Gateway Drive [Member]
Initial Cost
Land	17,126,000
Building and Land Improvements	46,994,000
Costs Capitalized Subsequent to Acquisition	9,813,000
Gross Amounts Carried at Close of Period
Land	17,126,000
Building and Land Improvements	56,807,000
Total	73,933,000
Accumulated Depreciation		(16,632,000)
7130 Columbia Gateway Drive [Member]
Initial Cost
Land	1,350,000
Building and Land Improvements	4,359,000
Costs Capitalized Subsequent to Acquisition	1,906,000
Gross Amounts Carried at Close of Period
Land	1,350,000
Building and Land Improvements	6,265,000
Total	7,615,000
Accumulated Depreciation		(2,692,000)
7134 Columbia Gateway Drive [Member]
Initial Cost
Land	704,000
Building and Land Improvements	2,139,000
Costs Capitalized Subsequent to Acquisition	314,000
Gross Amounts Carried at Close of Period
Land	704,000
Building and Land Improvements	2,453,000
Total	3,157,000
Accumulated Depreciation		(1,234,000)
7138 Columbia Gateway Drive [Member]
Initial Cost
Land	1,104,000
Building and Land Improvements	3,518,000
Costs Capitalized Subsequent to Acquisition	2,118,000
Gross Amounts Carried at Close of Period
Land	1,104,000
Building and Land Improvements	5,636,000
Total	6,740,000
Accumulated Depreciation		(3,139,000)
7142 Columbia Gateway Drive [Member]
Initial Cost
Land	1,342,000
Building and Land Improvements	3,978,000
Costs Capitalized Subsequent to Acquisition	2,502,000
Gross Amounts Carried at Close of Period
Land	1,342,000
Building and Land Improvements	6,480,000
Total	7,822,000
Accumulated Depreciation		(2,322,000)
7150 Columbia Gateway Drive [Member]
Initial Cost
Land	1,032,000
Building and Land Improvements	3,429,000
Costs Capitalized Subsequent to Acquisition	665,000
Gross Amounts Carried at Close of Period
Land	1,032,000
Building and Land Improvements	4,094,000
Total	5,126,000
Accumulated Depreciation		(1,170,000)
7150 Riverwood Drive [Member]
Initial Cost
Land	1,821,000
Building and Land Improvements	4,388,000
Costs Capitalized Subsequent to Acquisition	1,713,000
Gross Amounts Carried at Close of Period
Land	1,821,000
Building and Land Improvements	6,101,000
Total	7,922,000
Accumulated Depreciation		(1,933,000)
7160 Riverwood Drive [Member]
Initial Cost
Land	2,732,000
Building and Land Improvements	7,006,000
Costs Capitalized Subsequent to Acquisition	2,275,000
Gross Amounts Carried at Close of Period
Land	2,732,000
Building and Land Improvements	9,281,000
Total	12,013,000
Accumulated Depreciation		(3,265,000)
7170 Riverwood Drive [Member]
Initial Cost
Land	1,283,000
Building and Land Improvements	3,096,000
Costs Capitalized Subsequent to Acquisition	692,000
Gross Amounts Carried at Close of Period
Land	1,283,000
Building and Land Improvements	3,788,000
Total	5,071,000
Accumulated Depreciation		(1,386,000)
7175 Riverwood Drive [Member]
Initial Cost
Land	1,788,000
Building and Land Improvements	7,269,000
Gross Amounts Carried at Close of Period
Land	1,788,000
Building and Land Improvements	7,269,000
Total	9,057,000
Accumulated Depreciation		(389,000)
7200 Redstone Gateway [Member]
Real Estate and Accumulated Depreciation
Encumbrances	6,649,000
Initial Cost
Land	0
Building and Land Improvements	8,347,000
Gross Amounts Carried at Close of Period
Land	0
Building and Land Improvements	8,347,000
Total	8,347,000
Accumulated Depreciation		(328,000)
7200 Riverwood Road [Member]
Initial Cost
Land	4,089,000
Building and Land Improvements	22,544,000
