COPT Defense Properties (CDP) 10-K16 Feb 182017 FY Annual reportFinancial data

Company Profile

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2017	Jan. 31, 2018	Jun. 30, 2017
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, 2017
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			$ 3,500
Entity Common Stock, Shares Outstanding		101,283,508
Document Fiscal Year Focus	2,017
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, 2017
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			$ 108.9
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Properties, net:
Operating properties, net	$ 2,737,611	$ 2,671,831
Projects in development or held for future development	403,494	401,531
Total properties, net	3,141,105	3,073,362
Assets held for sale, net	42,226	94,654
Cash and cash equivalents	12,261	209,863
Restricted cash and marketable securities	7,186	8,193
Investment in unconsolidated real estate joint venture	25,066	25,548
Accounts receivable (net of allowance for doubtful accounts of $607 and $603, respectively)	31,802	34,438
Deferred rent receivable (net of allowance of $364 and $373, respectively)	86,710	90,219
Intangible assets on real estate acquisitions, net	59,092	78,351
Deferred leasing costs (net of accumulated amortization of $29,560 and $65,988, respectively)	48,322	41,214
Investing receivables	57,493	52,279
Prepaid expenses and other assets, net	67,221	72,764
Total assets	3,578,484	3,780,885
Liabilities:
Debt, net	1,828,333	1,904,001
Accounts payable and accrued expenses	108,137	108,682
Rents received in advance and security deposits	25,648	29,798
Dividends and distributions payable	28,921	31,335
Deferred revenue associated with operating leases	11,682	12,666
Redeemable preferred units of general partner, 531,667 units outstanding at December 31, 2016	0	26,583
Deferred property sale	43,377	0
Capital lease obligation	15,853	0
Other liabilities	41,822	50,177
Total liabilities	2,103,773	2,163,242
Commitments and contingencies (Note 19)
Redeemable noncontrolling interests	23,125	22,979
Corporate Office Properties Trust’s shareholders’ equity:
Preferred Shares of beneficial interest at liquidation preference	0	172,500
Common Shares of beneficial interest ($0.01 par value; shares authorized of 150,000,000 at December 31, 2017 and 125,000,000 at December 31, 2016; shares issued and outstanding of 101,292,299 at December 31, 2017 and 98,498,651 at December 31, 2016)	1,013	985
Additional paid-in capital	2,201,047	2,116,581
Cumulative distributions in excess of net income	(818,190)	(765,276)
Accumulated other comprehensive income (loss)	2,167	(1,731)
Total Corporate Office Properties Trust’s shareholders’ equity	1,386,037	1,523,059
Noncontrolling interests in subsidiaries:
Common units in COPLP	44,481	49,228
Preferred units in COPLP	8,800	8,800
Other consolidated entities	12,268	13,577
Noncontrolling interests in subsidiaries	65,549	71,605
Total equity	1,451,586	1,594,664
Total liabilities, redeemable noncontrolling interests and equity	3,578,484	3,780,885
Corporate Office Properties, L.P. [Member]
Properties, net:
Operating properties, net	2,737,611	2,671,831
Projects in development or held for future development	403,494	401,531
Total properties, net	3,141,105	3,073,362
Assets held for sale, net	42,226	94,654
Cash and cash equivalents	12,261	209,863
Restricted cash and marketable securities	2,570	2,756
Investment in unconsolidated real estate joint venture	25,066	25,548
Accounts receivable (net of allowance for doubtful accounts of $607 and $603, respectively)	31,802	34,438
Deferred rent receivable (net of allowance of $364 and $373, respectively)	86,710	90,219
Intangible assets on real estate acquisitions, net	59,092	78,351
Deferred leasing costs (net of accumulated amortization of $29,560 and $65,988, respectively)	48,322	41,214
Investing receivables	57,493	52,279
Prepaid expenses and other assets, net	67,221	72,764
Total assets	3,573,868	3,775,448
Liabilities:
Debt, net	1,828,333	1,904,001
Accounts payable and accrued expenses	108,137	108,682
Rents received in advance and security deposits	25,648	29,798
Dividends and distributions payable	28,921	31,335
Deferred revenue associated with operating leases	11,682	12,666
Redeemable preferred units of general partner, 531,667 units outstanding at December 31, 2016	0	26,583
Deferred property sale	43,377	0
Capital lease obligation	15,853	0
Other liabilities	37,206	44,740
Total liabilities	2,099,157	2,157,805
Commitments and contingencies (Note 19)
Redeemable noncontrolling interests	23,125	22,979
Corporate Office Properties Trust’s shareholders’ equity:
Common Shares of beneficial interest ($0.01 par value; shares authorized of 150,000,000 at December 31, 2017 and 125,000,000 at December 31, 2016; shares issued and outstanding of 101,292,299 at December 31, 2017 and 98,498,651 at December 31, 2016)	1,428,301	1,401,597
Accumulated other comprehensive income (loss)	2,173	(1,854)
Total Corporate Office Properties Trust’s shareholders’ equity	1,439,274	1,581,043
Noncontrolling interests in subsidiaries:
Noncontrolling interests in subsidiaries	12,312	13,621
Total equity	1,451,586	1,594,664
Total liabilities, redeemable noncontrolling interests and equity	3,573,868	3,775,448
Corporate Office Properties, L.P. [Member] \| General Partner [Member]
Corporate Office Properties Trust’s shareholders’ equity:
Preferred partners' capital accounts	0	172,500
Corporate Office Properties, L.P. [Member] \| Limited Partner [Member]
Corporate Office Properties Trust’s shareholders’ equity:
Preferred partners' capital accounts	$ 8,800	$ 8,800

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Allowance for doubtful accounts- AR	$ 607	$ 603
Allowance for deferred rent receivable	364	373
Accumulated amortization of deferred leasing costs	$ 29,560	$ 65,988
Common Shares of beneficial interest, par value (in dollars per share)	$ 0.01	$ 0.01
Common Shares of beneficial interest, shares authorized	150,000,000	125,000,000
Common Shares of beneficial interest, shares issued	101,292,299	98,498,651
Common Shares of beneficial interest, shares outstanding	101,292,299	98,498,651
Corporate Office Properties, L.P. [Member]
Allowance for doubtful accounts- AR	$ 607	$ 603
Allowance for deferred rent receivable	364	373
Accumulated amortization of deferred leasing costs	$ 29,560	$ 65,988
Redeemable preferred units of general partner outstanding, in units		531,667
General Partner [Member] \| Corporate Office Properties, L.P. [Member]
Preferred Units, Outstanding	0	6,900,000
Common Shares of beneficial interest, shares outstanding	101,292,299	98,498,651
Limited Partner [Member] \| Corporate Office Properties, L.P. [Member]
Preferred Units, Outstanding	352,000	352,000
Common Shares of beneficial interest, shares outstanding	3,250,878	3,590,391

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues
Rental revenue		$ 405,722	$ 417,711	$ 420,340
Tenant recoveries and other real estate operations revenue		104,258	108,253	98,724
Construction contract and other service revenues		102,840	48,364	106,402
Total revenues		612,820	574,328	625,466
Expenses
Property operating expenses		190,964	197,530	194,494
Depreciation and amortization associated with real estate operations		134,228	132,719	140,025
Construction contract and other service expenses		99,618	45,481	102,696
Impairment losses		15,123	101,391	23,289
General, administrative and leasing expenses		30,837	36,553	31,361
Business development expenses and land carry costs		6,213	8,244	13,507
Total operating expenses		476,983	521,918	505,372
Operating income		135,837	52,410	120,094
Interest expense		(76,983)	(83,163)	(89,074)
Interest and other income		6,318	5,444	4,517
(Loss) gain on early extinguishment of debt		(513)	(1,110)	85,275
Income (loss) from continuing operations before equity in income of unconsolidated entities and income taxes		64,659	(26,419)	120,812
Equity in income of unconsolidated entities		2,882	1,332	62
Income tax expense		(1,098)	(244)	(199)
Income (loss) from continuing operations		66,443	(25,331)	120,675
Discontinued operations		0	0	156
Income (loss) before gain on sales of real estate		66,443	(25,331)	120,831
Gain on sales of real estate		9,890	40,986	68,047
Net income		76,333	15,655	188,878
Net (income) loss attributable to noncontrolling interests:
Common units in COPLP		(1,936)	155	(6,403)
Preferred units in COPLP		(660)	(660)	(660)
Other consolidated entities		(3,646)	(3,711)	(3,515)
Net income attributable to COPT		70,091	11,439	178,300
Preferred share/ unit dividends/ distributions		(6,219)	(14,297)	(14,210)
Issuance costs associated with redeemed preferred shares		(6,847)	(17)	0
Net income (loss) attributable to COPT common shareholders		57,025	(2,875)	164,090
Net income attributable to COPT:
Income from continuing operations		70,091	11,439	178,147
Discontinued operations, net		0	0	153
Net income attributable to COPT		$ 70,091	$ 11,439	$ 178,300
Basic earnings per common share
Income (loss) from continuing operations (in dollars per share/unit)	[1]	$ 0.57	$ (0.03)	$ 1.74
Net income (loss) attributable to COPT common shareholders (in dollars per share/unit)	[1]	0.57	(0.03)	1.74
Diluted earnings per common share
Income (loss) from continuing operations (in dollars per share/unit)	[1]	0.57	(0.03)	1.74
Net income (loss) attributable to COPT common shareholders (in dollars per share/unit)	[1]	$ 0.57	$ (0.03)	$ 1.74
Corporate Office Properties, L.P. [Member]
Revenues
Rental revenue		$ 405,722	$ 417,711	$ 420,340
Tenant recoveries and other real estate operations revenue		104,258	108,253	98,724
Construction contract and other service revenues		102,840	48,364	106,402
Total revenues		612,820	574,328	625,466
Expenses
Property operating expenses		190,964	197,530	194,494
Depreciation and amortization associated with real estate operations		134,228	132,719	140,025
Construction contract and other service expenses		99,618	45,481	102,696
Impairment losses		15,123	101,391	23,289
General, administrative and leasing expenses		30,837	36,553	31,361
Business development expenses and land carry costs		6,213	8,244	13,507
Total operating expenses		476,983	521,918	505,372
Operating income		135,837	52,410	120,094
Interest expense		(76,983)	(83,163)	(89,074)
Interest and other income		6,318	5,444	4,517
(Loss) gain on early extinguishment of debt		(513)	(1,110)	85,275
Income (loss) from continuing operations before equity in income of unconsolidated entities and income taxes		64,659	(26,419)	120,812
Equity in income of unconsolidated entities		2,882	1,332	62
Income tax expense		(1,098)	(244)	(199)
Income (loss) from continuing operations		66,443	(25,331)	120,675
Discontinued operations		0	0	156
Income (loss) before gain on sales of real estate		66,443	(25,331)	120,831
Gain on sales of real estate		9,890	40,986	68,047
Net income		76,333	15,655	188,878
Net income attributable to noncontrolling interests in consolidated entities		(3,646)	(3,715)	(3,520)
Net (income) loss attributable to noncontrolling interests:
Net income attributable to COPT		72,687	11,940	185,358
Preferred share/ unit dividends/ distributions		(6,879)	(14,957)	(14,870)
Issuance costs associated with redeemed preferred shares		(6,847)	(17)	0
Net income (loss) attributable to COPT common shareholders		58,961	(3,034)	170,488
Net income attributable to COPT:
Income from continuing operations		72,687	11,940	185,199
Discontinued operations, net		0	0	159
Net income attributable to COPT		$ 72,687	$ 11,940	$ 185,358
Basic earnings per common share
Income (loss) from continuing operations (in dollars per share/unit)	[2]	$ 0.57	$ (0.04)	$ 1.74
Net income (loss) attributable to COPT common shareholders (in dollars per share/unit)	[2]	0.57	(0.04)	1.74
Diluted earnings per common share
Income (loss) from continuing operations (in dollars per share/unit)	[2]	0.57	(0.04)	1.74
Net income (loss) attributable to COPT common shareholders (in dollars per share/unit)	[2]	$ 0.57	$ (0.04)	$ 1.74

[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, 2017	Dec. 31, 2016	Dec. 31, 2015
Net income	$ 76,333	$ 15,655	$ 188,878
Other comprehensive income (loss)
Unrealized gain (loss) on interest rate derivatives	684	(2,915)	(4,739)
Loss on interest rate derivatives recognized in interest expense (effective portion)	3,216	4,230	3,599
Loss on interest rate derivatives recognized in interest expense (ineffective portion)	88	0	0
Equity in other comprehensive income (loss) of equity method investee	39	(184)	(264)
Other comprehensive income (loss)	4,027	1,131	(1,404)
Comprehensive income	80,360	16,786	187,474
Comprehensive income attributable to noncontrolling interests	(6,371)	(4,240)	(10,715)
Comprehensive income attributable to COPT	73,989	12,546	176,759
Corporate Office Properties, L.P. [Member]
Net income	76,333	15,655	188,878
Other comprehensive income (loss)
Unrealized gain (loss) on interest rate derivatives	684	(2,915)	(4,739)
Loss on interest rate derivatives recognized in interest expense (effective portion)	3,216	4,230	3,599
Loss on interest rate derivatives recognized in interest expense (ineffective portion)	88	0	0
Equity in other comprehensive income (loss) of equity method investee	39	(184)	(264)
Other comprehensive income (loss)	4,027	1,131	(1,404)
Comprehensive income	80,360	16,786	187,474
Comprehensive income attributable to noncontrolling interests	(3,646)	(3,715)	(3,720)
Comprehensive income attributable to COPT	$ 76,714	$ 13,071	$ 183,754

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 [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 [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, 2014	$ 1,520,884		$ 199,083	$ 933	$ 1,969,968		$ (717,264)	$ (1,297)	$ 69,461	$ 1,520,884	$ 1,305,219	$ (1,381)	$ 9,163		$ 8,800		$ 199,083
Balance (in units/ shares) at Dec. 31, 2014	93,255,284										97,092,835
Balance (preferred units) at Dec. 31, 2014															352,000		7,431,667
Increase (Decrease) in Shareholders' Equity
Conversion of common units to common shares (160,160, 87,000 and 339,513 shares in 2015, 2016 and 2017, 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 (890,241, 3,721,227 and 591,042 shares in 2015, 2016 and 2017, respectively)		$ 26,535		9		$ 26,526				26,535	$ 26,535
Exercise of share options (in units/shares)	76,474										76,474
Exercise of share options (76,474 and 5,000 shares in 2015 and 2017 respectively)	$ 2,008				2,008					2,008	$ 2,008
Share-based compensation (149,353, 158,912 and 179,180 shares issued, net of redemptions in 2015, 2016 and 2017, respectively)	$ 7,398			1	7,397					7,398	$ 7,398
Share-based compensation (in shares/units)	149,353										149,353
Redemption of vested equity awards	$ (2,462)				(2,462)					(2,462)	$ (2,462)
Adjustments to noncontrolling interests resulting from changes in ownership of COPLP	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 consolidated entities	300								300	300			300
Distributions to noncontrolling interests in other consolidated entities	(35)								(35)	(35)			(35)
Adjustment to arrive at fair value of redeemable noncontrolling interests	116				116					116	116
Tax loss from share-based compensation	$ (513)				(513)					(513)	$ (513)
Balance (preferred units) at Dec. 31, 2015															352,000		7,431,667
Balance (in units/ shares) at Dec. 31, 2015	94,531,512										98,208,903
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
Increase (Decrease) in Shareholders' Equity
Redemption of preferred shares/units			(531,667)														(531,667)
Redemption of preferred shares/units (531,667 and 6,900,000 shares in 2016 and 2017, respectively)	(26,583)		$ (26,583)		17		(17)			(26,583)							$ (26,583)
Conversion of common units to common shares (160,160, 87,000 and 339,513 shares in 2015, 2016 and 2017, respectively)	0			1	1,166				(1,167)
Issuance of common units resulting from common shares issued under at-the-market program (in units)											3,721,227
Issuance of common units resulting from common shares issued under at-the-market program (890,241, 3,721,227 and 591,042 shares in 2015, 2016 and 2017, respectively)	109,053			37	109,016					109,053	$ 109,053
Share-based compensation (149,353, 158,912 and 179,180 shares issued, net of redemptions in 2015, 2016 and 2017, respectively)	$ 7,453			2	7,451					7,453	$ 7,453
Share-based compensation (in shares/units)	158,912										158,912
Redemption of vested equity awards	$ (2,466)				(2,466)					(2,466)	$ (2,466)
Adjustments to noncontrolling interests resulting from changes in ownership of COPLP	0				(2,158)				2,158
Comprehensive income	14,543						11,439	1,107	1,997	14,543	(3,017)	1,131	1,472		660		14,297
Dividends/Distributions	(119,526)						(119,526)			(124,176)	(109,219)				$ (660)		$ (14,297)
Distributions to owners of common and preferred units in COPLP	(4,650)								(4,650)
Contributions from noncontrolling interests in other consolidated entities	1,244				0				1,244	1,244			1,244
Distributions to noncontrolling interests in other consolidated entities	(16)								(16)	(16)			(16)
Adjustment to arrive at fair value of redeemable noncontrolling interests	(621)				(621)					(621)	(621)
Tax loss from share-based compensation	$ (331)				(331)					(331)	$ (331)
Balance (preferred units) at Dec. 31, 2016														352,000	352,000	6,900,000	6,900,000
Balance (in units/ shares) at Dec. 31, 2016	98,498,651										102,089,042			3,590,391		98,498,651
Balance at Dec. 31, 2016	$ 1,594,664		$ 172,500	985	2,116,581		(765,276)	(1,731)	71,605	1,594,664	$ 1,401,597	(1,854)	13,621		$ 8,800		$ 172,500
Increase (Decrease) in Shareholders' Equity
Redemption of preferred shares/units			(6,900,000)														(6,900,000)
Redemption of preferred shares/units (531,667 and 6,900,000 shares in 2016 and 2017, respectively)	(172,500)		$ (172,500)		6,847		(6,847)			(172,500)							$ (172,500)
Conversion of common units to common shares (160,160, 87,000 and 339,513 shares in 2015, 2016 and 2017, respectively)	0			3	4,633				(4,636)
Common shares issued under forward equity sale agreements (1,678,913 shares)	49,944			17	49,927					49,944	$ (49,944)
Issuance of common units resulting from COPT forward equity sale agreements (in shares)											1,678,913
Issuance of common units resulting from common shares issued under at-the-market program (in units)											591,042
Issuance of common units resulting from common shares issued under at-the-market program (890,241, 3,721,227 and 591,042 shares in 2015, 2016 and 2017, respectively)	$ 19,668			6	19,662					19,668	$ 19,668
Exercise of share options (in units/shares)	5,000										5,000
Exercise of share options (76,474 and 5,000 shares in 2015 and 2017 respectively)	$ 150				150					150	$ 150
Share-based compensation (149,353, 158,912 and 179,180 shares issued, net of redemptions in 2015, 2016 and 2017, respectively)	$ 6,095			2	6,093					6,095	$ 6,095
Share-based compensation (in shares/units)	179,180										179,180
Redemption of vested equity awards	$ (1,973)				(1,973)					(1,973)	$ (1,973)
Adjustments to noncontrolling interests resulting from changes in ownership of COPLP	0				(1,486)				1,486
Comprehensive income	78,022						70,091	3,898	4,033	78,022	65,808	4,027	1,308		660		6,219
Dividends/Distributions	(116,158)						(116,158)			(120,480)	(113,601)				$ (660)		$ (6,219)
Distributions to owners of common and preferred units in COPLP	(4,322)								(4,322)
Distributions to noncontrolling interests in other consolidated entities	(2,617)								(2,617)	(2,617)			(2,617)
Adjustment to arrive at fair value of redeemable noncontrolling interests	626				626					626	626
Tax loss from share-based compensation	$ (13)				(13)					(13)	$ (13)
Balance (preferred units) at Dec. 31, 2017														352,000	352,000	0	0
Balance (in units/ shares) at Dec. 31, 2017	101,292,299										104,543,177			3,250,878		101,292,299
Balance at Dec. 31, 2017	$ 1,451,586		$ 0	$ 1,013	$ 2,201,047		$ (818,190)	$ 2,167	$ 65,549	$ 1,451,586	$ 1,428,301	$ 2,173	$ 12,312		$ 8,800		$ 0

Consolidated Statements of Equ7

Consolidated Statements of Equity (Parenthetical) - shares	12 Months Ended
Consolidated Statements of Equity (Parenthetical) - shares	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Balance (in units/ shares)	101,292,299	98,498,651	94,531,512
Conversion of common units to common shares (in shares/units)	339,513	87,000	160,160
Exercise of share options (in units/shares)	5,000		76,474
Restricted common units/shares net of redemptions	179,180	158,912	149,353
Preferred Shares [Member]
Stock redeemed or called during period (in units/ shares)	(6,900,000)	(531,667)
Forward Equity Sale Agreement [Member]
Shares issued to the public (in units/shares)	1,678,913
Common Stock Issued to Public Under At-the-Market Program [Member]
Shares issued to the public (in units/shares)	591,042	3,721,227	890,241

Consolidated Statements of Cash

Consolidated Statements of Cash Flows $ in Thousands	12 Months Ended
Consolidated Statements of Cash Flows $ in Thousands	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Cash flows from operating activities
Revenues from real estate operations received	$ 510,985	$ 514,098	$ 503,503
Construction contract and other service revenues received	102,531	76,824	117,107
Property operating expenses paid	(186,478)	(197,254)	(190,281)
Construction contract and other service expenses paid	(82,707)	(46,318)	(124,481)
General, administrative, leasing, business development and land carry costs paid	(32,673)	(34,877)	(38,113)
Interest expense paid	(73,079)	(78,158)	(65,816)
Lease incentives paid	(9,725)	(2,760)	(1,724)
Income taxes paid	(31)	(5)	(8)
Other	1,831	988	3,821
Net cash provided by operating activities	230,654	232,538	204,008
Cash flows from investing activities
Construction, development and redevelopment	(200,504)	(161,519)	(234,346)
Acquisitions of operating properties and related intangible assets	0	0	(202,866)
Tenant improvements on operating properties	(33,409)	(34,275)	(29,413)
Other capital improvements on operating properties	(22,882)	(26,345)	(23,147)
Proceeds from dispositions of properties	180,839	262,866	193,735
Proceeds from partial sales of properties, net of related debt	0	43,681	0
Leasing costs paid	(14,581)	(10,296)	(13,710)
Other	827	(2,663)	2,215
Net cash (used in) provided by investing activities	(89,710)	71,449	(307,532)
Proceeds from debt
Revolving Credit Facility	352,000	495,500	522,000
Unsecured senior notes	0	0	296,580
Other debt proceeds	0	255,000	164,000
Repayments of debt
Revolving Credit Facility	(226,000)	(539,000)	(561,500)
Scheduled principal amortization	(3,862)	(5,395)	(6,728)
Other debt repayments	(200,200)	(323,107)	(155,307)
Deferred financing costs paid	(500)	(825)	(7,522)
Net proceeds from issuance of common shares	69,534	109,069	28,567
Redemption of preferred shares	(199,083)	0	0
Common share/unit dividends/distributions paid	(109,174)	(104,135)	(103,638)
Preferred share/unit dividends/distributions paid	(9,305)	(14,210)	(14,210)
Distributions paid to noncontrolling interests in COPLP	(4,426)	(4,619)	(4,752)
Distributions paid to redeemable noncontrolling interests	(8,215)	(15,206)	(2,993)
Redemption of vested equity awards	(1,973)	(2,466)	(2,462)
Other	2,658	(5,040)	5,722
Net cash (used in) provided by financing activities	(338,546)	(154,434)	157,757
Net (decrease) increase in cash and cash equivalents	(197,602)	149,553	54,233
Cash and cash equivalents
Beginning of year	209,863	60,310	6,077
End of year	12,261	209,863	60,310
Reconciliation of net income to net cash provided by operating activities:
Net income	76,333	15,655	188,878
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and other amortization	136,501	134,870	142,231
Impairment losses	15,116	101,341	23,523
Amortization of deferred financing costs and net debt discounts	4,307	5,885	5,588
Increase in deferred rent receivable	(2,651)	(145)	(14,969)
Gain on sales of real estate	(9,890)	(40,986)	(68,047)
Share-based compensation	5,615	6,843	6,574
Loss (gain) on early extinguishment of debt	513	456	(86,028)
Other	(6,121)	(4,295)	528
Changes in operating assets and liabilities:
Decrease (increase) in accounts receivable	2,783	(5,262)	1,331
Decrease (increase) in restricted cash and marketable securities	2,015	(365)	(1,241)
Decrease (increase) in prepaid expenses and other assets, net	5,737	(17,272)	2,853
Increase (decrease) in accounts payable, accrued expenses and other liabilities	4,309	43,163	(3,620)
(Decrease) increase in rents received in advance and security deposits	(3,913)	(7,350)	6,407
Net cash provided by operating activities	230,654	232,538	204,008
Supplemental schedule of non-cash investing and financing activities:
(Decrease) increase in accrued capital improvements, leasing and other investing activity costs	(10,654)	5,950	(14,797)
Increase in property in connection with capital lease obligation	16,127	0	0
Increase in property and redeemable noncontrolling interests in connection with property contribution by redeemable noncontrolling interests in a joint venture	0	22,600	0
Decrease in redeemable noncontrolling interests and increase in other liabilities in connection with distribution payable to redeemable noncontrolling interests	0	6,675	0
Decrease in properties, net	0	(114,597)	0
Increase in investment in unconsolidated real estate joint venture	0	25,680	0
Decrease in debt	0	59,534	0
Other net decreases in assets and liabilities	0	3,619	0
Debt assumed on acquisition of operating property	0	0	55,490
Other liabilities assumed on acquisition of operating properties	0	0	5,179
Decrease in property in connection with surrender of property in settlement of debt	0	0	(82,738)
Decrease in debt in connection with surrender of property in settlement of debt	0	0	(150,000)
Increase in property and redeemable noncontrolling interests in connection with property contribution by redeemable noncontrolling interests in a joint venture	0	0	1,415
Increase (decrease) in fair value of derivatives applied to accumulated other comprehensive income (loss) and noncontrolling interests	3,845	1,315	(1,140)
Equity in other comprehensive income (loss) of an equity method investee	39	(184)	(264)
Reclassification of preferred shares to be redeemed to liability	0	26,583	0
Dividends/distribution payable	28,921	31,335	30,178
Decrease in noncontrolling interests and increase in shareholders’ equity in connection with the conversion of common units into common shares	4,636	1,167	2,151
Adjustments to noncontrolling interests resulting from changes in COPLP ownership	1,486	2,158	682
(Decrease) increase in redeemable noncontrolling interests and (increase) decrease in equity to carry redeemable noncontrolling interests at fair value	(626)	621	(116)
Corporate Office Properties, L.P. [Member]
Cash flows from operating activities
Revenues from real estate operations received	510,985	514,098	503,503
Construction contract and other service revenues received	102,531	76,824	117,107
Property operating expenses paid	(186,478)	(197,254)	(190,281)
Construction contract and other service expenses paid	(82,707)	(46,318)	(124,481)
General, administrative, leasing, business development and land carry costs paid	(32,673)	(34,877)	(38,113)
Interest expense paid	(73,079)	(78,158)	(65,816)
Lease incentives paid	(9,725)	(2,760)	(1,724)
Income taxes paid	(31)	(5)	(8)
Other	1,831	988	3,821
Net cash provided by operating activities	230,654	232,538	204,008
Cash flows from investing activities
Construction, development and redevelopment	(200,504)	(161,519)	(234,346)
Acquisitions of operating properties and related intangible assets	0	0	(202,866)
Tenant improvements on operating properties	(33,409)	(34,275)	(29,413)
Other capital improvements on operating properties	(22,882)	(26,345)	(23,147)
Proceeds from dispositions of properties	180,839	262,866	193,735
Proceeds from partial sales of properties, net of related debt	0	43,681	0
Leasing costs paid	(14,581)	(10,296)	(13,710)
Other	827	(2,663)	2,215
Net cash (used in) provided by investing activities	(89,710)	71,449	(307,532)
Proceeds from debt
Revolving Credit Facility	352,000	495,500	522,000
Unsecured senior notes	0	0	296,580
Other debt proceeds	0	255,000	164,000
Repayments of debt
Revolving Credit Facility	(226,000)	(539,000)	(561,500)
Scheduled principal amortization	(3,862)	(5,395)	(6,728)
Other debt repayments	(200,200)	(323,107)	(155,307)
Deferred financing costs paid	(500)	(825)	(7,522)
Net proceeds from issuance of common shares	69,534	109,069	28,567
Redemption of preferred shares	(199,083)	0	0
Common share/unit dividends/distributions paid	(112,940)	(108,094)	(107,730)
Preferred share/unit dividends/distributions paid	(9,965)	(14,870)	(14,870)
Distributions paid to redeemable noncontrolling interests	(8,215)	(15,206)	0
Redemption of vested equity awards	(1,973)	(2,466)	(2,462)
Other	2,658	(5,040)	2,729
Net cash (used in) provided by financing activities	(338,546)	(154,434)	157,757
Net (decrease) increase in cash and cash equivalents	(197,602)	149,553	54,233
Cash and cash equivalents
Beginning of year	209,863	60,310	6,077
End of year	12,261	209,863	60,310
Reconciliation of net income to net cash provided by operating activities:
Net income	76,333	15,655	188,878
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and other amortization	136,501	134,870	142,231
Impairment losses	15,116	101,341	23,523
Amortization of deferred financing costs and net debt discounts	4,307	5,885	5,588
Increase in deferred rent receivable	(2,651)	(145)	(14,969)
Gain on sales of real estate	(9,890)	(40,986)	(68,047)
Share-based compensation	5,615	6,843	6,574
Loss (gain) on early extinguishment of debt	513	456	(86,028)
Other	(6,121)	(4,295)	528
Changes in operating assets and liabilities:
Decrease (increase) in accounts receivable	2,783	(5,262)	1,331
Decrease (increase) in restricted cash and marketable securities	1,194	(691)	(1,360)
Decrease (increase) in prepaid expenses and other assets, net	5,737	(17,272)	2,853
Increase (decrease) in accounts payable, accrued expenses and other liabilities	5,130	43,489	(3,501)
(Decrease) increase in rents received in advance and security deposits	(3,913)	(7,350)	6,407
Net cash provided by operating activities	230,654	232,538	204,008
Supplemental schedule of non-cash investing and financing activities:
(Decrease) increase in accrued capital improvements, leasing and other investing activity costs	(10,654)	5,950	(14,797)
Increase in property in connection with capital lease obligation	16,127	0	0
Increase in property and redeemable noncontrolling interests in connection with property contribution by redeemable noncontrolling interests in a joint venture	0	22,600	0
Decrease in redeemable noncontrolling interests and increase in other liabilities in connection with distribution payable to redeemable noncontrolling interests	0	6,675	0
Decrease in properties, net	0	(114,597)	0
Increase in investment in unconsolidated real estate joint venture	0	25,680	0
Decrease in debt	0	59,534	0
Other net decreases in assets and liabilities	0	3,619	0
Debt assumed on acquisition of operating property	0	0	55,490
Other liabilities assumed on acquisition of operating properties	0	0	5,179
Decrease in property in connection with surrender of property in settlement of debt	0	0	(82,738)
Decrease in debt in connection with surrender of property in settlement of debt	0	0	(150,000)
Increase in property and redeemable noncontrolling interests in connection with property contribution by redeemable noncontrolling interests in a joint venture	0	0	1,415
Increase (decrease) in fair value of derivatives applied to accumulated other comprehensive income (loss) and noncontrolling interests	3,845	1,315	(1,140)
Equity in other comprehensive income (loss) of an equity method investee	39	(184)	(264)
Reclassification of preferred shares to be redeemed to liability	0	26,583	0
Dividends/distribution payable	28,921	31,335	30,178
(Decrease) increase in redeemable noncontrolling interests and (increase) decrease in equity to carry redeemable noncontrolling interests at fair value	$ (626)	$ 621	$ (116)

Organization

Organization	12 Months Ended
Organization	Dec. 31, 2017
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 the United States Government and its 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 portfolio of office properties located in select urban/urban-like submarkets in the Greater Washington, DC/Baltimore region with durable Class-A office fundamentals and characteristics (“Regional Office”). As of December 31, 2017 , our properties included the following (all references to number of properties, square footage, acres and megawatts are unaudited): • 159 properties totaling 17.3 million square feet comprised of 144 office properties and 15 single-tenant data center shell properties (“data center shells”). We owned six of these data center shells through an unconsolidated real estate joint venture; • a wholesale data center with a critical load of 19.25 megawatts; • ten properties under construction or redevelopment ( seven office properties and three data center shells) that we estimate will total approximately 1.1 million square feet upon completion, including two partially operational properties and two properties completed and held for future lease to the United States Government; and • approximately 1,000 acres of land controlled for future development that we believe could be developed into approximately 13.0 million square feet and 150 acres of other land. COPLP owns real estate 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, 2017 , COPT owned 96.9% of the outstanding COPLP common units (“common units”); the remaining common units and all of the outstanding COPLP preferred units (“preferred units”) were owned by third parties. Common units not owned by COPT carry certain redemption rights. The number of common units owned by COPT is equivalent to the number of outstanding common shares of beneficial interest (“common shares”) of COPT, and the entitlement of all common units to quarterly distributions and payments in liquidation is substantially the same as those of COPT common shareholders. Similarly, in the case of any series of preferred units 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; similarly, although 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
Summary of Significant Accounting Policies	Dec. 31, 2017
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. 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 Operating Properties Upon completion of operating 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 operating 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 direct and indirect project costs (including related compensation and other indirect costs), interest expense and real estate taxes associated with properties, or portions thereof, undergoing construction, development and redevelopment activities. In capitalizing interest expense, if there is a specific borrowing for a 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 properties for indicators of impairment quarterly or when circumstances indicate that a property may be impaired. 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. When we dispose of, or classify as held for sale, a component or group of components that represents a strategic shift having a major effect on our operations and financial results (such as a major geographical area of operations, a major line of business or a major equity method investment), we classify the associated results of operations as discontinued operations. We had no properties newly classified as discontinued operations in the last three years. 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 collectability 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. We classify leasing costs paid as an investing activity on our statements of cash flows since such costs are necessary in order for us to generate long-term future cash flows from our properties. 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 amortize deferred costs of line-of-credit arrangements ratably over the terms of such arrangements. We expense any unamortized loan costs when loans are retired early. We present deferred costs of financing arrangements as a direct deduction from the related debt liability, except for costs attributable to line-of-credit arrangements and interest rate derivatives, which we present in the balance sheet in the line entitled “prepaid expenses and other assets, net”. Noncontrolling Interests COPT’s consolidated noncontrolling interests are comprised of interests in COPLP not owned by COPT (discussed further in Note 14) and interests in consolidated real estate joint ventures not owned by us (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 noncancelable term of tenant leases. A lease term 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 noncancelable term of a lease includes periods when a tenant: (1) may not terminate its lease obligation early without incurring a penalty in such an amount that the continuation of the lease appears reasonably assured; (2) 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 (3) 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 reported in liabilities as 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 accumulated other comprehensive income (“AOCI”) or loss (“AOCL”) 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 or AOCL, 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 the section below entitled “Recent Accounting Pronouncements” for disclosure pertaining to the effect of new hedge accounting guidance that we adopted effective January 1, 2018 and Note 11 for additional disclosure pertaining to our 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, 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 and director and officers) and leasing prospects, as well as associated labor and indirect costs attributable to these expenses. Share-Based Compensation We issue three forms of share-based compensation: restricted COPT common shares (“restricted shares”), deferred share awards (also known as restricted share units) and performance share units (also known as performance share awards) (“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 PSUs using a Monte Carlo model. Significant assumptions used for that model include the following: the baseline common share value is 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; and 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, 2017 2016 2015 2017 2016 2015 Ordinary income 86.5 % 48.0 % 38.3 % 100.0 % 100 % 38.3 % Long-term capital gain 0.0 % 0.0 % 61.7 % 0.0 % 0.0 % 61.7 % Return of capital 13.5 % 52.0 % 0.0 % 0.0 % 0.0 % 0.0 % However, 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 2017 , 2016 and 2015 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 $150 million higher than the amount reported on our consolidated balance sheet as of December 31, 2017 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. Reclassification We reclassified certain amounts from prior periods to conform to the current period presentation of our consolidated financial statements with no effect on previously reported net income or equity. Recent Accounting Pronouncements We adopted guidance issued by the Financial Accounting Standards Board (“FASB”) effective January 1, 2017 intended to simplify various aspects related to the accounting and presentation for employee share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities and classification on the consolidated statement of cash flows. In connection with our adoption of this policy, we made an entity-wide accounting policy election to continue to account for potential future award forfeitures by estimating the number of awards that are expected to vest. Our adoption of this guidance did not have a material impact on our consolidated financial statements. We adopted guidance issued by the FASB prospectively effective January 1, 2017 that clarifies the definition of a business used by entities in determining whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. The definition of a business affects many areas of accounting including acquisitions, disposals, goodwill and consolidation. Under the new guidance, we expect that the majority of our future operating property acquisitions will be accounted for as asset acquisitions, whereas under the previous guidance our recent acquisitions were accounted for as business combinations; we believe that the primary effect of this change will be that transaction costs associated with future acquisitions will be capitalized rather than expensed as incurred. This guidance had no effect on our consolidated

