Corporate Office Properties Trust 10-K 21 Feb 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Jan. 31, 2019	Jun. 30, 2018
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, 2018
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$ 2,700
Entity Common Stock, Shares Outstanding		110,263,078
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
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, 2018
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			$ 84.4
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Properties, net:
Operating properties, net	$ 2,847,265	$ 2,737,611
Projects in development or held for future development	403,361	403,494
Total properties, net	3,250,626	3,141,105
Assets held for sale, net	0	42,226
Cash and cash equivalents	8,066	12,261
Investment in unconsolidated real estate joint venture	39,845	41,787
Accounts receivable (net of allowance for doubtful accounts of $830 and $607, respectively)	26,277	31,802
Deferred rent receivable (net of allowance of $264 and $364, respectively)	89,350	86,710
Intangible assets on real estate acquisitions, net	43,470	59,092
Deferred leasing costs (net of accumulated amortization of $31,994 and $29,560, respectively)	50,191	48,322
Investing receivables	56,982	57,493
Interest rate derivatives	5,617	3,073
Prepaid expenses and other assets, net	85,581	71,334
Total assets	3,656,005	3,595,205
Liabilities:
Debt, net	1,823,909	1,828,333
Accounts payable and accrued expenses	92,855	108,137
Rents received in advance and security deposits	30,079	25,648
Dividends and distributions payable	30,856	28,921
Deferred revenue associated with operating leases	9,125	11,682
Deferred property sale	0	43,377
Capital lease obligation	660	15,853
Other liabilities	15,213	41,822
Total liabilities	2,002,697	2,103,773
Commitments and contingencies (Note 20)
Redeemable noncontrolling interests	26,260	23,125
Corporate Office Properties Trust’s shareholders’ equity:
Common Shares of beneficial interest	1,102	1,013
Additional paid-in capital	2,431,355	2,201,047
Cumulative distributions in excess of net income	(846,808)	(802,085)
Accumulated other comprehensive (loss) income	(238)	2,167
Total Corporate Office Properties Trust’s shareholders’ equity	1,585,411	1,402,142
Noncontrolling interests in subsidiaries:
Common units in COPLP	19,168	45,097
Preferred units in COPLP	8,800	8,800
Other consolidated entities	13,669	12,268
Noncontrolling interests in subsidiaries	41,637	66,165
Total equity	1,627,048	1,468,307
Total liabilities, redeemable noncontrolling interests and equity	3,656,005	3,595,205
Corporate Office Properties, L.P. [Member]
Properties, net:
Operating properties, net	2,847,265	2,737,611
Projects in development or held for future development	403,361	403,494
Total properties, net	3,250,626	3,141,105
Assets held for sale, net	0	42,226
Cash and cash equivalents	8,066	12,261
Investment in unconsolidated real estate joint venture	39,845	41,787
Accounts receivable (net of allowance for doubtful accounts of $830 and $607, respectively)	26,277	31,802
Deferred rent receivable (net of allowance of $264 and $364, respectively)	89,350	86,710
Intangible assets on real estate acquisitions, net	43,470	59,092
Deferred leasing costs (net of accumulated amortization of $31,994 and $29,560, respectively)	50,191	48,322
Investing receivables	56,982	57,493
Interest rate derivatives	5,617	3,073
Prepaid expenses and other assets, net	81,713	66,718
Total assets	3,652,137	3,590,589
Liabilities:
Debt, net	1,823,909	1,828,333
Accounts payable and accrued expenses	92,855	108,137
Rents received in advance and security deposits	30,079	25,648
Dividends and distributions payable	30,856	28,921
Deferred revenue associated with operating leases	9,125	11,682
Deferred property sale	0	43,377
Capital lease obligation	660	15,853
Other liabilities	11,345	37,206
Total liabilities	1,998,829	2,099,157
Commitments and contingencies (Note 20)
Redeemable noncontrolling interests	26,260	23,125
Corporate Office Properties Trust’s shareholders’ equity:
Common Shares of beneficial interest	1,604,655	1,445,022
Accumulated other comprehensive (loss) income	(121)	2,173
Total Corporate Office Properties Trust’s shareholders’ equity	1,613,334	1,455,995
Noncontrolling interests in subsidiaries:
Noncontrolling interests in subsidiaries	13,714	12,312
Total equity	1,627,048	1,468,307
Total liabilities, redeemable noncontrolling interests and equity	3,652,137	3,590,589
Corporate Office Properties, L.P. [Member] | Limited Partner [Member]
Corporate Office Properties Trust’s shareholders’ equity:
Preferred units held by limited partner, 352,000 preferred units outstanding at December 31, 2018 and 2017	$ 8,800	$ 8,800

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Allowance for doubtful accounts- AR	$ 830	$ 607
Allowance for deferred rent receivable	264	364
Accumulated amortization of deferred leasing costs	$ 31,994	$ 29,560
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	150,000,000