Costs Capitalized Subsequent to Acquisition	3,994,000
Gross Amounts Carried at Close of Period
Land	4,089,000
Building and Land Improvements	26,538,000
Total	30,627,000
Accumulated Depreciation		(8,761,000)
7205 Riverwood Drive [Member]
Initial Cost
Land	1,367,000
Building and Land Improvements	21,484,000
Gross Amounts Carried at Close of Period
Land	1,367,000
Building and Land Improvements	21,484,000
Total	22,851,000
Accumulated Depreciation		(1,310,000)
7272 Park Circle Drive [Member]
Real Estate and Accumulated Depreciation
Encumbrances	4,747,000
Initial Cost
Land	1,479,000
Building and Land Improvements	6,300,000
Costs Capitalized Subsequent to Acquisition	3,883,000
Gross Amounts Carried at Close of Period
Land	1,479,000
Building and Land Improvements	10,183,000
Total	11,662,000
Accumulated Depreciation		(2,956,000)
7318 Parkway Drive [Member]
Initial Cost
Land	972,000
Building and Land Improvements	3,888,000
Costs Capitalized Subsequent to Acquisition	1,002,000
Gross Amounts Carried at Close of Period
Land	972,000
Building and Land Improvements	4,890,000
Total	5,862,000
Accumulated Depreciation		(2,084,000)
7320 Parkway Drive [Member]
Initial Cost
Land	905,000
Building and Land Improvements	3,570,000
Costs Capitalized Subsequent to Acquisition	5,048,000
Gross Amounts Carried at Close of Period
Land	905,000
Building and Land Improvements	8,618,000
Total	9,523,000
Accumulated Depreciation		(2,883,000)
7400 Redstone Gateway [Member]
Real Estate and Accumulated Depreciation
Encumbrances	7,292,000
Initial Cost
Land	0
Building and Land Improvements	9,012,000
Costs Capitalized Subsequent to Acquisition	0
Gross Amounts Carried at Close of Period
Land	0
Building and Land Improvements	9,012,000
Total	9,012,000
Accumulated Depreciation		(121,000)
7467 Ridge Road [Member]
Initial Cost
Land	1,629,000
Building and Land Improvements	6,517,000
Costs Capitalized Subsequent to Acquisition	3,645,000
Gross Amounts Carried at Close of Period
Land	1,629,000
Building and Land Improvements	10,162,000
Total	11,791,000
Accumulated Depreciation		(4,439,000)
7740 Milestone Parkway [Member]
Real Estate and Accumulated Depreciation
Encumbrances	18,987,000
Initial Cost
Land	3,825,000
Building and Land Improvements	34,176,000
Costs Capitalized Subsequent to Acquisition	404,000
Gross Amounts Carried at Close of Period
Land	3,825,000
Building and Land Improvements	34,580,000
Total	38,405,000
Accumulated Depreciation	(4,692,000)
7770 Backlick Road [Member]
Initial Cost
Land	6,387,000
Building and Land Improvements	74,325,000
Costs Capitalized Subsequent to Acquisition	141,000
Gross Amounts Carried at Close of Period
Land	6,387,000
Building and Land Improvements	74,466,000
Total	80,853,000
Accumulated Depreciation	(5,292,000)
7880 Milestone Parkway [Member]
Initial Cost
Land	4,857,000
Building and Land Improvements	23,647,000
Costs Capitalized Subsequent to Acquisition	0
Gross Amounts Carried at Close of Period
Land	4,857,000
Building and Land Improvements	23,647,000
Total	28,504,000
Accumulated Depreciation	(215,000)
8003 Corporate Drive [Member]
Initial Cost
Land	611,000
Building and Land Improvements	1,126,000
Costs Capitalized Subsequent to Acquisition	685,000
Gross Amounts Carried at Close of Period
Land	611,000
Building and Land Improvements	1,811,000
Total	2,422,000
Accumulated Depreciation	(99,000)
8007 Corporate Drive [Member]
Initial Cost
Land	1,434,000
Building and Land Improvements	350,000