Fair Value Measurements

Fair Value Measurements	12 Months Ended
Fair Value Measurements	Dec. 31, 2017
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 Trustees and 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 $4.6 million as of December 31, 2017 and $5.4 million as of December 31, 2016 , 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, 2017 and 2016 , 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. 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 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 as of 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, 2017 and 2016 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, 2017: Assets: Marketable securities in deferred compensation plan (1) Mutual funds $ 4,547 $ — $ — $ 4,547 Other 69 — — 69 Interest rate derivatives (2) — 3,073 — 3,073 Total assets $ 4,616 $ 3,073 $ — $ 7,689 Liabilities: Deferred compensation plan liability (3) $ — $ 4,616 $ — $ 4,616 December 31, 2016: Assets: Marketable securities in deferred compensation plan (1) Mutual funds $ 5,346 $ — $ — $ 5,346 Other 91 — — 91 Interest rate derivatives (2) — 158 — 158 Total assets $ 5,437 $ 158 $ — $ 5,595 Liabilities: Deferred compensation plan liability (3) $ — $ 5,437 $ — $ 5,437 Interest rate derivatives — 1,572 — 1,572 Redeemable preferred shares of beneficial interest (4) — 26,583 — 26,583 Total liabilities $ — $ 33,592 $ — $ 33,592 (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, net” on COPT ’ s consolidated balance sheet. (3) Included in the line entitled “other liabilities” on COPT ’ s consolidated balance sheet. (4) See disclosure regarding our Series K Preferred Shares in Note 13. 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, 2017 and 2016 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, 2017: Assets: Interest rate derivatives (1) $ — $ 3,073 $ — $ 3,073 December 31, 2016: Assets: Interest rate derivatives (1) $ — $ 158 $ — $ 158 Liabilities: Interest rate derivatives $ — $ 1,572 $ — $ 1,572 Redeemable preferred units of general partner (2) — 26,583 — 26,583 Total liabilities $ — $ 28,155 $ — $ 28,155 (1) Included in the line entitled “prepaid expenses and other assets, net” on COPLP ’ s consolidated balance sheet. (2) See disclosure regarding our Series K Preferred Units in Note 14. 2017 Nonrecurring Fair Value Measurements As part of our closing process for each of the four quarters in 2017, we conducted our review of our portfolio of long-lived assets to be held and used for indicators of impairment and found there to be no impairment losses in the first, second and third quarters. In the fourth quarter of 2017, our assessment of weakening leasing prospects and expected enduring vacancy in our Aberdeen, Maryland (“Aberdeen”) portfolio indicated that these properties could be impaired. We have performed recovery analyses on the properties considering weakening tenant demand, high vacancy and low investor demand for office properties in the surrounding submarkets and concluded that the carrying values of these properties were not likely to be recovered from the expected undiscounted cash flows from the operation and eventual disposition of these properties. Accordingly, we recognized $9.0 million of impairment losses on the operating properties in Aberdeen (included in our Other segment). In addition, and also considering these conditions, we determined that we would not likely recover the carrying amount of land in this submarket and recognized a $4.7 million impairment loss on it. We previously recognized impairment losses on these properties in the second quarter of 2016 as discussed below. We determined that the declines in values that have occurred since the initial losses were recognized were due to declining market conditions. For the respective quarters in 2017, we also performed recoverability analyses for our properties classified as held for sale, which resulted in impairment losses of $1.6 million in the second quarter of 2017. These impairment losses were primarily on properties in White Marsh, Maryland (“White Marsh”) (included in our Regional Office and Other segments) that we reclassified to held for sale during the period and adjusted to fair value less costs to sell. These properties were sold in the third quarter. Changes in the expected future cash flows due to changes in our plans for specific properties (especially our expected holding period) could result in the recognition of impairment losses. In addition, because properties held for sale are carried at the lower of carrying value or estimated fair values less costs to sell, declines in their estimated fair values due to market conditions and other factors could result in the recognition of impairment losses. The table below sets forth the fair value hierarchy of the valuation technique we used to determine nonrecurring fair value measurements of properties as of December 31, 2017 (in thousands): Fair Values as of December 31, 2017 Quoted Prices in Significant Active Markets for Significant Other Unobservable Identical Assets Observable Inputs Inputs Description (Level 1) (Level 2) (Level 3) Total Assets: Operating properties, net $ — $ — $ 3,850 $ 3,850 Projects in development or held for future development $ — $ — $ 1,755 $ 1,755 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, 2017 (dollars in thousands): Valuation Technique Fair Values on Measurement Date Unobservable Input Range (Weighted Average) Discounted cash flow $ 3,850 Discount rate 14% - 16% (14%) Terminal capitalization rate 12% (1) Comparable sales analysis $ 1,755 Comparable sales prices N/A (1) Only one fair value applied for this unobservable input. 2016 Nonrecurring Fair Value Measurements In the first quarter of 2016, we set a goal to raise cash from sales of properties in 2016 considerably in excess of the $96.8 million in assets held for sale at December 31, 2015. The specific properties we would sell to achieve this goal had not been identified when the goal was established. Throughout 2016, we engaged in the process of identifying properties we would sell. In the first quarter of 2016, we reclassified: most of our properties in Greater Philadelphia (included in our Regional Office segment); two properties in the Fort Meade/BW Corridor sub-segment; and our remaining land holdings in Colorado Springs, Colorado (“Colorado Springs”) to held for sale and recognized $2.4 million of impairment losses. As of March 31, 2016, we had $225.9 million of assets held for sale. During the second quarter of 2016, as part of our closing process, we conducted our quarterly review of our portfolio for indicators of impairment considering the refined investment strategy of our then newly-appointed Chief Executive Officer and the goals of the asset sales program and concluded that we would: (1) not hold our operating properties in Aberdeen; (2) not develop commercial properties on land in Frederick, Maryland; (3) sell specific properties in our Northern Virginia Defense/IT and Fort Meade/BW Corridor sub-segments; and (4) sell the remaining operating property in Greater Philadelphia that had not previously been classified as held for sale. Accordingly, we performed recoverability analyses for each of these properties and recorded the following impairment losses: • $34.4 million on operating properties in Aberdeen. After shortening our estimated holding period for these properties, we determined that the carrying amount of the properties would not likely be recovered from the operation and eventual dispositions of the properties during the shortened holding period. Accordingly, we adjusted the properties to their estimated fair values; • $4.4 million on land in Aberdeen. In performing our analysis related to the operating properties in Aberdeen, we determined that the weakening leasing and overall commercial real estate conditions in that market indicated that our land holdings in the market may be impaired. As a result, we determined that the carrying amount of the land was not recoverable and adjusted the land to its estimated fair value; • $8.2 million on land in Frederick, Maryland. We determined that the carrying amount of the land would not likely be recovered from its sale and adjusted the land to its estimated fair value; • $14.1 million on operating properties in our Northern Virginia and Fort Meade/BW Corridor sub-segments that we reclassified to held for sale during the period whose carrying amounts exceeded their estimated fair values less costs to sell; • $6.2 million on the property in Greater Philadelphia that we reclassified to held for sale during the period and adjusted to fair value less costs to sell; and • $2.4 million primarily on land in Colorado Springs and operating properties in White Marsh (included in our Regional Office Segment) classified as held for sale whose carrying amounts exceeded their estimated fair values less costs to sell based on updated negotiations with prospective buyers. There were no property sales in the second quarter of 2016 and as of June 30, 2016, we had $300.6 million of assets held for sale. During the third quarter of 2016, as part of our closing process, we conducted our quarterly review of our portfolio for indicators of impairment considering refinements to our disposition strategy made during the third quarter of 2016 to sell an additional operating property in our Northern Virginia Defense/IT sub-segment, an additional operating property in our Fort Meade/BW Corridor sub-segment and our remaining operating properties and land in White Marsh that had not previously been classified as held for sale. In connection with our determinations that we planned to sell these properties, we performed recoverability analyses for each of these properties and recorded the following impairment losses: • $13.3 million on the operating property in our Northern Virginia Defense/IT sub-segment. Communication with a major tenant in the building during the quarter led us to conclude that there was significant uncertainty with respect to the tenant renewing its lease expiring in 2019. As a result of this information and continuing sub-market weakness, we determined that this property no longer met our long-term hold strategy and we placed it into our asset sales program. Accordingly, we adjusted the carrying amount of the property to its estimated fair value less costs to sell; and • $2.9 million on the other properties that we reclassified as held for sale, primarily associated with a land parcel in White Marsh. As of June 30, 2016, this land was under a sales contract subject to a re-zoning contingency. During the third quarter, we were denied favorable re-zoning and the contract was canceled. As a result, we determined this property will be sold as is, reclassified it to held for sale and adjusted its carrying value to its estimated fair value less costs to sell. During our review we also recognized additional impairment losses of $11.5 million on properties previously classified as held for sale. Approximately $10.0 million of these losses pertained to properties in White Marsh due to our assessment that certain significant tenants will likely exercise lease termination rights and to reflect market conditions. The remainder of these losses pertained primarily to properties in San Antonio, Texas (included in our Other segment), where prospective purchasers reduced offering prices late in the third quarter. We executed property sales of $210.7 million in the third quarter of 2016 (discussed further in Note 5), and had $161.5 million of assets held for sale as of September 30, 2016. We executed property sales of $54.1 million in the fourth quarter of 2016 (discussed further in Note 5), and had $94.7 million of assets held for sale as of December 31, 2016. As part of our closing process for the fourth quarter, we conducted our quarterly review of our portfolio for indicators of impairment and found there to be no impairment losses for the quarter other than additional impairment losses of $1.3 million on properties previously classified as held for sale in White Marsh, where prospective purchasers reduced offering prices, and $0.3 million of losses on properties that were sold during the period. Changes in the expected future cash flows due to changes in our plans for specific properties (especially our expected holding period) could result in the recognition of additional impairment losses. In addition, because properties held for sale are carried at the lower of carrying value or estimated fair values less costs to sell, declines in their estimated fair values due to market conditions and other factors could result in the recognition of additional impairment losses. 2015 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. 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. 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 (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.

Concentration of Revenue

Concentration of Revenue	12 Months Ended
Concentration of Revenue	Dec. 31, 2017
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 22% of our rental revenue in 2017 , 21% in 2016 and 20% in 2015 (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/BW Corridor, Lackland Air Force Base and Navy Support Locations business sub-segments (see Note 17). No other individual tenants accounted for 10% or more of our revenue from real estate operations. We also derived more than 80% of our construction contract revenue from the United States Government in 2017 , 2016 and 2015 . 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
Properties, Net	Dec. 31, 2017
Real Estate [Abstract]
Properties, Net	Properties, Net Operating properties, net consisted of the following (in thousands): December 31, 2017 2016 Land $ 455,680 $ 433,311 Buildings and improvements 3,068,124 2,944,905 Less: Accumulated depreciation (786,193 ) (706,385 ) Operating properties, net $ 2,737,611 $ 2,671,831 Projects we had in development or held for future development consisted of the following (in thousands): December 31, 2017 2016 Land $ 240,825 $ 195,521 Development in progress, excluding land 162,669 206,010 Projects in development or held for future development $ 403,494 $ 401,531 Our properties held for sale included: • as of December 31, 2017 : 11751 Meadowville Lane, a property in our Data Center Shells sub-segment, the sale of which was not recognized for accounting purposes, as discussed below; and • as of December 31, 2016 : eight operating properties in White Marsh (included primarily in our Regional Office segment); one operating property in our Northern Virginia Defense/IT sub-segment; and land in White Marsh and Northern Virginia. The table below sets forth the components of assets held for sale (in thousands): December 31, 2017 2016 Properties, net $ 38,670 $ 85,402 Deferred rent receivable 3,237 4,241 Intangible assets on real estate acquisitions, net — 338 Deferred leasing costs, net 319 3,636 Lease incentives, net — 1,037 Assets held for sale, net $ 42,226 $ 94,654 2017 Dispositions In 2017, we sold 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 Sale 3120 Fairview Park Drive Falls Church, VA Northern Virginia Defense/IT 2/15/2017 1 190,000 $ 39,000 $ — 1334 Ashton Road Hanover, MD Fort Meade/BW Corridor 6/9/2017 1 37,000 2,300 — Remaining White Marsh Properties (1) White Marsh, MD Regional Office and Other 7/28/2017 8 412,000 47,500 1,180 201 Technology Drive Lebanon, VA Data Center Shells 10/27/2017 1 103,000 29,500 3,625 7320 Parkway Drive Hanover, MD Fort Meade/BW Corridor 12/15/2017 1 57,000 7,529 831 12 799,000 $ 125,829 $ 5,636 (1) This sale also included land. We also sold: • 11751 Meadowville Lane, an operating property totaling 193,000 square feet in Chester, Virginia (in our Data Center Shells sub-segment), for $44.0 million on October 27, 2017. We provided a financial guaranty to the buyer under which we provided an indemnification for up to $20 million in losses it could incur related to a potential defined capital event occurring on the property by June 30, 2019. We accounted for this transaction as a financing arrangement. Accordingly, we did not recognize the sale of this property for accounting purposes (and will not until the guaranty expires) and we reported the sales proceeds as a liability on the balance sheet as of December 31, 2017 on the line entitled deferred property sale. In addition, we reported this property as held for sale as of December 31, 2017. We do not expect to incur any losses under this financial guaranty; and • other land for $14.3 million and recognized a gain on sale of $4.2 million . 2017 Construction Activities In 2017 , we placed into service 1.1 million square feet in eight newly constructed properties (including a partially operational property) and 94,000 square feet in three redeveloped properties. As of December 31, 2017 , we had nine properties under construction, or for which we were contractually committed to construct, that we estimate will total 1.1 million square feet upon completion (including a partially operational property and two properties completed and held for future lease to the United States Government) and one partially operational property under redevelopment that we estimate will total 22,000 square feet upon completion. 2016 Dispositions In 2016, we sold 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 Sale Arborcrest Corporate Campus (1) Philadelphia, PA Regional Office 8/4/2016 4 654,000 $ 142,800 $ 4,742 8003 Corporate Drive White Marsh, MD Regional Office 8/17/2016 1 18,000 2,400 — 1341 & 1343 Ashton Road Hanover, MD Fort Meade/BW Corridor 9/9/2016 2 25,000 2,900 848 8007, 8013, 8015, 8019 and 8023-8027 Corporate Drive (1) White Marsh, MD Regional Office 9/21/2016 5 130,000 14,513 1,906 1302, 1304 and 1306 Concourse Drive Linthicum, MD Fort Meade/BW Corridor 9/29/2016 3 299,000 48,100 8,375 2900 Towerview Road Herndon, VA Northern Virginia Defense/IT 10/19/2016 1 151,000 12,100 — 4940 Campbell Boulevard White Marsh, MD Regional Office 11/17/2016 1 50,000 5,200 — 1560 A and B Cable Ranch Road San Antonio, TX Other 11/30/2016 2 120,000 10,300 — 1331 Ashton Road Hanover, MD Fort Meade/BW Corridor 12/19/2016 1 29,000 2,625 — 900 Elkridge Landing Road Linthicum, MD Fort Meade/BW Corridor 12/22/2016 1 101,000 7,800 — 21 1,577,000 $ 248,738 $ 15,871 (1) This sale also included land. We also sold: • a 50% interest in six triple-net leased, single-tenant data center properties in Virginia by contributing them into a newly-formed joint venture, GI-COPT DC Partnership LLC (“GI-COPT”), for an aggregate property value of $147.6 million on July 21, 2016. We obtained $60.0 million in non-recourse mortgage loans on the properties through the joint venture immediately prior to the sale of our interest and received the net proceeds. Our partner in the joint venture acquired the 50% interest in the joint venture from us for $44.3 million . We account for our 50% interest in the joint venture using the equity method of accounting as described further in Note 6. We recognized a gain on the sale of our interest of $17.9 million ; and • other land for $21.8 million and recognized a gain on sale of $7.2 million . 2016 Construction Activities In 2016, we placed into service 639,000 square feet in six newly constructed properties and 61,000 square feet in three redeveloped properties. 2015 Acquisitions In 2015, we acquired the following operating properties: • 250 W. Pratt Street, a 367,000 square foot office property in Baltimore, Maryland (included in our Regional Office segment) 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 (included in our Northern Virginia Defense/IT segment) 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 (included in our Regional Office segment) 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 $38.1 million in 2017 , $36.9 million in 2016 and $20.2 million in 2015; and • net income from continuing operations of $1.9 million in 2017 , $2.2 million in 2016 and $1.2 million in 2015. We expensed operating property acquisition costs of $4.1 million in 2015 that were 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, 2017 . 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 to the year ended December 31, 2014 from the 2015 periods in which they were actually incurred. 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 (Unaudited) Pro forma total revenues $ 641,982 Pro forma net income attributable to COPT common shareholders $ 167,079 Pro forma EPS: Basic $ 1.77 Diluted $ 1.77 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 Virginia 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 897,000 square feet in seven newly constructed properties and 170,000 square feet in two redeveloped properties.

Real Estate Joint Ventures

Real Estate Joint Ventures	12 Months Ended
Real Estate Joint Ventures	Dec. 31, 2017
Equity Method Investments and Joint Ventures [Abstract]
Real Estate Joint Ventures	Real Estate Joint Ventures Consolidated Real Estate Joint Ventures We consolidate the real estate joint ventures described below because of our: (1) power to direct the matters that most significantly impact their activities, including development, leasing and management of the properties constructed by the VIEs; and (2) 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. The table below sets forth information pertaining to our investments in consolidated real estate joint ventures as of December 31, 2017 (dollars in thousands): Nominal Ownership December 31, 2017 (1) Date % as of Total Encumbered Total Acquired 12/31/2017 Nature of Activity Assets Assets Liabilities LW Redstone Company, LLC 3/23/2010 85% Development and operation of real estate (2) $ 158,891 $ 75,569 $ 51,180 M Square Associates, LLC 6/26/2007 50% Development and operation of real estate (3) 73,116 45,384 45,745 Stevens Investors, LLC 8/11/2015 95% Development of real estate (4) 71,976 — 19,905 $ 303,983 $ 120,953 $ 116,830 (1) Excludes amounts eliminated in consolidation. (2) This joint venture’s properties are in Huntsville, Alabama. (3) This joint venture’s properties are in College Park, Maryland. (4) This joint venture’s property is in Washington, DC. In January 2016, our partner in Stevens Investors, LLC contributed to the joint venture, for a value of $22.6 million , interests in contracts controlling land to be developed (including a purchase agreement and a ground lease). Our partner subsequently received cash distributions from the joint venture that we funded of $6.7 million in 2017 and $13.4 million in 2016. 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, 2017 , we had advanced $37.8 million to the City to fund such costs. 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 sheets. 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 674,000 square feet of construction commencement through December 31, 2017 ; • 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 sheets. 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. Our commitments and contingencies pertaining to our real estate joint ventures are disclosed in Note 19. Unconsolidated Real Estate Joint Venture As described further in Note 5, on July 21, 2016, we sold a 50% interest in six triple-net leased, single-tenant data center properties in Virginia by contributing them into GI-COPT, a newly-formed joint venture. We account for our 50% interest in the joint venture using the equity method of accounting. We had an investment balance in GI-COPT of $25.1 million as of December 31, 2017 and $25.5 million as of December 31, 2016 . Our balance was lower than our share of the joint venture’s equity by $16.7 million as of December 31, 2017 and $18.1 million as of December 31, 2016 due to a difference between our cost basis and our share of the underlying equity in the net assets upon formation of the joint venture. In 2016 and 2017, we amortized this basis difference into equity in income from unconsolidated entities based on the lives of the underlying assets. In connection with our adoption of guidance pertaining to the accounting for partial sales of nonfinancial assets using the full retrospective method, effective January 1, 2018, we will retrospectively restate each reporting period presented to: fully recognize in 2016 the difference between our cost basis and our share of the underlying equity in the net assets upon formation of the joint venture; and remove the amortization of this basis difference into equity in income from consolidated entities that we recognized in 2016 and 2017. Under the terms of the joint venture agreement, we and our partner receive returns in proportion to our investments in the joint venture.

Intangible Assets on Real Estat

Intangible Assets on Real Estate Acquisitions	12 Months Ended
Intangible Assets on Real Estate Acquisitions	Dec. 31, 2017
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, 2017 December 31, 2016 Gross Carrying Amount Accumulated Amortization Net Carrying Amount Gross Carrying Amount Accumulated Amortization Net Carrying Amount In-place lease value $ 132,276 $ 110,814 $ 21,462 $ 132,647 $ 99,940 $ 32,707 Tenant relationship value 60,028 32,198 27,830 60,028 26,253 33,775 Below-market cost arrangements 15,102 7,507 7,595 15,102 7,285 7,817 Above-market leases 13,944 12,092 1,852 13,944 10,259 3,685 Other 1,333 980 353 1,333 966 367 $ 222,683 $ 163,591 $ 59,092 $ 223,054 $ 144,703 $ 78,351 Amortization of the intangible asset categories set forth above totaled $ 19.3 million in 2017 , $ 20.0 million in 2016 and $18.5 million in 2015 . The approximate weighted average amortization periods of the categories set forth above follow: in-place lease value: six years; tenant relationship value: eight years; below-market cost arrangements: 34 years; above-market leases: three years; and other: 25 years. The approximate weighted average amortization period for all of the categories combined is 11 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: $14.5 million for 2018 ; $8.1 million for 2019 ; $5.7 million for 2020 ; $5.5 million for 2021 ; and $4.1 million for 2022 .

Investing Receivables

Investing Receivables	12 Months Ended
Investing Receivables	Dec. 31, 2017
Receivables [Abstract]
Investing Receivables	Investing Receivables Investing receivables, including accrued interest thereon, consisted of the following (in thousands): December 31, 2017 2016 Notes receivable from City of Huntsville $ 54,472 $ 49,258 Other investing loans receivable 3,021 3,021 $ 57,493 $ 52,279 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, 2017 or December 31, 2016 . The fair value of these receivables approximated their carrying amounts as of December 31, 2017 and December 31, 2016 .

Prepaid Expenses and Other Asse

Prepaid Expenses and Other Assets, Net	12 Months Ended
Prepaid Expenses and Other Assets, Net	Dec. 31, 2017
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, 2017 2016 Prepaid expenses $ 24,670 $ 24,432 Lease incentives, net 19,011 18,276 Furniture, fixtures and equipment, net 5,256 5,204 Construction contract costs incurred in excess of billings 4,884 10,350 Interest rate derivatives 3,073 158 Non-real estate equity method investments 2,412 2,355 Deferred tax asset, net 1,892 3,036 Deferred financing costs, net (1) 1,202 3,128 Other assets 4,821 5,825 Prepaid expenses and other assets, net $ 67,221 $ 72,764 (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, 2017 2016 Operating loss carry forward $ 3,209 $ 5,084 Share-based compensation 7 13 Accrued payroll 49 101 Property 43 (100 ) Valuation allowance (1,416 ) (2,062 ) Deferred tax asset, net $ 1,892 $ 3,036 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, 2017 net deferred tax asset.

Debt, Net

Debt, Net	12 Months Ended
Debt, Net	Dec. 31, 2017
Debt Disclosure [Abstract]
Debt, Net	Debt, Net 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, 2017 December 31, 2017 Mortgage and Other Secured Debt: Fixed rate mortgage debt (2) $ 150,723 $ 154,143 3.82% - 7.87% (3) 2019-2026 Variable rate secured loans 13,115 13,448 LIBOR + 1.85% (4) October 2020 Total mortgage and other secured debt 163,838 167,591 Revolving Credit Facility (5) 126,000 — LIBOR + 0.875% to 1.60% May 2019 Term Loan Facilities (6) 347,959 547,494 LIBOR + 0.90% to 1.85% (7) 2020-2022 Unsecured Senior Notes (5) 3.600%, $350,000 aggregate principal 347,551 347,128 3.60% (8) May 2023 5.250%, $250,000 aggregate principal 246,645 246,176 5.25% (9) February 2024 3.700%, $300,000 aggregate principal 298,322 297,843 3.70% (10) June 2021 5.000%, $300,000 aggregate principal 296,731 296,368 5.00% (11) July 2025 Unsecured note payable 1,287 1,401 0% (12) May 2026 Total debt, net $ 1,828,333 $ 1,904,001 (1) The carrying values of our debt other than the Revolving Credit Facility reflect net deferred financing costs of $5.0 million as of December 31, 2017 and $6.1 million as of December 31, 2016 . (2) Certain of the fixed rate mortgages carry interest rates that, upon assumption, were above or below market rates 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 $349,000 as of December 31, 2017 and $422,000 as of December 31, 2016 . (3) The weighted average interest rate on our fixed rate mortgage debt was 4.19% as of December 31, 2017 . (4) The interest rate on our variable rate secured debt was 3.21% as of December 31, 2017 . (5) Refer to the paragraphs below for further disclosure. (6) As discussed below, we have the ability to borrow an additional $350.0 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 2.73% as of December 31, 2017 . (8) The carrying value of these notes reflects an unamortized discount totaling $1.7 million as of December 31, 2017 and $2.0 million as of December 31, 2016 . 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 $3.0 million as of December 31, 2017 and $3.4 million as of December 31, 2016 . The effective interest rate under the notes, including amortization of the issuance costs, was 5.49% . (10) The carrying value of these notes reflects an unamortized discount totaling $1.3 million as of December 31, 2017 and $1.7 million as of December 31, 2016 . The effective interest rate under the notes, including amortization of the issuance costs, was 3.85% . (11) Refer to the paragraphs below for further disclosure. (12) This note carries an interest rate that, upon assumption, was below market rates and it therefore was recorded at its fair value based on applicable effective interest rates. The carrying value of this note reflects an unamortized discount totaling $373,000 as of December 31, 2017 and $460,000 as of December 31, 2016 . 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, 2017 , we were within the compliance requirements of these financial covenants. Our debt matures on the following schedule (in thousands): 2018 $ 4,241 2019 130,387 2020 116,156 2021 303,875 2022 254,033 Thereafter 1,033,475 Total $ 1,842,167 (1) (1) Represents scheduled principal amortization and maturities only and therefore excludes net discounts and deferred financing costs of $13.8 million . We capitalized interest costs of $5.2 million in 2017 , $5.7 million in 2016 and $7.2 million in 2015 . The following table sets forth information pertaining to the fair value of our debt (in thousands): December 31, 2017 December 31, 2016 Carrying Estimated Carrying Estimated Amount Fair Value Amount Fair Value Fixed-rate debt Unsecured Senior Notes $ 1,189,249 $ 1,229,398 $ 1,187,515 $ 1,220,282 Other fixed-rate debt 152,010 152,485 155,544 156,887 Variable-rate debt 487,074 485,694 560,942 558,437 $ 1,828,333 $ 1,867,577 $ 1,904,001 $ 1,935,606 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. 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 Investors 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, 2017 , the maximum borrowing capacity under this facility totaled $800.0 million , of which $674.0 million was available. Weighted average borrowings under our Revolving Credit Facility totaled $97.8 million in 2017 and $90.3 million in 2016 . The weighted average interest rate on our Revolving Credit Facility was 2.44% in 2017 and 1.64% in 2016 . 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, subject to certain conditions. We repaid this term loan by $200.0 million in May 2017. The term loan matures on May 6, 2020, and carries a variable interest rate 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 December 17, 2015, we entered into an unsecured term loan agreement with an initial commitment of $250.0 million ; we borrowed $100.0 million under this loan on December 17, 2015 and $150.0 million on December 28, 2016. 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, and carries a variable interest rate based on the LIBOR rate (customarily the 30-day rate) plus 0.90% to 1.75% , as determined by the credit ratings assigned to COPLP by the Ratings Agencies. In addition to the term loans discussed above, we also had the following term loans that were repaid prior to December 31, 2017 : • for a term loan originating in 2012, we repaid the remaining balance of $120.0 million in 2016; and • for a term loan originating in 2011, we repaid the remaining balance of $150.0 million in 2015. Unsecured Senior Notes On June 29, 2015, we issued $300.0 million of 5.000% Senior Notes at an initial offering price of 99.510% of their face value, 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 $2.7 million at December 31, 2017 and $3.0 million as of December 31, 2016 . 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. (Losses) Gains on Early Extinguishment of Debt Our (losses) gains on early extinguishment of debt included 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
Interest Rate Derivatives	Dec. 31, 2017
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, each of which was designated as a cash flow hedge (dollars in thousands): Fair Value at Notional Effective Expiration December 31, Amount Fixed Rate Floating Rate Index Date Date 2017 2016 $ 100,000 1.7300 % One-Month LIBOR 9/1/2015 8/1/2019 $ 252 $ (848 ) 13,217 (1) 1.3900 % One-Month LIBOR 10/13/2015 10/1/2020 213 100 100,000 1.9013 % One-Month LIBOR 9/1/2016 12/1/2022 1,046 (23 ) 100,000 1.9050 % One-Month LIBOR 9/1/2016 12/1/2022 1,051 48 50,000 1.9079 % One-Month LIBOR 9/1/2016 12/1/2022 511 10 100,000 (2) 1.6730 % One-Month LIBOR 9/1/2015 8/1/2019 — (701 ) $ 3,073 $ (1,414 ) (1) The notional amount of this instrument is scheduled to amortize to $12.1 million . (2) We cash settled this derivative and interest accrued thereon for $460,000 on May 1, 2017. Since the hedged transactions associated with this derivative were still probable to occur as of the settlement date, amounts in accumulated other comprehensive loss (“AOCL”) associated with this derivative will be reclassified to interest expense through August 2019. 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 2017 2016 Interest rate swaps designated as cash flow hedges Prepaid expenses and other assets $ 3,073 $ 158 Interest rate swaps designated as cash flow hedges Other liabilities — (1,572 ) 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, 2017 2016 2015 Unrealized gain (loss) recognized in AOCL (effective portion) $ 684 $ (2,915 ) $ (4,739 ) Loss reclassified from AOCL into interest expense (effective portion) (3,216 ) (4,230 ) (3,599 ) Gain (loss) on derivatives recognized in interest expense (ineffective portion) 323 378 (386 ) Loss reclassified from AOCL into interest expense (ineffective portion) (1) (88 ) — — (1) Represents a loss recognized on certain interest rate swaps from the accelerated reclassification of amounts in AOCL on May 1, 2017, when we concluded that hedged forecasted transactions were probable not to occur. Over the next 12 months, we estimate that approximately $460,000 of losses will be reclassified from AOCL as an increase to interest expense. We have agreements with each of our interest rate derivative counterparties that contain provisions under which, if we default or are capable of being declared in default on defined levels of our indebtedness, we could also be declared in default on our derivative obligations. 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. We are not in default with any of these provisions. As of December 31, 2017 , we did not have any derivatives in liability positions. As of December 31, 2017 , we had not posted any collateral related to these agreements.

Redeemable Noncontrolling Inter

Redeemable Noncontrolling Interests	12 Months Ended
Redeemable Noncontrolling Interests	Dec. 31, 2017
Noncontrolling Interest [Abstract]
Redeemable Noncontrolling Interests	Redeemable Noncontrolling Interests 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 sheets. The table below sets forth the activity in these redeemable noncontrolling interests (in thousands): For the Years Ended December 31, 2017 2016 2015 Beginning balance $ 22,979 $ 19,218 $ 18,417 Contributions from noncontrolling interests — 22,779 1,654 Distributions to noncontrolling interests (1,566 ) (21,881 ) (2,964 ) Net income attributable to noncontrolling interests 2,338 2,242 2,227 Adjustment to arrive at fair value of interests (626 ) 621 (116 ) Ending balance $ 23,125 $ 22,979 $ 19,218 We determine the fair value of these 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.

Equity - COPT and Subsidiaries

Equity - COPT and Subsidiaries	12 Months Ended
Equity - COPT and Subsidiaries	Dec. 31, 2017
Stockholders' Equity Note [Abstract]
Equity - COPT and Subsidiaries	Equity - COPT and Subsidiaries Preferred Shares As of December 31, 2017 , COPT had 25.0 million preferred shares authorized and unissued at $0.01 par value per share. In 2017, COPT redeemed all of its outstanding preferred shares, including: • the 5.600% Series K Cumulative Redeemable Preferred Shares (the “Series K Preferred Shares”) redeemed effective January 21, 2017 at a price of $50.00 per share, or $26.6 million in the aggregate, plus accrued and unpaid dividends thereon through the date of redemption. Concurrently with this redemption, COPLP redeemed its Series K Preferred Units on the same terms. Since we made an irrevocable notification to holders of the Series K Preferred Shares in December 2016 of our intention to redeem such shares, we presented the liquidation preference of the shares as a liability on COPT’s consolidated balance sheet as of December 31, 2016; we also recognized a $17,000 decrease to net income available to common shareholders in 2016 pertaining to the original issuance costs incurred on the shares. The liability associated with these shares as of December 31, 2016 is classified in Level 2 of the fair value hierarchy; and • the 7.375% Series L Cumulative Preferred Shares (the “Series L Preferred Shares”) redeemed effective June 27, 2017 at a price of $25.00 per share, or $172.5 million in the aggregate, plus accrued and unpaid dividends thereon up to but not including the date of redemption. Concurrently with this redemption, COPLP redeemed its Series L Preferred Units on the same terms. We also recognized a $6.8 million decrease to net income available to common shareholders in 2017 pertaining to the original issuance costs incurred on the shares. Common Shares In October 2012, COPT established an at-the-market (“ATM”) stock offering program under which it could, 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 . COPT issued 890,241 common shares under this program 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. As discussed below, this program was replaced by a new ATM program established in 2016. In September 2016, COPT established a new 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 $200.0 million . This program replaced the ATM stock offering program that we previously had in place. COPT issued the following common shares under this ATM program: • 3.72 million common shares in the three months ended December 31, 2016 at a weighted average price of $29.56 per share. Net proceeds from the shares issued totaled $109.1 million , after payment of $0.9 million in commissions to sales agents; and • 591,000 common shares in 2017 at a weighted average price of $33.84 per share. Net proceeds from the shares issued totaled $19.7 million , after payment of $0.3 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 this ATM program as of December 31, 2017 was an aggregate gross sales price of $70.0 million in common share sales. On November 2, 2017, COPT entered into forward equity sale agreements to issue 9.2 million common shares at an initial gross offering price of $285.2 million , or $31.00 per share, before underwriting discounts, commissions and offering expenses. The forward sale price that we expect to receive upon physical settlement of the agreements will be subject to adjustment on a daily basis based on a floating interest rate factor equal to the overnight bank funding rate less a spread, and will be decreased on each of certain dates specified in the agreements during the term of the agreements. On December 27, 2017, COPT issued 1.7 million common shares under the agreements for net proceeds of $50.0 million . COPT contributed the net proceeds from these issuances to COPLP in exchange for an equal number of units in COPLP. 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 339,513 in 2017 , 87,000 in 2016 and 160,160 in 2015 . COPT declared dividends per common share of $1.10 in 2017 , 2016 and 2015 . 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
Equity - COPLP and Subsidiaries	Dec. 31, 2017
Class of Stock [Line Items]
Equity - COPLP and Subsidiaries	Equity - COPT and Subsidiaries Preferred Shares As of December 31, 2017 , COPT had 25.0 million preferred shares authorized and unissued at $0.01 par value per share. In 2017, COPT redeemed all of its outstanding preferred shares, including: • the 5.600% Series K Cumulative Redeemable Preferred Shares (the “Series K Preferred Shares”) redeemed effective January 21, 2017 at a price of $50.00 per share, or $26.6 million in the aggregate, plus accrued and unpaid dividends thereon through the date of redemption. Concurrently with this redemption, COPLP redeemed its Series K Preferred Units on the same terms. Since we made an irrevocable notification to holders of the Series K Preferred Shares in December 2016 of our intention to redeem such shares, we presented the liquidation preference of the shares as a liability on COPT’s consolidated balance sheet as of December 31, 2016; we also recognized a $17,000 decrease to net income available to common shareholders in 2016 pertaining to the original issuance costs incurred on the shares. The liability associated with these shares as of December 31, 2016 is classified in Level 2 of the fair value hierarchy; and • the 7.375% Series L Cumulative Preferred Shares (the “Series L Preferred Shares”) redeemed effective June 27, 2017 at a price of $25.00 per share, or $172.5 million in the aggregate, plus accrued and unpaid dividends thereon up to but not including the date of redemption. Concurrently with this redemption, COPLP redeemed its Series L Preferred Units on the same terms. We also recognized a $6.8 million decrease to net income available to common shareholders in 2017 pertaining to the original issuance costs incurred on the shares. Common Shares In October 2012, COPT established an at-the-market (“ATM”) stock offering program under which it could, 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 . COPT issued 890,241 common shares under this program 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. As discussed below, this program was replaced by a new ATM program established in 2016. In September 2016, COPT established a new 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 $200.0 million . This program replaced the ATM stock offering program that we previously had in place. COPT issued the following common shares under this ATM program: • 3.72 million common shares in the three months ended December 31, 2016 at a weighted average price of $29.56 per share. Net proceeds from the shares issued totaled $109.1 million , after payment of $0.9 million in commissions to sales agents; and • 591,000 common shares in 2017 at a weighted average price of $33.84 per share. Net proceeds from the shares issued totaled $19.7 million , after payment of $0.3 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 this ATM program as of December 31, 2017 was an aggregate gross sales price of $70.0 million in common share sales. On November 2, 2017, COPT entered into forward equity sale agreements to issue 9.2 million common shares at an initial gross offering price of $285.2 million , or $31.00 per share, before underwriting discounts, commissions and offering expenses. The forward sale price that we expect to receive upon physical settlement of the agreements will be subject to adjustment on a daily basis based on a floating interest rate factor equal to the overnight bank funding rate less a spread, and will be decreased on each of certain dates specified in the agreements during the term of the agreements. On December 27, 2017, COPT issued 1.7 million common shares under the agreements for net proceeds of $50.0 million . COPT contributed the net proceeds from these issuances to COPLP in exchange for an equal number of units in COPLP. 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 339,513 in 2017 , 87,000 in 2016 and 160,160 in 2015 . COPT declared dividends per common share of $1.10 in 2017 , 2016 and 2015 . 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 In 2017, COPLP redeemed all of the outstanding units of the following series of preferred units held by COPT: • the 5.600% Series K Preferred Shares effective on January 21, 2017. Since notification of this redemption occurred in December 2016, we present the liquidation preference of the related units as a liability on COPLP’s consolidated balance sheet as of December 31, 2016; we also recognized at a price of $50.00 per unit, or $26.6 million in the aggregate, plus accrued and unpaid distributions thereon through the date of redemption, and recognized a $17,000 decrease to net income available to common unitholders pertaining to the units’ original issuance costs at the time of redemption; and • the 7.375% Series L Cumulative Preferred Units on June 27, 2017 at a price of $25.00 per unit, or $172.5 million in the aggregate, plus accrued and unpaid distributions thereon through the date of redemption, and recognized a $6.8 million decrease to net income available to common unitholders pertaining to the units’ original issuance costs at the time of redemption. With the completion of these redemptions in 2017, no preferred units in COPLP are held by COPT. 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 96.9% of COPLP’s common units as of December 31, 2017 and 96.5% as of December 31, 2016 . From 2015 through 2017, COPT acquired additional common units through the following common share issuances under ATM programs: • 591,042 common shares in 2017 at a weighted average price of $33.84 per share. Net proceeds from the shares issued totaled $19.7 million , after payment of $0.3 million in commissions to sales agents; • 3.72 million common shares issued in 2016 at a weighted average price of $29.56 per share. Net proceeds from the shares issued totaled $109.1 million , after payment of $0.9 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. In December 2017, COPT also acquired additional common units from COPT’s issuance of 1.7 million common shares under its forward equity sale agreements for net proceeds of $50.0 million . 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 339,513 in 2017 , 87,000 in 2016 and 160,160 in 2015 . We declared distributions per common unit of $1.10 in 2017 , 2016 and 2015 .

Share-Based Compensation and Ot

Share-Based Compensation and Other Compensation Matters	12 Months Ended
Share-Based Compensation and Other Compensation Matters	Dec. 31, 2017
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 2017, COPT adopted the 2017 Omnibus Equity and Incentive Plan following the approval of such plan by our common shareholders. 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 3.4 million common shares in COPT to be issued in the form of options, share appreciation rights, restricted share unit awards, restricted share awards, unrestricted share awards, dividend equivalent rights and other equity-based awards and for the granting of cash-based awards. This plan expires on May 11, 2027. In May 2010, COPT adopted the Amended and Restated 2008 Omnibus Equity and Incentive Plan following the approval of such plan by our common shareholders. This plan, which was replaced by the 2017 Plan in May 2017, provided for the award 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. In March 1998, COPT adopted a long-term incentive plan for our Trustees and employees following the approval of such plan by our common shareholders. 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 table below sets forth our reporting for share based compensation cost (in thousands): For the Years Ended December 31, 2017 2016 2015 General, administrative and leasing expenses $ 4,649 $ 5,816 $ 5,574 Property operating expenses 966 1,027 1,000 Capitalized to development activities 480 610 824 Share-based compensation cost $ 6,095 $ 7,453 $ 7,398 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, 2017 , unrecognized compensation costs related to unvested awards included: • $8.1 million on restricted shares expected to be recognized over a weighted average period of approximately three years; • $1.3 million on PSUs expected to be recognized over a weighted average performance period of approximately two years and • $120,000 on deferred share awards expected to be recognized through May 2018. Our TRS is subject to Federal and state income taxes. We realized a windfall tax loss of $13,000 in 2017 , $331,000 in 2016 and $513,000 in 2015 on options exercised and vesting restricted shares in connection with employees of that subsidiary. Restricted Shares The following table summarizes restricted shares under the share-based compensation plans for 2015 , 2016 and 2017 : Shares Weighted Average Grant Date Fair Value Unvested as of December 31, 2014 390,507 $ 26.19 Granted 201,024 28.69 Forfeited (10,550 ) 26.05 Vested (202,781 ) 26.07 Unvested as of December 31, 2015 378,200 27.58 Granted 231,937 24.77 Forfeited (22,907 ) 25.31 Vested (215,983 ) 27.19 Unvested as of December 31, 2016 371,247 26.20 Granted 239,479 33.84 Forfeited (27,056 ) 27.80 Vested (158,044 ) 26.27 Unvested as of December 31, 2017 425,626 $ 30.37 Unvested shares as of December 31, 2017 that are expected to vest 402,870 $ 30.31 The aggregate intrinsic value of restricted shares that vested was $5.3 million in 2017 , $5.4 million in 2016 and $4.9 million in 2015 . PSUs We made the following grants of PSUs to executives from 2013 through 2017 (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, 2017 3/1/2013 69,579 1/1/2013 12/31/2015 $ 1,867 — 3/6/2014 49,103 1/1/2014 12/31/2016 $ 1,723 — 3/5/2015 45,656 1/1/2015 12/31/2017 $ 1,678 15,767 3/1/2016 26,299 1/1/2016 12/31/2018 $ 1,000 24,850 1/1/2017 39,351 1/1/2017 12/31/2019 $ 1,400 39,351 In 2017, we modified certain provisions of the PSUs granted in 2015, 2016 and 2017, resulting in incremental compensation cost totaling $236,000 based on the difference between the pre-modification and post-modification award fair values on the date of modification. 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 2013 and 2014 PSUs issued to 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; • for the 2013 PSUs that vested on December 31, 2015, there was no payout value in connection with the vesting; • for the 2014 and 2015 PSUs issued to Wayne H. Lingafelter, our former Executive Vice President, Development & Construction Services, who departed on March 31, 2016, we issued 10,326 common shares on May 30, 2016 in settlement of such PSUs; • for the 2014 and 2015 PSUs issued to Roger A. Waesche, Jr., our former Chief Executive Officer, who departed on May 12, 2016, we issued 20,569 common shares on July 12, 2016 in settlement of such PSUs; • for the 2014, 2015 and 2016 PSUs issued to Karen M. Singer, our former General Counsel and Secretary, who departed on August 31, 2016, we issued 2,248 common shares on October 30, 2016 in settlement of such PSUs; and • for the 2014 PSUs issued to Steven E. Budorick, our Chief Executive Officer, that vested on December 31, 2016, we issued 9,763 common shares in settlement of the PSUs on February 7, 2017. We computed grant date fair values for PSUs using Monte Carlo models and are recognizing these values over the performance periods. The grant date fair value and certain of the assumptions used in the Monte Carlo models for the PSUs granted in 2015 , 2016 and 2017 are set forth below: Grant Date Grant Date Fair Value Baseline Common Share Value Expected Volatility of Common Shares Risk-free Interest Rate 3/5/2015 $ 36.76 $ 29.28 19.9 % 0.99 % 3/1/2016 $ 38.21 $ 23.90 20.4 % 0.96 % 1/1/2017 $ 38.43 $ 31.22 19.0 % 1.47 % Deferred Share Awards We made the following grants of deferred share awards to nonemployee members of our Board of Trustees in 2015, 2016 and 2017 (dollars in thousands, except per share amounts): Year of Grant Number of Deferred Share Awards Granted Aggregate Grant Date Fair Value Grant Date Fair Value Per Share 2015 24,056 $ 642 $ 26.70 2016 24,944 $ 671 $ 26.89 2017 10,032 $ 326 $ 32.47 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). We issued the following common shares in settlement of deferred shares in 2015 , 2016 and 2017 (dollars in thousands, except per share amounts): For the Years Ended December 31, 2017 2016 2015 Number of common shares issued 15,590 12,028 15,485 Grant date fair value $ 26.89 $ 26.70 $ 26.77 Aggregate intrinsic value $ 508 $ 322 $ 413 Options We have not issued options since 2009, and all of our options were vested and fully expensed prior to 2017. The table below sets forth information regarding our outstanding options as of the following dates (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, 2014 559,736 $39.60 2 $ 167 December 31, 2015 425,347 $42.75 1 $ — December 31, 2016 201,100 $43.35 1 $ 31 December 31, 2017 60,000 $35.17 1 $ — The aggregate intrinsic value of options exercised was $18,000 in 2017 and $300,000 in 2015 . No options were exercised in 2016. Executive Transition Costs 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 President and Chief Executive Officer, left the Company to pursue other interests, and he was not nominated for reelection as a Trustee. The Board appointed Mr. Budorick to our Board of Trustees after the 2016 Annual Meeting of Shareholders. In addition, our Executive Vice President, Development & Construction Services, Wayne H. Lingafelter, and our Senior Vice President, General Counsel and Secretary, Karen M. Singer, departed the Company to pursue other interests effective March 31, 2016 and August 31, 2016, respectively. We recognized executive transition costs of approximately $6.5 million in 2016 primarily for termination benefits in connection with the departures of Mr. Waesche, Mr. Lingafelter and Ms. Singer.

Operating Leases

Operating Leases	12 Months Ended
Operating Leases	Dec. 31, 2017
Leases, Operating [Abstract]
Operating Leases	Operating Leases We lease our properties to tenants under operating leases with various expiration dates extending to the year 2033. Gross minimum future rentals on noncancelable leases in our properties as of December 31, 2017 were as follows (in thousands): Year Ending December 31, 2018 $ 372,420 2019 329,760 2020 260,238 2021 207,727 2022 175,123 Thereafter 484,444 $ 1,829,712

Information by Business Segment

Information by Business Segment	12 Months Ended
Information by Business Segment	Dec. 31, 2017
Segment Reporting [Abstract]
Information by Business Segment	Information by Business Segment We have the following reportable segments: Defense/IT Locations; Regional Office; Wholesale Data Center; and Other. We also report on 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); 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). As of December 31, 2017, our Regional Office segment included properties located in select urban/urban-like submarkets in the Greater Washington, DC/Baltimore region with durable Class-A office fundamentals and characteristics; in prior reporting periods, this segment also included suburban properties that did not meet these characteristics (that were since disposed). We measure the performance of our segments through the measure we define as net operating income from real estate operations (“NOI from real estate operations”), which includes: real estate revenues and property operating expenses from continuing and discontinued operations; and the net of revenues and property operating expenses of real estate operations owned through unconsolidated real estate joint ventures (“UJVs”) that is allocable to COPT’s ownership interest (“UJV NOI allocable to COPT”). Amounts reported for segment assets represent long-lived assets associated with consolidated operating properties (including the carrying value of properties, intangible assets, deferred leasing costs, deferred rents receivable and lease incentives) and the carrying value of investments in UJVs owning operating properties. Amounts reported as additions to long-lived assets represent additions to existing consolidated operating properties, excluding transfers from non-operating properties, which we report separately. The table below reports segment financial information for our reportable segments (in thousands): Operating Property Segments Defense/Information Technology Locations Fort Meade/BW Corridor Northern Virginia Defense/IT Lackland Air Force Base Navy Support Locations Redstone Arsenal Data Center Shells Total Defense/IT Locations Regional Office Operating Wholesale Data Center Other Total Year Ended December 31, 2017 Revenues from real estate operations $ 245,613 $ 47,118 $ 47,209 $ 29,540 $ 14,322 $ 24,320 $ 408,122 $ 68,262 $ 28,875 $ 4,721 $ 509,980 Property operating expenses (80,697 ) (16,938 ) (27,812 ) (12,619 ) (5,783 ) (2,709 ) (146,558 ) (28,982 ) (13,551 ) (1,873 ) (190,964 ) UJV NOI allocable to COPT — — — — — 5,188 5,188 — — — 5,188 NOI from real estate operations $ 164,916 $ 30,180 $ 19,397 $ 16,921 $ 8,539 $ 26,799 $ 266,752 $ 39,280 $ 15,324 $ 2,848 $ 324,204 Additions to long-lived assets $ 26,659 $ 8,115 $ 71 $ 8,451 $ 1,056 $ — $ 44,352 $ 25,299 $ 3,580 $ 110 $ 73,341 Transfers from non-operating properties $ 43,370 $ 48,328 $ — $ 474 $ 2,159 $ 107,854 $ 202,185 $ — $ 8 $ 18 $ 202,211 Segment assets at December 31, 2017 $ 1,263,567 $ 402,076 $ 128,755 $ 194,476 $ 108,119 $ 285,275 $ 2,382,268 $ 400,512 $ 224,422 $ 4,082 $ 3,011,284 Year Ended December 31, 2016 Revenues from real estate operations $ 245,354 $ 48,964 $ 46,803 $ 28,197 $ 13,056 $ 23,836 $ 406,210 $ 85,805 $ 26,869 $ 7,080 $ 525,964 Property operating expenses (83,684 ) (17,824 ) (27,357 ) (12,690 ) (4,476 ) (2,674 ) (148,705 ) (34,095 ) (11,512 ) (3,218 ) (197,530 ) UJV NOI allocable to COPT — — — — — 2,305 2,305 — — — 2,305 NOI from real estate operations $ 161,670 $ 31,140 $ 19,446 $ 15,507 $ 8,580 $ 23,467 $ 259,810 $ 51,710 $ 15,357 $ 3,862 $ 330,739 Additions to long-lived assets $ 26,267 $ 17,344 $ — $ 9,168 $ 4,352 $ — $ 57,131 $ 12,559 $ 299 $ 335 $ 70,324 Transfers from non-operating properties $ 49,937 $ 28,230 $ 240 $ — $ 3,169 $ 103,367 $ 184,943 $ 82 $ (377 ) $ (8 ) $ 184,640 Segment assets at December 31, 2016 $ 1,255,230 $ 404,438 $ 131,957 $ 196,486 $ 110,395 $ 209,683 $ 2,308,189 $ 442,811 $ 231,954 $ 21,293 $ 3,004,247 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 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, 2017 2016 2015 Segment revenues from real estate operations $ 509,980 $ 525,964 $ 519,068 Construction contract and other service revenues 102,840 48,364 106,402 Less: Revenues from discontinued operations — — (4 ) Total revenues $ 612,820 $ 574,328 $ 625,466 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, 2017 2016 2015 Segment property operating expenses $ 190,964 $ 197,530 $ 194,488 Less: Property operating expenses from discontinued operations — — 6 Total property operating expenses $ 190,964 $ 197,530 $ 194,494 The following table reconciles UJV NOI allocable to COPT to equity in income of unconsolidated entities as reported on our consolidated statements of operations (in thousands): For the Years Ended December 31, 2017 2016 2015 UJV NOI allocable to COPT $ 5,188 $ 2,305 $ — Less: Income from UJV allocable to COPT attributable to depreciation and amortization expense and interest expense (2,301 ) (993 ) — Add: Equity in (loss) income of unconsolidated non-real estate entities (5 ) 20 62 Equity in income of unconsolidated entities $ 2,882 $ 1,332 $ 62 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, 2017 2016 2015 Construction contract and other service revenues $ 102,840 $ 48,364 $ 106,402 Construction contract and other service expenses (99,618 ) (45,481 ) (102,696 ) NOI from service operations $ 3,222 $ 2,883 $ 3,706 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, 2017 2016 2015 NOI from real estate operations $ 324,204 $ 330,739 $ 324,580 NOI from service operations 3,222 2,883 3,706 Interest and other income 6,318 5,444 4,517 Equity in income of unconsolidated entities 2,882 1,332 62 Income tax expense (1,098 ) (244 ) (199 ) Depreciation and other amortization associated with real estate operations (134,228 ) (132,719 ) (140,025 ) Impairment losses (15,123 ) (101,391 ) (23,289 ) General, administrative and leasing expenses (30,837 ) (36,553 ) (31,361 ) Business development expenses and land carry costs (6,213 ) (8,244 ) (13,507 ) Interest expense (76,983 ) (83,163 ) (89,074 ) NOI from discontinued operations — — (10 ) Less: UJV NOI allocable to COPT included in equity in income of unconsolidated entities (5,188 ) (2,305 ) — (Loss) gain on early extinguishment of debt (513 ) (1,110 ) 85,275 COPT consolidated income (loss) from continuing operations $ 66,443 $ (25,331 ) $ 120,675 The following table reconciles our segment assets to the consolidated total assets of COPT and subsidiaries (in thousands): As of December 31, 2017 2016 Segment assets $ 3,011,284 $ 3,004,247 Non-operating property assets 411,041 418,171 Other assets 156,159 358,467 Total COPT consolidated assets $ 3,578,484 $ 3,780,885 The accounting policies of the segments are the same as those used to prepare our consolidated financial statements, except that discontinued operations and UJV NOI allocable to COPT 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) gain on early extinguishment of debt, gain on sales of real estate and equity in income of unconsolidated entities not included in NOI 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, administrative and leasing expenses, business development expenses and land carry costs, interest and other income, income taxes and noncontrolling interests because these items represent general corporate or non-operating property items not attributable to segments.

Earnings Per Share ("EPS") and

Earnings Per Share ("EPS") and Earnings Per Unit (“EPU”)	12 Months Ended
Earnings Per Share ("EPS") and Earnings Per Unit (“EPU”)	Dec. 31, 2017
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 our forward equity sale agreements and 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, 2017 2016 2015 Numerator: Income (loss) from continuing operations $ 66,443 $ (25,331 ) $ 120,675 Gain on sales of real estate 9,890 40,986 68,047 Preferred share dividends (6,219 ) (14,297 ) (14,210 ) Issuance costs associated with redeemed preferred shares (6,847 ) (17 ) — Income from continuing operations attributable to noncontrolling interests (6,242 ) (4,216 ) (10,575 ) Income from continuing operations attributable to share-based compensation awards (449 ) (419 ) (706 ) Numerator for basic EPS from continuing operations attributable to COPT common shareholders $ 56,576 $ (3,294 ) $ 163,231 Dilutive effect of common units in COPLP on diluted EPS from continuing operations — — 6,397 Numerator for diluted EPS from continuing operations attributable to COPT common shareholders $ 56,576 $ (3,294 ) $ 169,628 Numerator for basic EPS from continuing operations attributable to COPT common shareholders $ 56,576 $ (3,294 ) $ 163,231 Discontinued operations — — 156 Discontinued operations attributable to noncontrolling interests — — (3 ) Numerator for basic EPS on net income (loss) attributable to COPT common shareholders $ 56,576 $ (3,294 ) $ 163,384 Dilutive effect of common units in COPLP — — 6,403 Numerator for diluted EPS on net income (loss) attributable to COPT common shareholders $ 56,576 $ (3,294 ) $ 169,787 Denominator (all weighted averages): Denominator for basic EPS (common shares) 98,969 94,502 93,914 Dilutive effect of forward equity sale agreements and share-based compensation awards 186 — 61 Dilutive effect of common units — — 3,692 Denominator for diluted EPS (common shares) 99,155 94,502 97,667 Basic EPS: Income (loss) from continuing operations attributable to COPT common shareholders $ 0.57 $ (0.03 ) $ 1.74 Net income (loss) attributable to COPT common shareholders $ 0.57 $ (0.03 ) $ 1.74 Diluted EPS: Income (loss) from continuing operations attributable to COPT common shareholders $ 0.57 $ (0.03 ) $ 1.74 Net income (loss) attributable to COPT common shareholders $ 0.57 $ (0.03 ) $ 1.74 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, 2017 2016 2015 Conversion of common units 3,362 3,633 — Conversion of Series I preferred units 176 176 176 Conversion of Series K preferred shares — 434 434 The following share-based compensation securities were excluded from the computation of diluted EPS because their effect was antidilutive: • weighted average restricted shares and deferred share awards of 433,000 for 2017 , 385,000 for 2016 and 410,000 for 2015 ; and • weighted average options of 70,000 for 2017 , 285,000 for 2016 and 469,000 for 2015 . We had outstanding senior notes, which we redeemed in April 2015, 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 our forward equity sale agreements and 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, 2017 2016 2015 Numerator: Income (loss) from continuing operations $ 66,443 $ (25,331 ) $ 120,675 Gain on sales of real estate, net 9,890 40,986 68,047 Preferred unit distributions (6,879 ) (14,957 ) (14,870 ) Issuance costs associated with redeemed preferred units (6,847 ) (17 ) — Income from continuing operations attributable to noncontrolling interests (3,646 ) (3,715 ) (3,523 ) Income from continuing operations attributable to share-based compensation awards (449 ) (419 ) (706 ) Numerator for basic and diluted EPU from continuing operations attributable to COPLP common unitholders $ 58,512 $ (3,453 ) $ 169,623 Discontinued operations — — 156 Discontinued operations attributable to noncontrolling interests — — 3 Numerator for basic and diluted EPU on net income (loss) attributable to COPLP common unitholders $ 58,512 $ (3,453 ) $ 169,782 Denominator (all weighted averages): Denominator for basic EPU (common units) 102,331 98,135 97,606 Dilutive effect of forward equity sale agreements and share-based compensation awards 186 — 61 Denominator for diluted EPU (common units) 102,517 98,135 97,667 Basic EPU: Income (loss) from continuing operations attributable to COPLP common unitholders $ 0.57 $ (0.04 ) $ 1.74 Net income (loss) attributable to COPLP common unitholders $ 0.57 $ (0.04 ) $ 1.74 Diluted EPU: Income (loss) from continuing operations attributable to COPLP common unitholders $ 0.57 $ (0.04 ) $ 1.74 Net income (loss) attributable to COPLP common unitholders $ 0.57 $ (0.04 ) $ 1.74 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, 2017 2016 2015 Conversion of Series I preferred units 176 176 176 Conversion of Series K preferred units — 434 434 The following share-based compensation securities were excluded from the computation of diluted EPU because their effect was antidilutive: • weighted average restricted units and deferred share awards of 433,000 for 2017 , 385,000 for 2016 and 410,000 for 2015 ; and • weighted average options of 70,000 for 2017 , 285,000 for 2016 and 469,000 for 2015 . We had outstanding senior notes, which we redeemed in April 2015, 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
Commitments and Contingencies	Dec. 31, 2017
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 $981,000 liability through December 31, 2017 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, 2017 follow (in thousands): Year Ending December 31, 2018 $ 1,283 2019 1,267 2020 1,259 2021 1,263 2022 1,149 Thereafter 84,611 $ 90,832 Capital Lease On May 25, 2017, we entered into a ground lease on land under development in Washington, DC for our Stevens Investors, LLC joint venture. The lease has a 99 -year term, and we possess an option to purchase the property for one dollar (estimated to occur between 2019 and 2020). Upon inception of the lease, we recorded a $16.1 million capital lease liability on our consolidated balance sheets based on the present value of the future minimum rental payments. Future minimum rental payments due under the term of this lease as of December 31, 2017 follow (in thousands): Year Ending December 31, 2018 $ 15,829 2020 135 2022 75 Total minimum rental payments 16,039 Less: Amount representing interest (186 ) Capital lease obligation $ 15,853 Contractual Obligations We had amounts remaining to be incurred under various contractual obligations as of December 31, 2017 that included the following (excluding amounts incurred and therefore reflected as liabilities reported on our consolidated balance sheets): • new development and redevelopment obligations of $22.8 million ; • capital expenditures for operating properties of $44.2 million ; • third party construction and development of $35.8 million ; and • other obligations of $0.9 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 limited to $19 million in the aggregate. We have insurance coverage in place to mitigate much of any potential future losses that may result from these indemnification agreements.

Quarterly Data (Unaudited)

Quarterly Data (Unaudited)	12 Months Ended
Quarterly Data (Unaudited)	Dec. 31, 2017
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, 2017 and 2016 (in thousands, except per share/unit data). For the Year Ended December 31, 2017 For the Year Ended December 31, 2016 First Quarter Second Quarter Third Quarter Fourth Quarter First Quarter Second Quarter Third Quarter Fourth Quarter COPT and Subsidiaries Revenues $ 139,801 $ 151,435 $ 157,017 $ 164,567 $ 144,307 $ 145,927 $ 142,103 $ 141,991 Operating income $ 35,433 $ 36,618 $ 38,939 $ 24,847 $ 30,464 $ (27,021 ) $ 11,525 $ 37,442 Income (loss) from continuing operations $ 18,850 $ 19,195 $ 21,494 $ 6,904 $ 8,096 $ (48,316 ) $ (4,829 ) $ 19,718 Net income (loss) $ 23,088 $ 19,207 $ 22,682 $ 11,356 $ 8,096 $ (48,316 ) $ 29,272 $ 26,603 Net (income) loss attributable to noncontrolling interests (1,733 ) (1,345 ) (1,766 ) (1,398 ) (1,270 ) 897 (1,973 ) (1,870 ) Net income (loss) attributable to COPT 21,355 17,862 20,916 9,958 6,826 (47,419 ) 27,299 24,733 Preferred share dividends (3,180 ) (3,039 ) — — (3,552 ) (3,553 ) (3,552 ) (3,640 ) Issuance costs associated with redeemed preferred shares — (6,847 ) — — — — — (17 ) Net income (loss) attributable to COPT common shareholders $ 18,175 $ 7,976 $ 20,916 $ 9,958 $ 3,274 $ (50,972 ) $ 23,747 $ 21,076 Basic EPS $ 0.18 $ 0.08 $ 0.21 $ 0.10 $ 0.03 $ (0.54 ) $ 0.25 $ 0.22 Diluted EPS $ 0.18 $ 0.08 $ 0.21 $ 0.10 $ 0.03 $ (0.54 ) $ 0.25 $ 0.22 COPLP and Subsidiaries Revenues $ 139,801 $ 151,435 $ 157,017 $ 164,567 $ 144,307 $ 145,927 $ 142,103 $ 141,991 Operating income $ 35,433 $ 36,618 $ 38,939 $ 24,847 $ 30,464 $ (27,021 ) $ 11,525 $ 37,442 Income (loss) from continuing operations $ 18,850 $ 19,195 $ 21,494 $ 6,904 $ 8,096 $ (48,316 ) $ (4,829 ) $ 19,718 Net income (loss) $ 23,088 $ 19,207 $ 22,682 $ 11,356 $ 8,096 $ (48,316 ) $ 29,272 $ 26,603 Net income attributable to noncontrolling interests (934 ) (907 ) (897 ) (908 ) (979 ) (911 ) (913 ) (912 ) Net income (loss) attributable to COPLP 22,154 18,300 21,785 10,448 7,117 (49,227 ) 28,359 25,691 Preferred unit distributions (3,345 ) (3,204 ) (165 ) (165 ) (3,717 ) (3,718 ) (3,717 ) (3,805 ) Issuance costs associated with redeemed preferred units — (6,847 ) — — — — — (17 ) Net income (loss) attributable to COPLP common unitholders $ 18,809 $ 8,249 $ 21,620 $ 10,283 $ 3,400 $ (52,945 ) $ 24,642 $ 21,869 Basic EPU $ 0.18 $ 0.08 $ 0.21 $ 0.10 $ 0.03 $ (0.54 ) $ 0.25 $ 0.22 Diluted EPU $ 0.18 $ 0.08 $ 0.21 $ 0.10 $ 0.03 $ (0.54 ) $ 0.25 $ 0.22