Costs Capitalized Subsequent to Acquisition	1,218,000
Gross Amounts Carried at Close of Period
Land	1,434,000
Building and Land Improvements	1,568,000
Total	3,002,000
Accumulated Depreciation	(278,000)
8010 Corporate Drive [Member]
Initial Cost
Land	1,349,000
Building and Land Improvements	3,262,000
Costs Capitalized Subsequent to Acquisition	1,744,000
Gross Amounts Carried at Close of Period
Land	1,349,000
Building and Land Improvements	5,006,000
Total	6,355,000
Accumulated Depreciation	(1,448,000)
8013 Corporate Drive [Member]
Initial Cost
Land	642,000
Building and Land Improvements	1,072,000
Costs Capitalized Subsequent to Acquisition	1,808,000
Gross Amounts Carried at Close of Period
Land	642,000
Building and Land Improvements	2,880,000
Total	3,522,000
Accumulated Depreciation	(519,000)
8015 Corporate Drive [Member]
Initial Cost
Land	446,000
Building and Land Improvements	1,116,000
Costs Capitalized Subsequent to Acquisition	466,000
Gross Amounts Carried at Close of Period
Land	446,000
Building and Land Improvements	1,582,000
Total	2,028,000
Accumulated Depreciation	(468,000)
8019 Corporate Drive [Member]
Initial Cost
Land	680,000
Building and Land Improvements	1,898,000
Costs Capitalized Subsequent to Acquisition	825,000
Gross Amounts Carried at Close of Period
Land	680,000
Building and Land Improvements	2,723,000
Total	3,403,000
Accumulated Depreciation	(827,000)
8020 Corporate Drive [Member]
Initial Cost
Land	2,184,000
Building and Land Improvements	3,767,000
Costs Capitalized Subsequent to Acquisition	2,205,000
Gross Amounts Carried at Close of Period
Land	2,184,000
Building and Land Improvements	5,972,000
Total	8,156,000
Accumulated Depreciation	(1,703,000)
8023 Corporate Drive [Member]
Initial Cost
Land	651,000
Building and Land Improvements	1,164,000
Costs Capitalized Subsequent to Acquisition	5,000
Gross Amounts Carried at Close of Period
Land	651,000
Building and Land Improvements	1,169,000
Total	1,820,000
Accumulated Depreciation	(47,000)
8094 Sandpiper Circle [Member]
Initial Cost
Land	1,960,000
Building and Land Improvements	3,716,000
Costs Capitalized Subsequent to Acquisition	1,129,000
Gross Amounts Carried at Close of Period
Land	1,960,000
Building and Land Improvements	4,845,000
Total	6,805,000
Accumulated Depreciation	(1,267,000)
8098 Sandpiper Circle [Member]
Initial Cost
Land	1,797,000
Building and Land Improvements	3,651,000
Costs Capitalized Subsequent to Acquisition	639,000
Gross Amounts Carried at Close of Period
Land	1,797,000
Building and Land Improvements	4,290,000
Total	6,087,000
Accumulated Depreciation	(929,000)
8110 Corporate Drive [Member]
Initial Cost
Land	2,285,000
Building and Land Improvements	10,117,000
Costs Capitalized Subsequent to Acquisition	989,000
Gross Amounts Carried at Close of Period
Land	2,285,000
Building and Land Improvements	11,106,000
Total	13,391,000
Accumulated Depreciation	(3,133,000)
8140 Corporate Drive [Member]
Initial Cost
Land	2,158,000
Building and Land Improvements	8,457,000
Costs Capitalized Subsequent to Acquisition	3,163,000
Gross Amounts Carried at Close of Period
Land	2,158,000
Building and Land Improvements	11,620,000
Total	13,778,000
Accumulated Depreciation	(4,117,000)
8621 Robert Fulton Drive [Member]
Initial Cost
Land	2,317,000
Building and Land Improvements	12,642,000
Costs Capitalized Subsequent to Acquisition	344,000
Gross Amounts Carried at Close of Period