Schedule II - Valuation and Qua

Schedule II - Valuation and Qualifying Accounts (Notes)	12 Months Ended
Schedule II - Valuation and Qualifying Accounts (Notes)	Dec. 31, 2017
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, 2017 , 2016 and 2015 (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, 2017 $ 603 $ 368 $ (36 ) $ (328 ) $ 607 Year ended December 31, 2016 $ 1,525 $ (17 ) $ 235 $ (1,140 ) $ 603 Year ended December 31, 2015 $ 717 $ 1,125 $ 98 $ (415 ) $ 1,525 Allowance for Deferred Rent Receivable Year ended December 31, 2017 $ 373 $ (9 ) $ — $ — $ 364 Year ended December 31, 2016 $ 1,962 $ (1,589 ) $ — $ — $ 373 Year ended December 31, 2015 $ 1,418 $ — $ 544 $ — $ 1,962 Allowance for Deferred Tax Asset Year ended December 31, 2017 $ 2,062 $ (646 ) $ — $ — $ 1,416 Year ended December 31, 2016 $ 2,062 $ — $ — $ — $ 2,062 Year ended December 31, 2015 $ 2,062 $ — $ — $ — $ 2,062 (1) Amounts charged to costs and expenses are net of recoveries. Reduction in allowance for deferred tax asset was the result of a decrease in the corporate tax rate. (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
Schedule III - Real Estate and Accumulated Depreciation	Dec. 31, 2017
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, 2017 (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 $ 49,378 $ 26,715 $ 58,343 $ 5,286 $ 26,715 $ 63,629 $ 90,344 $ (9,467 ) 1973 8/7/2015 1000 Redstone Gateway (O) Huntsville, AL 10,730 — 20,533 5 — 20,538 20,538 (2,465 ) 2013 3/23/2010 1100 Redstone Gateway (O) Huntsville, AL 11,222 — 19,593 — — 19,593 19,593 (1,945 ) 2014 3/23/2010 114 National Business Parkway (O) Annapolis Junction, MD — 364 3,109 118 364 3,227 3,591 (1,309 ) 2002 6/30/2000 11751 Meadowville Lane (O) (9) Richmond, VA — 1,305 52,098 112 1,305 52,210 53,515 (14,845 ) 2007 9/15/2006 1200 Redstone Gateway (O) Huntsville, AL 12,973 — 22,389 — — 22,389 22,389 (2,264 ) 2013 3/23/2010 1201 M Street (O) Washington, DC — — 49,785 8,590 — 58,375 58,375 (12,800 ) 2001 9/28/2010 1201 Winterson Road (O) Linthicum, MD — 1,288 16,433 460 1,288 16,893 18,181 (4,279 ) 1985/2017 4/30/1998 1220 12th Street, SE (O) Washington, DC — — 42,464 5,820 — 48,284 48,284 (11,719 ) 2003 9/28/2010 1243 Winterson Road (L) Linthicum, MD — 630 — — 630 — 630 — (7) 12/19/2001 131 National Business Parkway (O) Annapolis Junction, MD — 1,906 7,623 3,868 1,906 11,491 13,397 (6,286 ) 1990 9/28/1998 132 National Business Parkway (O) Annapolis Junction, MD — 2,917 12,259 4,124 2,917 16,383 19,300 (8,715 ) 2000 5/28/1999 133 National Business Parkway (O) Annapolis Junction, MD — 2,517 10,068 5,544 2,517 15,612 18,129 (9,234 ) 1997 9/28/1998 134 National Business Parkway (O) Annapolis Junction, MD — 3,684 7,517 3,691 3,684 11,208 14,892 (5,419 ) 1999 11/13/1998 1340 Ashton Road (O) Hanover, MD — 905 3,620 1,470 905 5,090 5,995 (2,854 ) 1989 4/28/1999 13450 Sunrise Valley Road (O) Herndon, VA — 1,386 5,576 4,553 1,386 10,129 11,515 (4,808 ) 1998 7/25/2003 13454 Sunrise Valley Road (O) Herndon, VA — 2,899 11,986 7,071 2,899 19,057 21,956 (9,262 ) 1998 7/25/2003 135 National Business Parkway (O) Annapolis Junction, MD — 2,484 9,750 6,075 2,484 15,825 18,309 (7,970 ) 1998 12/30/1998 1362 Mellon Road (O) Hanover, MD — 950 3,864 206 950 4,070 5,020 (192 ) 2006 2/10/2006 13857 McLearen Road (O) Herndon, VA — 3,507 30,177 1,768 3,507 31,945 35,452 (10,100 ) 2007 7/11/2012 140 National Business Parkway (O) Annapolis Junction, MD — 3,407 24,167 1,487 3,407 25,654 29,061 (8,907 ) 2003 12/31/2003 141 National Business Parkway (O) Annapolis Junction, MD — 2,398 9,538 4,815 2,398 14,353 16,751 (7,409 ) 1990 9/28/1998 14280 Park Meadow Drive (O) Chantilly, VA — 3,731 15,953 2,628 3,731 18,581 22,312 (7,326 ) 1999 9/29/2004 1460 Dorsey Road (L) Hanover, MD — 1,577 33 — 1,577 33 1,610 — (7) 2/28/2006 14840 Conference Center Drive (O) Chantilly, VA — 1,572 8,175 3,092 1,572 11,267 12,839 (5,410 ) 2000 7/25/2003 14850 Conference Center Drive (O) Chantilly, VA — 1,615 8,358 3,072 1,615 11,430 13,045 (5,857 ) 2000 7/25/2003 14900 Conference Center Drive (O) Chantilly, VA — 3,436 14,402 6,239 3,436 20,641 24,077 (10,027 ) 1999 7/25/2003 1501 South Clinton Street (O) Baltimore, MD — 27,964 51,990 13,670 27,964 65,660 93,624 (19,942 ) 2006 10/27/2009 15049 Conference Center Drive (O) Chantilly, VA — 4,415 20,365 14,994 4,415 35,359 39,774 (12,097 ) 1997 8/14/2002 15059 Conference Center Drive (O) Chantilly, VA — 5,753 13,615 3,645 5,753 17,260 23,013 (7,907 ) 2000 8/14/2002 1550 West Nursery Road (O) Linthicum, MD — 14,071 16,930 — 14,071 16,930 31,001 (4,802 ) 2009 10/28/2009 1560 West Nursery Road (O) Linthicum, MD — 1,441 113 — 1,441 113 1,554 (10 ) 2014 10/28/2009 1610 West Nursery Road (O) Linthicum, MD — 259 246 — 259 246 505 (5 ) 2016 4/30/1998 1616 West Nursery Road (O) Linthicum, MD — 393 2,919 — 393 2,919 3,312 (13 ) 2017 4/30/1998 1622 West Nursery Road (O) Linthicum, MD — 393 2,477 — 393 2,477 2,870 (53 ) 2016 4/30/1998 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) 16442 Commerce Drive (O) Dahlgren, VA — 613 2,582 891 613 3,473 4,086 (1,538 ) 2002 12/21/2004 16480 Commerce Drive (O) Dahlgren, VA — 1,856 7,425 2,068 1,856 9,493 11,349 (3,279 ) 2000 12/28/2004 16501 Commerce Drive (O) Dahlgren, VA — 522 2,090 727 522 2,817 3,339 (975 ) 2002 12/21/2004 16539 Commerce Drive (O) Dahlgren, VA — 688 2,860 1,892 688 4,752 5,440 (2,325 ) 1990 12/21/2004 16541 Commerce Drive (O) Dahlgren, VA — 773 3,094 1,757 773 4,851 5,624 (2,026 ) 1996 12/21/2004 16543 Commerce Drive (O) Dahlgren, VA — 436 1,742 716 436 2,458 2,894 (839 ) 2002 12/21/2004 1751 Pinnacle Drive (O) McLean, VA — 10,486 42,339 27,048 10,486 69,387 79,873 (29,582 ) 1989/1995 9/23/2004 1753 Pinnacle Drive (O) McLean, VA — 8,275 34,353 16,648 8,275 51,001 59,276 (18,757 ) 1976/2004 9/23/2004 206 Research Boulevard (O) Aberdeen, MD — — 132 — — 132 132 (132 ) 2012 9/14/2007 209 Research Boulevard (O) Aberdeen, MD — 134 1,711 175 134 1,886 2,020 (283 ) 2010 9/14/2007 210 Research Boulevard (O) Aberdeen, MD — 113 1,402 86 113 1,488 1,601 (174 ) 2010 9/14/2007 2100 L Street (L) Washington, DC — 55,615 9,073 — 55,615 9,073 64,688 — (7) 8/11/2015 2100 Rideout Road (O) Huntsville, AL — — 5,003 2,881 — 7,884 7,884 (438 ) 2016 3/23/2010 22289 Exploration Drive (O) Lexington Park, MD — 1,422 5,719 1,829 1,422 7,548 8,970 (3,415 ) 2000 3/24/2004 22299 Exploration Drive (O) Lexington Park, MD — 1,362 5,791 2,308 1,362 8,099 9,461 (3,768 ) 1998 3/24/2004 22300 Exploration Drive (O) Lexington Park, MD — 1,094 5,038 1,489 1,094 6,527 7,621 (2,432 ) 1997 11/9/2004 22309 Exploration Drive (O) Lexington Park, MD — 2,243 10,419 7,967 2,243 18,386 20,629 (6,087 ) 1984/1997 3/24/2004 23535 Cottonwood Parkway (O) California, MD — 692 3,051 537 692 3,588 4,280 (1,527 ) 1984 3/24/2004 250 W Pratt St (O) Baltimore, MD — 8,057 34,588 6,942 8,057 41,530 49,587 (6,972 ) 1985 3/19/2015 2500 Riva Road (O) Annapolis, MD — 2,791 12,145 1 2,791 12,146 14,937 (5,105 ) 2000 3/4/2003 2600 Park Tower Drive (O) Vienna, VA — 20,304 34,443 517 20,304 34,960 55,264 (3,708 ) 1999 4/15/2015 2691 Technology Drive (O) Annapolis Junction, MD — 2,098 17,334 5,563 2,098 22,897 24,995 (9,630 ) 2005 5/26/2000 2701 Technology Drive (O) Annapolis Junction, MD — 1,737 15,266 4,398 1,737 19,664 21,401 (9,676 ) 2001 5/26/2000 2711 Technology Drive (O) Annapolis Junction, MD — 2,251 21,611 1,899 2,251 23,510 25,761 (11,727 ) 2002 11/13/2000 2720 Technology Drive (O) Annapolis Junction, MD — 3,863 29,272 1,218 3,863 30,490 34,353 (10,261 ) 2004 1/31/2002 2721 Technology Drive (O) Annapolis Junction, MD — 4,611 14,597 1,270 4,611 15,867 20,478 (7,484 ) 2000 10/21/1999 2730 Hercules Road (O) Annapolis Junction, MD — 8,737 31,612 8,697 8,737 40,309 49,046 (18,673 ) 1990 9/28/1998 30 Light Street (O) Baltimore, MD 4,153 — 12,101 629 — 12,730 12,730 (753 ) 2009 8/7/2015 300 Sentinel Drive (O) Annapolis Junction, MD — 1,517 59,165 925 1,517 60,090 61,607 (11,562 ) 2009 11/14/2003 302 Sentinel Drive (O) Annapolis Junction, MD — 2,648 29,687 468 2,648 30,155 32,803 (7,579 ) 2007 11/14/2003 304 Sentinel Drive (O) Annapolis Junction, MD — 3,411 24,917 202 3,411 25,119 28,530 (7,610 ) 2005 11/14/2003 306 Sentinel Drive (O) Annapolis Junction, MD — 3,260 22,592 961 3,260 23,553 26,813 (6,631 ) 2006 11/14/2003 308 Sentinel Drive (O) Annapolis Junction, MD — 1,422 26,208 2,396 1,422 28,604 30,026 (4,370 ) 2010 11/14/2003 310 Sentinel Way (O) Annapolis Junction, MD — 2,372 38,865 — 2,372 38,865 41,237 (1,977 ) 2016 (8) 11/14/2003 310 The Bridge Street (O) Huntsville, AL — 261 26,531 3,762 261 30,293 30,554 (7,430 ) 2009 8/9/2011 312 Sentinel Way (O) Annapolis Junction, MD — 3,138 27,797 — 3,138 27,797 30,935 (2,304 ) 2014 11/14/2003 314 Sentinel Way (O) Annapolis Junction, MD — 1,254 7,741 — 1,254 7,741 8,995 (548 ) 2008 11/14/2003 316 Sentinel Way (O) Annapolis Junction, MD — 2,748 38,156 145 2,748 38,301 41,049 (5,507 ) 2011 11/14/2003 318 Sentinel Way (O) Annapolis Junction, MD — 2,185 28,426 560 2,185 28,986 31,171 (8,416 ) 2005 11/14/2003 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) 320 Sentinel Way (O) Annapolis Junction, MD — 2,067 21,623 — 2,067 21,623 23,690 (5,391 ) 2007 11/14/2003 322 Sentinel Way (O) Annapolis Junction, MD — 2,605 22,827 1,900 2,605 24,727 27,332 (6,286 ) 2006 11/14/2003 324 Sentinel Way (O) Annapolis Junction, MD — 1,656 23,018 — 1,656 23,018 24,674 (4,229 ) 2010 6/29/2006 4000 Market Street (O) Huntsville, AL — — 466 — — 466 466 — (8) 3/23/2010 4100 Market Street (O) Huntsville, AL — — 1,013 — — 1,013 1,013 — (8) 3/23/2010 410 National Business Parkway (O) Annapolis Junction, MD — 1,831 23,257 119 1,831 23,376 25,207 (2,904 ) 2012 6/29/2006 420 National Business Parkway (O) Annapolis Junction, MD — 2,370 27,750 108 2,370 27,858 30,228 (2,635 ) 2013 6/29/2006 430 National Business Parkway (O) Annapolis Junction, MD — 1,852 21,563 126 1,852 21,689 23,541 (3,127 ) 2011 6/29/2006 44408 Pecan Court (O) California, MD — 817 1,583 1,490 817 3,073 3,890 (838 ) 1986 3/24/2004 44414 Pecan Court (O) California, MD — 405 1,619 1,033 405 2,652 3,057 (954 ) 1986 3/24/2004 44417 Pecan Court (O) California, MD — 434 3,822 148 434 3,970 4,404 (1,448 ) 1989/2015 3/24/2004 44420 Pecan Court (O) California, MD — 344 890 168 344 1,058 1,402 (368 ) 1989 11/9/2004 44425 Pecan Court (O) California, MD — 1,309 3,506 1,881 1,309 5,387 6,696 (2,419 ) 1997 5/5/2004 45310 Abell House Lane (O) California, MD — 2,272 13,808 147 2,272 13,955 16,227 (2,094 ) 2011 8/30/2010 46579 Expedition Drive (O) Lexington Park, MD — 1,406 5,796 1,680 1,406 7,476 8,882 (3,421 ) 2002 3/24/2004 46591 Expedition Drive (O) Lexington Park, MD — 1,200 7,199 1,226 1,200 8,425 9,625 (2,652 ) 2005 3/24/2004 4851 Stonecroft Boulevard (O) Chantilly, VA — 1,878 11,558 21 1,878 11,579 13,457 (3,827 ) 2004 8/14/2002 540 National Business Parkway (O) Annapolis Junction, MD — 2,035 29,490 — 2,035 29,490 31,525 (260 ) 2017 (8) 6/29/2006 5520 Research Park Drive (O) Catonsville, MD — — 20,072 1,018 — 21,090 21,090 (4,260 ) 2009 4/4/2006 5522 Research Park Drive (O) Catonsville, MD — — 4,550 210 — 4,760 4,760 (1,185 ) 2007 3/8/2006 5801 University Research Court (O) College Park, MD — — 9,423 — — 9,423 9,423 (36 ) (8) 11/9/2016 5825 University Research Court (O) College Park, MD 21,284 — 22,771 666 — 23,437 23,437 (5,020 ) 2008 1/29/2008 5850 University Research Court (O) College Park, MD 22,517 — 31,906 405 — 32,311 32,311 (6,306 ) 2008 1/29/2008 6700 Alexander Bell Drive (O) Columbia, MD — 1,755 7,019 6,866 1,755 13,885 15,640 (6,916 ) 1988 5/14/2001 6708 Alexander Bell Drive (O) Columbia, MD — 897 11,984 1,605 897 13,589 14,486 (3,839 ) 1988/2016 5/14/2001 6711 Columbia Gateway Drive (O) Columbia, MD — 2,683 23,239 1,278 2,683 24,517 27,200 (6,817 ) 2006-2007 9/28/2000 6716 Alexander Bell Drive (O) Columbia, MD — 1,242 4,969 3,754 1,242 8,723 9,965 (5,129 ) 1990 12/31/1998 6721 Columbia Gateway Drive (O) Columbia, MD — 1,753 34,090 102 1,753 34,192 35,945 (7,514 ) 2009 9/28/2000 6724 Alexander Bell Drive (O) Columbia, MD — 449 5,039 1,374 449 6,413 6,862 (2,670 ) 2001 5/14/2001 6731 Columbia Gateway Drive (O) Columbia, MD — 2,807 19,098 4,872 2,807 23,970 26,777 (10,176 ) 2002 3/29/2000 6740 Alexander Bell Drive (O) Columbia, MD — 1,424 5,696 3,321 1,424 9,017 10,441 (5,698 ) 1992 12/31/1998 6741 Columbia Gateway Drive (O) Columbia, MD — 675 1,711 124 675 1,835 2,510 (466 ) 2008 9/28/2000 6750 Alexander Bell Drive (O) Columbia, MD — 1,263 12,461 3,959 1,263 16,420 17,683 (8,761 ) 2001 12/31/1998 6760 Alexander Bell Drive (O) Columbia, MD — 890 3,561 3,358 890 6,919 7,809 (3,932 ) 1991 12/31/1998 6940 Columbia Gateway Drive (O) Columbia, MD — 3,545 9,916 7,095 3,545 17,011 20,556 (8,164 ) 1999 11/13/1998 6950 Columbia Gateway Drive (O) Columbia, MD — 3,596 14,269 3,238 3,596 17,507 21,103 (8,787 ) 1998 10/22/1998 7000 Columbia Gateway Drive (O) Columbia, MD — 3,131 12,103 5,085 3,131 17,188 20,319 (6,178 ) 1999 5/31/2002 7005 Columbia Gateway Drive (O) Columbia, MD — 3,036 753 — 3,036 753 3,789 — (7) 6/26/2014 7015 Albert Einstein Drive (O) Columbia, MD 829 2,058 6,093 2,178 2,058 8,271 10,329 (3,343 ) 1999 12/1/2005 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) 7061 Columbia Gateway Drive (O) Columbia, MD — 729 3,094 2,018 729 5,112 5,841 (2,191 ) 2000 8/30/2001 7063 Columbia Gateway Drive (O) Columbia, MD — 902 3,684 3,151 902 6,835 7,737 (3,113 ) 2000 8/30/2001 7065 Columbia Gateway Drive (O) Columbia, MD — 919 3,763 3,095 919 6,858 7,777 (3,637 ) 2000 8/30/2001 7067 Columbia Gateway Drive (O) Columbia, MD — 1,829 11,823 3,051 1,829 14,874 16,703 (7,037 ) 2001 8/30/2001 7125 Columbia Gateway Drive (L) Columbia, MD — 3,361 2,354 279 3,361 2,633 5,994 — 1973/1999 (7) 6/29/2006 7125 Columbia Gateway Drive (O) Columbia, MD — 17,126 46,994 15,786 17,126 62,780 79,906 (20,876 ) 1973/1999 6/29/2006 7130 Columbia Gateway Drive (O) Columbia, MD — 1,350 4,359 2,534 1,350 6,893 8,243 (3,192 ) 1989 9/19/2005 7134 Columbia Gateway Drive (O) Columbia, MD — 704 4,707 353 704 5,060 5,764 (1,422 ) 1990/2016 9/19/2005 7138 Columbia Gateway Drive (O) Columbia, MD — 1,104 3,518 2,729 1,104 6,247 7,351 (3,459 ) 1990 9/19/2005 7142 Columbia Gateway Drive (O) Columbia, MD — 1,342 4,657 2,608 1,342 7,265 8,607 (2,796 ) 1994 (8) 9/19/2005 7150 Columbia Gateway Drive (O) Columbia, MD — 1,032 3,429 813 1,032 4,242 5,274 (1,439 ) 1991 9/19/2005 7150 Riverwood Drive (O) Columbia, MD — 1,821 4,388 1,774 1,821 6,162 7,983 (2,343 ) 2000 1/10/2007 7160 Riverwood Drive (O) Columbia, MD — 2,732 7,006 2,455 2,732 9,461 12,193 (3,778 ) 2000 1/10/2007 7170 Riverwood Drive (O) Columbia, MD — 1,283 3,096 1,465 1,283 4,561 5,844 (1,798 ) 2000 1/10/2007 7175 Riverwood Drive (O) Columbia, MD — 1,788 7,269 — 1,788 7,269 9,057 (752 ) 1996/2013 7/27/2005 7200 Redstone Gateway (O) Huntsville, AL 6,303 — 8,348 5 — 8,353 8,353 (752 ) 2013 3/23/2010 7200 Riverwood Road (O) Columbia, MD — 4,089 22,630 4,532 4,089 27,162 31,251 (10,180 ) 1986 10/13/1998 7205 Riverwood Drive (O) Columbia, MD — 1,367 21,419 — 1,367 21,419 22,786 (2,381 ) 2013 7/27/2005 7272 Park Circle Drive (O) Hanover, MD — 1,479 6,300 4,578 1,479 10,878 12,357 (3,898 ) 1991/1996 1/10/2007 7318 Parkway Drive (O) Hanover, MD — 972 3,888 1,239 972 5,127 6,099 (2,398 ) 1984 4/16/1999 7400 Redstone Gateway (O) Huntsville, AL 6,914 — 9,223 — — 9,223 9,223 (582 ) 2015 3/23/2010 7467 Ridge Road (O) Hanover, MD — 1,565 3,116 4,264 1,565 7,380 8,945 (1,930 ) 1990 4/28/1999 7740 Milestone Parkway (O) Hanover, MD 18,203 3,825 34,176 567 3,825 34,743 38,568 (6,482 ) 2009 7/2/2007 7770 Backlick Road (O) Springfield, VA — 6,387 76,315 142 6,387 76,457 82,844 (9,075 ) 2012 3/10/2010 7880 Milestone Parkway (O) Hanover, MD — 4,857 24,677 62 4,857 24,739 29,596 (1,382 ) 2015 9/17/2013 8621 Robert Fulton Drive (O) Columbia, MD — 2,317 12,642 537 2,317 13,179 15,496 (4,097 ) 2005-2006 6/10/2005 8661 Robert Fulton Drive (O) Columbia, MD — 1,510 3,764 2,453 1,510 6,217 7,727 (2,655 ) 2002 12/30/2003 8671 Robert Fulton Drive (O) Columbia, MD — 1,718 4,280 4,052 1,718 8,332 10,050 (3,688 ) 2002 12/30/2003 870 Elkridge Landing Road (O) Linthicum, MD — 2,003 9,442 8,735 2,003 18,177 20,180 (9,370 ) 1981 8/3/2001 891 Elkridge Landing Road (O) Linthicum, MD — 1,165 4,772 3,466 1,165 8,238 9,403 (4,186 ) 1984 7/2/2001 901 Elkridge Landing Road (O) Linthicum, MD — 1,156 4,437 2,704 1,156 7,141 8,297 (3,608 ) 1984 7/2/2001 911 Elkridge Landing Road (O) Linthicum, MD — 1,215 4,861 2,191 1,215 7,052 8,267 (3,899 ) 1985 4/30/1998 938 Elkridge Landing Road (O) Linthicum, MD — 922 4,748 1,446 922 6,194 7,116 (2,621 ) 1984 7/2/2001 939 Elkridge Landing Road (O) Linthicum, MD — 939 3,756 4,438 939 8,194 9,133 (4,254 ) 1983 4/30/1998 940 Elkridge Landing Road (L) Linthicum, MD — 842 4 — 842 4 846 — (7) 7/2/2001 9651 Hornbaker Road (D) Manassas, VA — 6,050 249,142 3,868 6,050 253,010 259,060 (39,294 ) 2010 9/14/2010 Arundel Preserve (L) Hanover, MD — 13,401 9,578 — 13,401 9,578 22,979 — (7) 7/2/2007 Bethlehem Tech. Park - DC 18 (O) Manassas, VA — 3,599 25,992 — 3,599 25,992 29,591 (306 ) 2017 6/17/2016 Bethlehem Tech. Park - DC 19 (O) Manassas, VA — 3,708 16,362 — 3,708 16,362 20,070 (455 ) 2016 6/9/2016 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) Bethlehem Tech. Park - DC 20 (O) Manassas, VA — 3,599 23,625 — 3,599 23,625 27,224 (370 ) 2017 6/9/2016 Bethlehem Tech. Park - DC 23 (O) Manassas, VA — — 479 — — 479 479 — (8) 6/9/2016 BLC 1 (O) Northern Virginia — 12,035 368 — 12,035 368 12,403 — (8) 12/28/2017 BLC 2 (O) Northern Virginia — 12,035 292 — 12,035 292 12,327 — (8) 12/28/2017 Canton Crossing Land (L) Baltimore, MD — 16,085 2,698 — 16,085 2,698 18,783 — (7) 10/27/2009 Canton Crossing Util Distr Ctr (O) Baltimore, MD — 7,300 15,556 986 7,300 16,542 23,842 (4,475 ) 2006 10/27/2009 Columbia Gateway - Southridge (L) Columbia, MD — 6,387 3,719 — 6,387 3,719 10,106 — (7) 9/20/2004 Dahlgren Technology Center (L) Dahlgren, VA — 978 178 — 978 178 1,156 — (7) 3/16/2005 Expedition VII (L) Lexington Park, MD — 705 729 — 705 729 1,434 — (7) 3/24/2004 Innovation Park (L) Manassas, VA — 4,443 120 — 4,443 120 4,563 — (7) 9/1/2016 M Square Research Park (L) College Park, MD — — 3,571 — — 3,571 3,571 — (7) 1/29/2008 MR Land (L) Northern Virginia — 18,827 293 — 18,827 293 19,120 — (7) 11/20/2017 National Business Park North (L) Annapolis Junction, MD — 28,066 47,802 — 28,066 47,802 75,868 — (7) 6/29/2006 North Gate Business Park (L) Aberdeen, MD — 1,755 — — 1,755 — 1,755 — (7) 9/14/2007 Northwest Crossroads (L) San Antonio, TX — 7,430 847 — 7,430 847 8,277 — (7) 1/20/2006 NOVA Office A (O) (10) Chantilly, VA — 2,096 46,835 — 2,096 46,835 48,931 (3,403 ) 2015 7/31/2002 NOVA Office B (O) (10) Chantilly, VA — 739 27,128 — 739 27,128 27,867 (1,128 ) 2016 (8) 7/31/2002 NOVA Office D (O) Chantilly, VA — 6,587 38,758 — 6,587 38,758 45,345 (437 ) 2017 7/31/2002 Old Annapolis Road (O) Columbia, MD — 1,637 5,500 5,241 1,637 10,741 12,378 (3,509 ) 1974/1985 12/14/2000 Paragon Park - DC 21 (O) Sterling, VA — 7,828 17,992 — 7,828 17,992 25,820 (100 ) 2017 5/8/2017 Paragon Park - DC 22 (O) Sterling, VA — 7,828 17,445 — 7,828 17,445 25,273 (68 ) 2017 5/8/2017 Patriot Point - DC 15 (O) Ashburn, VA — 12,156 17,069 — 12,156 17,069 29,225 (752 ) 2016 10/15/2015 Patriot Point - DC 16 (O) Ashburn, VA — 12,156 16,973 — 12,156 16,973 29,129 (709 ) 2016 10/15/2015 Patriot Point - DC 17 (O) Ashburn, VA — 6,078 16,347 — 6,078 16,347 22,425 (520 ) 2016 10/15/2015 Patriot Ridge (L) Springfield, VA — 18,517 14,467 — 18,517 14,467 32,984 — (7) 3/10/2010 Redstone Gateway (L) Huntsville, AL — — 19,152 — — 19,152 19,152 — (7) 3/23/2010 Route 15/Biggs Ford Road (L) Frederick, MD — 1,129 — — 1,129 — 1,129 — (7) 8/28/2008 Sentry Gateway (L) San Antonio, TX — 8,275 3,704 — 8,275 3,704 11,979 — (7) 3/30/2005 Sentry Gateway - T (O) San Antonio, TX — 14,020 38,804 13 14,020 38,817 52,837 (10,561 ) 1982/1985 3/30/2005 Sentry Gateway - V (O) San Antonio, TX — — 1,066 — — 1,066 1,066 (241 ) 2007 3/30/2005 Sentry Gateway - W (O) San Antonio, TX — — 1,884 71 — 1,955 1,955 (392 ) 2009 3/30/2005 Sentry Gateway - X (O) San Antonio, TX — 1,964 21,178 — 1,964 21,178 23,142 (3,787 ) 2010 1/20/2006 Sentry Gateway - Y (O) San Antonio, TX — 1,964 21,298 — 1,964 21,298 23,262 (3,810 ) 2010 1/20/2006 Sentry Gateway - Z (O) San Antonio, TX — 1,964 30,573 — 1,964 30,573 32,537 (2,144 ) 2015 6/14/2005 Westfields - Park Center (L) Chantilly, VA — 16,418 11,264 — 16,418 11,264 27,682 — (7) 7/18/2002 Westfields Corporate Center (L) Chantilly, VA — 7,141 1,649 — 7,141 1,649 8,790 — (7) 7/31/2002 Other Developments, including intercompany eliminations (V) Various — 4 283 256 4 539 543 (58 ) Various Various $ 164,506 $ 697,810 $ 2,902,516 $ 380,487 $ 697,810 $ 3,283,003 $ 3,980,813 $ (801,038 ) (1) A legend for the Property Type follows: (O) = Office or Data Center Shell Property; (L) = Land held or pre-construction; (D) = Wholesale Data Center; and (V) = Various. (2) Excludes our term loan facilities of $348.0 million , unsecured senior notes of $1.2 billion , unsecured notes payable of $1.3 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, 2017 . (4) As discussed in Note 3 to our Consolidated Financial Statements, we recognized impairment losses of $15.1 million primarily in connection with certain of our land and operating properties, including $13.7 million related to land and operating properties still owned as of December 31, 2017 . (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) Held or under pre-construction at December 31, 2017 . (8) Under construction or redevelopment at December 31, 2017 . (9) Classified as held for sale as of December 31, 2017. (10) The carrying amounts of these properties under construction exclude allocated costs of the garage being constructed to support the properties. The following table summarizes our changes in cost of properties for the years ended December 31, 2017, 2016 and 2015 (in thousands): 2017 2016 2015 Beginning balance $ 3,874,715 $ 4,158,616 $ 4,014,336 Acquisitions of operating properties — — 194,616 Improvements and other additions 259,548 251,960 273,761 Sales (138,216 ) (268,038 ) (172,628 ) Impairments (15,116 ) (143,502 ) (29,548 ) Other dispositions (118 ) (124,321 ) (121,921 ) Ending balance $ 3,980,813 $ 3,874,715 $ 4,158,616 The following table summarizes our changes in accumulated depreciation for the same time periods (in thousands): 2017 2016 2015 Beginning balance $ 715,951 $ 718,680 $ 703,083 Depreciation expense 107,772 105,763 112,695 Sales (22,567 ) (56,607 ) (49,614 ) Impairments — (42,161 ) (6,092 ) Other dispositions (118 ) (9,724 ) (41,392 ) Ending balance $ 801,038 $ 715,951 $ 718,680

Summary of Significant Accoun31

Summary of Significant Accounting Policies (Policies)	12 Months Ended
Summary of Significant Accounting Policies (Policies)	Dec. 31, 2017
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.
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 Operating Properties	Acquisitions of Operating Properties Upon completion of operating 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 operating 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 direct and indirect project costs (including related compensation and other indirect costs), interest expense and real estate taxes associated with properties, or portions thereof, undergoing construction, development and redevelopment activities. In capitalizing interest expense, if there is a specific borrowing for a 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 properties for indicators of impairment quarterly or when circumstances indicate that a property may be impaired. 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. When we dispose of, or classify as held for sale, a component or group of components that represents a strategic shift having a major effect on our operations and financial results (such as a major geographical area of operations, a major line of business or a major equity method investment), we classify the associated results of operations as discontinued operations. We had no properties newly classified as discontinued operations in the last three years.
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 collectability 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. We classify leasing costs paid as an investing activity on our statements of cash flows since such costs are necessary in order for us to generate long-term future cash flows from our properties. 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 amortize deferred costs of line-of-credit arrangements ratably over the terms of such arrangements. We expense any unamortized loan costs when loans are retired early. We present deferred costs of financing arrangements as a direct deduction from the related debt liability, except for costs attributable to line-of-credit arrangements and interest rate derivatives, which we present in the balance sheet in the line entitled “prepaid expenses and other assets, net”.
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 interests in consolidated real estate joint ventures not owned by us (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 noncancelable term of tenant leases. A lease term 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 noncancelable term of a lease includes periods when a tenant: (1) may not terminate its lease obligation early without incurring a penalty in such an amount that the continuation of the lease appears reasonably assured; (2) 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 (3) 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 reported in liabilities as 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 accumulated other comprehensive income (“AOCI”) or loss (“AOCL”) 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 or AOCL, 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 the section below entitled “Recent Accounting Pronouncements” for disclosure pertaining to the effect of new hedge accounting guidance that we adopted effective January 1, 2018 and Note 11 for additional disclosure pertaining to our 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, 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 and director and officers) and leasing prospects, as well as associated labor and indirect costs attributable to these expenses.
Share-Based Compensation	Share-Based Compensation We issue three forms of share-based compensation: restricted COPT common shares (“restricted shares”), deferred share awards (also known as restricted share units) and performance share units (also known as performance share awards) (“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 PSUs using a Monte Carlo model. Significant assumptions used for that model include the following: the baseline common share value is 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; and 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, 2017 2016 2015 2017 2016 2015 Ordinary income 86.5 % 48.0 % 38.3 % 100.0 % 100 % 38.3 % Long-term capital gain 0.0 % 0.0 % 61.7 % 0.0 % 0.0 % 61.7 % Return of capital 13.5 % 52.0 % 0.0 % 0.0 % 0.0 % 0.0 % However, 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 2017 , 2016 and 2015 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 $150 million higher than the amount reported on our consolidated balance sheet as of December 31, 2017 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.
Reclassification	Reclassification We reclassified certain amounts from prior periods to conform to the current period presentation of our consolidated financial statements with no effect on previously reported net income or equity.
Recent Accounting Pronouncements	Recent Accounting Pronouncements We adopted guidance issued by the Financial Accounting Standards Board (“FASB”) effective January 1, 2017 intended to simplify various aspects related to the accounting and presentation for employee share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities and classification on the consolidated statement of cash flows. In connection with our adoption of this policy, we made an entity-wide accounting policy election to continue to account for potential future award forfeitures by estimating the number of awards that are expected to vest. Our adoption of this guidance did not have a material impact on our consolidated financial statements. We adopted guidance issued by the FASB prospectively effective January 1, 2017 that clarifies the definition of a business used by entities in determining whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. The definition of a business affects many areas of accounting including acquisitions, disposals, goodwill and consolidation. Under the new guidance, we expect that the majority of our future operating property acquisitions will be accounted for as asset acquisitions, whereas under the previous guidance our recent acquisitions were accounted for as business combinations; we believe that the primary effect of this change will be that transaction costs associated with future acquisitions will be capitalized rather than expensed as incurred. This guidance had no effect on our consolidated financial statements upon adoption. 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. This guidance also requires improved disclosures regarding the nature, amount, timing and uncertainty of revenue and cash flows arising from contracts with customers. We adopted this guidance effective January 1, 2018 using the modified retrospective method, under which the cumulative effect of initially applying the guidance is recognized at the date of initial application. Our adoption of this guidance beginning on January 1, 2018 will not have a material effect on our consolidated financial statements. However, as discussed further below, once the new guidance setting forth principles for the recognition, measurement, presentation and disclosure of leases goes into effect on January 1, 2019, the new revenue standard may apply to executory costs and other components of revenue due under leases that are deemed to be non-lease components (such as common area maintenance and provision of utilities), which could affect our recognition pattern for such revenue. In January 2016, the FASB issued guidance that requires entities to measure equity investments at fair value through net income, except for those that result in consolidation or are accounted for under the equity method of accounting. For equity investments without readily determinable fair values, the guidance permits the application of a measurement alternative using the cost of the investment, less any impairments, plus or minus changes resulting from observable price changes for an identical or similar investment of the same issuer. We adopted this guidance effective January 1, 2018. We do not expect the adoption of this guidance to have a material impact on our consolidated financial statements. In February 2016, the FASB issued guidance that sets forth principles for the recognition, measurement, presentation and disclosure of leases. This guidance requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase of the leased asset by the lessee. The resulting classification determines whether the lease expense is recognized based on an effective interest method or straight-line basis over the term of the lease. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases. The guidance requires lessors of real estate to account for leases using an approach that is substantially equivalent to existing guidance for sales-type leases, direct financing leases and operating leases. This guidance is effective for reporting periods beginning January 1, 2019 using a modified retrospective transition approach at the time of adoption. Early adoption is also permitted for this guidance. In addition, the guidance permits lessees and lessors to elect to apply a package of practical expedients that allow them not to reassess upon adoption: the lease classification for any expired or existing leases; their deferred recognition of incremental direct costs of leasing for any expired or existing leases; and whether any expired or existing contracts are, or contain, leases. While we are still completing our assessment of the impact of this guidance, below is a summary of the anticipated primary effects of this guidance on our accounting and reporting. • Real estate leases in which we are the lessor: ◦ Balance sheet reporting: We believe that we will apply an approach under the new guidance that is similar to the current accounting for operating leases, in which we will continue to recognize the underlying leased asset as property on our balance sheet. ◦ Deferral of non-incremental lease costs: Under the new lease guidance, we will no longer be able to defer the recognition of non-incremental costs in connection with new or extended tenant leases; these deferrals totaled $1.1 million in 2017 , $1.1 million in 2016 and $1.2 million in 2015 . Upon adoption of the new guidance, we would expense previously deferred non-incremental lease costs for existing leases unless we elect the package of practical expedients, in which case such costs would remain deferred and amortized over the remaining lease terms. ◦ Lease revenue reporting: We believe that the new revenue standard will apply to executory costs and other components of revenue deemed to be non-lease components (such as common area maintenance and provision of utilities), even when the revenue for such activities is not separately stipulated in the lease. In that case, we would separate the lease components of revenue due under leases from the non-lease components, and the revenue from the non-lease components previously recognized on a straight-line basis under current lease guidance would be recognized under the new revenue guidance as the related services are delivered. As a result, while the total revenue recognized over time would not differ under the new guidance, the recognition pattern could be different. We are in the process of evaluating the significance of the difference in the recognition pattern that would result from this change. • Leases in which we are the lessee: ◦ Our most significant leases as lessee are ground leases we have for certain properties; as of December 31, 2017 , our future minimum rental payments under these leases totaled $89.9 million , with various expiration dates extending to the year 2100. While we are still in the process of evaluating these leases under the new guidance, we believe that we will be required to recognize a right-of-use asset and a lease liability for the present value of these minimum lease payments. We also believe that these types of leases most likely would be classified as finance leases under the new guidance which would result in the interest component of each lease payment being recorded as interest expense and the right-of-use asset being amortized into expense using the straight-line method over the life of the lease; however, if we elect to apply the package of practical expedients, we will continue to account for our existing ground leases as operating leases upon adoption of the guidance. In June 2016, the FASB issued guidance that changes how entities measure credit losses for most financial assets and certain other instruments that are not measured at fair value through net income. The guidance replaces the current incurred loss model with an expected loss approach, resulting in a more timely recognition of such losses. The guidance will apply to most financial assets measured at amortized cost and certain other instruments, including trade and other receivables, loans, held-to-maturity debt securities, net investments in leases, and off-balance-sheet credit exposures (e.g. loan commitments). Under the new guidance, an entity will recognize its estimate of expected credit losses as an allowance, as the guidance requires that financial assets be measured on an amortized cost basis and to be presented at the net amount expected to be collected. The guidance is effective for us beginning January 1, 2020, with early adoption permitted after December 2018. We are currently assessing the financial impact of this guidance on our consolidated financial statements. In August 2016, the FASB issued guidance that clarifies how entities should classify certain cash receipts and cash payments on the statement of cash flows with the objective of reducing the existing diversity in practice related to eight specific cash flow issues. The areas addressed in the new guidance relate to debt prepayment costs, settlement of zero-coupon debt instruments, contingent consideration payments made after a business combination, proceeds from the settlement of insurance claims, proceeds from the settlement of corporate-owned and bank-owned life insurance policies, distributions received from equity method investments, beneficial interest in securitization transactions and separately identifiable cash flows and application of the predominance principle. We adopted this guidance effective January 1, 2018 using a retrospective transition method. We do not expect the adoption of this guidance to have a material impact on our consolidated financial statements. In November 2016, the FASB issued guidance that requires the statement of cash flows to explain the change during the period in the total of cash, cash equivalents and amounts described as restricted cash or restricted cash equivalents. Under the new guidance, amounts described as restricted cash and restricted cash equivalents will be included with cash and cash equivalents when reconciling the beginning of period and end of period total amounts shown on the statement of cash flows. Our restricted cash primarily consists of cash escrowed under mortgage debt for capital improvements and real estate taxes and cash restricted in connection with our deferred compensation plan and certain tenant security deposits. We adopted this guidance effective January 1, 2018 using a retrospective transition method. Upon our adoption of this guidance, changes in our restricted cash that we currently report as either operating or investing activities on our statements of cash flows will no longer be reported as such. We do not expect the adoption of this guidance to have a material impact on our consolidated financial statements. In February 2017, the FASB issued guidance clarifying the scope of asset derecognition provisions and accounting for partial sales of nonfinancial assets. The new guidance requires recognition of a sale of real estate and resulting gain or loss when control transfers and the buyer has the ability to direct use of, or obtain substantially all of the remaining benefit from, the asset (which generally will occur on the closing date); the factor of continuing involvement is no longer a specific consideration for the timing of recognition. The new guidance eliminates the need to consider adequacy of buyer investment, which was replaced by additional judgments regarding collectability and intent and/or ability to pay. The new guidance also requires an entity to derecognize nonfinancial assets and in-substance nonfinancial assets once it transfers control of such assets. When an entity transfers its controlling interest in a nonfinancial asset, but retains a noncontrolling ownership interest, the entity is required to measure any non-controlling interest it receives or retains at fair value and recognize a full gain or loss on the transaction; as a result, sales and partial sales of real estate assets will now be subject to the same derecognition model as all other nonfinancial assets. We had a transaction in July 2016 accounted for as a partial sale under existing guidance that would meet the criteria for immediate full gain recognition under the new guidance; this would result in an additional $18 million in income being recognized in 2016 that is currently being amortized into income in subsequent periods under existing guidance. We do not believe that the recognition pattern for our other sales of real estate will be changed by the new guidance. We adopted this guidance effective January 1, 2018, and expect to use the full retrospective method, under which we would retrospectively restate each reporting period presented at the time of adoption. In August 2017, the FASB issued guidance that makes targeted improvements to hedge accounting. This new guidance simplifies the application of hedge accounting and better aligns financial reporting for hedging activities with companies’ economic objectives in undertaking those activities. Under the new guidance, all changes in the fair value of highly effective cash flow hedges will be recorded in other comprehensive income instead of income. The new guidance also eases the administrative burden of hedge documentation requirements and assessing hedge effectiveness. While we are required to adopt this guidance no later than January 1, 2019, we expect to adopt this guidance effective January 1, 2018. We do not expect the adoption of this guidance to have a material impact on our consolidated financial statements.

Summary of Significant Accoun32

Summary of Significant Accounting Policies (Tables)	12 Months Ended
Summary of Significant Accounting Policies (Tables)	Dec. 31, 2017
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, 2017 2016 2015 2017 2016 2015 Ordinary income 86.5 % 48.0 % 38.3 % 100.0 % 100 % 38.3 % Long-term capital gain 0.0 % 0.0 % 61.7 % 0.0 % 0.0 % 61.7 % Return of capital 13.5 % 52.0 % 0.0 % 0.0 % 0.0 % 0.0 %

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
Fair Value Measurements (Tables)	Dec. 31, 2017
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, 2017 and 2016 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, 2017: Assets: Marketable securities in deferred compensation plan (1) Mutual funds $ 4,547 $ — $ — $ 4,547 Other 69 — — 69 Interest rate derivatives (2) — 3,073 — 3,073 Total assets $ 4,616 $ 3,073 $ — $ 7,689 Liabilities: Deferred compensation plan liability (3) $ — $ 4,616 $ — $ 4,616 December 31, 2016: Assets: Marketable securities in deferred compensation plan (1) Mutual funds $ 5,346 $ — $ — $ 5,346 Other 91 — — 91 Interest rate derivatives (2) — 158 — 158 Total assets $ 5,437 $ 158 $ — $ 5,595 Liabilities: Deferred compensation plan liability (3) $ — $ 5,437 $ — $ 5,437 Interest rate derivatives — 1,572 — 1,572 Redeemable preferred shares of beneficial interest (4) — 26,583 — 26,583 Total liabilities $ — $ 33,592 $ — $ 33,592 (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, net” on COPT ’ s consolidated balance sheet. (3) Included in the line entitled “other liabilities” on COPT ’ s consolidated balance sheet. (4) See disclosure regarding our Series K Preferred Shares in Note 13. 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, 2017 and 2016 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, 2017: Assets: Interest rate derivatives (1) $ — $ 3,073 $ — $ 3,073 December 31, 2016: Assets: Interest rate derivatives (1) $ — $ 158 $ — $ 158 Liabilities: Interest rate derivatives $ — $ 1,572 $ — $ 1,572 Redeemable preferred units of general partner (2) — 26,583 — 26,583 Total liabilities $ — $ 28,155 $ — $ 28,155 (1) Included in the line entitled “prepaid expenses and other assets, net” on COPLP ’ s consolidated balance sheet. (2) See disclosure regarding our Series K Preferred Units in Note 14.
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 value measurements of properties as of December 31, 2017 (in thousands): Fair Values as of December 31, 2017 Quoted Prices in Significant Active Markets for Significant Other Unobservable Identical Assets Observable Inputs Inputs Description (Level 1) (Level 2) (Level 3) Total Assets: Operating properties, net $ — $ — $ 3,850 $ 3,850 Projects in development or held for future development $ — $ — $ 1,755 $ 1,755
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, 2017 (dollars in thousands): Valuation Technique Fair Values on Measurement Date Unobservable Input Range (Weighted Average) Discounted cash flow $ 3,850 Discount rate 14% - 16% (14%) Terminal capitalization rate 12% (1) Comparable sales analysis $ 1,755 Comparable sales prices N/A (1) Only one fair value applied for this unobservable input.

Properties, Net (Tables)

Properties, Net (Tables)	12 Months Ended
Properties, Net (Tables)	Dec. 31, 2017
Real Estate [Abstract]
Schedule of operating properties, net	Operating properties, net consisted of the following (in thousands): December 31, 2017 2016 Land $ 455,680 $ 433,311 Buildings and improvements 3,068,124 2,944,905 Less: Accumulated depreciation (786,193 ) (706,385 ) Operating properties, net $ 2,737,611 $ 2,671,831
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, 2017 2016 Land $ 240,825 $ 195,521 Development in progress, excluding land 162,669 206,010 Projects in development or held for future development $ 403,494 $ 401,531
Components of assets held for sale	The table below sets forth the components of assets held for sale (in thousands): December 31, 2017 2016 Properties, net $ 38,670 $ 85,402 Deferred rent receivable 3,237 4,241 Intangible assets on real estate acquisitions, net — 338 Deferred leasing costs, net 319 3,636 Lease incentives, net — 1,037 Assets held for sale, net $ 42,226 $ 94,654
Schedule of operating property dispositions	In 2017, we sold 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 Sale 3120 Fairview Park Drive Falls Church, VA Northern Virginia Defense/IT 2/15/2017 1 190,000 $ 39,000 $ — 1334 Ashton Road Hanover, MD Fort Meade/BW Corridor 6/9/2017 1 37,000 2,300 — Remaining White Marsh Properties (1) White Marsh, MD Regional Office and Other 7/28/2017 8 412,000 47,500 1,180 201 Technology Drive Lebanon, VA Data Center Shells 10/27/2017 1 103,000 29,500 3,625 7320 Parkway Drive Hanover, MD Fort Meade/BW Corridor 12/15/2017 1 57,000 7,529 831 12 799,000 $ 125,829 $ 5,636 (1) This sale also included land 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 Virginia 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. In 2016, we sold 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 Sale Arborcrest Corporate Campus (1) Philadelphia, PA Regional Office 8/4/2016 4 654,000 $ 142,800 $ 4,742 8003 Corporate Drive White Marsh, MD Regional Office 8/17/2016 1 18,000 2,400 — 1341 & 1343 Ashton Road Hanover, MD Fort Meade/BW Corridor 9/9/2016 2 25,000 2,900 848 8007, 8013, 8015, 8019 and 8023-8027 Corporate Drive (1) White Marsh, MD Regional Office 9/21/2016 5 130,000 14,513 1,906 1302, 1304 and 1306 Concourse Drive Linthicum, MD Fort Meade/BW Corridor 9/29/2016 3 299,000 48,100 8,375 2900 Towerview Road Herndon, VA Northern Virginia Defense/IT 10/19/2016 1 151,000 12,100 — 4940 Campbell Boulevard White Marsh, MD Regional Office 11/17/2016 1 50,000 5,200 — 1560 A and B Cable Ranch Road San Antonio, TX Other 11/30/2016 2 120,000 10,300 — 1331 Ashton Road Hanover, MD Fort Meade/BW Corridor 12/19/2016 1 29,000 2,625 — 900 Elkridge Landing Road Linthicum, MD Fort Meade/BW Corridor 12/22/2016 1 101,000 7,800 — 21 1,577,000 $ 248,738 $ 15,871 (1) This sale also included land.
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 (Unaudited) Pro forma total revenues $ 641,982 Pro forma net income attributable to COPT common shareholders $ 167,079 Pro forma EPS: Basic $ 1.77 Diluted $ 1.77

Real Estate Joint Ventures (Tab

Real Estate Joint Ventures (Tables)	12 Months Ended
Real Estate Joint Ventures (Tables)	Dec. 31, 2017
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, 2017 (dollars in thousands): Nominal Ownership December 31, 2017 (1) Date % as of Total Encumbered Total Acquired 12/31/2017 Nature of Activity Assets Assets Liabilities LW Redstone Company, LLC 3/23/2010 85% Development and operation of real estate (2) $ 158,891 $ 75,569 $ 51,180 M Square Associates, LLC 6/26/2007 50% Development and operation of real estate (3) 73,116 45,384 45,745 Stevens Investors, LLC 8/11/2015 95% Development of real estate (4) 71,976 — 19,905 $ 303,983 $ 120,953 $ 116,830 (1) Excludes amounts eliminated in consolidation. (2) This joint venture’s properties are in Huntsville, Alabama. (3) This joint venture’s properties are in College Park, Maryland. (4) This joint venture’s property is in Washington, DC.

Intangible Assets on Real Est36

Intangible Assets on Real Estate Acquisitions (Tables)	12 Months Ended
Intangible Assets on Real Estate Acquisitions (Tables)	Dec. 31, 2017
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, 2017 December 31, 2016 Gross Carrying Amount Accumulated Amortization Net Carrying Amount Gross Carrying Amount Accumulated Amortization Net Carrying Amount In-place lease value $ 132,276 $ 110,814 $ 21,462 $ 132,647 $ 99,940 $ 32,707 Tenant relationship value 60,028 32,198 27,830 60,028 26,253 33,775 Below-market cost arrangements 15,102 7,507 7,595 15,102 7,285 7,817 Above-market leases 13,944 12,092 1,852 13,944 10,259 3,685 Other 1,333 980 353 1,333 966 367 $ 222,683 $ 163,591 $ 59,092 $ 223,054 $ 144,703 $ 78,351

Investing Receivables (Tables)

Investing Receivables (Tables)	12 Months Ended
Investing Receivables (Tables)	Dec. 31, 2017
Receivables [Abstract]
Schedule of investing receivables	Investing receivables, including accrued interest thereon, consisted of the following (in thousands): December 31, 2017 2016 Notes receivable from City of Huntsville $ 54,472 $ 49,258 Other investing loans receivable 3,021 3,021 $ 57,493 $ 52,279

Prepaid Expenses and Other As38

Prepaid Expenses and Other Assets, Net (Tables)	12 Months Ended
Prepaid Expenses and Other Assets, Net (Tables)	Dec. 31, 2017
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, 2017 2016 Prepaid expenses $ 24,670 $ 24,432 Lease incentives, net 19,011 18,276 Furniture, fixtures and equipment, net 5,256 5,204 Construction contract costs incurred in excess of billings 4,884 10,350 Interest rate derivatives 3,073 158 Non-real estate equity method investments 2,412 2,355 Deferred tax asset, net 1,892 3,036 Deferred financing costs, net (1) 1,202 3,128 Other assets 4,821 5,825 Prepaid expenses and other assets, net $ 67,221 $ 72,764 (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, 2017 2016 Operating loss carry forward $ 3,209 $ 5,084 Share-based compensation 7 13 Accrued payroll 49 101 Property 43 (100 ) Valuation allowance (1,416 ) (2,062 ) Deferred tax asset, net $ 1,892 $ 3,036

Debt, Net (Tables)

Debt, Net (Tables)	12 Months Ended
Debt, Net (Tables)	Dec. 31, 2017
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, 2017 December 31, 2017 Mortgage and Other Secured Debt: Fixed rate mortgage debt (2) $ 150,723 $ 154,143 3.82% - 7.87% (3) 2019-2026 Variable rate secured loans 13,115 13,448 LIBOR + 1.85% (4) October 2020 Total mortgage and other secured debt 163,838 167,591 Revolving Credit Facility (5) 126,000 — LIBOR + 0.875% to 1.60% May 2019 Term Loan Facilities (6) 347,959 547,494 LIBOR + 0.90% to 1.85% (7) 2020-2022 Unsecured Senior Notes (5) 3.600%, $350,000 aggregate principal 347,551 347,128 3.60% (8) May 2023 5.250%, $250,000 aggregate principal 246,645 246,176 5.25% (9) February 2024 3.700%, $300,000 aggregate principal 298,322 297,843 3.70% (10) June 2021 5.000%, $300,000 aggregate principal 296,731 296,368 5.00% (11) July 2025 Unsecured note payable 1,287 1,401 0% (12) May 2026 Total debt, net $ 1,828,333 $ 1,904,001 (1) The carrying values of our debt other than the Revolving Credit Facility reflect net deferred financing costs of $5.0 million as of December 31, 2017 and $6.1 million as of December 31, 2016 . (2) Certain of the fixed rate mortgages carry interest rates that, upon assumption, were above or below market rates 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 $349,000 as of December 31, 2017 and $422,000 as of December 31, 2016 . (3) The weighted average interest rate on our fixed rate mortgage debt was 4.19% as of December 31, 2017 . (4) The interest rate on our variable rate secured debt was 3.21% as of December 31, 2017 . (5) Refer to the paragraphs below for further disclosure. (6) As discussed below, we have the ability to borrow an additional $350.0 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 2.73% as of December 31, 2017 . (8) The carrying value of these notes reflects an unamortized discount totaling $1.7 million as of December 31, 2017 and $2.0 million as of December 31, 2016 . 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 $3.0 million as of December 31, 2017 and $3.4 million as of December 31, 2016 . The effective interest rate under the notes, including amortization of the issuance costs, was 5.49% . (10) The carrying value of these notes reflects an unamortized discount totaling $1.3 million as of December 31, 2017 and $1.7 million as of December 31, 2016 . The effective interest rate under the notes, including amortization of the issuance costs, was 3.85% . (11) Refer to the paragraphs below for further disclosure. (12) This note carries an interest rate that, upon assumption, was below market rates and it therefore was recorded at its fair value based on applicable effective interest rates. The carrying value of this note reflects an unamortized discount totaling $373,000 as of December 31, 2017 and $460,000 as of December 31, 2016 .
Schedule of debt maturities	Our debt matures on the following schedule (in thousands): 2018 $ 4,241 2019 130,387 2020 116,156 2021 303,875 2022 254,033 Thereafter 1,033,475 Total $ 1,842,167 (1) (1) Represents scheduled principal amortization and maturities only and therefore excludes net discounts and deferred financing costs of $13.8 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, 2017 December 31, 2016 Carrying Estimated Carrying Estimated Amount Fair Value Amount Fair Value Fixed-rate debt Unsecured Senior Notes $ 1,189,249 $ 1,229,398 $ 1,187,515 $ 1,220,282 Other fixed-rate debt 152,010 152,485 155,544 156,887 Variable-rate debt 487,074 485,694 560,942 558,437 $ 1,828,333 $ 1,867,577 $ 1,904,001 $ 1,935,606

Interest Rate Derivatives (Tabl

Interest Rate Derivatives (Tables)	12 Months Ended
Interest Rate Derivatives (Tables)	Dec. 31, 2017
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, each of which was designated as a cash flow hedge (dollars in thousands): Fair Value at Notional Effective Expiration December 31, Amount Fixed Rate Floating Rate Index Date Date 2017 2016 $ 100,000 1.7300 % One-Month LIBOR 9/1/2015 8/1/2019 $ 252 $ (848 ) 13,217 (1) 1.3900 % One-Month LIBOR 10/13/2015 10/1/2020 213 100 100,000 1.9013 % One-Month LIBOR 9/1/2016 12/1/2022 1,046 (23 ) 100,000 1.9050 % One-Month LIBOR 9/1/2016 12/1/2022 1,051 48 50,000 1.9079 % One-Month LIBOR 9/1/2016 12/1/2022 511 10 100,000 (2) 1.6730 % One-Month LIBOR 9/1/2015 8/1/2019 — (701 ) $ 3,073 $ (1,414 ) (1) The notional amount of this instrument is scheduled to amortize to $12.1 million . (2) We cash settled this derivative and interest accrued thereon for $460,000 on May 1, 2017. Since the hedged transactions associated with this derivative were still probable to occur as of the settlement date, amounts in accumulated other comprehensive loss (“AOCL”) associated with this derivative will be reclassified to interest expense through August 2019.
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 2017 2016 Interest rate swaps designated as cash flow hedges Prepaid expenses and other assets $ 3,073 $ 158 Interest rate swaps designated as cash flow hedges Other liabilities — (1,572 )
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, 2017 2016 2015 Unrealized gain (loss) recognized in AOCL (effective portion) $ 684 $ (2,915 ) $ (4,739 ) Loss reclassified from AOCL into interest expense (effective portion) (3,216 ) (4,230 ) (3,599 ) Gain (loss) on derivatives recognized in interest expense (ineffective portion) 323 378 (386 ) Loss reclassified from AOCL into interest expense (ineffective portion) (1) (88 ) — — (1) Represents a loss recognized on certain interest rate swaps from the accelerated reclassification of amounts in AOCL on May 1, 2017, when we concluded that hedged forecasted transactions were probable not to occur.

Redeemable Noncontrolling Int41

Redeemable Noncontrolling Interests (Tables)	12 Months Ended
Redeemable Noncontrolling Interests (Tables)	Dec. 31, 2017
Noncontrolling Interest [Abstract]
Schedule of activity for redeemable noncontrolling interest	The table below sets forth the activity in these redeemable noncontrolling interests (in thousands): For the Years Ended December 31, 2017 2016 2015 Beginning balance $ 22,979 $ 19,218 $ 18,417 Contributions from noncontrolling interests — 22,779 1,654 Distributions to noncontrolling interests (1,566 ) (21,881 ) (2,964 ) Net income attributable to noncontrolling interests 2,338 2,242 2,227 Adjustment to arrive at fair value of interests (626 ) 621 (116 ) Ending balance $ 23,125 $ 22,979 $ 19,218

Share-Based Compensation and 42

Share-Based Compensation and Other Compensation Matters (Tables)	12 Months Ended
	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
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, 2017 2016 2015 General, administrative and leasing expenses $ 4,649 $ 5,816 $ 5,574 Property operating expenses 966 1,027 1,000 Capitalized to development activities 480 610 824 Share-based compensation cost $ 6,095 $ 7,453 $ 7,398
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 2015 , 2016 and 2017 : Shares Weighted Average Grant Date Fair Value Unvested as of December 31, 2014 390,507 $ 26.19 Granted 201,024 28.69 Forfeited (10,550 ) 26.05 Vested (202,781 ) 26.07 Unvested as of December 31, 2015 378,200 27.58 Granted 231,937 24.77 Forfeited (22,907 ) 25.31 Vested (215,983 ) 27.19 Unvested as of December 31, 2016 371,247 26.20 Granted 239,479 33.84 Forfeited (27,056 ) 27.80 Vested (158,044 ) 26.27 Unvested as of December 31, 2017 425,626 $ 30.37 Unvested shares as of December 31, 2017 that are expected to vest 402,870 $ 30.31
Schedule of PSU's Granted	We made the following grants of PSUs to executives from 2013 through 2017 (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, 2017 3/1/2013 69,579 1/1/2013 12/31/2015 $ 1,867 — 3/6/2014 49,103 1/1/2014 12/31/2016 $ 1,723 — 3/5/2015 45,656 1/1/2015 12/31/2017 $ 1,678 15,767 3/1/2016 26,299 1/1/2016 12/31/2018 $ 1,000 24,850 1/1/2017 39,351 1/1/2017 12/31/2019 $ 1,400 39,351
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 2015 , 2016 and 2017 are set forth below: Grant Date Grant Date Fair Value Baseline Common Share Value Expected Volatility of Common Shares Risk-free Interest Rate 3/5/2015 $ 36.76 $ 29.28 19.9 % 0.99 % 3/1/2016 $ 38.21 $ 23.90 20.4 % 0.96 % 1/1/2017 $ 38.43 $ 31.22 19.0 % 1.47 %
Deferred share awards	We made the following grants of deferred share awards to nonemployee members of our Board of Trustees in 2015, 2016 and 2017 (dollars in thousands, except per share amounts): Year of Grant Number of Deferred Share Awards Granted Aggregate Grant Date Fair Value Grant Date Fair Value Per Share 2015 24,056 $ 642 $ 26.70 2016 24,944 $ 671 $ 26.89 2017 10,032 $ 326 $ 32.47
Deferred share awards vested activity	We issued the following common shares in settlement of deferred shares in 2015 , 2016 and 2017 (dollars in thousands, except per share amounts): For the Years Ended December 31, 2017 2016 2015 Number of common shares issued 15,590 12,028 15,485 Grant date fair value $ 26.89 $ 26.70 $ 26.77 Aggregate intrinsic value $ 508 $ 322 $ 413
Summary of stock option transactions under the entity's share-based compensation plans	The table below sets forth information regarding our outstanding options as of the following dates (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, 2014 559,736 $39.60 2 $ 167 December 31, 2015 425,347 $42.75 1 $ — December 31, 2016 201,100 $43.35 1 $ 31 December 31, 2017 60,000 $35.17 1 $ —

Operating Leases (Tables)

Operating Leases (Tables)	12 Months Ended
Operating Leases (Tables)	Dec. 31, 2017
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, 2017 were as follows (in thousands): Year Ending December 31, 2018 $ 372,420 2019 329,760 2020 260,238 2021 207,727 2022 175,123 Thereafter 484,444 $ 1,829,712

Information by Business Segme44

Information by Business Segment (Tables)	12 Months Ended
Information by Business Segment (Tables)	Dec. 31, 2017
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): Operating Property Segments Defense/Information Technology Locations Fort Meade/BW Corridor Northern Virginia Defense/IT Lackland Air Force Base Navy Support Locations Redstone Arsenal Data Center Shells Total Defense/IT Locations Regional Office Operating Wholesale Data Center Other Total Year Ended December 31, 2017 Revenues from real estate operations $ 245,613 $ 47,118 $ 47,209 $ 29,540 $ 14,322 $ 24,320 $ 408,122 $ 68,262 $ 28,875 $ 4,721 $ 509,980 Property operating expenses (80,697 ) (16,938 ) (27,812 ) (12,619 ) (5,783 ) (2,709 ) (146,558 ) (28,982 ) (13,551 ) (1,873 ) (190,964 ) UJV NOI allocable to COPT — — — — — 5,188 5,188 — — — 5,188 NOI from real estate operations $ 164,916 $ 30,180 $ 19,397 $ 16,921 $ 8,539 $ 26,799 $ 266,752 $ 39,280 $ 15,324 $ 2,848 $ 324,204 Additions to long-lived assets $ 26,659 $ 8,115 $ 71 $ 8,451 $ 1,056 $ — $ 44,352 $ 25,299 $ 3,580 $ 110 $ 73,341 Transfers from non-operating properties $ 43,370 $ 48,328 $ — $ 474 $ 2,159 $ 107,854 $ 202,185 $ — $ 8 $ 18 $ 202,211 Segment assets at December 31, 2017 $ 1,263,567 $ 402,076 $ 128,755 $ 194,476 $ 108,119 $ 285,275 $ 2,382,268 $ 400,512 $ 224,422 $ 4,082 $ 3,011,284 Year Ended December 31, 2016 Revenues from real estate operations $ 245,354 $ 48,964 $ 46,803 $ 28,197 $ 13,056 $ 23,836 $ 406,210 $ 85,805 $ 26,869 $ 7,080 $ 525,964 Property operating expenses (83,684 ) (17,824 ) (27,357 ) (12,690 ) (4,476 ) (2,674 ) (148,705 ) (34,095 ) (11,512 ) (3,218 ) (197,530 ) UJV NOI allocable to COPT — — — — — 2,305 2,305 — — — 2,305 NOI from real estate operations $ 161,670 $ 31,140 $ 19,446 $ 15,507 $ 8,580 $ 23,467 $ 259,810 $ 51,710 $ 15,357 $ 3,862 $ 330,739 Additions to long-lived assets $ 26,267 $ 17,344 $ — $ 9,168 $ 4,352 $ — $ 57,131 $ 12,559 $ 299 $ 335 $ 70,324 Transfers from non-operating properties $ 49,937 $ 28,230 $ 240 $ — $ 3,169 $ 103,367 $ 184,943 $ 82 $ (377 ) $ (8 ) $ 184,640 Segment assets at December 31, 2016 $ 1,255,230 $ 404,438 $ 131,957 $ 196,486 $ 110,395 $ 209,683 $ 2,308,189 $ 442,811 $ 231,954 $ 21,293 $ 3,004,247 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
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, 2017 2016 2015 Segment revenues from real estate operations $ 509,980 $ 525,964 $ 519,068 Construction contract and other service revenues 102,840 48,364 106,402 Less: Revenues from discontinued operations — — (4 ) Total revenues $ 612,820 $ 574,328 $ 625,466
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, 2017 2016 2015 Segment property operating expenses $ 190,964 $ 197,530 $ 194,488 Less: Property operating expenses from discontinued operations — — 6 Total property operating expenses $ 190,964 $ 197,530 $ 194,494
Schedule of reconciliation of unconsolidated joint venture net operating income to equity in income of unconsolidated entities	The following table reconciles UJV NOI allocable to COPT to equity in income of unconsolidated entities as reported on our consolidated statements of operations (in thousands): For the Years Ended December 31, 2017 2016 2015 UJV NOI allocable to COPT $ 5,188 $ 2,305 $ — Less: Income from UJV allocable to COPT attributable to depreciation and amortization expense and interest expense (2,301 ) (993 ) — Add: Equity in (loss) income of unconsolidated non-real estate entities (5 ) 20 62 Equity in income of unconsolidated entities $ 2,882 $ 1,332 $ 62
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, 2017 2016 2015 Construction contract and other service revenues $ 102,840 $ 48,364 $ 106,402 Construction contract and other service expenses (99,618 ) (45,481 ) (102,696 ) NOI from service operations $ 3,222 $ 2,883 $ 3,706
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, 2017 2016 2015 NOI from real estate operations $ 324,204 $ 330,739 $ 324,580 NOI from service operations 3,222 2,883 3,706 Interest and other income 6,318 5,444 4,517 Equity in income of unconsolidated entities 2,882 1,332 62 Income tax expense (1,098 ) (244 ) (199 ) Depreciation and other amortization associated with real estate operations (134,228 ) (132,719 ) (140,025 ) Impairment losses (15,123 ) (101,391 ) (23,289 ) General, administrative and leasing expenses (30,837 ) (36,553 ) (31,361 ) Business development expenses and land carry costs (6,213 ) (8,244 ) (13,507 ) Interest expense (76,983 ) (83,163 ) (89,074 ) NOI from discontinued operations — — (10 ) Less: UJV NOI allocable to COPT included in equity in income of unconsolidated entities (5,188 ) (2,305 ) — (Loss) gain on early extinguishment of debt (513 ) (1,110 ) 85,275 COPT consolidated income (loss) from continuing operations $ 66,443 $ (25,331 ) $ 120,675
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, 2017 2016 Segment assets $ 3,011,284 $ 3,004,247 Non-operating property assets 411,041 418,171 Other assets 156,159 358,467 Total COPT consolidated assets $ 3,578,484 $ 3,780,885

Earnings Per Share ("EPS") an45

Earnings Per Share ("EPS") and Earnings Per Unit (“EPU”) (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 2016 2015 Numerator: Income (loss) from continuing operations $ 66,443 $ (25,331 ) $ 120,675 Gain on sales of real estate 9,890 40,986 68,047 Preferred share dividends (6,219 ) (14,297 ) (14,210 ) Issuance costs associated with redeemed preferred shares (6,847 ) (17 ) — Income from continuing operations attributable to noncontrolling interests (6,242 ) (4,216 ) (10,575 ) Income from continuing operations attributable to share-based compensation awards (449 ) (419 ) (706 ) Numerator for basic EPS from continuing operations attributable to COPT common shareholders $ 56,576 $ (3,294 ) $ 163,231 Dilutive effect of common units in COPLP on diluted EPS from continuing operations — — 6,397 Numerator for diluted EPS from continuing operations attributable to COPT common shareholders $ 56,576 $ (3,294 ) $ 169,628 Numerator for basic EPS from continuing operations attributable to COPT common shareholders $ 56,576 $ (3,294 ) $ 163,231 Discontinued operations — — 156 Discontinued operations attributable to noncontrolling interests — — (3 ) Numerator for basic EPS on net income (loss) attributable to COPT common shareholders $ 56,576 $ (3,294 ) $ 163,384 Dilutive effect of common units in COPLP — — 6,403 Numerator for diluted EPS on net income (loss) attributable to COPT common shareholders $ 56,576 $ (3,294 ) $ 169,787 Denominator (all weighted averages): Denominator for basic EPS (common shares) 98,969 94,502 93,914 Dilutive effect of forward equity sale agreements and share-based compensation awards 186 — 61 Dilutive effect of common units — — 3,692 Denominator for diluted EPS (common shares) 99,155 94,502 97,667 Basic EPS: Income (loss) from continuing operations attributable to COPT common shareholders $ 0.57 $ (0.03 ) $ 1.74 Net income (loss) attributable to COPT common shareholders $ 0.57 $ (0.03 ) $ 1.74 Diluted EPS: Income (loss) from continuing operations attributable to COPT common shareholders $ 0.57 $ (0.03 ) $ 1.74 Net income (loss) attributable to COPT common shareholders $ 0.57 $ (0.03 ) $ 1.74
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, 2017 2016 2015 Conversion of common units 3,362 3,633 — Conversion of Series I preferred units 176 176 176 Conversion of Series K preferred shares — 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, 2017 2016 2015 Numerator: Income (loss) from continuing operations $ 66,443 $ (25,331 ) $ 120,675 Gain on sales of real estate, net 9,890 40,986 68,047 Preferred unit distributions (6,879 ) (14,957 ) (14,870 ) Issuance costs associated with redeemed preferred units (6,847 ) (17 ) — Income from continuing operations attributable to noncontrolling interests (3,646 ) (3,715 ) (3,523 ) Income from continuing operations attributable to share-based compensation awards (449 ) (419 ) (706 ) Numerator for basic and diluted EPU from continuing operations attributable to COPLP common unitholders $ 58,512 $ (3,453 ) $ 169,623 Discontinued operations — — 156 Discontinued operations attributable to noncontrolling interests — — 3 Numerator for basic and diluted EPU on net income (loss) attributable to COPLP common unitholders $ 58,512 $ (3,453 ) $ 169,782 Denominator (all weighted averages): Denominator for basic EPU (common units) 102,331 98,135 97,606 Dilutive effect of forward equity sale agreements and share-based compensation awards 186 — 61 Denominator for diluted EPU (common units) 102,517 98,135 97,667 Basic EPU: Income (loss) from continuing operations attributable to COPLP common unitholders $ 0.57 $ (0.04 ) $ 1.74 Net income (loss) attributable to COPLP common unitholders $ 0.57 $ (0.04 ) $ 1.74 Diluted EPU: Income (loss) from continuing operations attributable to COPLP common unitholders $ 0.57 $ (0.04 ) $ 1.74 Net income (loss) attributable to COPLP common unitholders $ 0.57 $ (0.04 ) $ 1.74
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, 2017 2016 2015 Conversion of Series I preferred units 176 176 176 Conversion of Series K preferred units — 434 434

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
Commitments and Contingencies (Tables)	Dec. 31, 2017
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, 2017 follow (in thousands): Year Ending December 31, 2018 $ 1,283 2019 1,267 2020 1,259 2021 1,263 2022 1,149 Thereafter 84,611 $ 90,832
Schedule of future minimum lease payments for capital leases	Future minimum rental payments due under the term of this lease as of December 31, 2017 follow (in thousands): Year Ending December 31, 2018 $ 15,829 2020 135 2022 75 Total minimum rental payments 16,039 Less: Amount representing interest (186 ) Capital lease obligation $ 15,853

Quarterly Data (Unaudited) (Tab

Quarterly Data (Unaudited) (Tables)	12 Months Ended
Quarterly Data (Unaudited) (Tables)	Dec. 31, 2017
Quarterly Data [Line Items]
Schedule of selected quarterly information	The tables below set forth selected quarterly information for the years ended December 31, 2017 and 2016 (in thousands, except per share/unit data). For the Year Ended December 31, 2017 For the Year Ended December 31, 2016 First Quarter Second Quarter Third Quarter Fourth Quarter First Quarter Second Quarter Third Quarter Fourth Quarter COPT and Subsidiaries Revenues $ 139,801 $ 151,435 $ 157,017 $ 164,567 $ 144,307 $ 145,927 $ 142,103 $ 141,991 Operating income $ 35,433 $ 36,618 $ 38,939 $ 24,847 $ 30,464 $ (27,021 ) $ 11,525 $ 37,442 Income (loss) from continuing operations $ 18,850 $ 19,195 $ 21,494 $ 6,904 $ 8,096 $ (48,316 ) $ (4,829 ) $ 19,718 Net income (loss) $ 23,088 $ 19,207 $ 22,682 $ 11,356 $ 8,096 $ (48,316 ) $ 29,272 $ 26,603 Net (income) loss attributable to noncontrolling interests (1,733 ) (1,345 ) (1,766 ) (1,398 ) (1,270 ) 897 (1,973 ) (1,870 ) Net income (loss) attributable to COPT 21,355 17,862 20,916 9,958 6,826 (47,419 ) 27,299 24,733 Preferred share dividends (3,180 ) (3,039 ) — — (3,552 ) (3,553 ) (3,552 ) (3,640 ) Issuance costs associated with redeemed preferred shares — (6,847 ) — — — — — (17 ) Net income (loss) attributable to COPT common shareholders $ 18,175 $ 7,976 $ 20,916 $ 9,958 $ 3,274 $ (50,972 ) $ 23,747 $ 21,076 Basic EPS $ 0.18 $ 0.08 $ 0.21 $ 0.10 $ 0.03 $ (0.54 ) $ 0.25 $ 0.22 Diluted EPS $ 0.18 $ 0.08 $ 0.21 $ 0.10 $ 0.03 $ (0.54 ) $ 0.25 $ 0.22
Corporate Office Properties, L.P. [Member]
Quarterly Data [Line Items]
Schedule of selected quarterly information	COPLP and Subsidiaries Revenues $ 139,801 $ 151,435 $ 157,017 $ 164,567 $ 144,307 $ 145,927 $ 142,103 $ 141,991 Operating income $ 35,433 $ 36,618 $ 38,939 $ 24,847 $ 30,464 $ (27,021 ) $ 11,525 $ 37,442 Income (loss) from continuing operations $ 18,850 $ 19,195 $ 21,494 $ 6,904 $ 8,096 $ (48,316 ) $ (4,829 ) $ 19,718 Net income (loss) $ 23,088 $ 19,207 $ 22,682 $ 11,356 $ 8,096 $ (48,316 ) $ 29,272 $ 26,603 Net income attributable to noncontrolling interests (934 ) (907 ) (897 ) (908 ) (979 ) (911 ) (913 ) (912 ) Net income (loss) attributable to COPLP 22,154 18,300 21,785 10,448 7,117 (49,227 ) 28,359 25,691 Preferred unit distributions (3,345 ) (3,204 ) (165 ) (165 ) (3,717 ) (3,718 ) (3,717 ) (3,805 ) Issuance costs associated with redeemed preferred units — (6,847 ) — — — — — (17 ) Net income (loss) attributable to COPLP common unitholders $ 18,809 $ 8,249 $ 21,620 $ 10,283 $ 3,400 $ (52,945 ) $ 24,642 $ 21,869 Basic EPU $ 0.18 $ 0.08 $ 0.21 $ 0.10 $ 0.03 $ (0.54 ) $ 0.25 $ 0.22 Diluted EPU $ 0.18 $ 0.08 $ 0.21 $ 0.10 $ 0.03 $ (0.54 ) $ 0.25 $ 0.22

Organization (Details)

Organization (Details) - Dec. 31, 2017 ft² in Millions	a	ft²	MW	Property
Operating properties [Member]
Investments in real estate
Number of real estate properties				159
Area of real estate property (in sqft or acres) \| ft²		17.3
Operating properties [Member] \| Single-tenant data centers [Member]
Investments in real estate
Number of real estate properties				15
Operating properties [Member] \| Office Properties [Member]
Investments in real estate
Number of real estate properties				144
Office properties under, or contractually committed for, construction or approved for redevelopment [Member]
Investments in real estate
Number of real estate properties				10
Area of real estate property (in sqft or acres) \| ft²		1.1
Office properties under, or contractually committed for, construction or approved for redevelopment [Member] \| Single-tenant data centers [Member]
Investments in real estate
Number of real estate properties				3
Office properties under, or contractually committed for, construction or approved for redevelopment [Member] \| Office Properties [Member]
Investments in real estate
Number of real estate properties				7
Office properties under, or contractually committed for, construction or approved for redevelopment [Member] \| Partially operational properties [Member]
Investments in real estate
Number of real estate properties				2
Office properties under, or contractually committed for, construction or approved for redevelopment [Member] \| Properties completed Held-for-future lease [Member]
Investments in real estate
Number of real estate properties				2
Land controlled for future development [Member]
Investments in real estate
Area of real estate property (in sqft or acres)	1,000	13
Other land [Member]
Investments in real estate
Area of real estate property (in sqft or acres) \| a	150
Operating wholesale data centers [Member]
Investments in real estate
Critical load (in megawatts) \| MW			19.25
GI-COPT [Member] \| Operating properties [Member] \| Single-tenant data centers [Member]
Investments in real estate
Number of real estate properties				6

Organization (Details 2)

Organization (Details 2)	12 Months Ended
Organization (Details 2)	Dec. 31, 2017	Dec. 31, 2016
Corporate Office Properties, L.P. [Member] \| Common Units [Member]
Forms of ownership in Operating Partnership and ownership percentage by the entity
Percentage ownership in operating partnership	96.90%	96.50%

Summary of Significant Accoun50

Summary of Significant Accounting Policies (Details)	12 Months Ended
Summary of Significant Accounting Policies (Details)	Dec. 31, 2017
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 recovery analysis for 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 Accoun51

Summary of Significant Accounting Policies (Details 1)	12 Months Ended
Summary of Significant Accounting Policies (Details 1)	Dec. 31, 2017Form
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 Accoun52

Summary of Significant Accounting Policies (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in Accounting Estimate [Line Items]
Approximate amount by which basis of assets and liabilities for tax reporting purposes is higher than amount reported on consolidated balance sheet	$ 150,000
Deferred lease related compensation costs	1,100	$ 1,100	$ 1,200
Future minimum payments due	$ 90,832
Common Shares [Member]
Change in Accounting Estimate [Line Items]
Ordinary income (as a percent)	86.50%	48.00%	38.30%
Long-term capital gain (as a percent)	0.00%	0.00%	61.70%
Return of capital (as a percent)	13.50%	52.00%	0.00%
Preferred Shares [Member]
Change in Accounting Estimate [Line Items]
Ordinary income (as a percent)	100.00%	100.00%	38.30%
Long-term capital gain (as a percent)	0.00%	0.00%	61.70%
Return of capital (as a percent)	0.00%	0.00%	0.00%
Ground Leases [Member]
Change in Accounting Estimate [Line Items]
Future minimum payments due	$ 89,900
Accounting Standards Update 2017-05 [Member]
Change in Accounting Estimate [Line Items]
Effect of change on net income		$ 18,000

Fair Value Measurements (Detail

Fair Value Measurements (Details) - Deferred Compensation, Excluding Share-based Payments and Retirement Benefits [Member] - Management [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
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	$ 4.6	$ 5.4

Fair Value Measurements (Deta54

Fair Value Measurements (Details 2) - Fair value measurement on a recurring basis [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets:
Interest rate derivatives	$ 3,073	$ 158
Assets	7,689	5,595
Liabilities:
Deferred compensation plan liability	4,616	5,437
Interest rate derivatives		1,572
Redeemable preferred shares of beneficial interest		26,583
Liabilities		33,592
Mutual funds [Member]
Assets:
Marketable securities in deferred compensation plan	4,547	5,346
Other [Member]
Assets:
Marketable securities in deferred compensation plan	69	91
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets:
Assets	4,616	5,437
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] \| Mutual funds [Member]
Assets:
Marketable securities in deferred compensation plan	4,547	5,346
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] \| Other [Member]
Assets:
Marketable securities in deferred compensation plan	69	91
Significant Other Observable Inputs (Level 2) [Member]
Assets:
Interest rate derivatives	3,073	158
Assets	3,073	158
Liabilities:
Deferred compensation plan liability	4,616	5,437
Interest rate derivatives		1,572
Redeemable preferred shares of beneficial interest		26,583
Liabilities		33,592
Corporate Office Properties, L.P. [Member]
Assets:
Interest rate derivatives	3,073	158
Liabilities:
Interest rate derivatives		1,572
Redeemable preferred shares of beneficial interest		26,583
Liabilities		28,155
Corporate Office Properties, L.P. [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Assets:
Interest rate derivatives	$ 3,073	158
Liabilities:
Interest rate derivatives		1,572
Redeemable preferred shares of beneficial interest		26,583
Liabilities		$ 28,155

Fair Value Measurements (Deta55

Fair Value Measurements (Details 3) $ in Thousands	3 Months Ended						12 Months Ended
Fair Value Measurements (Details 3) $ in Thousands	Dec. 31, 2017USD ($)	Jun. 30, 2017USD ($)	Dec. 31, 2016USD ($)	Sep. 30, 2016USD ($)	Jun. 30, 2016USD ($)	Mar. 31, 2016USD ($)Property	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Fair value of impaired properties and other assets
Assets held for sale, net	$ 42,226		$ 94,654				$ 42,226	$ 94,654
Impairment losses							15,123	101,391	$ 23,289
Held-for-sale [Member]
Fair value of impaired properties and other assets
Assets held for sale, net	42,226		94,654				42,226	94,654
Fort Meade/BW Corridor [Member]
Fair value of impaired properties and other assets
Number of properties \| Property						2
Fair value measurement on a nonrecurring basis [Member] \| Held-for-sale [Member]
Fair value of impaired properties and other assets
Assets held for sale, net			94,700	$ 161,500	$ 300,600	$ 225,900		$ 94,700	96,800
Impairment losses				11,500		$ 2,400
Fair value measurement on a nonrecurring basis [Member] \| Disposed of by sale [Member]
Fair value of impaired properties and other assets
Impairment losses			300
Transaction value of operating property dispositions			54,100	210,700
Fair value measurement on a nonrecurring basis [Member] \| Operating properties, net [Member]
Fair value of impaired properties and other assets
Assets	3,850						3,850
Fair value measurement on a nonrecurring basis [Member] \| Operating properties, net [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair value of impaired properties and other assets
Assets	3,850						3,850
Fair value measurement on a nonrecurring basis [Member] \| Projects in development or held for future development [Member]
Fair value of impaired properties and other assets
Assets	1,755						1,755
Fair value measurement on a nonrecurring basis [Member] \| Projects in development or held for future development [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair value of impaired properties and other assets
Assets	1,755						$ 1,755
Fair value measurement on a nonrecurring basis [Member] \| Northern Virginia Defense/IT and Fort Meade/Baltimore Washington Corridor [Member] \| Held-for-sale [Member]
Fair value of impaired properties and other assets
Impairment losses					14,100
Fair value measurement on a nonrecurring basis [Member] \| Defense/Information Technology Sector [Member] \| Northern Virginia Defense/IT [Member] \| Held-for-sale [Member]
Fair value of impaired properties and other assets
Impairment losses				13,300
Fair value measurement on a nonrecurring basis [Member] \| Aberdeen, Maryland [Member] \| Land in development or held for future development [Member]
Fair value of impaired properties and other assets
Impairment of Long-Lived Assets to be Disposed of	4,700
Impairment losses					4,400
Fair value measurement on a nonrecurring basis [Member] \| Aberdeen, Maryland [Member] \| Other Segments [Member]
Fair value of impaired properties and other assets
Impairment of Long-Lived Assets to be Disposed of	$ 9,000
Fair value measurement on a nonrecurring basis [Member] \| Frederick, Maryland [Member] \| Land in development or held for future development [Member]
Fair value of impaired properties and other assets
Impairment losses					8,200
Fair value measurement on a nonrecurring basis [Member] \| Greater Philadelphia [Member] \| Regional Office [Member] \| Held-for-sale [Member]
Fair value of impaired properties and other assets
Impairment losses					6,200
Fair value measurement on a nonrecurring basis [Member] \| Colorado Springs, Colorado and White Marsh, Maryland [Member] \| Regional Office [Member] \| Held-for-sale [Member]
Fair value of impaired properties and other assets
Impairment losses					2,400
Fair value measurement on a nonrecurring basis [Member] \| Colorado Springs [Member] \| Held-for-sale [Member] \| Land in development or held for future development [Member]
Fair value of impaired properties and other assets
Impairment losses									12,800
Fair value measurement on a nonrecurring basis [Member] \| White Marsh, Maryland [Member] \| Held-for-sale [Member]
Fair value of impaired properties and other assets
Impairment of Long-Lived Assets to be Disposed of		$ 1,600
Impairment losses			$ 1,300	10,000
Fair value measurement on a nonrecurring basis [Member] \| White Marsh, Maryland [Member] \| Held-for-sale [Member] \| Land in development or held for future development [Member]
Fair value of impaired properties and other assets
Impairment losses				$ 2,900
Fair value measurement on a nonrecurring basis [Member] \| White Marsh, Maryland [Member] \| Regional Office [Member] \| Held-for-sale [Member]
Fair value of impaired properties and other assets
Impairment losses									2,600
Operating properties, net [Member] \| Fair value measurement on a nonrecurring basis [Member] \| Aberdeen, Maryland [Member] \| Land in development or held for future development [Member]
Fair value of impaired properties and other assets
Impairment losses									6,600
Operating properties, net [Member] \| Fair value measurement on a nonrecurring basis [Member] \| Aberdeen, Maryland [Member] \| Other Segments [Member]
Fair value of impaired properties and other assets
Impairment losses					$ 34,400
Operating properties, net [Member] \| Fair value measurement on a nonrecurring basis [Member] \| Northern Virginia [Member] \| Regional Office [Member]
Fair value of impaired properties and other assets
Impairment losses									$ 1,300

Fair Value Measurements (Deta56

Fair Value Measurements (Details 4) - Fair value measurement on a nonrecurring basis [Member] - Fair Value, Inputs, Level 3 [Member] $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)
Discounted cash flow [Member]
Assets and liabilities measured at fair value on a recurring basis
Fair Values on Measurement Date	$ 3,850
Discounted cash flow [Member] \| Minimum [Member]
Assets and liabilities measured at fair value on a recurring basis
Discount rate (percent)	14.00%
Discounted cash flow [Member] \| Maximum [Member]
Assets and liabilities measured at fair value on a recurring basis
Discount rate (percent)	16.00%
Discounted cash flow [Member] \| Weighted Average [Member]
Assets and liabilities measured at fair value on a recurring basis
Discount rate (percent)	14.00%
Terminal capitalization rate (percent)	12.00%
Comparable sales analysis [Member]
Assets and liabilities measured at fair value on a recurring basis
Fair Values on Measurement Date	$ 1,755

Concentration of Revenue (Detai

Concentration of Revenue (Details) - Tenants [Member] - United States Government [Member]	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Rental revenue [Member]
Concentration Risk
Percentage of revenue	22.00%	21.00%	20.00%
Construction contract revenue [Member] \| Minimum [Member]
Concentration Risk
Percentage of revenue	80.00%	80.00%	80.00%

Properties, Net (Details)

Properties, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Properties
Gross	$ 3,980,813	$ 3,874,715	$ 4,158,616	$ 4,014,336
Operating properties, net	2,737,611	2,671,831
Operating properties, net [Member]
Properties
Less: accumulated depreciation	(786,193)	(706,385)
Operating properties, net	2,737,611	2,671,831
Operating properties, net [Member] \| Land [Member]
Properties
Gross	455,680	433,311
Operating properties, net [Member] \| Buildings and improvements [Member]
Properties
Gross	$ 3,068,124	$ 2,944,905

Properties, Net (Details 2)

Properties, Net (Details 2) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Properties
Projects in development or held for future development	$ 403,494	$ 401,531
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	240,825	195,521
Projects in development or held for future development [Member] \| Development in progress, excluding land [Member]
Properties
Projects in development or held for future development	$ 162,669	$ 206,010

Properties, Net (Details 3)

Properties, Net (Details 3) $ in Thousands	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)Property
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets held for sale, net	$ 42,226	$ 94,654
Held-for-sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Properties, net	38,670	85,402
Deferred rent receivable	3,237	4,241
Intangible assets on real estate acquisitions, net	0	338
Deferred leasing costs, net	319	3,636
Lease incentives, net	0	1,037
Assets held for sale, net	$ 42,226	$ 94,654
Held-for-sale [Member] \| Regional Office and Other [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		8
Held-for-sale [Member] \| Defense/Information Technology Sector [Member] \| Northern Virginia Defense/IT [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of land parcels held-for-sale \| Property		1

Properties, Net (Details 4)