Land	2,317,000
Building and Land Improvements	12,986,000
Total	15,303,000
Accumulated Depreciation	(3,378,000)
8661 Robert Fulton Drive [Member]
Initial Cost
Land	1,510,000
Building and Land Improvements	3,764,000
Costs Capitalized Subsequent to Acquisition	2,420,000
Gross Amounts Carried at Close of Period
Land	1,510,000
Building and Land Improvements	6,184,000
Total	7,694,000
Accumulated Depreciation	(2,117,000)
8671 Robert Fulton Drive [Member]
Initial Cost
Land	1,718,000
Building and Land Improvements	4,280,000
Costs Capitalized Subsequent to Acquisition	3,931,000
Gross Amounts Carried at Close of Period
Land	1,718,000
Building and Land Improvements	8,211,000
Total	9,929,000
Accumulated Depreciation	(3,060,000)
870 Elkridge Landing Road [Member]
Initial Cost
Land	2,003,000
Building and Land Improvements	9,442,000
Costs Capitalized Subsequent to Acquisition	7,616,000
Gross Amounts Carried at Close of Period
Land	2,003,000
Building and Land Improvements	17,058,000
Total	19,061,000
Accumulated Depreciation	(8,411,000)
891 Elkridge Landing Road [Member]
Initial Cost
Land	1,165,000
Building and Land Improvements	4,772,000
Costs Capitalized Subsequent to Acquisition	2,928,000
Gross Amounts Carried at Close of Period
Land	1,165,000
Building and Land Improvements	7,700,000
Total	8,865,000
Accumulated Depreciation	(3,488,000)
900 Elkridge Landing Road [Member]
Initial Cost
Land	1,993,000
Building and Land Improvements	7,972,000
Costs Capitalized Subsequent to Acquisition	3,603,000
Gross Amounts Carried at Close of Period
Land	1,993,000
Building and Land Improvements	11,575,000
Total	13,568,000
Accumulated Depreciation	(6,009,000)
901 Elkridge Landing Road [Member]
Initial Cost
Land	1,156,000
Building and Land Improvements	4,437,000
Costs Capitalized Subsequent to Acquisition	2,390,000
Gross Amounts Carried at Close of Period
Land	1,156,000
Building and Land Improvements	6,827,000
Total	7,983,000
Accumulated Depreciation	(3,015,000)
911 Elkridge Landing Road [Member]
Initial Cost
Land	1,215,000
Building and Land Improvements	4,861,000
Costs Capitalized Subsequent to Acquisition	2,024,000
Gross Amounts Carried at Close of Period
Land	1,215,000
Building and Land Improvements	6,885,000
Total	8,100,000
Accumulated Depreciation	(3,462,000)
921 Elkridge Landing Road [Member]
Initial Cost
Land	1,044,000
Building and Land Improvements	4,727,000
Costs Capitalized Subsequent to Acquisition	748,000
Gross Amounts Carried at Close of Period
Land	1,044,000
Building and Land Improvements	5,475,000
Total	6,519,000
Accumulated Depreciation	(4,924,000)
938 Elkridge Landing Road [Member]
Initial Cost
Land	922,000
Building and Land Improvements	4,748,000
Costs Capitalized Subsequent to Acquisition	1,263,000
Gross Amounts Carried at Close of Period
Land	922,000
Building and Land Improvements	6,011,000
Total	6,933,000
Accumulated Depreciation	(2,116,000)
939 Elkridge Landing Road [Member]
Initial Cost
Land	939,000
Building and Land Improvements	3,756,000
Costs Capitalized Subsequent to Acquisition	4,245,000
Gross Amounts Carried at Close of Period
Land	939,000
Building and Land Improvements	8,001,000
Total	8,940,000
Accumulated Depreciation	(3,337,000)
940 Elkridge Landing Road [Member]
Initial Cost
Land	842,000
Building and Land Improvements	4,000
Costs Capitalized Subsequent to Acquisition	0
Gross Amounts Carried at Close of Period
Land	842,000
Building and Land Improvements	4,000