Properties, Net (Details 4) $ in Thousands	Jul. 21, 2016USD ($)	Aug. 28, 2015USD ($)	Dec. 31, 2017USD ($)ft²builldingProperty	Dec. 31, 2016USD ($)ft²builldingProperty	Dec. 31, 2015USD ($)ft²builldingProperty	Oct. 27, 2017USD ($)ft²
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Increase in property and redeemable noncontrolling interests in connection with property contribution by redeemable noncontrolling interests in a joint venture			$ 0	$ 22,600	$ 0
(Loss) gain on early extinguishment of debt			$ (513)	$ (1,110)	$ 85,275
Operating properties [Member] \| 15000 and 15010 Conference Center Drive [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Debt settled through the surrender of real estate assets		$ 150,000
(Loss) gain on early extinguishment of debt		$ 84,800
Newly-constructed properties placed in service [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buildings \| Property			8	6	7
Total Rentable Square Feet \| ft²			1,100,000	639,000	897,000
Newly redeveloped properties placed In service [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buildings \| Property			3	3	2
Total Rentable Square Feet \| ft²			94,000	61,000	170,000
Properties under construction or contractually committed for construction [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buildings \| Property			9
Total Rentable Square Feet \| ft²			1,100,000
Properties under redevelopment [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buildings \| Property			1
Total Rentable Square Feet \| ft²			22,000
Office properties under, or contractually committed for, construction or approved for redevelopment [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buildings \| Property			10
Total Rentable Square Feet \| ft²			1,100,000
Office properties under, or contractually committed for, construction or approved for redevelopment [Member] \| Single-tenant data centers [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buildings \| Property			3
Office properties under, or contractually committed for, construction or approved for redevelopment [Member] \| Properties completed Held-for-future lease [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buildings \| Property			2
Operating office properties [Member] \| Single-tenant data centers [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Transaction Value				$ 44,300
Ownership percentage sold (percent)	50.00%
Increase in property and redeemable noncontrolling interests in connection with property contribution by redeemable noncontrolling interests in a joint venture	$ 147,600
Initial amount borrowed				60,000
Ownership percentage	50.00%
Operating office properties [Member] \| Single-tenant data centers [Member] \| Operating properties [Member] \| Data Center Shells [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on Sale				$ 17,900
Number of properties contributed \| Property				6
Held-for-sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of land held-for-investment			$ 14,300	$ 21,800	$ 18,100
Gains on sale of other land			$ 4,200	$ 7,200	$ 4,000
Held-for-sale [Member] \| Operating properties [Member] \| Defense/Information Technology Sector [Member] \| Data Center Shells [Member] \| 11751 Meadowville Lane [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total Rentable Square Feet \| ft²						193,000
Transaction Value						$ 44,000
Undiscounted indemnification amount						$ 20,000
Disposed of by sale [Member] \| Operating properties [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buildings \| buillding			12	21	9
Total Rentable Square Feet \| ft²			799,000	1,577,000	1,597,000
Transaction Value			$ 125,829	$ 248,738	$ 347,785
Gain on Sale			$ 5,636	$ 15,871	$ 64,033
Disposed of by sale [Member] \| Operating properties [Member] \| Regional Office [Member] \| Arborcrest Corporate Campus [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buildings \| buillding				4
Total Rentable Square Feet \| ft²				654,000
Transaction Value				$ 142,800
Gain on Sale				$ 4,742
Disposed of by sale [Member] \| Operating properties [Member] \| Regional Office [Member] \| 8003 Corporate Drive [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buildings \| buillding				1
Total Rentable Square Feet \| ft²				18,000
Transaction Value				$ 2,400
Gain on Sale				$ 0
Disposed of by sale [Member] \| Operating properties [Member] \| Regional Office [Member] \| 8007, 8013, 8015, 8019 and 8023-8027 Corporate Drive [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buildings \| buillding				5
Total Rentable Square Feet \| ft²				130,000
Transaction Value				$ 14,513
Gain on Sale				$ 1,906
Disposed of by sale [Member] \| Operating properties [Member] \| Regional Office [Member] \| 4940 Campbell Drive [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buildings \| buillding				1
Total Rentable Square Feet \| ft²				50,000
Transaction Value				$ 5,200
Gain on Sale				$ 0
Disposed of by sale [Member] \| Operating properties [Member] \| Regional Office [Member] \| 1550 Westbranch Drive [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buildings \| buillding					1
Total Rentable Square Feet \| ft²					160,000
Transaction Value					$ 27,800
Gain on Sale					$ 0
Disposed of by sale [Member] \| Operating properties [Member] \| Regional Office [Member] \| 13200 Woodland Park Road [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buildings \| buillding					1
Total Rentable Square Feet \| ft²					397,000
Transaction Value					$ 84,000
Gain on Sale					$ 42,515
Disposed of by sale [Member] \| Operating properties [Member] \| Regional Office [Member] \| 9900, 9910, 9920 Franklin Square Drive [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buildings \| buillding					3
Total Rentable Square Feet \| ft²					135,000
Transaction Value					$ 24,150
Gain on Sale					$ 6,468
Disposed of by sale [Member] \| Operating properties [Member] \| Regional Office [Member] \| 9690 Deereco Road and 375 W Padonia Road [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buildings \| buillding					2
Total Rentable Square Feet \| ft²					240,000
Transaction Value					$ 44,500
Gain on Sale					$ 15,050
Disposed of by sale [Member] \| Operating properties [Member] \| Fort Meade/BW Corridor [Member] \| 1341 and 1343 Ashton Road [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buildings \| buillding				2
Total Rentable Square Feet \| ft²				25,000
Transaction Value				$ 2,900
Gain on Sale				$ 848
Disposed of by sale [Member] \| Operating properties [Member] \| Fort Meade/BW Corridor [Member] \| 1302, 1304 & 1306 Concourse Drive [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buildings \| buillding				3
Total Rentable Square Feet \| ft²				299,000
Transaction Value				$ 48,100
Gain on Sale				$ 8,375
Disposed of by sale [Member] \| Operating properties [Member] \| Fort Meade/BW Corridor [Member] \| 1331 Ashton Road [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buildings \| buillding				1
Total Rentable Square Feet \| ft²				29,000
Transaction Value				$ 2,625
Gain on Sale				$ 0
Disposed of by sale [Member] \| Operating properties [Member] \| Fort Meade/BW Corridor [Member] \| 900 Elkridge Landing Road [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buildings \| buillding				1
Total Rentable Square Feet \| ft²				101,000
Transaction Value				$ 7,800
Gain on Sale				$ 0
Disposed of by sale [Member] \| Operating properties [Member] \| Defense/Information Technology Sector [Member] \| Fort Meade/BW Corridor [Member] \| 1334 Ashton Road [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buildings \| buillding			1
Total Rentable Square Feet \| ft²			37,000
Transaction Value			$ 2,300
Gain on Sale			$ 0
Disposed of by sale [Member] \| Operating properties [Member] \| Defense/Information Technology Sector [Member] \| Fort Meade/BW Corridor [Member] \| 7320 Parkway Drive [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buildings \| buillding			1
Total Rentable Square Feet \| ft²			57,000
Transaction Value			$ 7,529
Gain on Sale			$ 831
Disposed of by sale [Member] \| Operating properties [Member] \| Defense/Information Technology Sector [Member] \| Data Center Shells [Member] \| 201 Technology Drive [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buildings \| buillding			1
Total Rentable Square Feet \| ft²			103,000
Transaction Value			$ 29,500
Gain on Sale			$ 3,625
Disposed of by sale [Member] \| Operating properties [Member] \| Defense/Information Technology Sector [Member] \| Northern Virginia Defense/IT [Member] \| 3120 Fairview Park Drive [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buildings \| buillding			1
Total Rentable Square Feet \| ft²			190,000
Transaction Value			$ 39,000
Gain on Sale			$ 0
Disposed of by sale [Member] \| Operating properties [Member] \| Defense/Information Technology Sector [Member] \| Northern Virginia Defense/IT [Member] \| 2900 Towerview Road [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buildings \| buillding				1
Total Rentable Square Feet \| ft²				151,000
Transaction Value				$ 12,100
Gain on Sale				$ 0
Disposed of by sale [Member] \| Operating properties [Member] \| Defense/Information Technology Sector [Member] \| Northern Virginia Defense/IT [Member] \| 15000 and 15010 Conference Center Drive [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buildings \| buillding					2
Total Rentable Square Feet \| ft²					665,000
Transaction Value					$ 167,335
Gain on Sale					$ 0
Disposed of by sale [Member] \| Operating properties [Member] \| Other Segments [Member] \| 1560 A and B Cable Ranch Road [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buildings \| buillding				2
Total Rentable Square Feet \| ft²				120,000
Transaction Value				$ 10,300
Gain on Sale				$ 0
White Marsh, Maryland [Member] \| Disposed of by sale [Member] \| Operating properties [Member] \| Regional Office and Other [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buildings \| buillding			8
Total Rentable Square Feet \| ft²			412,000
Transaction Value			$ 47,500
Gain on Sale			$ 1,180

Properties, Net (Details 5)

Properties, Net (Details 5) $ / shares in Units, $ in Thousands	Aug. 07, 2015USD ($)ft²	Apr. 15, 2015USD ($)ft²	Mar. 19, 2015USD ($)ft²	Dec. 31, 2017USD ($)ft²Property	Dec. 31, 2016USD ($)ft²Property	Dec. 31, 2015USD ($)ft²Property$ / shares
Business Combination
Weighted average amortization period of intangible assets				11 years
Pro Forma Information
Pro forma total revenues						$ 641,982
Pro forma net income attributable to COPT common shareholders						$ 167,079
Basic (in dollars per share) \| $ / shares						$ 1.77
Diluted (in dollars per share) \| $ / shares						$ 1.77
Tenant relationship value [Member]
Business Combination
Weighted average amortization period of intangible assets				8 years
Below-market cost arrangements [Member]
Business Combination
Weighted average amortization period of intangible assets				34 years
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				$ 38,100	$ 36,900	$ 20,200
Net income from continuing operations since acquisition date				$ 1,900	2,200	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] \| In-place lease value [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
Newly-constructed properties placed in service [Member]
Business Acquisition [Line Items]
Number of real estate properties \| Property				8	6	7
Square footage of real estate properties (in square feet) \| ft²				1,100,000	639,000	897,000
Newly redeveloped properties placed In service [Member]
Business Acquisition [Line Items]
Number of real estate properties \| Property				3	3	2
Square footage of real estate properties (in square feet) \| ft²				94,000	61,000	170,000
Properties under construction or contractually committed for construction [Member]
Business Acquisition [Line Items]
Number of real estate properties \| Property				9
Square footage of real estate properties (in square feet) \| ft²				1,100,000
Properties under redevelopment [Member]
Business Acquisition [Line Items]
Number of real estate properties \| Property				1
Square footage of real estate properties (in square feet) \| ft²				22,000
Office properties under, or contractually committed for, construction or approved for redevelopment [Member]
Business Acquisition [Line Items]
Number of real estate properties \| Property				10
Square footage of real estate properties (in square feet) \| ft²				1,100,000
Office properties under, or contractually committed for, construction or approved for redevelopment [Member] \| Properties completed Held-for-future lease [Member]
Business Acquisition [Line Items]
Number of real estate properties \| Property				2
Regional Office [Member] \| 250 W Pratt St [Member] \| Baltimore, Maryland [Member]
Business Acquisition [Line Items]
Square footage of real estate properties (in square feet) \| ft²			367,000
Acquired property percentage leased			96.20%
Purchase price			$ 61,800
Regional Office [Member] \| 100 and 30 Light Street [Member] \| Baltimore, Maryland [Member]
Business Acquisition [Line Items]
Square footage of real estate properties (in square feet) \| ft²	558,000
Acquired property percentage leased	93.50%
Purchase price	$ 121,200
Liabilities incurred	55,000
Fair value assumption of debt	$ 55,500
Northern Virginia [Member] \| Defense/Information Technology Sector [Member] \| 2600 Park Tower Drive [Member] \| Vienna, Virginia [Member]
Business Acquisition [Line Items]
Square footage of real estate properties (in square feet) \| ft²		237,000
Acquired property percentage leased		100.00%
Purchase price		$ 80,500

Real Estate Joint Ventures (Det

Real Estate Joint Ventures (Details) $ in Thousands	Jul. 21, 2016USD ($)	Jan. 31, 2016USD ($)	Dec. 31, 2017USD ($)ft²	Dec. 31, 2016USD ($)Property	Dec. 31, 2015USD ($)
Investments in consolidated real estate joint ventures
Increase in property and redeemable noncontrolling interests in connection with property contribution by redeemable noncontrolling interests in a joint venture			$ 0	$ 22,600	$ 0
Distributions paid to redeemable noncontrolling interests			8,215	15,206	$ 2,993
Non-real estate equity method investments			2,412	$ 2,355
Consolidated real estate joint ventures [Member]
Investments in consolidated real estate joint ventures
Total Assets			303,983
Encumbered Assets			120,953
Total Liabilities			116,830
Notes receivable from City of Huntsville [Member]
Investments in consolidated real estate joint ventures
Amount of advanced			37,800
Single-tenant data centers [Member] \| Operating office properties [Member]
Investments in consolidated real estate joint ventures
Increase in property and redeemable noncontrolling interests in connection with property contribution by redeemable noncontrolling interests in a joint venture	$ 147,600
Ownership percentage sold (percent)	50.00%
Ownership percentage	50.00%
Data Center Shells [Member] \| Operating properties, net [Member] \| Single-tenant data centers [Member] \| Operating office properties [Member]
Investments in consolidated real estate joint ventures
Number of properties contributed \| Property				6
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			674,000
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%
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%
M Square Associates, LLC [Member]
Investments in consolidated real estate joint ventures
Percentage of residual amounts distributed to each member			50.00%
GI-COPT [Member]
Investments in consolidated real estate joint ventures
Non-real estate equity method investments			$ 25,100	$ 25,500
Difference between cost basis and share of underlying equity			$ 16,700	18,100
LW Redstone Company, LLC [Member] \| Primary Beneficiary [Member]
Investments in consolidated real estate joint ventures
Ownership (as a percent)			85.00%
Total Assets			$ 158,891
Encumbered Assets			75,569
Total Liabilities			$ 51,180
M Square Associates, LLC [Member] \| Primary Beneficiary [Member]
Investments in consolidated real estate joint ventures
Ownership (as a percent)			50.00%
Total Assets			$ 73,116
Encumbered Assets			45,384
Total Liabilities			$ 45,745
Stevens Investors, LLC [Member] \| Primary Beneficiary [Member]
Investments in consolidated real estate joint ventures
Ownership (as a percent)			95.00%
Total Assets			$ 71,976
Encumbered Assets			0
Total Liabilities			19,905
Increase in property and redeemable noncontrolling interests in connection with property contribution by redeemable noncontrolling interests in a joint venture		$ 22,600
Distributions paid to redeemable noncontrolling interests			$ 6,700	$ 13,400

Intangible Assets on Real Est64

Intangible Assets on Real Estate Acquisitions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Intangible assets on real estate acquisitions
Gross Carrying Amount	$ 222,683	$ 223,054
Accumulated Amortization	163,591	144,703
Net Carrying Amount	59,092	78,351
Amortization of the intangible assets	$ 19,300	20,000	$ 18,500
Weighted average amortization period of intangible assets	11 years
Estimated future amortization expense associated with the intangible asset categories for the next five years
2,018	$ 14,500
2,019	8,100
2,020	5,700
2,021	5,500
2,022	4,100
In-place lease value [Member]
Intangible assets on real estate acquisitions
Gross Carrying Amount	132,276	132,647
Accumulated Amortization	110,814	99,940
Net Carrying Amount	$ 21,462	32,707
Weighted average amortization period of intangible assets	6 years
Tenant relationship value [Member]
Intangible assets on real estate acquisitions
Gross Carrying Amount	$ 60,028	60,028
Accumulated Amortization	32,198	26,253
Net Carrying Amount	$ 27,830	33,775
Weighted average amortization period of intangible assets	8 years
Below-market cost arrangements [Member]
Intangible assets on real estate acquisitions
Gross Carrying Amount	$ 15,102	15,102
Accumulated Amortization	7,507	7,285
Net Carrying Amount	$ 7,595	7,817
Weighted average amortization period of intangible assets	34 years
Above-market leases [Member]
Intangible assets on real estate acquisitions
Gross Carrying Amount	$ 13,944	13,944
Accumulated Amortization	12,092	10,259
Net Carrying Amount	$ 1,852	3,685
Weighted average amortization period of intangible assets	3 years
Other [Member]
Intangible assets on real estate acquisitions
Gross Carrying Amount	$ 1,333	1,333
Accumulated Amortization	980	966
Net Carrying Amount	$ 353	$ 367
Weighted average amortization period of intangible assets	25 years

Investing Receivables (Details)

Investing Receivables (Details) - USD ($) $ in Thousands	12 Months Ended
Investing Receivables (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Investing receivables	$ 57,493	$ 52,279
Notes receivable from City of Huntsville [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Investing receivables	$ 54,472	49,258
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	$ 3,021	$ 3,021

Prepaid Expenses and Other As66

Prepaid Expenses and Other Assets, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Prepaid Expense and Other Assets [Abstract]
Prepaid expenses	$ 24,670	$ 24,432
Lease incentives, net	19,011	18,276
Furniture, fixtures and equipment, net	5,256	5,204
Construction contract costs incurred in excess of billings	4,884	10,350
Interest rate derivatives	3,073	158
Non-real estate equity method investments	2,412	2,355
Deferred tax asset, net	1,892	3,036
Deferred financing costs, net	1,202	3,128
Other assets	4,821	5,825
Prepaid expenses and other assets, net	$ 67,221	$ 72,764

Prepaid Expenses and Other As67

Prepaid Expenses and Other Assets, Net (Details 2) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Tax Credit Carryforward [Line Items]
Deferred tax asset, net	$ 1,892	$ 3,036
TRS [Member]
Tax Credit Carryforward [Line Items]
Operating loss carry forward	3,209	5,084
Share-based compensation	7	13
Accrued payroll	49	101
Property	43
Property		(100)
Valuation allowance	(1,416)	(2,062)
Deferred tax asset, net	$ 1,892	$ 3,036

Debt, Net (Details)

Debt, Net (Details) - USD ($) $ in Thousands	12 Months Ended
Debt, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Jun. 29, 2015
Debt
Debt, net	$ 1,828,333	$ 1,904,001
Deferred financing costs, net	1,202	3,128
Revolving Credit Facility [Member]
Debt
Debt, net	126,000	0
Term Loan Facilities [Member]
Debt
Debt, net	$ 347,959	547,494
Weighted average interest rate (as a percent)	2.73%
Aggregate additional borrowing capacity	$ 350,000
Unsecured notes payable [Member]
Debt
Debt, net	$ 1,287	1,401
Stated interest rate (as a percent)	0.00%
Unamortized discount included in carrying value	$ 373	460
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)	1.85%
Mortgage and other secured debt [Member]
Debt
Debt, net	$ 163,838	167,591
Mortgage and other secured debt [Member] \| Fixed rate mortgage loans [Member]
Debt
Debt, net	150,723	154,143
Unamortized premium included in carrying value	$ 349	422
Weighted average interest rate (as a percent)	4.19%
Mortgage and other secured debt [Member] \| Fixed rate mortgage loans [Member] \| Minimum [Member]
Debt
Stated interest rate (as a percent)	3.82%
Mortgage and other secured debt [Member] \| Fixed rate mortgage loans [Member] \| Maximum [Member]
Debt
Stated interest rate (as a percent)	7.87%
Mortgage and other secured debt [Member] \| Variable rate secured loans [Member]
Debt
Debt, net	$ 13,115	13,448
Stated interest rate (as a percent)	3.21%
Mortgage and other secured debt [Member] \| London Interbank Offered Rate (LIBOR) [Member] \| Variable rate secured loans [Member]
Debt
Variable rate, spread (as a percent)	1.85%
Senior Notes [Member] \| 3.60% Senior Notes [Member]
Debt
Debt, net	$ 347,551	347,128
Stated interest rate (as a percent)	3.60%
Interest rate on debt (as a percent)	3.70%
Unamortized discount included in carrying value	$ 1,700	2,000
Senior Notes [Member] \| 5.250% Senior Notes [Member]
Debt
Debt, net	$ 246,645	246,176
Stated interest rate (as a percent)	5.25%
Interest rate on debt (as a percent)	5.49%
Unamortized discount included in carrying value	$ 3,000	3,400
Senior Notes [Member] \| 3.70% Senior Notes [Member]
Debt
Debt, net	$ 298,322	297,843
Stated interest rate (as a percent)	3.70%
Interest rate on debt (as a percent)	3.85%
Unamortized discount included in carrying value	$ 1,300	1,700
Senior Notes [Member] \| 5.0% Senior Notes [Member]
Debt
Debt, net	$ 296,731	296,368
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	$ 2,700	3,000
Loans Payable [Member]
Debt
Deferred financing costs, net	$ 5,000	$ 6,100

Debt, Net (Details 2)

Debt, Net (Details 2) - USD ($) $ in Thousands	12 Months Ended
Debt, Net (Details 2) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule on basis of which debt matures
2,018	$ 4,241
2,019	130,387
2,020	116,156
2,021	303,875
2,022	254,033
Thereafter	1,033,475
Total	1,842,167
Net discounts and deferred financing costs	13,800
Capitalized interest costs	$ 5,200	$ 5,700	$ 7,200

Debt, Net (Details 3)

Debt, Net (Details 3) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Carrying Amount [Member]
Debt Instrument [Line Items]
Long-term debt	$ 1,828,333	$ 1,904,001
Carrying Amount [Member] \| Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Long-term debt	1,189,249	1,187,515
Carrying Amount [Member] \| Other fixed-rate debt [Member]
Debt Instrument [Line Items]
Long-term debt	152,010	155,544
Carrying Amount [Member] \| Variable-rate debt [Member]
Debt Instrument [Line Items]
Long-term debt	487,074	560,942
Estimated Fair Value [Member]
Debt Instrument [Line Items]
Long-term debt	1,867,577	1,935,606
Estimated Fair Value [Member] \| Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Long-term debt	1,229,398	1,220,282
Estimated Fair Value [Member] \| Other fixed-rate debt [Member]
Debt Instrument [Line Items]
Long-term debt	152,485	156,887
Estimated Fair Value [Member] \| Variable-rate debt [Member]
Debt Instrument [Line Items]
Long-term debt	$ 485,694	$ 558,437

Debt, Net (Details 4)

Debt, Net (Details 4)	May 01, 2017USD ($)	Dec. 28, 2016USD ($)	Dec. 17, 2015USD ($)	Aug. 28, 2015USD ($)	Jun. 29, 2015USD ($)	May 06, 2015USD ($)	Feb. 14, 2012USD ($)	Dec. 31, 2017USD ($)extension	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Debt Instrument [Line Items]
(Loss) gain on early extinguishment of debt								$ 513,000	$ 1,110,000	$ (85,275,000)
Term Credit Facility Effective February 2012 [Member]
Debt Instrument [Line Items]
Initial amount borrowed							$ 250,000,000
Term Loan Facilities Maturing 2020 [Member]
Debt Instrument [Line Items]
Repayments of term loan	$ 200,000,000
Term Credit Facility Effective August 2012 [Member]
Debt Instrument [Line Items]
Repayments of debt									120,000,000
Term Credit Facility Effective September 2011 [Member]
Debt Instrument [Line Items]
Repayments of debt										$ 150,000,000
Fixed rate mortgage loans [Member]
Debt Instrument [Line Items]
Debt settled through the surrender of real estate assets				$ 150,000,000
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%
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%
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
Line of credit facility, extension fee percentage								0.075%
Remaining borrowing capacity								$ 674,000,000
Weighted average borrowings								$ 97,800,000	$ 90,300,000
Weighted average interest rate (as a percent)								2.44%	1.64%
Line of Credit [Member] \| Term Credit Facility Effective February 2012 [Member]
Debt Instrument [Line Items]
Maximum availability						$ 300,000,000
Increase to maximum borrowing capacity						$ 200,000,000
Line of Credit [Member] \| Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Number of extensions \| extension								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%
Unsecured Debt [Member] \| Term Credit Facility Effective December 2015 [Member]
Debt Instrument [Line Items]
Maximum availability			$ 250,000,000
Remaining borrowing capacity								$ 150,000,000
Initial amount borrowed			$ 100,000,000
Unsecured Debt [Member] \| Term Credit Facility Effective December 2016 [Member]
Debt Instrument [Line Items]
Initial amount borrowed		$ 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)			0.90%
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)			1.75%
Senior Notes [Member]
Debt Instrument [Line Items]
Redemption price (percent)								100.00%
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								$ 2,700,000	$ 3,000,000
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								0.45%
Senior Notes [Member] \| 3.70% Senior Notes [Member]
Debt Instrument [Line Items]
Stated interest rate (as a percent)								3.70%
Unamortized discount included in carrying value								$ 1,300,000	1,700,000
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								0.25%
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								$ 1,700,000	2,000,000
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								0.30%
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								$ 3,000,000	$ 3,400,000
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								0.40%
Operating properties, net [Member] \| 15000 and 15010 Conference Center Drive [Member]
Debt Instrument [Line Items]
(Loss) gain 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	May 01, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair value of interest rate derivatives and balance sheet classification
Interest rate swaps designated as cash flow hedges		$ 3,073	$ 158
Effect of interest rate derivatives on consolidated statements of operations and comprehensive income
Unrealized gain (loss) recognized in AOCL (effective portion)		684	(2,915)	$ (4,739)
Interest rate swaps
Effect of interest rate derivatives on consolidated statements of operations and comprehensive income
Unrealized gain (loss) recognized in AOCL (effective portion)		684	(2,915)	(4,739)
Approximate amount of losses to be reclassified from AOCL to interest expense over the next 12 months		460
Interest rate swaps \| Interest expense [Member]
Effect of interest rate derivatives on consolidated statements of operations and comprehensive income
Loss reclassified from AOCL into interest expense (effective portion)		(3,216)	(4,230)	(3,599)
Gain (loss) on derivatives recognized in interest expense (ineffective portion)		323	378	(386)
Loss reclassified from AOCL into interest expense (ineffective portion)		(88)	0	$ 0
Interest rate swaps \| Prepaid expenses and other assets [Member]
Fair value of interest rate derivatives and balance sheet classification
Interest rate swaps designated as cash flow hedges		3,073	158
Interest rate swaps \| Other liabilities [Member]
Fair value of interest rate derivatives and balance sheet classification
Interest rate swaps designated as cash flow hedges		0	(1,572)
Designated [Member]
Fair values of interest rate swap derivatives
Fair value of interest rate swaps		3,073	(1,414)
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		$ 252	(848)
Designated [Member] \| Interest rate swap, effective October 13, 2015 [Member]
Fair values of interest rate swap derivatives
Notional Amount		$ 13,217
Fixed Rate (as a percent)		1.39%
Fair value of interest rate swaps		$ 213	100
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		$ 1,046	(23)
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		$ 1,051	48
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		$ 511	10
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		$ 0	$ (701)
Interest rate swap cash settled	$ 460

Redeemable Noncontrolling Int73

Redeemable Noncontrolling Interests (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)joint_venture	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Noncontrolling Interest [Abstract]
Number of joint ventures with redeemable noncontrolling interest \| joint_venture	2
Redeemable Noncontrolling Interest [Roll Forward]
Beginning balance	$ 22,979	$ 19,218	$ 18,417
Contributions from noncontrolling interests	0	22,779	1,654
Distributions to noncontrolling interests	(1,566)	(21,881)	(2,964)
Net income attributable to noncontrolling interests	2,338	2,242	2,227
Adjustment to arrive at fair value of interests	(626)	621	(116)
Ending balance	$ 23,125	$ 22,979	$ 19,218

Equity - COPT and Subsidiaries

Equity - COPT and Subsidiaries (Details) - USD ($)	Dec. 27, 2017	Nov. 02, 2017	Jun. 27, 2017	Jan. 21, 2017	Dec. 31, 2017	Sep. 30, 2016	Oct. 31, 2012	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Preferred Stock
Stock redeemed or called during period value									$ 172,500,000	$ 26,583,000
Common Shares
Common shares issued under forward equity sale agreements									$ 49,944,000
Number of common shares for each converted common unit (in shares)									1
Number of operating partnerships units converted into common shares (in units)									339,513	87,000	160,160
Dividends declared per common share (in dollars per share)									$ 1.1	$ 1.10	$ 1.10
Common Stock Issued to Public Under At-the-Market Program [Member]
Common Shares
At-market-stock, offering program established, aggregate value						$ 200,000,000	$ 150,000,000
Shares issued to the public								3,720,000	591,042	3,720,000	890,241
Net proceeds from shares issued								$ 109,100,000	$ 19,700,000		$ 26,600,000
Payments of stock issuance costs								$ 900,000	300,000	$ 900,000	$ 400,000
At-market stock, offering program established, remaining capacity					$ 70,000,000				$ 70,000,000
Issuance of stock, weighted average price per share (in dollars per share)					$ 33.84			$ 29.56	$ 33.84	$ 29.56	$ 30.29
Common Shares [Member]
Common Shares
Common shares issued under forward equity sale agreements									$ 17,000
Forward Equity Sale Agreement [Member]
Common Shares
Shares issued to the public	1,700,000				1,700,000
Forward equity sale agreements number of shares available for issuance		9,200,000
Net proceeds from issue of shares after underwriter discounts but before offering expenses		$ 285,200,000
Initial Gross Offering Price Per Share		$ 31
Common shares issued under forward equity sale agreements	$ 50,000,000				$ 50,000,000
Preferred Shares [Member]
Preferred Stock
Number of preferred shares authorized					25,000,000				25,000,000
Number of preferred shares of beneficial interest authorized (in dollars per share)					$ 0.01				$ 0.01
Series K Preferred Shares [Member]
Preferred Stock
Annual dividend yield				5.60%
Issuance costs associated with redeemed preferred shares								$ 17,000
Series L [Member]
Preferred Stock
Annual dividend yield			7.375%
Issuance costs associated with redeemed preferred shares									$ 6,800,000

Equity - COPLP and Subsidiari75

Equity - COPLP and Subsidiaries (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 27, 2017	Jun. 27, 2017	Jan. 21, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Preferred Stock
Stock redeemed or called during period value						$ 172,500	$ 26,583
Preferred units in COPLP				$ 8,800	$ 8,800	8,800	$ 8,800
Common Shares
Common shares issued under forward equity sale agreements						$ 49,944
Number of common shares for each converted common unit (in shares)						1
Number of operating partnerships units converted into common shares (in units)						339,513	87,000	160,160
Dividends declared per common share (in dollars per share)						$ 1.1	$ 1.10	$ 1.10
Common Stock Issued to Public Under At-the-Market Program [Member]
Common Shares
Shares issued to the public					3,720,000	591,042	3,720,000	890,241
Issuance of stock, weighted average price per share (in dollars per share)				$ 33.84	$ 29.56	$ 33.84	$ 29.56	$ 30.29
Net proceeds from issue of shares after underwriter discounts but before offering expenses						$ 19,700	$ 109,100	$ 26,600
Payments of stock issuance costs					$ 900	300	900	$ 400
Forward Equity Sale Agreement [Member]
Common Shares
Shares issued to the public	1,700,000			1,700,000
Common shares issued under forward equity sale agreements	$ 50,000			$ 50,000
Series K Preferred Shares [Member]
Preferred Stock
Annual dividend yield			5.60%
Issuance costs associated with redeemed preferred shares					17
Series L [Member]
Preferred Stock
Annual dividend yield		7.375%
Issuance costs associated with redeemed preferred shares						6,800
Corporate Office Properties, L.P. [Member]
Preferred Stock
Stock redeemed or called during period value						172,500	$ 26,583
Common Shares
Common shares issued under forward equity sale agreements						$ 49,944
Number of common shares for each converted common unit (in shares)						1
Number of operating partnerships units converted into common shares (in units)						339,513	87,000	160,160
Dividends declared per common share (in dollars per share)						$ 1.1	$ 1.10	$ 1.10
Corporate Office Properties, L.P. [Member] \| Common Units [Member]
Common Shares
Percentage ownership in operating partnership						96.90%	96.50%
Corporate Office Properties, L.P. [Member] \| Conversion of Series I preferred units [Member]
Preferred Stock
Annual dividend yield						7.50%
Limited partners' capital account, units issued				352,000		352,000
Preferred units in COPLP				$ 8,800		$ 8,800
Preferred stock, liquidation preference per share				$ 25		$ 25
Annual cumulative preferred return increment frequency						5 years
Common units conversion basis units issuable						0.5
Corporate Office Properties, L.P. [Member] \| Series K Preferred Shares [Member]
Preferred Stock
Annual dividend yield			5.60%
Preferred stock, redemption price per share (in dollars per share)			$ 50
Stock redeemed or called during period value			$ 26,600
Issuance costs associated with redeemed preferred shares					$ 17
Corporate Office Properties, L.P. [Member] \| Series L [Member]
Preferred Stock
Annual dividend yield		7.375%
Preferred stock, redemption price per share (in dollars per share)		$ 25
Stock redeemed or called during period value		$ 172,500
Issuance costs associated with redeemed preferred shares						$ 6,800

Share-Based Compensation and 76

Share-Based Compensation and Other Compensation Matters (Details 1) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-Based Compensation
Share-based compensation cost	$ 6,095	$ 7,453	$ 7,398
General, adminstrative and leasing expenses [Member]
Share-Based Compensation
Share-based compensation cost	4,649	5,816	5,574
Property operating expenses [Member]
Share-Based Compensation
Share-based compensation cost	966	1,027	1,000
Capitalized to development activities [Member]
Share-Based Compensation
Share-based compensation cost	$ 480	$ 610	$ 824

Share-Based Compensation and 77

Share-Based Compensation and Other Compensation Matters (Details 2) - Restricted shares [Member] - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Shares
Unvested at the beginning of the period (in shares)	371,247	378,200	390,507
Stock awards granted (in shares or units)	239,479	231,937	201,024
Forfeited (in shares)	(27,056)	(22,907)	(10,550)
Vested (in shares)	(158,044)	(215,983)	(202,781)
Unvested at the end of the period (in shares)	425,626	371,247	378,200
Restricted shares expected to vest (in shares)	402,870
Weighted Average Grant Date Fair Value
Unvested at the beginning of the period (in dollars per share)	$ 26.20	$ 27.58	$ 26.19
Grant date fair value (in dollars per share)	33.84	24.77	28.69
Forfeited (in dollars per share)	27.80	25.31	26.05
Vested (in dollars per share)	26.27	27.19	26.07
Unvested at the end of the period (in dollars per share)	30.37	$ 26.20	$ 27.58
Restricted shares expected to vest (in dollars per share)	$ 30.31

Share-Based Compensation and 78

Share-Based Compensation and Other Compensation Matters (Details 3) - Performance share units [Member] - USD ($) $ / shares in Units, $ in Thousands	Jan. 01, 2017	Mar. 01, 2016	Mar. 05, 2015	Mar. 06, 2014	Mar. 01, 2013	Dec. 31, 2017
2013 PSU Grants [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock awards granted (in shares or units)					69,579
Grant date fair value					$ 1,867
Unvested at the end of the period (in shares)						0
2014 PSU Grants [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock awards granted (in shares or units)				49,103
Grant date fair value				$ 1,723
Unvested at the end of the period (in shares)						0
2015 PSU Grants [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock awards granted (in shares or units)			45,656
Grant date fair value			$ 1,678
Unvested at the end of the period (in shares)						15,767
Grant date fair value (in dollars per share)			$ 36.76
2016 PSU Grants [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock awards granted (in shares or units)		26,299
Grant date fair value		$ 1,000
Unvested at the end of the period (in shares)						24,850
Grant date fair value (in dollars per share)		$ 38.21
2017 PSU Grants [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock awards granted (in shares or units)	39,351
Grant date fair value	$ 1,400
Unvested at the end of the period (in shares)						39,351
Grant date fair value (in dollars per share)	$ 38.43

Share-Based Compensation and 79

Share-Based Compensation and Other Compensation Matters (Details 4) - Performance share units [Member] - $ / shares	Jan. 01, 2017	Mar. 01, 2016	Mar. 05, 2015	Dec. 31, 2017
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%
2015 PSU Grants [Member]
Assumptions used to value stock awards
Grant date fair value (in dollars per share)			$ 36.76
Baseline Common Share Value (in dollars per share)			$ 29.28
Expected Volatility of Common Shares			19.90%
Risk-free Interest Rate			0.99%
2016 PSU Grants [Member]
Assumptions used to value stock awards
Grant date fair value (in dollars per share)		$ 38.21
Baseline Common Share Value (in dollars per share)		$ 23.90
Expected Volatility of Common Shares		20.40%
Risk-free Interest Rate		0.96%
2017 PSU Grants [Member]
Assumptions used to value stock awards
Grant date fair value (in dollars per share)	$ 38.43
Baseline Common Share Value (in dollars per share)	$ 31.22
Expected Volatility of Common Shares	19.00%
Risk-free Interest Rate	1.47%

Share-Based Compensation and 80

Share-Based Compensation and Other Compensation Matters (Details 5) - Deferred Share Award [Member] - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Deferred Share Awards Granted (in shares or units)	10,032	24,944	24,056
Aggregate Grant Date Fair Value	$ 326	$ 671	$ 642
Grant Date Fair Value Per Share (in dollars per share)	$ 32.47	$ 26.89	$ 26.70
Number of common shares issued (in shares)	15,590	12,028	15,485
Grant date fair value (in dollars per share)	$ 26.89	$ 26.70	$ 26.77
Aggregate intrinsic value	$ 508	$ 322	$ 413

Share-Based Compensation and 81

Share-Based Compensation and Other Compensation Matters (Details 6) - Options [Member] - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options Outstanding and Exercisable (in shares)	60,000	201,100	425,347	559,736
Weighted Average Exercise Price Per Share (in dollars per share)	$ 35.17	$ 43.35	$ 42.75	$ 39.60
Weighted Average Remaining Contractual Term (years)	1 year	1 year	1 year	2 years
Aggregate Intrinsic Value	$ 0	$ 31	$ 0	$ 167

Share-Based Compensation and 82

Share-Based Compensation and Other Compensation Matters (Details 7) $ / shares in Units, $ in Thousands	Feb. 07, 2017shares	Jan. 01, 2017Percentile_Rank$ / shares	Oct. 30, 2016shares	Jul. 12, 2016shares	May 30, 2016shares	Mar. 05, 2015shares	Dec. 31, 2017USD ($)$ / shares	Dec. 31, 2016USD ($)$ / shares	Dec. 31, 2015USD ($)$ / shares	May 31, 2017shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Decrease in tax benefit from share-based compensation							$ 13	$ 331	$ 513
Executive transition costs								6,500
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period							10 years
Performance share units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Estimated pre-vesting forfeitures							0.00%
Unrecognized compensation cost							$ 1,300
Expected weighted average period during which unrecognized compensation cost will be recognized							2 years
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							$ 236
Deferred Share Award [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Estimated pre-vesting forfeitures							0.00%
Unrecognized compensation cost							$ 120
Aggregate intrinsic value							$ 508	$ 322	$ 413
Grant date fair value (in dollars per share) \| $ / shares							$ 32.47	$ 26.89	$ 26.70
Restricted shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost							$ 8,100
Expected weighted average period during which unrecognized compensation cost will be recognized							3 years
Aggregate intrinsic value							$ 5,300	$ 5,400	$ 4,900
Grant date fair value (in dollars per share) \| $ / shares							$ 33.84	$ 24.77	$ 28.69
Restricted shares [Member] \| Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Estimated pre-vesting forfeitures							0.00%
Restricted shares [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Estimated pre-vesting forfeitures							5.00%
Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate intrinsic value of options exercised							$ 18		$ 300
2017 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										3,400,000
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
2014 and 2015 Performance Share Unit Grants [Member] \| Performance share units [Member] \| Executive Vice President [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued for PSU Awards Vested in Period (in shares) \| shares					10,326
2014 and 2015 Performance Share Unit Grants [Member] \| Performance share units [Member] \| Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued for PSU Awards Vested in Period (in shares) \| shares				20,569
2014, 2015 and 2016 Performance Share Unit Grants [Member] \| Performance share units [Member] \| General Counsel [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued for PSU Awards Vested in Period (in shares) \| shares			2,248
2017 PSU Grants [Member] \| Performance share units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant date fair value (in dollars per share) \| $ / shares		$ 38.43
2014 PSU Grants [Member] \| Performance share units [Member] \| Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued for PSU Awards Vested in Period (in shares) \| shares	9,763

Operating Leases (Details)

Operating Leases (Details) $ in Thousands	Dec. 31, 2017USD ($)
Gross minimum future rentals
2,018	$ 372,420
2,019	329,760
2,020	260,238
2,021	207,727
2,022	175,123
Thereafter	484,444
Total	$ 1,829,712

Information by Business Segme84

Information by Business Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment financial information for real estate operations
Revenues from real estate operations	$ 509,980	$ 525,964	$ 519,068
Property operating expenses	(190,964)	(197,530)	(194,488)
UJV NOI allocable to COPT	5,188	2,305	0
NOI from real estate operations	324,204	330,739	324,580
Additions to long-lived assets	73,341	70,324	335,269
Transfers from non-operating properties	202,211	184,640	307,120
Segment assets	3,578,484	3,780,885
Defense/Information Technology Sector [Member]
Segment financial information for real estate operations
Revenues from real estate operations	408,122	406,210	394,283
Property operating expenses	(146,558)	(148,705)	(144,444)
UJV NOI allocable to COPT	5,188	2,305
NOI from real estate operations	266,752	259,810	249,839
Additions to long-lived assets	44,352	57,131	130,670
Transfers from non-operating properties	202,185	184,943	194,647
Defense/Information Technology Sector [Member] \| Fort Meade/BW Corridor [Member]
Segment financial information for real estate operations
Revenues from real estate operations	245,613	245,354	244,274
Property operating expenses	(80,697)	(83,684)	(83,309)
UJV NOI allocable to COPT	0	0
NOI from real estate operations	164,916	161,670	160,965
Additions to long-lived assets	26,659	26,267	31,883
Transfers from non-operating properties	43,370	49,937	45,560
Defense/Information Technology Sector [Member] \| Northern Virginia Defense/IT [Member]
Segment financial information for real estate operations
Revenues from real estate operations	47,118	48,964	49,199
Property operating expenses	(16,938)	(17,824)	(20,107)
UJV NOI allocable to COPT	0	0
NOI from real estate operations	30,180	31,140	29,092
Additions to long-lived assets	8,115	17,344	90,248
Transfers from non-operating properties	48,328	28,230	50,690
Defense/Information Technology Sector [Member] \| Lackland Air Force Base [Member]
Segment financial information for real estate operations
Revenues from real estate operations	47,209	46,803	39,659
Property operating expenses	(27,812)	(27,357)	(22,004)
UJV NOI allocable to COPT	0	0
NOI from real estate operations	19,397	19,446	17,655
Additions to long-lived assets	71	0	0
Transfers from non-operating properties	0	240	32,307
Defense/Information Technology Sector [Member] \| Navy Support Locations [Member]
Segment financial information for real estate operations
Revenues from real estate operations	29,540	28,197	28,177
Property operating expenses	(12,619)	(12,690)	(13,229)
UJV NOI allocable to COPT	0	0
NOI from real estate operations	16,921	15,507	14,948
Additions to long-lived assets	8,451	9,168	7,656
Transfers from non-operating properties	474	0	1,408
Defense/Information Technology Sector [Member] \| Redstone Arsenal [Member]
Segment financial information for real estate operations
Revenues from real estate operations	14,322	13,056	11,228
Property operating expenses	(5,783)	(4,476)	(3,497)
UJV NOI allocable to COPT	0	0
NOI from real estate operations	8,539	8,580	7,731
Additions to long-lived assets	1,056	4,352	883
Transfers from non-operating properties	2,159	3,169	13,190
Defense/Information Technology Sector [Member] \| Data Center Shells [Member]
Segment financial information for real estate operations
Revenues from real estate operations	24,320	23,836	21,746
Property operating expenses	(2,709)	(2,674)	(2,298)
UJV NOI allocable to COPT	5,188	2,305
NOI from real estate operations	26,799	23,467	19,448
Additions to long-lived assets	0	0	0
Transfers from non-operating properties	107,854	103,367	51,492
Regional Office [Member]
Segment financial information for real estate operations
Revenues from real estate operations	68,262	85,805	98,165
Property operating expenses	(28,982)	(34,095)	(36,165)
UJV NOI allocable to COPT	0	0
NOI from real estate operations	39,280	51,710	62,000
Additions to long-lived assets	25,299	12,559	204,139
Transfers from non-operating properties	0	82	22,313
Operating wholesale data centers [Member]
Segment financial information for real estate operations
Revenues from real estate operations	28,875	26,869	19,032
Property operating expenses	(13,551)	(11,512)	(10,402)
UJV NOI allocable to COPT	0	0
NOI from real estate operations	15,324	15,357	8,630
Additions to long-lived assets	3,580	299	132
Transfers from non-operating properties	8	(377)	89,745
Other [Member]
Segment financial information for real estate operations
Revenues from real estate operations	4,721	7,080	7,588
Property operating expenses	(1,873)	(3,218)	(3,477)
UJV NOI allocable to COPT	0	0
NOI from real estate operations	2,848	3,862	4,111
Additions to long-lived assets	110	335	328
Transfers from non-operating properties	18	(8)	415
Segment assets [Member]
Segment financial information for real estate operations
Segment assets	3,011,284	3,004,247	3,265,469
Segment assets [Member] \| Defense/Information Technology Sector [Member]
Segment financial information for real estate operations
Segment assets	2,382,268	2,308,189	2,342,746
Segment assets [Member] \| Defense/Information Technology Sector [Member] \| Fort Meade/BW Corridor [Member]
Segment financial information for real estate operations
Segment assets	1,263,567	1,255,230	1,290,028
Segment assets [Member] \| Defense/Information Technology Sector [Member] \| Northern Virginia Defense/IT [Member]
Segment financial information for real estate operations
Segment assets	402,076	404,438	411,196
Segment assets [Member] \| Defense/Information Technology Sector [Member] \| Lackland Air Force Base [Member]
Segment financial information for real estate operations
Segment assets	128,755	131,957	134,381
Segment assets [Member] \| Defense/Information Technology Sector [Member] \| Navy Support Locations [Member]
Segment financial information for real estate operations
Segment assets	194,476	196,486	196,090
Segment assets [Member] \| Defense/Information Technology Sector [Member] \| Redstone Arsenal [Member]
Segment financial information for real estate operations
Segment assets	108,119	110,395	108,038
Segment assets [Member] \| Defense/Information Technology Sector [Member] \| Data Center Shells [Member]
Segment financial information for real estate operations
Segment assets	285,275	209,683	203,013
Segment assets [Member] \| Regional Office [Member]
Segment financial information for real estate operations
Segment assets	400,512	442,811	608,471
Segment assets [Member] \| Operating wholesale data centers [Member]
Segment financial information for real estate operations
Segment assets	224,422	231,954	243,338
Segment assets [Member] \| Other [Member]
Segment financial information for real estate operations
Segment assets	$ 4,082	$ 21,293	$ 70,914

Information by Business Segme85

Information by Business Segment (Details 2) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of segment revenues to total revenues
Segment revenues from real estate operations									$ 509,980	$ 525,964	$ 519,068
Construction contract and other service revenues									102,840	48,364	106,402
Less: Revenues from discontinued operations									0	0	(4)
Total revenues	$ 164,567	$ 157,017	$ 151,435	$ 139,801	$ 141,991	$ 142,103	$ 145,927	$ 144,307	612,820	574,328	625,466
Reconciliation of segment property operating expenses to property operating expenses
Segment property operating expenses									190,964	197,530	194,488
Less: Property operating expenses from discontinued operations									0	0	6
Total property operating expenses									190,964	197,530	194,494
Reconciliation of Operatiing Income from Unconsolidated Joint Ventures to Equity Method Investments [Abstract]
UJV NOI allocable to COPT									5,188	2,305	0
Less: Income from UJV allocable to COPT attributable to depreciation and amortization expense and interest expense									(2,301)	(993)	0
Add: Equity in (loss) income of unconsolidated non-real estate entities									(5)	20	62
Equity in income of unconsolidated entities									2,882	1,332	62
Computation of net operating income from service operations
Construction contract and other service revenues									102,840	48,364	106,402
Construction contract and other service expenses									(99,618)	(45,481)	(102,696)
NOI from service operations									$ 3,222	$ 2,883	$ 3,706

Information by Business Segme86

Information by Business Segment (Details 3) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting [Abstract]
NOI from real estate operations									$ 324,204	$ 330,739	$ 324,580
NOI from service operations									3,222	2,883	3,706
Interest and other income									6,318	5,444	4,517
Equity in income of unconsolidated entities									2,882	1,332	62
Income tax expense									(1,098)	(244)	(199)
Depreciation and other amortization associated with real estate operations									(134,228)	(132,719)	(140,025)
Impairment losses									(15,123)	(101,391)	(23,289)
General, administrative and leasing expenses									(30,837)	(36,553)	(31,361)
Business development expenses and land carry costs									(6,213)	(8,244)	(13,507)
Interest expense									(76,983)	(83,163)	(89,074)
NOI from discontinued operations									0	0	(10)
UJV NOI allocable to COPT									5,188	2,305	0
(Loss) gain on early extinguishment of debt									(513)	(1,110)	85,275
COPT consolidated income (loss) from continuing operations	$ 6,904	$ 21,494	$ 19,195	$ 18,850	$ 19,718	$ (4,829)	$ (48,316)	$ 8,096	$ 66,443	$ (25,331)	$ 120,675

Information by Business Segme87

Information by Business Segment (Details 4) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of segment assets to total assets
Total COPT consolidated assets	$ 3,578,484	$ 3,780,885
Segment assets [Member]
Reconciliation of segment assets to total assets
Total COPT consolidated assets	3,011,284	3,004,247	$ 3,265,469
Non-operating property assets [Member]
Reconciliation of segment assets to total assets
Total COPT consolidated assets	411,041	418,171
Other assets [Member]
Reconciliation of segment assets to total assets
Total COPT consolidated assets	$ 156,159	$ 358,467

Earnings Per Share ("EPS") an88

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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Numerator:
Income (loss) from continuing operations		$ 6,904	$ 21,494	$ 19,195	$ 18,850	$ 19,718	$ (4,829)	$ (48,316)	$ 8,096	$ 66,443		$ (25,331)		$ 120,675
Gain on sales of real estate										9,890		40,986		68,047
Preferred share/ unit dividends/ distributions		0	0	(3,039)	(3,180)	(3,640)	(3,552)	(3,553)	(3,552)	(6,219)		(14,297)		(14,210)
Issuance costs associated with redeemed preferred shares		$ 0	$ 0	$ (6,847)	$ 0	$ (17)	$ 0	$ 0	$ 0	(6,847)		(17)		0
Income from continuing operations attributable to noncontrolling interests										(6,242)		(4,216)		(10,575)
Income from continuing operations attributable to share-based compensation awards										(449)		(419)		(706)
Numerator for basic EPS/EPU from continuing operations attributable to COPT/COPLP common shareholders/unitholders										56,576		(3,294)		163,231
Dilutive effect of common units in COPLP on diluted EPS from continuing operations										0		0		6,397
Numerator for diluted EPS/EPU from continuing operations attributable to COPT/COPLP common shareholders/unitholders										56,576		(3,294)		169,628
Discontinued operations										0		0		156
Discontinued operations attributable to noncontrolling interests										0		0		(3)
Numerator for basic EPS on net income (loss) attributable to COPT common shareholders										56,576		(3,294)		163,384
Dilutive effect of common units in COPLP										0		0		6,403
Numerator for diluted EPS on net income (loss) attributable to COPT common shareholders										$ 56,576		$ (3,294)		$ 169,787
Denominator (all weighted averages):
Denominator for basic EPS (common shares)										98,969		94,502		93,914
Dilutive effect of forward equity sale agreements and share-based compensation awards										186		0		61
Dilutive effect of common units										0		0		3,692
Denominator for basic and diluted EPS (common shares)										99,155		94,502		97,667
Basic EPS:
Income (loss) from continuing operations (in dollars per share/unit)	[1]									$ 0.57		$ (0.03)		$ 1.74
Net income (loss) attributable to COPT common shareholders (in dollars per share/unit)		$ 0.10	$ 0.21	$ 0.08	$ 0.18	$ 0.22	$ 0.25	$ (0.54)	$ 0.03	0.57	[1]	(0.03)	[1]	1.74	[1]
Diluted EPS:
Income (loss) from continuing operations (in dollars per share/unit)	[1]									0.57		(0.03)		1.74
Net income (loss) attributable to COPT common shareholders (in dollars per share/unit)		$ 0.10	$ 0.21	$ 0.08	$ 0.18	$ 0.22	$ 0.25	$ (0.54)	$ 0.03	$ 0.57	[1]	$ (0.03)	[1]	$ 1.74	[1]
Corporate Office Properties, L.P. [Member]
Numerator:
Income (loss) from continuing operations		$ 6,904	$ 21,494	$ 19,195	$ 18,850	$ 19,718	$ (4,829)	$ (48,316)	$ 8,096	$ 66,443		$ (25,331)		$ 120,675
Gain on sales of real estate										9,890		40,986		68,047
Preferred share/ unit dividends/ distributions		(165)	(165)	(3,204)	(3,345)	(3,805)	(3,717)	(3,718)	(3,717)	(6,879)		(14,957)		(14,870)
Issuance costs associated with redeemed preferred shares		$ 0	$ 0	$ (6,847)	$ 0	$ (17)	$ 0	$ 0	$ 0	(6,847)		(17)		0
Income from continuing operations attributable to noncontrolling interests										(3,646)		(3,715)		(3,523)
Income from continuing operations attributable to share-based compensation awards										(449)		(419)		(706)
Numerator for basic and diluted EPS/EPU from continuing operations attributable to COPT/COPLP common shareholders/unitholders										58,512		(3,453)		169,623
Discontinued operations										0		0		156
Discontinued operations attributable to noncontrolling interests										0		0		3
Numerator for basic and diluted EPU on net income (loss) attributable to COPLP common unitholders										$ 58,512		$ (3,453)		$ 169,782
Denominator (all weighted averages):
Denominator for basic EPS (common shares)										102,331		98,135		97,606
Dilutive effect of forward equity sale agreements and share-based compensation awards										186		0		61
Denominator for basic and diluted EPS (common shares)										102,517		98,135		97,667
Basic EPS:
Income (loss) from continuing operations (in dollars per share/unit)	[2]									$ 0.57		$ (0.04)		$ 1.74
Net income (loss) attributable to COPT common shareholders (in dollars per share/unit)		$ 0.10	$ 0.21	$ 0.08	$ 0.18	$ 0.22	$ 0.25	$ (0.54)	$ 0.03	0.57	[2]	(0.04)	[2]	1.74	[2]
Diluted EPS:
Income (loss) from continuing operations (in dollars per share/unit)	[2]									0.57		(0.04)		1.74
Net income (loss) attributable to COPT common shareholders (in dollars per share/unit)		$ 0.10	$ 0.21	$ 0.08	$ 0.18	$ 0.22	$ 0.25	$ (0.54)	$ 0.03	$ 0.57	[2]	$ (0.04)	[2]	$ 1.74	[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") an89

Earnings Per Share ("EPS") and Earnings Per Unit (“EPU”) (Details 2) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Conversion of common units [Member]
Antidilutive securities
Weighted average antidilutive securities excluded from computation of diluted earnings per share (in shares)	3,362	3,633	0
Conversion of Series I preferred units [Member]
Antidilutive securities
Weighted average antidilutive securities excluded from computation of diluted earnings per share (in shares)	176	176	176
Conversion of Series K preferred shares [Member]
Antidilutive securities
Weighted average antidilutive securities excluded from computation of diluted earnings per share (in shares)	0	434	434
Restricted Stock and Deferred Shares [Member]
Antidilutive securities
Weighted average antidilutive securities excluded from computation of diluted earnings per share (in shares)	433	385	410
Stock Options [Member]
Antidilutive securities
Weighted average antidilutive securities excluded from computation of diluted earnings per share (in shares)	70	285	469
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)	433	385	410
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)	70	285	469
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	176	176
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)	0	434	434

Commitments and Contingencies -

Commitments and Contingencies - Narrative (Details)	May 25, 2017USD ($)	Dec. 31, 2017USD ($)Property	Dec. 31, 2016USD ($)	Aug. 31, 2010USD ($)
Capital Lease
Capital lease term	99 years
Purchase option	$ 1
Capital lease obligation	$ 16,100,000	$ 15,853,000	$ 0
Environmental Indemnity Agreement
Number of lease properties which were provided environmental indemnifications \| Property		3
Maximum environmental indemnification to the tenant against consequential damages after acquisition of property		$ 19,000,000
Specialty Tax Guarantee [Member]
Tax incremental financing obligation
Liability recognized with regard to tax incremental financing obligation at end of current period		981,000
New Development and Redevelopment Obligations [Member]
Capital Lease
Contractual obligation		22,800,000
Capital Expenditures For Operating Properties [Member]
Capital Lease
Contractual obligation		44,200,000
Third Party Construction and Development [Member]
Capital Lease
Contractual obligation		35,800,000
Other Obligations [Member]
Capital Lease
Contractual obligation		$ 900,000
Tax Incremental Financing Bond [Member] \| Anne Arundel County, Maryland [Member] \| Specialty Tax Guarantee [Member]
Commitments and Contingencies
Loan amount				$ 30,000,000

Commitments and Contingencies91

Commitments and Contingencies - Operating Leases Future Minimum Payments (Details) $ in Thousands	Dec. 31, 2017USD ($)
Commitments and Contingencies Disclosure [Abstract]
2,018	$ 1,283
2,019	1,267
2,020	1,259
2,021	1,263
2,022	1,149
Thereafter	84,611
Total	$ 90,832

Commitments and Contingencies92

Commitments and Contingencies - Capital Leases Future Minimum Payments (Details) $ in Thousands	Dec. 31, 2017USD ($)
Commitments and Contingencies Disclosure [Abstract]
2,018	$ 15,829
2,020	135
2,022	75
Total minimum rental payments	16,039
Less: Amount representing interest	(186)
Capital lease obligation	$ 15,853

Quarterly Data (Unaudited) (Det

Quarterly Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Quarterly Data [Line Items]
Revenues	$ 164,567	$ 157,017	$ 151,435	$ 139,801	$ 141,991	$ 142,103	$ 145,927	$ 144,307	$ 612,820		$ 574,328		$ 625,466
Operating income	24,847	38,939	36,618	35,433	37,442	11,525	(27,021)	30,464	135,837		52,410		120,094
Income (loss) from continuing operations	6,904	21,494	19,195	18,850	19,718	(4,829)	(48,316)	8,096	66,443		(25,331)		120,675
Net income (loss)	11,356	22,682	19,207	23,088	26,603	29,272	(48,316)	8,096	76,333		15,655		188,878
Net (income) loss attributable to noncontrolling interests	(1,398)	(1,766)	(1,345)	(1,733)	(1,870)	(1,973)	897	(1,270)
Net income attributable to COPT	9,958	20,916	17,862	21,355	24,733	27,299	(47,419)	6,826	70,091		11,439		178,300
Preferred share/ unit dividends/ distributions	0	0	(3,039)	(3,180)	(3,640)	(3,552)	(3,553)	(3,552)	(6,219)		(14,297)		(14,210)
Issuance costs associated with redeemed preferred shares	0	0	(6,847)	0	(17)	0	0	0	(6,847)		(17)		0
Net income (loss) attributable to COPT common shareholders	$ 9,958	$ 20,916	$ 7,976	$ 18,175	$ 21,076	$ 23,747	$ (50,972)	$ 3,274	$ 57,025		$ (2,875)		$ 164,090
Basic earnings per common share/unit (in dollars per share/unit)	$ 0.10	$ 0.21	$ 0.08	$ 0.18	$ 0.22	$ 0.25	$ (0.54)	$ 0.03	$ 0.57	[1]	$ (0.03)	[1]	$ 1.74	[1]
Diluted earnings per common share/unit (in dollars per share/unit)	$ 0.10	$ 0.21	$ 0.08	$ 0.18	$ 0.22	$ 0.25	$ (0.54)	$ 0.03	$ 0.57	[1]	$ (0.03)	[1]	$ 1.74	[1]
Corporate Office Properties, L.P. [Member]
Quarterly Data [Line Items]
Revenues	$ 164,567	$ 157,017	$ 151,435	$ 139,801	$ 141,991	$ 142,103	$ 145,927	$ 144,307	$ 612,820		$ 574,328		$ 625,466
Operating income	24,847	38,939	36,618	35,433	37,442	11,525	(27,021)	30,464	135,837		52,410		120,094
Income (loss) from continuing operations	6,904	21,494	19,195	18,850	19,718	(4,829)	(48,316)	8,096	66,443		(25,331)		120,675
Net income (loss)	11,356	22,682	19,207	23,088	26,603	29,272	(48,316)	8,096	76,333		15,655		188,878
Net (income) loss attributable to noncontrolling interests	(908)	(897)	(907)	(934)	(912)	(913)	(911)	(979)	(3,646)		(3,715)		(3,520)
Net income attributable to COPT	10,448	21,785	18,300	22,154	25,691	28,359	(49,227)	7,117	72,687		11,940		185,358
Preferred share/ unit dividends/ distributions	(165)	(165)	(3,204)	(3,345)	(3,805)	(3,717)	(3,718)	(3,717)	(6,879)		(14,957)		(14,870)
Issuance costs associated with redeemed preferred shares	0	0	(6,847)	0	(17)	0	0	0	(6,847)		(17)		0
Net income (loss) attributable to COPT common shareholders	$ 10,283	$ 21,620	$ 8,249	$ 18,809	$ 21,869	$ 24,642	$ (52,945)	$ 3,400	$ 58,961		$ (3,034)		$ 170,488
Basic earnings per common share/unit (in dollars per share/unit)	$ 0.10	$ 0.21	$ 0.08	$ 0.18	$ 0.22	$ 0.25	$ (0.54)	$ 0.03	$ 0.57	[2]	$ (0.04)	[2]	$ 1.74	[2]
Diluted earnings per common share/unit (in dollars per share/unit)	$ 0.10	$ 0.21	$ 0.08	$ 0.18	$ 0.22	$ 0.25	$ (0.54)	$ 0.03	$ 0.57	[2]	$ (0.04)	[2]	$ 1.74	[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 Q94

Schedule II - Valuation and Qualifying Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts Receivables-Allowance for doubtful accounts [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	$ 603	$ 1,525	$ 717
Charged to Costs and Expenses	368	(17)	1,125
Charged to Other Accounts	(36)	235	98
Deductions	(328)	(1,140)	(415)
Balance at End of Year	607	603	1,525
Allowance for Deferred Rent Receivable [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	373	1,962	1,418
Charged to Costs and Expenses	(9)	(1,589)	0
Charged to Other Accounts	0	0	544
Deductions	0	0	0
Balance at End of Year	364	373	1,962
Allowance for Deferred Tax Asset [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	2,062	2,062	2,062
Charged to Costs and Expenses	(646)	0	0
Charged to Other Accounts	0	0	0
Deductions	0	0	0
Balance at End of Year	$ 1,416	$ 2,062	$ 2,062