Total	846,000
Accumulated Depreciation	0
9651 Hornbaker Road [Member]
Initial Cost
Land	6,050,000
Building and Land Improvements	249,706,000
Costs Capitalized Subsequent to Acquisition	985,000
Gross Amounts Carried at Close of Period
Land	6,050,000
Building and Land Improvements	250,691,000
Total	256,741,000
Accumulated Depreciation	(18,119,000)
Aerotech Commerce [Member]
Initial Cost
Land	900,000
Gross Amounts Carried at Close of Period
Land	900,000
Total	900,000
Arborcrest [Member]
Initial Cost
Land	21,968,000
Building and Land Improvements	122,759,000
Costs Capitalized Subsequent to Acquisition	1,301,000
Gross Amounts Carried at Close of Period
Land	21,968,000
Building and Land Improvements	124,060,000
Total	146,028,000
Accumulated Depreciation	(25,901,000)
Arundel Preserve [Member]
Initial Cost
Land	13,401,000
Building and Land Improvements	9,331,000
Gross Amounts Carried at Close of Period
Land	13,401,000
Building and Land Improvements	9,331,000
Total	22,732,000
Ashburn Crossing - DC 10 [Member]
Initial Cost
Land	4,408,000
Building and Land Improvements	10,689,000
Gross Amounts Carried at Close of Period
Land	4,408,000
Building and Land Improvements	10,689,000
Total	15,097,000
Accumulated Depreciation	(225,000)
Canton Crossing Land [Member]
Initial Cost
Land	16,085,000
Building and Land Improvements	2,134,000
Gross Amounts Carried at Close of Period
Land	16,085,000
Building and Land Improvements	2,134,000
Total	18,219,000
Canton Crossing Util Distr Ctr [Member]
Initial Cost
Land	7,300,000
Building and Land Improvements	15,556,000
Costs Capitalized Subsequent to Acquisition	942,000
Gross Amounts Carried at Close of Period
Land	7,300,000
Building and Land Improvements	16,498,000
Total	23,798,000
Accumulated Depreciation	(3,326,000)
Columbia Gateway - Southridge [Member]
Initial Cost
Land	6,387,000
Building and Land Improvements	3,539,000
Gross Amounts Carried at Close of Period
Land	6,387,000
Building and Land Improvements	3,539,000
Total	9,926,000
Dahlgren Technology Center [Member]
Initial Cost
Land	978,000
Building and Land Improvements	178,000
Gross Amounts Carried at Close of Period
Land	978,000
Building and Land Improvements	178,000
Total	1,156,000
Expedition VII [Member]
Initial Cost
Land	705,000
Building and Land Improvements	728,000
Gross Amounts Carried at Close of Period
Land	705,000
Building and Land Improvements	728,000
Total	1,433,000
InterQuest [Member]
Initial Cost
Land	8,201,000
Building and Land Improvements	0
Gross Amounts Carried at Close of Period
Land	8,201,000
Building and Land Improvements	0
Total	8,201,000
M Square Research Park [Member]
Initial Cost
Building and Land Improvements	2,986,000
Gross Amounts Carried at Close of Period
Building and Land Improvements	2,986,000
Total	2,986,000
National Business Park North [Member]
Initial Cost
Land	28,066,000
Building and Land Improvements	41,936,000
Gross Amounts Carried at Close of Period
Land	28,066,000
Building and Land Improvements	41,936,000
Total	70,002,000
North Gate Business Park [Member]
Initial Cost
Land	6,309,000
Building and Land Improvements	4,540,000
Gross Amounts Carried at Close of Period
Land	6,309,000
Building and Land Improvements	4,540,000
Total	10,849,000
Northwest Crossroads [Member]
Initial Cost
Land	7,430,000
Building and Land Improvements	847,000
Gross Amounts Carried at Close of Period
Land	7,430,000
Building and Land Improvements	847,000
Total	8,277,000