Schedule III - Real Estate an95

Schedule III - Real Estate and Accumulated Depreciation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Real Estate and Accumulated Depreciation
Encumbrances	$ 164,506
Initial Cost
Land	697,810
Building and Land Improvements	2,902,516
Costs Capitalized Subsequent to Acquisition	380,487
Gross Amounts Carried At Close of Period
Land	697,810
Building and Land Improvements	3,283,003
Total	3,980,813	$ 3,874,715	$ 4,158,616	$ 4,014,336
Accumulated Depreciation	(801,038)	(715,951)	(718,680)	$ (703,083)
Additional information
Debt excluded from encumbrances	1,828,333	1,904,001
Net discounts and deferred financing costs	13,800
Aggregate cost of assets for federal income tax purposes	3,500,000
Impairment losses	$ 15,123	101,391	$ 23,289
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
Term Loan Facilities [Member]
Additional information
Debt excluded from encumbrances	$ 347,959	$ 547,494
Unsecured Senior Notes [Member]
Additional information
Debt excluded from encumbrances	1,200,000
Unsecured notes payable [Member]
Additional information
Debt excluded from encumbrances	1,300
Fixed rate mortgage loans [Member]
Additional information
Net discounts and deferred financing costs	668
100 Light Street [Member]
Real Estate and Accumulated Depreciation
Encumbrances	49,378
Initial Cost
Land	26,715
Building and Land Improvements	58,343
Costs Capitalized Subsequent to Acquisition	5,286
Gross Amounts Carried At Close of Period
Land	26,715
Building and Land Improvements	63,629
Total	90,344
Accumulated Depreciation	(9,467)
1000 Redstone Gateway [Member]
Real Estate and Accumulated Depreciation
Encumbrances	10,730
Initial Cost
Land	0
Building and Land Improvements	20,533
Costs Capitalized Subsequent to Acquisition	5
Gross Amounts Carried At Close of Period
Land	0
Building and Land Improvements	20,538
Total	20,538
Accumulated Depreciation	(2,465)
1100 Redstone Gateway [Member]
Real Estate and Accumulated Depreciation
Encumbrances	11,222
Initial Cost
Land	0
Building and Land Improvements	19,593
Gross Amounts Carried At Close of Period
Land	0
Building and Land Improvements	19,593
Total	19,593
Accumulated Depreciation	(1,945)
114 National Business Parkway [Member]
Initial Cost
Land	364
Building and Land Improvements	3,109
Costs Capitalized Subsequent to Acquisition	118
Gross Amounts Carried At Close of Period
Land	364
Building and Land Improvements	3,227
Total	3,591
Accumulated Depreciation	(1,309)
11751 Meadowville Lane [Member]
Initial Cost
Land	1,305
Building and Land Improvements	52,098
Costs Capitalized Subsequent to Acquisition	112
Gross Amounts Carried At Close of Period
Land	1,305
Building and Land Improvements	52,210
Total	53,515
Accumulated Depreciation	(14,845)
1200 Redstone Gateway [Member]
Real Estate and Accumulated Depreciation
Encumbrances	12,973
Initial Cost
Land	0
Building and Land Improvements	22,389
Gross Amounts Carried At Close of Period
Land	0
Building and Land Improvements	22,389
Total	22,389
Accumulated Depreciation	(2,264)
1201 M Street [Member]
Initial Cost
Land	0
Building and Land Improvements	49,785
Costs Capitalized Subsequent to Acquisition	8,590
Gross Amounts Carried At Close of Period
Land	0
Building and Land Improvements	58,375
Total	58,375
Accumulated Depreciation	(12,800)
1201 Winterson Road [Member]
Initial Cost
Land	1,288
Building and Land Improvements	16,433
Costs Capitalized Subsequent to Acquisition	460
Gross Amounts Carried At Close of Period
Land	1,288
Building and Land Improvements	16,893
Total	18,181
Accumulated Depreciation	(4,279)
1220 12th Street, SE [Member]
Initial Cost
Land	0
Building and Land Improvements	42,464
Costs Capitalized Subsequent to Acquisition	5,820
Gross Amounts Carried At Close of Period
Land	0
Building and Land Improvements	48,284
Total	48,284
Accumulated Depreciation	(11,719)
1243 Winterson Road [Member]
Initial Cost
Land	630
Building and Land Improvements	0
Gross Amounts Carried At Close of Period
Land	630
Building and Land Improvements	0
Total	630
131 National Business Parkway [Member]
Initial Cost
Land	1,906
Building and Land Improvements	7,623
Costs Capitalized Subsequent to Acquisition	3,868
Gross Amounts Carried At Close of Period
Land	1,906
Building and Land Improvements	11,491
Total	13,397
Accumulated Depreciation	(6,286)
132 National Business Parkway [Member]
Initial Cost
Land	2,917
Building and Land Improvements	12,259
Costs Capitalized Subsequent to Acquisition	4,124
Gross Amounts Carried At Close of Period
Land	2,917
Building and Land Improvements	16,383
Total	19,300
Accumulated Depreciation	(8,715)
133 National Business Parkway [Member]
Initial Cost
Land	2,517
Building and Land Improvements	10,068
Costs Capitalized Subsequent to Acquisition	5,544
Gross Amounts Carried At Close of Period
Land	2,517
Building and Land Improvements	15,612
Total	18,129
Accumulated Depreciation	(9,234)
134 National Business Parkway [Member]
Initial Cost
Land	3,684
Building and Land Improvements	7,517
Costs Capitalized Subsequent to Acquisition	3,691
Gross Amounts Carried At Close of Period
Land	3,684
Building and Land Improvements	11,208
Total	14,892
Accumulated Depreciation	(5,419)
1340 Ashton Road [Member]
Initial Cost
Land	905
Building and Land Improvements	3,620
Costs Capitalized Subsequent to Acquisition	1,470
Gross Amounts Carried At Close of Period
Land	905
Building and Land Improvements	5,090
Total	5,995
Accumulated Depreciation	(2,854)
13450 Sunrise Valley Road [Member]
Initial Cost
Land	1,386
Building and Land Improvements	5,576
Costs Capitalized Subsequent to Acquisition	4,553
Gross Amounts Carried At Close of Period
Land	1,386
Building and Land Improvements	10,129
Total	11,515
Accumulated Depreciation	(4,808)
13454 Sunrise Valley Road [Member]
Initial Cost
Land	2,899
Building and Land Improvements	11,986
Costs Capitalized Subsequent to Acquisition	7,071
Gross Amounts Carried At Close of Period
Land	2,899
Building and Land Improvements	19,057
Total	21,956
Accumulated Depreciation	(9,262)
135 National Business Parkway [Member]
Initial Cost
Land	2,484
Building and Land Improvements	9,750
Costs Capitalized Subsequent to Acquisition	6,075
Gross Amounts Carried At Close of Period
Land	2,484
Building and Land Improvements	15,825
Total	18,309
Accumulated Depreciation	(7,970)
1362 Mellon Road [Member]
Initial Cost
Land	950
Building and Land Improvements	3,864
Costs Capitalized Subsequent to Acquisition	206
Gross Amounts Carried At Close of Period
Land	950
Building and Land Improvements	4,070
Total	5,020
Accumulated Depreciation	(192)
13857 McLearen Road [Member]
Initial Cost
Land	3,507
Building and Land Improvements	30,177
Costs Capitalized Subsequent to Acquisition	1,768
Gross Amounts Carried At Close of Period
Land	3,507
Building and Land Improvements	31,945
Total	35,452
Accumulated Depreciation	(10,100)
140 National Business Parkway [Member]
Initial Cost
Land	3,407
Building and Land Improvements	24,167
Costs Capitalized Subsequent to Acquisition	1,487
Gross Amounts Carried At Close of Period
Land	3,407
Building and Land Improvements	25,654
Total	29,061
Accumulated Depreciation	(8,907)
141 National Business Parkway [Member]
Initial Cost
Land	2,398
Building and Land Improvements	9,538
Costs Capitalized Subsequent to Acquisition	4,815
Gross Amounts Carried At Close of Period
Land	2,398
Building and Land Improvements	14,353
Total	16,751
Accumulated Depreciation	(7,409)
14280 Park Meadow Drive [Member]
Initial Cost
Land	3,731
Building and Land Improvements	15,953
Costs Capitalized Subsequent to Acquisition	2,628
Gross Amounts Carried At Close of Period
Land	3,731
Building and Land Improvements	18,581
Total	22,312
Accumulated Depreciation	(7,326)
1460 Dorsey Road [Member]
Initial Cost
Land	1,577
Building and Land Improvements	33
Gross Amounts Carried At Close of Period
Land	1,577
Building and Land Improvements	33
Total	1,610
14840 Conference Center Drive [Member]
Initial Cost
Land	1,572
Building and Land Improvements	8,175
Costs Capitalized Subsequent to Acquisition	3,092
Gross Amounts Carried At Close of Period
Land	1,572
Building and Land Improvements	11,267
Total	12,839
Accumulated Depreciation	(5,410)
14850 Conference Center Drive [Member]
Initial Cost
Land	1,615
Building and Land Improvements	8,358
Costs Capitalized Subsequent to Acquisition	3,072
Gross Amounts Carried At Close of Period
Land	1,615
Building and Land Improvements	11,430
Total	13,045
Accumulated Depreciation	(5,857)
14900 Conference Center Drive [Member]
Initial Cost
Land	3,436
Building and Land Improvements	14,402
Costs Capitalized Subsequent to Acquisition	6,239
Gross Amounts Carried At Close of Period
Land	3,436
Building and Land Improvements	20,641
Total	24,077
Accumulated Depreciation	(10,027)
1501 South Clinton Street [Member]
Initial Cost
Land	27,964
Building and Land Improvements	51,990
Costs Capitalized Subsequent to Acquisition	13,670
Gross Amounts Carried At Close of Period
Land	27,964
Building and Land Improvements	65,660
Total	93,624
Accumulated Depreciation	(19,942)
15049 Conference Center Drive [Member]
Initial Cost
Land	4,415
Building and Land Improvements	20,365
Costs Capitalized Subsequent to Acquisition	14,994
Gross Amounts Carried At Close of Period
Land	4,415
Building and Land Improvements	35,359
Total	39,774
Accumulated Depreciation	(12,097)
15059 Conference Center Drive [Member]
Initial Cost
Land	5,753
Building and Land Improvements	13,615
Costs Capitalized Subsequent to Acquisition	3,645
Gross Amounts Carried At Close of Period
Land	5,753
Building and Land Improvements	17,260
Total	23,013
Accumulated Depreciation	(7,907)
1550 West Nursery Road [Member]
Initial Cost
Land	14,071
Building and Land Improvements	16,930
Gross Amounts Carried At Close of Period
Land	14,071
Building and Land Improvements	16,930
Total	31,001
Accumulated Depreciation	(4,802)
1560 West Nursery Road [Member]
Initial Cost
Land	1,441
Building and Land Improvements	113
Gross Amounts Carried At Close of Period
Land	1,441
Building and Land Improvements	113
Total	1,554
Accumulated Depreciation	(10)
1610 West Nursery Road [Member]
Initial Cost
Land	259
Building and Land Improvements	246
Gross Amounts Carried At Close of Period
Land	259
Building and Land Improvements	246
Total	505
Accumulated Depreciation	(5)
1616 West Nursery Road [Member]
Initial Cost
Land	393
Building and Land Improvements	2,919
Gross Amounts Carried At Close of Period
Land	393
Building and Land Improvements	2,919
Total	3,312
Accumulated Depreciation	(13)
1622 West Nursery Road [Member]
Initial Cost
Land	393
Building and Land Improvements	2,477
Gross Amounts Carried At Close of Period
Land	393
Building and Land Improvements	2,477
Total	2,870
Accumulated Depreciation	(53)
16442 Commerce Drive [Member]
Initial Cost
Land	613
Building and Land Improvements	2,582
Costs Capitalized Subsequent to Acquisition	891
Gross Amounts Carried At Close of Period
Land	613
Building and Land Improvements	3,473
Total	4,086
Accumulated Depreciation	(1,538)
16480 Commerce Drive [Member]
Initial Cost
Land	1,856
Building and Land Improvements	7,425
Costs Capitalized Subsequent to Acquisition	2,068
Gross Amounts Carried At Close of Period
Land	1,856
Building and Land Improvements	9,493
Total	11,349
Accumulated Depreciation	(3,279)
16501 Commerce Drive [Member]
Initial Cost
Land	522
Building and Land Improvements	2,090
Costs Capitalized Subsequent to Acquisition	727
Gross Amounts Carried At Close of Period
Land	522
Building and Land Improvements	2,817
Total	3,339
Accumulated Depreciation	(975)
16539 Commerce Drive [Member]
Initial Cost
Land	688
Building and Land Improvements	2,860
Costs Capitalized Subsequent to Acquisition	1,892
Gross Amounts Carried At Close of Period
Land	688
Building and Land Improvements	4,752
Total	5,440
Accumulated Depreciation	(2,325)
16541 Commerce Drive [Member]
Initial Cost
Land	773
Building and Land Improvements	3,094
Costs Capitalized Subsequent to Acquisition	1,757
Gross Amounts Carried At Close of Period
Land	773
Building and Land Improvements	4,851
Total	5,624
Accumulated Depreciation	(2,026)
16543 Commerce Drive [Member]
Initial Cost
Land	436
Building and Land Improvements	1,742
Costs Capitalized Subsequent to Acquisition	716
Gross Amounts Carried At Close of Period
Land	436
Building and Land Improvements	2,458
Total	2,894
Accumulated Depreciation	(839)
1751 Pinnacle Drive [Member]
Initial Cost
Land	10,486
Building and Land Improvements	42,339
Costs Capitalized Subsequent to Acquisition	27,048
Gross Amounts Carried At Close of Period
Land	10,486
Building and Land Improvements	69,387
Total	79,873
Accumulated Depreciation	(29,582)
1753 Pinnacle Drive [Member]
Initial Cost
Land	8,275
Building and Land Improvements	34,353
Costs Capitalized Subsequent to Acquisition	16,648
Gross Amounts Carried At Close of Period
Land	8,275
Building and Land Improvements	51,001
Total	59,276
Accumulated Depreciation	(18,757)
206 Research Boulevard [Member]
Initial Cost
Land	0
Building and Land Improvements	132
Gross Amounts Carried At Close of Period
Land	0
Building and Land Improvements	132
Total	132
Accumulated Depreciation	(132)
209 Research Boulevard [Member]
Initial Cost
Land	134
Building and Land Improvements	1,711
Costs Capitalized Subsequent to Acquisition	175
Gross Amounts Carried At Close of Period
Land	134
Building and Land Improvements	1,886
Total	2,020
Accumulated Depreciation	(283)
210 Research Boulevard [Member]
Initial Cost
Land	113
Building and Land Improvements	1,402
Costs Capitalized Subsequent to Acquisition	86
Gross Amounts Carried At Close of Period
Land	113
Building and Land Improvements	1,488
Total	1,601
Accumulated Depreciation	(174)
2100 L Street [Member]
Initial Cost
Land	55,615
Building and Land Improvements	9,073
Gross Amounts Carried At Close of Period
Land	55,615
Building and Land Improvements	9,073
Total	64,688
2100 Rideout Road [Member]
Initial Cost
Land	0
Building and Land Improvements	5,003
Costs Capitalized Subsequent to Acquisition	2,881
Gross Amounts Carried At Close of Period
Land	0
Building and Land Improvements	7,884
Total	7,884
Accumulated Depreciation	(438)
22289 Exploration Drive [Member]
Initial Cost
Land	1,422
Building and Land Improvements	5,719
Costs Capitalized Subsequent to Acquisition	1,829
Gross Amounts Carried At Close of Period
Land	1,422
Building and Land Improvements	7,548
Total	8,970
Accumulated Depreciation	(3,415)
22299 Exploration Drive [Member]
Initial Cost
Land	1,362
Building and Land Improvements	5,791
Costs Capitalized Subsequent to Acquisition	2,308
Gross Amounts Carried At Close of Period
Land	1,362
Building and Land Improvements	8,099
Total	9,461
Accumulated Depreciation	(3,768)
22300 Exploration Drive [Member]
Initial Cost
Land	1,094
Building and Land Improvements	5,038
Costs Capitalized Subsequent to Acquisition	1,489
Gross Amounts Carried At Close of Period
Land	1,094
Building and Land Improvements	6,527
Total	7,621
Accumulated Depreciation	(2,432)
22309 Exploration Drive [Member]
Initial Cost
Land	2,243
Building and Land Improvements	10,419
Costs Capitalized Subsequent to Acquisition	7,967
Gross Amounts Carried At Close of Period
Land	2,243
Building and Land Improvements	18,386
Total	20,629
Accumulated Depreciation	(6,087)
23535 Cottonwood Parkway [Member]
Initial Cost
Land	692
Building and Land Improvements	3,051
Costs Capitalized Subsequent to Acquisition	537
Gross Amounts Carried At Close of Period
Land	692
Building and Land Improvements	3,588
Total	4,280
Accumulated Depreciation	(1,527)
250 W Pratt St [Member]
Initial Cost
Land	8,057
Building and Land Improvements	34,588
Costs Capitalized Subsequent to Acquisition	6,942
Gross Amounts Carried At Close of Period
Land	8,057
Building and Land Improvements	41,530
Total	49,587
Accumulated Depreciation	(6,972)
2500 Riva Road [Member]
Initial Cost
Land	2,791
Building and Land Improvements	12,145
Costs Capitalized Subsequent to Acquisition	1
Gross Amounts Carried At Close of Period
Land	2,791
Building and Land Improvements	12,146
Total	14,937
Accumulated Depreciation	(5,105)
2600 Park Tower Drive [Member]
Initial Cost
Land	20,304
Building and Land Improvements	34,443
Costs Capitalized Subsequent to Acquisition	517
Gross Amounts Carried At Close of Period
Land	20,304
Building and Land Improvements	34,960
Total	55,264
Accumulated Depreciation	(3,708)
2691 Technology Drive [Member]
Initial Cost
Land	2,098
Building and Land Improvements	17,334
Costs Capitalized Subsequent to Acquisition	5,563
Gross Amounts Carried At Close of Period
Land	2,098
Building and Land Improvements	22,897
Total	24,995
Accumulated Depreciation	(9,630)
2701 Technology Drive [Member]
Initial Cost
Land	1,737
Building and Land Improvements	15,266
Costs Capitalized Subsequent to Acquisition	4,398
Gross Amounts Carried At Close of Period
Land	1,737
Building and Land Improvements	19,664
Total	21,401
Accumulated Depreciation	(9,676)
2711 Technology Drive [Member]
Initial Cost
Land	2,251
Building and Land Improvements	21,611
Costs Capitalized Subsequent to Acquisition	1,899
Gross Amounts Carried At Close of Period
Land	2,251
Building and Land Improvements	23,510
Total	25,761
Accumulated Depreciation	(11,727)
2720 Technology Drive [Member]
Initial Cost
Land	3,863
Building and Land Improvements	29,272
Costs Capitalized Subsequent to Acquisition	1,218
Gross Amounts Carried At Close of Period
Land	3,863
Building and Land Improvements	30,490
Total	34,353
Accumulated Depreciation	(10,261)
2721 Technology Drive [Member]
Initial Cost
Land	4,611
Building and Land Improvements	14,597
Costs Capitalized Subsequent to Acquisition	1,270
Gross Amounts Carried At Close of Period
Land	4,611
Building and Land Improvements	15,867
Total	20,478
Accumulated Depreciation	(7,484)
2730 Hercules Road [Member]
Initial Cost
Land	8,737
Building and Land Improvements	31,612
Costs Capitalized Subsequent to Acquisition	8,697
Gross Amounts Carried At Close of Period
Land	8,737
Building and Land Improvements	40,309
Total	49,046
Accumulated Depreciation	(18,673)
30 Light Street [Member]
Real Estate and Accumulated Depreciation
Encumbrances	4,153
Initial Cost
Land	0
Building and Land Improvements	12,101
Costs Capitalized Subsequent to Acquisition	629
Gross Amounts Carried At Close of Period
Land	0
Building and Land Improvements	12,730
Total	12,730
Accumulated Depreciation	(753)
300 Sentinel Drive [Member]
Initial Cost
Land	1,517
Building and Land Improvements	59,165
Costs Capitalized Subsequent to Acquisition	925
Gross Amounts Carried At Close of Period
Land	1,517
Building and Land Improvements	60,090
Total	61,607
Accumulated Depreciation	(11,562)
302 Sentinel Drive [Member]
Initial Cost
Land	2,648
Building and Land Improvements	29,687
Costs Capitalized Subsequent to Acquisition	468
Gross Amounts Carried At Close of Period
Land	2,648
Building and Land Improvements	30,155
Total	32,803
Accumulated Depreciation	(7,579)
304 Sentinel Drive [Member]
Initial Cost
Land	3,411
Building and Land Improvements	24,917
Costs Capitalized Subsequent to Acquisition	202
Gross Amounts Carried At Close of Period
Land	3,411
Building and Land Improvements	25,119
Total	28,530
Accumulated Depreciation	(7,610)
306 Sentinel Drive [Member]
Initial Cost
Land	3,260
Building and Land Improvements	22,592
Costs Capitalized Subsequent to Acquisition	961
Gross Amounts Carried At Close of Period
Land	3,260
Building and Land Improvements	23,553
Total	26,813
Accumulated Depreciation	(6,631)
308 Sentinel Drive [Member]
Initial Cost
Land	1,422
Building and Land Improvements	26,208
Costs Capitalized Subsequent to Acquisition	2,396
Gross Amounts Carried At Close of Period
Land	1,422
Building and Land Improvements	28,604
Total	30,026
Accumulated Depreciation	(4,370)
310 Sentinel Way [Member]
Initial Cost
Land	2,372
Building and Land Improvements	38,865
Gross Amounts Carried At Close of Period
Land	2,372
Building and Land Improvements	38,865
Total	41,237
Accumulated Depreciation	(1,977)
310 The Bridge Street [Member]
Initial Cost
Land	261
Building and Land Improvements	26,531
Costs Capitalized Subsequent to Acquisition	3,762
Gross Amounts Carried At Close of Period
Land	261
Building and Land Improvements	30,293
Total	30,554
Accumulated Depreciation	(7,430)
312 Sentinel Way [Member]
Initial Cost
Land	3,138
Building and Land Improvements	27,797
Gross Amounts Carried At Close of Period
Land	3,138
Building and Land Improvements	27,797
Total	30,935
Accumulated Depreciation	(2,304)
314 Sentinel Way [Member]
Initial Cost
Land	1,254
Building and Land Improvements	7,741
Gross Amounts Carried At Close of Period
Land	1,254
Building and Land Improvements	7,741
Total	8,995
Accumulated Depreciation	(548)
316 Sentinel Way [Member]
Initial Cost
Land	2,748
Building and Land Improvements	38,156
Costs Capitalized Subsequent to Acquisition	145
Gross Amounts Carried At Close of Period
Land	2,748
Building and Land Improvements	38,301
Total	41,049
Accumulated Depreciation	(5,507)
318 Sentinel Way [Member]
Initial Cost
Land	2,185
Building and Land Improvements	28,426
Costs Capitalized Subsequent to Acquisition	560
Gross Amounts Carried At Close of Period
Land	2,185
Building and Land Improvements	28,986
Total	31,171
Accumulated Depreciation	(8,416)
320 Sentinel Way [Member]
Initial Cost
Land	2,067
Building and Land Improvements	21,623
Gross Amounts Carried At Close of Period
Land	2,067
Building and Land Improvements	21,623
Total	23,690
Accumulated Depreciation	(5,391)
322 Sentinel Way [Member]
Initial Cost
Land	2,605
Building and Land Improvements	22,827
Costs Capitalized Subsequent to Acquisition	1,900
Gross Amounts Carried At Close of Period
Land	2,605
Building and Land Improvements	24,727
Total	27,332
Accumulated Depreciation	(6,286)
324 Sentinel Way [Member]
Initial Cost
Land	1,656
Building and Land Improvements	23,018
Gross Amounts Carried At Close of Period
Land	1,656
Building and Land Improvements	23,018
Total	24,674
Accumulated Depreciation	(4,229)
4000 Market Street [Member]
Initial Cost
Land	0
Building and Land Improvements	466
Gross Amounts Carried At Close of Period
Land	0
Building and Land Improvements	466
Total	466
4100 Market Street [Member]
Initial Cost
Land	0
Building and Land Improvements	1,013
Gross Amounts Carried At Close of Period
Land	0
Building and Land Improvements	1,013
Total	1,013
410 National Business Parkway [Member]
Initial Cost
Land	1,831
Building and Land Improvements	23,257
Costs Capitalized Subsequent to Acquisition	119
Gross Amounts Carried At Close of Period
Land	1,831
Building and Land Improvements	23,376
Total	25,207
Accumulated Depreciation	(2,904)
420 National Business Parkway [Member]
Initial Cost
Land	2,370
Building and Land Improvements	27,750
Costs Capitalized Subsequent to Acquisition	108
Gross Amounts Carried At Close of Period
Land	2,370
Building and Land Improvements	27,858
Total	30,228
Accumulated Depreciation	(2,635)
430 National Business Parkway [Member]
Initial Cost
Land	1,852
Building and Land Improvements	21,563
Costs Capitalized Subsequent to Acquisition	126
Gross Amounts Carried At Close of Period
Land	1,852
Building and Land Improvements	21,689
Total	23,541
Accumulated Depreciation	(3,127)
44408 Pecan Court [Member]
Initial Cost
Land	817
Building and Land Improvements	1,583
Costs Capitalized Subsequent to Acquisition	1,490
Gross Amounts Carried At Close of Period
Land	817
Building and Land Improvements	3,073
Total	3,890
Accumulated Depreciation	(838)
44414 Pecan Court [Member]
Initial Cost
Land	405
Building and Land Improvements	1,619
Costs Capitalized Subsequent to Acquisition	1,033
Gross Amounts Carried At Close of Period
Land	405
Building and Land Improvements	2,652
Total	3,057
Accumulated Depreciation	(954)
44417 Pecan Court [Member]
Initial Cost
Land	434
Building and Land Improvements	3,822
Costs Capitalized Subsequent to Acquisition	148
Gross Amounts Carried At Close of Period
Land	434
Building and Land Improvements	3,970
Total	4,404
Accumulated Depreciation	(1,448)
44420 Pecan Court [Member]
Initial Cost
Land	344
Building and Land Improvements	890
Costs Capitalized Subsequent to Acquisition	168
Gross Amounts Carried At Close of Period
Land	344
Building and Land Improvements	1,058
Total	1,402
Accumulated Depreciation	(368)
44425 Pecan Court [Member]
Initial Cost
Land	1,309
Building and Land Improvements	3,506
Costs Capitalized Subsequent to Acquisition	1,881
Gross Amounts Carried At Close of Period
Land	1,309
Building and Land Improvements	5,387
Total	6,696
Accumulated Depreciation	(2,419)
45310 Abell House Lane [Member]
Initial Cost
Land	2,272
Building and Land Improvements	13,808
Costs Capitalized Subsequent to Acquisition	147
Gross Amounts Carried At Close of Period
Land	2,272
Building and Land Improvements	13,955
Total	16,227
Accumulated Depreciation	(2,094)
46579 Expedition Drive [Member]
Initial Cost
Land	1,406
Building and Land Improvements	5,796
Costs Capitalized Subsequent to Acquisition	1,680
Gross Amounts Carried At Close of Period
Land	1,406
Building and Land Improvements	7,476
Total	8,882
Accumulated Depreciation	(3,421)
46591 Expedition Drive [Member]
Initial Cost
Land	1,200
Building and Land Improvements	7,199
Costs Capitalized Subsequent to Acquisition	1,226
Gross Amounts Carried At Close of Period
Land	1,200
Building and Land Improvements	8,425
Total	9,625
Accumulated Depreciation	(2,652)
4851 Stonecroft Boulevard [Member]
Initial Cost
Land	1,878
Building and Land Improvements	11,558
Costs Capitalized Subsequent to Acquisition	21
Gross Amounts Carried At Close of Period
Land	1,878
Building and Land Improvements	11,579
Total	13,457
Accumulated Depreciation	(3,827)
540 National Business Parkway [Member]
Initial Cost
Land	2,035
Building and Land Improvements	29,490
Gross Amounts Carried At Close of Period
Land	2,035
Building and Land Improvements	29,490
Total	31,525
Accumulated Depreciation	(260)
5520 Research Park Drive [Member]
Initial Cost
Land	0
Building and Land Improvements	20,072
Costs Capitalized Subsequent to Acquisition	1,018
Gross Amounts Carried At Close of Period
Land	0
Building and Land Improvements	21,090
Total	21,090
Accumulated Depreciation	(4,260)
5522 Research Park Drive [Member]
Initial Cost
Land	0
Building and Land Improvements	4,550
Costs Capitalized Subsequent to Acquisition	210
Gross Amounts Carried At Close of Period
Land	0
Building and Land Improvements	4,760
Total	4,760
Accumulated Depreciation	(1,185)
5801 University Research Court [Member]
Initial Cost
Land	0
Building and Land Improvements	9,423
Gross Amounts Carried At Close of Period
Land	0
Building and Land Improvements	9,423
Total	9,423
Accumulated Depreciation	(36)
5825 University Research Court [Member]
Real Estate and Accumulated Depreciation
Encumbrances	21,284
Initial Cost
Land	0
Building and Land Improvements	22,771
Costs Capitalized Subsequent to Acquisition	666
Gross Amounts Carried At Close of Period
Land	0
Building and Land Improvements	23,437
Total	23,437
Accumulated Depreciation	(5,020)
5850 University Research Court [Member]
Real Estate and Accumulated Depreciation
Encumbrances	22,517
Initial Cost
Land	0
Building and Land Improvements	31,906
Costs Capitalized Subsequent to Acquisition	405
Gross Amounts Carried At Close of Period
Land	0
Building and Land Improvements	32,311
Total	32,311
Accumulated Depreciation	(6,306)
6700 Alexander Bell Drive [Member]
Initial Cost
Land	1,755
Building and Land Improvements	7,019
Costs Capitalized Subsequent to Acquisition	6,866
Gross Amounts Carried At Close of Period
Land	1,755
Building and Land Improvements	13,885
Total	15,640
Accumulated Depreciation	(6,916)
6708 Alexander Bell Drive [Member]
Initial Cost
Land	897
Building and Land Improvements	11,984
Costs Capitalized Subsequent to Acquisition	1,605
Gross Amounts Carried At Close of Period
Land	897
Building and Land Improvements	13,589
Total	14,486
Accumulated Depreciation	(3,839)
6711 Columbia Gateway Drive [Member]
Initial Cost
Land	2,683
Building and Land Improvements	23,239
Costs Capitalized Subsequent to Acquisition	1,278
Gross Amounts Carried At Close of Period
Land	2,683
Building and Land Improvements	24,517
Total	27,200
Accumulated Depreciation	(6,817)
6716 Alexander Bell Drive [Member]
Initial Cost
Land	1,242
Building and Land Improvements	4,969
Costs Capitalized Subsequent to Acquisition	3,754
Gross Amounts Carried At Close of Period
Land	1,242
Building and Land Improvements	8,723
Total	9,965
Accumulated Depreciation	(5,129)
6721 Columbia Gateway Drive [Member]
Initial Cost
Land	1,753
Building and Land Improvements	34,090
Costs Capitalized Subsequent to Acquisition	102
Gross Amounts Carried At Close of Period
Land	1,753
Building and Land Improvements	34,192
Total	35,945
Accumulated Depreciation	(7,514)
6724 Alexander Bell Drive [Member]
Initial Cost
Land	449
Building and Land Improvements	5,039
Costs Capitalized Subsequent to Acquisition	1,374
Gross Amounts Carried At Close of Period
Land	449
Building and Land Improvements	6,413
Total	6,862
Accumulated Depreciation	(2,670)
6731 Columbia Gateway Drive [Member]
Initial Cost
Land	2,807
Building and Land Improvements	19,098
Costs Capitalized Subsequent to Acquisition	4,872
Gross Amounts Carried At Close of Period
Land	2,807
Building and Land Improvements	23,970
Total	26,777
Accumulated Depreciation	(10,176)
6740 Alexander Bell Drive [Member]
Initial Cost
Land	1,424
Building and Land Improvements	5,696
Costs Capitalized Subsequent to Acquisition	3,321
Gross Amounts Carried At Close of Period
Land	1,424
Building and Land Improvements	9,017
Total	10,441
Accumulated Depreciation	(5,698)
6741 Columbia Gateway Drive [Member]
Initial Cost
Land	675
Building and Land Improvements	1,711
Costs Capitalized Subsequent to Acquisition	124
Gross Amounts Carried At Close of Period
Land	675
Building and Land Improvements	1,835
Total	2,510
Accumulated Depreciation	(466)
6750 Alexander Bell Drive [Member]
Initial Cost
Land	1,263
Building and Land Improvements	12,461
Costs Capitalized Subsequent to Acquisition	3,959
Gross Amounts Carried At Close of Period
Land	1,263
Building and Land Improvements	16,420
Total	17,683
Accumulated Depreciation	(8,761)
6760 Alexander Bell Drive [Member]
Initial Cost
Land	890
Building and Land Improvements	3,561
Costs Capitalized Subsequent to Acquisition	3,358
Gross Amounts Carried At Close of Period
Land	890
Building and Land Improvements	6,919
Total	7,809
Accumulated Depreciation	(3,932)
6940 Columbia Gateway Drive [Member]
Initial Cost
Land	3,545
Building and Land Improvements	9,916
Costs Capitalized Subsequent to Acquisition	7,095
Gross Amounts Carried At Close of Period
Land	3,545
Building and Land Improvements	17,011
Total	20,556
Accumulated Depreciation	(8,164)
6950 Columbia Gateway Drive [Member]
Initial Cost
Land	3,596
Building and Land Improvements	14,269
Costs Capitalized Subsequent to Acquisition	3,238
Gross Amounts Carried At Close of Period
Land	3,596
Building and Land Improvements	17,507
Total	21,103
Accumulated Depreciation	(8,787)
7000 Columbia Gateway Drive [Member]
Initial Cost
Land	3,131
Building and Land Improvements	12,103
Costs Capitalized Subsequent to Acquisition	5,085
Gross Amounts Carried At Close of Period
Land	3,131
Building and Land Improvements	17,188
Total	20,319
Accumulated Depreciation	(6,178)
7005 Columbia Gateway Drive [Member]
Initial Cost
Land	3,036
Building and Land Improvements	753
Gross Amounts Carried At Close of Period
Land	3,036
Building and Land Improvements	753
Total	3,789
7015 Albert Einstein Drive [Member]
Real Estate and Accumulated Depreciation
Encumbrances	829
Initial Cost
Land	2,058
Building and Land Improvements	6,093
Costs Capitalized Subsequent to Acquisition	2,178
Gross Amounts Carried At Close of Period
Land	2,058
Building and Land Improvements	8,271
Total	10,329
Accumulated Depreciation	(3,343)
7061 Columbia Gateway Drive [Member]
Initial Cost
Land	729
Building and Land Improvements	3,094
Costs Capitalized Subsequent to Acquisition	2,018
Gross Amounts Carried At Close of Period
Land	729
Building and Land Improvements	5,112
Total	5,841
Accumulated Depreciation	(2,191)
7063 Columbia Gateway Drive [Member]
Initial Cost
Land	902
Building and Land Improvements	3,684
Costs Capitalized Subsequent to Acquisition	3,151
Gross Amounts Carried At Close of Period
Land	902
Building and Land Improvements	6,835
Total	7,737
Accumulated Depreciation	(3,113)
7065 Columbia Gateway Drive [Member]
Initial Cost
Land	919
Building and Land Improvements	3,763
Costs Capitalized Subsequent to Acquisition	3,095
Gross Amounts Carried At Close of Period
Land	919
Building and Land Improvements	6,858
Total	7,777
Accumulated Depreciation	(3,637)
7067 Columbia Gateway Drive [Member]
Initial Cost
Land	1,829
Building and Land Improvements	11,823
Costs Capitalized Subsequent to Acquisition	3,051
Gross Amounts Carried At Close of Period
Land	1,829
Building and Land Improvements	14,874
Total	16,703
Accumulated Depreciation	(7,037)
7125 Columbia Gateway Drive [Member]
Initial Cost
Land	3,361
Building and Land Improvements	2,354
Costs Capitalized Subsequent to Acquisition	279
Gross Amounts Carried At Close of Period
Land	3,361
Building and Land Improvements	2,633
Total	5,994
7125 Columbia Gateway Drive [Member]
Initial Cost
Land	17,126
Building and Land Improvements	46,994
Costs Capitalized Subsequent to Acquisition	15,786
Gross Amounts Carried At Close of Period
Land	17,126
Building and Land Improvements	62,780
Total	79,906
Accumulated Depreciation	(20,876)
7130 Columbia Gateway Drive [Member]
Initial Cost
Land	1,350
Building and Land Improvements	4,359
Costs Capitalized Subsequent to Acquisition	2,534
Gross Amounts Carried At Close of Period
Land	1,350
Building and Land Improvements	6,893
Total	8,243
Accumulated Depreciation	(3,192)
7134 Columbia Gateway Drive [Member]
Initial Cost
Land	704
Building and Land Improvements	4,707
Costs Capitalized Subsequent to Acquisition	353
Gross Amounts Carried At Close of Period
Land	704
Building and Land Improvements	5,060
Total	5,764
Accumulated Depreciation	(1,422)
7138 Columbia Gateway Drive [Member]
Initial Cost
Land	1,104
Building and Land Improvements	3,518
Costs Capitalized Subsequent to Acquisition	2,729
Gross Amounts Carried At Close of Period
Land	1,104
Building and Land Improvements	6,247
Total	7,351
Accumulated Depreciation	(3,459)
7142 Columbia Gateway Drive [Member]
Initial Cost
Land	1,342
Building and Land Improvements	4,657
Costs Capitalized Subsequent to Acquisition	2,608
Gross Amounts Carried At Close of Period
Land	1,342
Building and Land Improvements	7,265
Total	8,607
Accumulated Depreciation	(2,796)
7150 Columbia Gateway Drive [Member]
Initial Cost
Land	1,032
Building and Land Improvements	3,429
Costs Capitalized Subsequent to Acquisition	813
Gross Amounts Carried At Close of Period
Land	1,032
Building and Land Improvements	4,242
Total	5,274
Accumulated Depreciation	(1,439)
7150 Riverwood Drive [Member]
Initial Cost
Land	1,821
Building and Land Improvements	4,388
Costs Capitalized Subsequent to Acquisition	1,774
Gross Amounts Carried At Close of Period
Land	1,821
Building and Land Improvements	6,162
Total	7,983
Accumulated Depreciation	(2,343)
7160 Riverwood Drive [Member]
Initial Cost
Land	2,732
Building and Land Improvements	7,006
Costs Capitalized Subsequent to Acquisition	2,455
Gross Amounts Carried At Close of Period
Land	2,732
Building and Land Improvements	9,461
Total	12,193
Accumulated Depreciation	(3,778)
7170 Riverwood Drive [Member]
Initial Cost
Land	1,283
Building and Land Improvements	3,096
Costs Capitalized Subsequent to Acquisition	1,465
Gross Amounts Carried At Close of Period
Land	1,283
Building and Land Improvements	4,561
Total	5,844
Accumulated Depreciation	(1,798)
7175 Riverwood Drive [Member]
Initial Cost
Land	1,788
Building and Land Improvements	7,269
Gross Amounts Carried At Close of Period
Land	1,788
Building and Land Improvements	7,269
Total	9,057
Accumulated Depreciation	(752)
7200 Redstone Gateway [Member]
Real Estate and Accumulated Depreciation
Encumbrances	6,303
Initial Cost
Land	0
Building and Land Improvements	8,348
Costs Capitalized Subsequent to Acquisition	5
Gross Amounts Carried At Close of Period
Land	0
Building and Land Improvements	8,353
Total	8,353
Accumulated Depreciation	(752)
7200 Riverwood Road [Member]
Initial Cost
Land	4,089
Building and Land Improvements	22,630
Costs Capitalized Subsequent to Acquisition	4,532
Gross Amounts Carried At Close of Period
Land	4,089
Building and Land Improvements	27,162
Total	31,251
Accumulated Depreciation	(10,180)
7205 Riverwood Drive [Member]
Initial Cost
Land	1,367
Building and Land Improvements	21,419
Gross Amounts Carried At Close of Period
Land	1,367
Building and Land Improvements	21,419
Total	22,786
Accumulated Depreciation	(2,381)
7272 Park Circle Drive [Member]
Initial Cost
Land	1,479
Building and Land Improvements	6,300
Costs Capitalized Subsequent to Acquisition	4,578
Gross Amounts Carried At Close of Period
Land	1,479
Building and Land Improvements	10,878
Total	12,357
Accumulated Depreciation	(3,898)
7318 Parkway Drive [Member]
Initial Cost
Land	972
Building and Land Improvements	3,888
Costs Capitalized Subsequent to Acquisition	1,239
Gross Amounts Carried At Close of Period
Land	972
Building and Land Improvements	5,127
Total	6,099
Accumulated Depreciation	(2,398)
7400 Redstone Gateway [Member]
Real Estate and Accumulated Depreciation
Encumbrances	6,914
Initial Cost
Land	0
Building and Land Improvements	9,223
Gross Amounts Carried At Close of Period
Land	0
Building and Land Improvements	9,223
Total	9,223
Accumulated Depreciation	(582)
7467 Ridge Road [Member]
Initial Cost
Land	1,565
Building and Land Improvements	3,116
Costs Capitalized Subsequent to Acquisition	4,264
Gross Amounts Carried At Close of Period
Land	1,565
Building and Land Improvements	7,380
Total	8,945
Accumulated Depreciation	(1,930)
7740 Milestone Parkway [Member]
Real Estate and Accumulated Depreciation
Encumbrances	18,203
Initial Cost
Land	3,825
Building and Land Improvements	34,176
Costs Capitalized Subsequent to Acquisition	567
Gross Amounts Carried At Close of Period
Land	3,825
Building and Land Improvements	34,743
Total	38,568
Accumulated Depreciation	(6,482)
7770 Backlick Road [Member]
Initial Cost
Land	6,387
Building and Land Improvements	76,315
Costs Capitalized Subsequent to Acquisition	142
Gross Amounts Carried At Close of Period
Land	6,387
Building and Land Improvements	76,457
Total	82,844
Accumulated Depreciation	(9,075)
7880 Milestone Parkway [Member]
Initial Cost
Land	4,857
Building and Land Improvements	24,677
Costs Capitalized Subsequent to Acquisition	62
Gross Amounts Carried At Close of Period
Land	4,857
Building and Land Improvements	24,739
Total	29,596
Accumulated Depreciation	(1,382)
8621 Robert Fulton Drive [Member]
Initial Cost
Land	2,317
Building and Land Improvements	12,642
Costs Capitalized Subsequent to Acquisition	537
Gross Amounts Carried At Close of Period
Land	2,317
Building and Land Improvements	13,179
Total	15,496
Accumulated Depreciation	(4,097)
8661 Robert Fulton Drive [Member]
Initial Cost
Land	1,510
Building and Land Improvements	3,764
Costs Capitalized Subsequent to Acquisition	2,453
Gross Amounts Carried At Close of Period
Land	1,510
Building and Land Improvements	6,217
Total	7,727
Accumulated Depreciation	(2,655)
8671 Robert Fulton Drive [Member]
Initial Cost
Land	1,718
Building and Land Improvements	4,280
Costs Capitalized Subsequent to Acquisition	4,052
Gross Amounts Carried At Close of Period
Land	1,718
Building and Land Improvements	8,332
Total	10,050
Accumulated Depreciation	(3,688)
870 Elkridge Landing Road [Member]
Initial Cost
Land	2,003
Building and Land Improvements	9,442
Costs Capitalized Subsequent to Acquisition	8,735
Gross Amounts Carried At Close of Period
Land	2,003
Building and Land Improvements	18,177
Total	20,180
Accumulated Depreciation	(9,370)
891 Elkridge Landing Road [Member]
Initial Cost
Land	1,165
Building and Land Improvements	4,772
Costs Capitalized Subsequent to Acquisition	3,466
Gross Amounts Carried At Close of Period
Land	1,165
Building and Land Improvements	8,238
Total	9,403
Accumulated Depreciation	(4,186)
901 Elkridge Landing Road [Member]
Initial Cost
Land	1,156
Building and Land Improvements	4,437
Costs Capitalized Subsequent to Acquisition	2,704
Gross Amounts Carried At Close of Period
Land	1,156
Building and Land Improvements	7,141
Total	8,297
Accumulated Depreciation	(3,608)
911 Elkridge Landing Road [Member]
Initial Cost
Land	1,215
Building and Land Improvements	4,861
Costs Capitalized Subsequent to Acquisition	2,191
Gross Amounts Carried At Close of Period
Land	1,215
Building and Land Improvements	7,052
Total	8,267
Accumulated Depreciation	(3,899)
938 Elkridge Landing Road [Member]
Initial Cost
Land	922
Building and Land Improvements	4,748
Costs Capitalized Subsequent to Acquisition	1,446
Gross Amounts Carried At Close of Period
Land	922
Building and Land Improvements	6,194
Total	7,116
Accumulated Depreciation	(2,621)
939 Elkridge Landing Road [Member]
Initial Cost
Land	939
Building and Land Improvements	3,756
Costs Capitalized Subsequent to Acquisition	4,438
Gross Amounts Carried At Close of Period
Land	939
Building and Land Improvements	8,194
Total	9,133
Accumulated Depreciation	(4,254)
940 Elkridge Landing Road [Member]
Initial Cost
Land	842
Building and Land Improvements	4
Gross Amounts Carried At Close of Period
Land	842
Building and Land Improvements	4
Total	846
9651 Hornbaker Road [Member]
Initial Cost
Land	6,050
Building and Land Improvements	249,142
Costs Capitalized Subsequent to Acquisition	3,868
Gross Amounts Carried At Close of Period
Land	6,050
Building and Land Improvements	253,010
Total	259,060
Accumulated Depreciation	(39,294)
Arundel Preserve [Member]
Initial Cost
Land	13,401
Building and Land Improvements	9,578
Gross Amounts Carried At Close of Period
Land	13,401
Building and Land Improvements	9,578
Total	22,979
Bethlehem Tech Park-DC18 [Member]
Initial Cost
Land	3,599
Building and Land Improvements	25,992
Gross Amounts Carried At Close of Period
Land	3,599
Building and Land Improvements	25,992
Total	29,591
Accumulated Depreciation	(306)
Bethlehem Tech Park-DC19 [Member]
Initial Cost
Land	3,708
Building and Land Improvements	16,362
Gross Amounts Carried At Close of Period
Land	3,708
Building and Land Improvements	16,362
Total	20,070
Accumulated Depreciation	(455)
Bethlehem Tech Park-DC20 [Member]
Initial Cost
Land	3,599
Building and Land Improvements	23,625
Gross Amounts Carried At Close of Period
Land	3,599
Building and Land Improvements	23,625
Total	27,224
Accumulated Depreciation	(370)
Bethlehem Tech Park - DC 23 [Member]
Initial Cost
Land	0
Building and Land Improvements	479
Gross Amounts Carried At Close of Period
Land	0
Building and Land Improvements	479
Total	479
BLC 1 [Member]
Initial Cost
Land	12,035
Building and Land Improvements	368
Gross Amounts Carried At Close of Period
Land	12,035
Building and Land Improvements	368
Total	12,403
BLC 2 [Member]
Initial Cost
Land	12,035
Building and Land Improvements	292
Gross Amounts Carried At Close of Period
Land	12,035
Building and Land Improvements	292
Total	12,327
Canton Crossing Land [Member]
Initial Cost
Land	16,085
Building and Land Improvements	2,698
Gross Amounts Carried At Close of Period
Land	16,085
Building and Land Improvements	2,698
Total	18,783
Canton Crossing Util Distr Ctr [Member]
Initial Cost
Land	7,300
Building and Land Improvements	15,556
Costs Capitalized Subsequent to Acquisition	986
Gross Amounts Carried At Close of Period
Land	7,300
Building and Land Improvements	16,542
Total	23,842
Accumulated Depreciation	(4,475)
Columbia Gateway - Southridge [Member]
Initial Cost
Land	6,387
Building and Land Improvements	3,719
Gross Amounts Carried At Close of Period
Land	6,387
Building and Land Improvements	3,719
Total	10,106
Dahlgren Technology Center [Member]
Initial Cost
Land	978
Building and Land Improvements	178
Gross Amounts Carried At Close of Period
Land	978
Building and Land Improvements	178
Total	1,156
Expedition VII [Member]
Initial Cost
Land	705
Building and Land Improvements	729
Gross Amounts Carried At Close of Period
Land	705
Building and Land Improvements	729
Total	1,434
Innovation Park [Member]
Initial Cost
Land	4,443
Building and Land Improvements	120
Gross Amounts Carried At Close of Period
Land	4,443
Building and Land Improvements	120
Total	4,563
M Square Research Park [Member]
Initial Cost
Land	0
Building and Land Improvements	3,571
Gross Amounts Carried At Close of Period
Land	0
Building and Land Improvements	3,571
Total	3,571
MR Land [Member]
Initial Cost
Land	18,827
Building and Land Improvements	293
Gross Amounts Carried At Close of Period
Land	18,827
Building and Land Improvements	293
Total	19,120
National Business Park North [Member]
Initial Cost
Land	28,066
Building and Land Improvements	47,802
Gross Amounts Carried At Close of Period
Land	28,066
Building and Land Improvements	47,802
Total	75,868
North Gate Business Park [Member]
Initial Cost
Land	1,755
Building and Land Improvements	0
Gross Amounts Carried At Close of Period
Land	1,755
Building and Land Improvements	0
Total	1,755
Northwest Crossroads [Member]
Initial Cost
Land	7,430
Building and Land Improvements	847
Gross Amounts Carried At Close of Period
Land	7,430
Building and Land Improvements	847
Total	8,277
NOVA Office A [Member]
Initial Cost
Land	2,096
Building and Land Improvements	46,835
Gross Amounts Carried At Close of Period
Land	2,096
Building and Land Improvements	46,835
Total	48,931
Accumulated Depreciation	(3,403)
NOVA Office B [Member]
Initial Cost
Land	739
Building and Land Improvements	27,128
Gross Amounts Carried At Close of Period
Land	739
Building and Land Improvements	27,128
Total	27,867
Accumulated Depreciation	(1,128)
NOVA Office D [Member]
Initial Cost
Land	6,587
Building and Land Improvements	38,758
Gross Amounts Carried At Close of Period
Land	6,587
Building and Land Improvements	38,758
Total	45,345
Accumulated Depreciation	(437)
Old Annapolis Road [Member]
Initial Cost
Land	1,637
Building and Land Improvements	5,500
Costs Capitalized Subsequent to Acquisition	5,241
Gross Amounts Carried At Close of Period
Land	1,637
Building and Land Improvements	10,741
Total	12,378
Accumulated Depreciation	(3,509)
Paragon Park - DC 21 [Member]
Initial Cost
Land	7,828
Building and Land Improvements	17,992
Gross Amounts Carried At Close of Period
Land	7,828
Building and Land Improvements	17,992
Total	25,820
Accumulated Depreciation	(100)
Paragon Park - DC 22 [Member]
Initial Cost
Land	7,828
Building and Land Improvements	17,445
Gross Amounts Carried At Close of Period
Land	7,828
Building and Land Improvements	17,445
Total	25,273
Accumulated Depreciation	(68)
Patriot Point - DC15 [Member]
Initial Cost
Land	12,156
Building and Land Improvements	17,069
Gross Amounts Carried At Close of Period
Land	12,156
Building and Land Improvements	17,069
Total	29,225
Accumulated Depreciation	(752)
Patriot Point - DC16 [Member]
Initial Cost
Land	12,156
Building and Land Improvements	16,973
Gross Amounts Carried At Close of Period
Land	12,156
Building and Land Improvements	16,973
Total	29,129
Accumulated Depreciation	(709)
Patriot Point - DC17 [Member]
Initial Cost
Land	6,078
Building and Land Improvements	16,347
Gross Amounts Carried At Close of Period
Land	6,078
Building and Land Improvements	16,347
Total	22,425
Accumulated Depreciation	(520)
Patriot Ridge [Member]
Initial Cost
Land	18,517
Building and Land Improvements	14,467
Gross Amounts Carried At Close of Period
Land	18,517
Building and Land Improvements	14,467
Total	32,984
Redstone Gateway [Member]
Initial Cost
Land	0
Building and Land Improvements	19,152
Gross Amounts Carried At Close of Period
Land	0
Building and Land Improvements	19,152
Total	19,152
Route 15/Biggs Ford Road [Member]
Initial Cost
Land	1,129
Building and Land Improvements	0
Gross Amounts Carried At Close of Period
Land	1,129
Building and Land Improvements	0
Total	1,129
Sentry Gateway [Member]
Initial Cost
Land	8,275
Building and Land Improvements	3,704
Gross Amounts Carried At Close of Period
Land	8,275
Building and Land Improvements	3,704
Total	11,979
Sentry Gateway - T [Member]
Initial Cost
Land	14,020
Building and Land Improvements	38,804
Costs Capitalized Subsequent to Acquisition	13
Gross Amounts Carried At Close of Period
Land	14,020
Building and Land Improvements	38,817
Total	52,837
Accumulated Depreciation	(10,561)
Sentry Gateway - V [Member]
Initial Cost
Land	0
Building and Land Improvements	1,066
Gross Amounts Carried At Close of Period
Land	0
Building and Land Improvements	1,066
Total	1,066
Accumulated Depreciation	(241)
Sentry Gateway - W [Member]
Initial Cost
Land	0
Building and Land Improvements	1,884
Costs Capitalized Subsequent to Acquisition	71
Gross Amounts Carried At Close of Period
Land	0
Building and Land Improvements	1,955
Total	1,955
Accumulated Depreciation	(392)
Sentry Gateway - X [Member]
Initial Cost
Land	1,964
Building and Land Improvements	21,178
Gross Amounts Carried At Close of Period
Land	1,964
Building and Land Improvements	21,178
Total	23,142
Accumulated Depreciation	(3,787)
Sentry Gateway - Y [Member]
Initial Cost
Land	1,964
Building and Land Improvements	21,298
Gross Amounts Carried At Close of Period
Land	1,964
Building and Land Improvements	21,298
Total	23,262
Accumulated Depreciation	(3,810)
Sentry Gateway - Z [Member]
Initial Cost
Land	1,964
Building and Land Improvements	30,573
Gross Amounts Carried At Close of Period
Land	1,964
Building and Land Improvements	30,573
Total	32,537
Accumulated Depreciation	(2,144)
Westfields - Park Center [Member]
Initial Cost
Land	16,418
Building and Land Improvements	11,264
Gross Amounts Carried At Close of Period
Land	16,418
Building and Land Improvements	11,264
Total	27,682
Westfields Corporate Center [Member]
Initial Cost
Land	7,141
Building and Land Improvements	1,649
Gross Amounts Carried At Close of Period
Land	7,141
Building and Land Improvements	1,649
Total	8,790
Other Developments, including intercompany eliminations [Member]
Initial Cost
Land	4
Building and Land Improvements	283
Costs Capitalized Subsequent to Acquisition	256
Gross Amounts Carried At Close of Period
Land	4
Building and Land Improvements	539
Total	543
Accumulated Depreciation	(58)
Fair value measurement on a nonrecurring basis [Member] \| Real Estate Held [Member]
Additional information
Impairment losses	13,700
Fair value measurement on a nonrecurring basis [Member] \| Real Estate Investment Properties, Net [Member]
Additional information
Impairment losses	$ 15,100

Schedule III - Real Estate an96

Schedule III - Real Estate and Accumulated Depreciation (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Changes in cost of properties
Beginning balance	$ 3,874,715	$ 4,158,616	$ 4,014,336
Acquisitions of operating properties	0	0	194,616
Improvements and other additions	259,548	251,960	273,761
Sales	(138,216)	(268,038)	(172,628)
Impairments	(15,116)	(143,502)	(29,548)
Other dispositions	(118)	(124,321)	(121,921)
Ending balance	3,980,813	3,874,715	4,158,616
Changes in accumulated depreciation
Beginning balance	715,951	718,680	703,083
Depreciation expense	107,772	105,763	112,695
Sales	(22,567)	(56,607)	(49,614)
Impairments	0	(42,161)	(6,092)
Other dispositions	(118)	(9,724)	(41,392)
Ending balance	$ 801,038	$ 715,951	$ 718,680