NOVA Office A [Member]
Initial Cost
Land	2,096,000
Building and Land Improvements	47,364,000
Gross Amounts Carried at Close of Period
Land	2,096,000
Building and Land Improvements	47,364,000
Total	49,460,000
Accumulated Depreciation	(1,046,000)
NOVA Office B [Member]
Initial Cost
Land	739,000
Building and Land Improvements	26,277,000
Gross Amounts Carried at Close of Period
Land	739,000
Building and Land Improvements	26,277,000
Total	27,016,000
NOVA Office D [Member]
Initial Cost
Land	6,587,000
Building and Land Improvements	103,000
Gross Amounts Carried at Close of Period
Land	6,587,000
Building and Land Improvements	103,000
Total	6,690,000
Old Annapolis Road [Member]
Initial Cost
Land	1,637,000
Building and Land Improvements	5,500,000
Costs Capitalized Subsequent to Acquisition	3,457,000
Gross Amounts Carried at Close of Period
Land	1,637,000
Building and Land Improvements	8,957,000
Total	10,594,000
Accumulated Depreciation	(2,983,000)
Patriot Park [Member]
Initial Cost
Land	2,328,000
Building and Land Improvements	1,000
Gross Amounts Carried at Close of Period
Land	2,328,000
Building and Land Improvements	1,000
Total	2,329,000
Patriot Point - DC15 [Member]
Initial Cost
Land	12,156,000
Building and Land Improvements	7,112,000
Gross Amounts Carried at Close of Period
Land	12,156,000
Building and Land Improvements	7,112,000
Total	19,268,000
Patriot Point - DC16 [Member]
Initial Cost
Land	12,156,000
Building and Land Improvements	4,110,000
Gross Amounts Carried at Close of Period
Land	12,156,000
Building and Land Improvements	4,110,000
Total	16,266,000
Patriot Point - DC17 [Member]
Initial Cost
Land	6,078,000
Building and Land Improvements	472,000
Gross Amounts Carried at Close of Period
Land	6,078,000
Building and Land Improvements	472,000
Total	6,550,000
Patriot Ridge [Member]
Initial Cost
Land	18,517,000
Building and Land Improvements	14,423,000
Gross Amounts Carried at Close of Period
Land	18,517,000
Building and Land Improvements	14,423,000
Total	32,940,000
Redstone Gateway [Member]
Initial Cost
Building and Land Improvements	14,250,000
Gross Amounts Carried at Close of Period
Building and Land Improvements	14,250,000
Total	14,250,000
Route 15/Biggs Ford Road [Member]
Initial Cost
Land	8,703,000
Building and Land Improvements	596,000
Gross Amounts Carried at Close of Period
Land	8,703,000
Building and Land Improvements	596,000
Total	9,299,000
Sentry Gateway [Member]
Initial Cost
Land	8,275,000
Building and Land Improvements	3,645,000
Gross Amounts Carried at Close of Period
Land	8,275,000
Building and Land Improvements	3,645,000
Total	11,920,000
Sentry Gateway - T [Member]
Initial Cost
Land	14,020,000
Building and Land Improvements	38,804,000
Costs Capitalized Subsequent to Acquisition	13,000
Gross Amounts Carried at Close of Period
Land	14,020,000
Building and Land Improvements	38,817,000
Total	52,837,000
Accumulated Depreciation	(8,618,000)
Sentry Gateway - V [Member]
Initial Cost
Building and Land Improvements	1,066,000
Gross Amounts Carried at Close of Period
Building and Land Improvements	1,066,000
Total	1,066,000
Accumulated Depreciation	(188,000)
Sentry Gateway - W [Member]
Initial Cost
Building and Land Improvements	1,884,000
Gross Amounts Carried at Close of Period
Building and Land Improvements	1,884,000
Total	1,884,000
Accumulated Depreciation	(296,000)
Sentry Gateway - X [Member]
Initial Cost
Land	1,964,000
Building and Land Improvements	21,178,000
Gross Amounts Carried at Close of Period
Land	1,964,000
Building and Land Improvements	21,178,000
Total	23,142,000
Accumulated Depreciation	(2,728,000)
Sentry Gateway - Y [Member]
Initial Cost
Land	1,964,000
Building and Land Improvements	21,298,000
Gross Amounts Carried at Close of Period
Land	1,964,000
Building and Land Improvements	21,298,000
Total	23,262,000
Accumulated Depreciation	(2,745,000)
Sentry Gateway - Z [Member]
Initial Cost
Land	1,964,000
Building and Land Improvements	30,335,000
Gross Amounts Carried at Close of Period
Land	1,964,000
Building and Land Improvements	30,335,000
Total	32,299,000
Accumulated Depreciation	(616,000)
Southpoint Manassas - DC12 [Member]
Initial Cost
Land	4,518,000
Building and Land Improvements	12,701,000
Gross Amounts Carried at Close of Period
Land	4,518,000
Building and Land Improvements	12,701,000
Total	17,219,000
Accumulated Depreciation	(127,000)
Southpoint Manassas - DC14 [Member]
Initial Cost
Land	4,518,000
Building and Land Improvements	13,009,000
Gross Amounts Carried at Close of Period
Land	4,518,000
Building and Land Improvements	13,009,000
Total	17,527,000
Accumulated Depreciation	(101,000)
Westfields - Park Center [Member]
Initial Cost
Land	16,418,000
Building and Land Improvements	7,488,000
Gross Amounts Carried at Close of Period
Land	16,418,000
Building and Land Improvements	7,488,000
Total	23,906,000
Westfields Corporate Center [Member]
Initial Cost
Land	7,141,000
Building and Land Improvements	1,423,000
Gross Amounts Carried at Close of Period
Land	7,141,000
Building and Land Improvements	1,423,000
Total	8,564,000
White Marsh [Member]
Initial Cost
Land	7,852,000
Building and Land Improvements	2,570,000
Gross Amounts Carried at Close of Period
Land	7,852,000
Building and Land Improvements	2,570,000
Total	10,422,000
Woodland Park [Member]
Initial Cost
Land	9,614,000
Building and Land Improvements	85,000
Gross Amounts Carried at Close of Period
Land	9,614,000
Building and Land Improvements	85,000
Total	9,699,000
Other Developments, including intercompany eliminations [Member]
Initial Cost
Land	94,000
Building and Land Improvements	34,000
Costs Capitalized Subsequent to Acquisition	431,000
Gross Amounts Carried at Close of Period
Land	94,000
Building and Land Improvements	465,000
Total	559,000
Accumulated Depreciation	(39,000)
Fair value measurement on a nonrecurring basis [Member] | Real Estate Held [Member]
Additional information
Impairment losses	22,000,000
Fair value measurement on a nonrecurring basis [Member] | Real Estate Investment Properties, Net [Member]
Additional information
Impairment losses	$ 23,500,000	$ 1,400,000

Schedule III - Real Estate an98

Schedule III - Real Estate and Accumulated Depreciation (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Changes in cost of properties
Beginning balance	$ 4,014,336	$ 3,811,950	$ 3,859,960
Acquisitions of operating properties	194,616	0	0
Building and land improvements	273,761	254,868	249,639
Sales	(172,628)	(48,466)	(141,045)
Impairments	(29,548)	(3,042)	(45,931)
Other dispositions	(121,921)	(974)	(110,673)
Ending balance	4,158,616	4,014,336	3,811,950
Changes in accumulated depreciation
Beginning balance	703,083	597,649	568,176
Depreciation expense	112,695	111,326	92,677
Sales	(49,614)	(3,129)	(9,542)
Impairments	(6,092)	(1,671)	(14,863)
Other dispositions	(41,392)	(1,092)	(38,799)
Ending balance	$ 718,680	$ 703,083	$ 597,649