Common Shares of beneficial interest, shares issued	110,241,868	101,292,299
Common Shares of beneficial interest, shares outstanding	110,241,868	101,292,299
Corporate Office Properties, L.P. [Member]
Allowance for doubtful accounts- AR	$ 830	$ 607
Allowance for deferred rent receivable	264	364
Accumulated amortization of deferred leasing costs	$ 31,994	$ 29,560
General Partner [Member] | Corporate Office Properties, L.P. [Member]
Common Shares of beneficial interest, shares outstanding	110,241,868	101,292,299
Limited Partner [Member] | Corporate Office Properties, L.P. [Member]
Preferred Units, Outstanding	352,000	352,000
Common Shares of beneficial interest, shares outstanding	1,332,886	3,250,878

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues
Rental revenue		$ 407,686	$ 405,722	$ 417,711
Construction contract and other service revenues		60,859	102,840	48,364
Total revenues		578,112	612,820	574,328
Operating expenses
Property operating expenses		201,035	190,964	197,530
Depreciation and amortization associated with real estate operations		137,116	134,228	132,719
Construction contract and other service expenses		58,326	99,618	45,481
Impairment losses		2,367	15,123	101,391
General, administrative and leasing expenses		28,900	30,837	36,553
Business development expenses and land carry costs		5,840	6,213	8,244
Total operating expenses		433,584	476,983	521,918
Interest expense		(75,385)	(76,983)	(83,163)
Interest and other income		4,358	6,318	5,444
Gain on sales of real estate		2,340	9,890	59,679
Loss on early extinguishment of debt		(258)	(513)	(1,110)
Income before equity in income of unconsolidated entities and income taxes		75,583	74,549	33,260
Equity in income of unconsolidated entities		2,697	1,490	752
Income tax benefit (expense)		363	(1,098)	(244)
Net income		78,643	74,941	33,768
Net income attributable to noncontrolling interests:
Common units in COPLP		(1,742)	(1,890)	(507)
Preferred units in COPLP		(660)	(660)	(660)
Other consolidated entities		(3,940)	(3,646)	(3,711)
Net income attributable to COPT		72,301	68,745	28,890
Preferred share/ unit dividends/ distributions		0	(6,219)	(14,297)
Issuance costs associated with redeemed preferred shares		0	(6,847)	(17)
Net income attributable to COPT common shareholders		$ 72,301	$ 55,679	$ 14,576
Earnings per common share: (1)
Net income attributable to COPT common shareholders - basic (in dollars per share/unit)		$ 0.69	$ 0.56	$ 0.15
Net income attributable to COPT common shareholders - diluted (in dollars per share/unit)		$ 0.69	$ 0.56	$ 0.15
Corporate Office Properties, L.P. [Member]
Revenues
Rental revenue		$ 407,686	$ 405,722	$ 417,711
Construction contract and other service revenues		60,859	102,840	48,364
Total revenues		578,112	612,820	574,328
Operating expenses
Property operating expenses		201,035	190,964	197,530
Depreciation and amortization associated with real estate operations		137,116	134,228	132,719
Construction contract and other service expenses		58,326	99,618	45,481
Impairment losses		2,367	15,123	101,391
General, administrative and leasing expenses		28,900	30,837	36,553
Business development expenses and land carry costs		5,840	6,213	8,244
Total operating expenses		433,584	476,983	521,918
Interest expense		(75,385)	(76,983)	(83,163)
Interest and other income		4,358	6,318	5,444
Gain on sales of real estate		2,340	9,890	59,679
Loss on early extinguishment of debt		(258)	(513)	(1,110)
Income before equity in income of unconsolidated entities and income taxes		75,583	74,549	33,260
Equity in income of unconsolidated entities		2,697	1,490	752
Income tax benefit (expense)		363	(1,098)	(244)
Net income		78,643	74,941	33,768
Net income attributable to noncontrolling interests in consolidated entities		(3,940)	(3,646)	(3,715)
Net income attributable to noncontrolling interests:
Net income attributable to COPT		74,703	71,295	30,053
Preferred share/ unit dividends/ distributions		(660)	(6,879)	(14,957)
Issuance costs associated with redeemed preferred shares		0	(6,847)	(17)
Net income attributable to COPT common shareholders		$ 74,043	$ 57,569	$ 15,079
Earnings per common share: (1)
Net income attributable to COPT common shareholders - basic (in dollars per share/unit)	[1]	$ 0.69	$ 0.56	$ 0.15
Net income attributable to COPT common shareholders - diluted (in dollars per share/unit)	[1]	$ 0.69	$ 0.56	$ 0.15
Tenant recoveries and other real estate operations [Member]
Revenues
Total revenues		$ 109,567	$ 104,258	$ 108,253
Tenant recoveries and other real estate operations [Member] | Corporate Office Properties, L.P. [Member]
Revenues
Total revenues		$ 109,567	$ 104,258	$ 108,253
[1]	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 - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net income	$ 78,643	$ 74,941	$ 33,768
Other comprehensive (loss) income
Unrealized (loss) gain on interest rate derivatives	(2,373)	684	(2,915)
(Gain) loss on interest rate derivatives recognized in interest expense	(407)	3,304	4,230
Equity in other comprehensive income (loss) of equity method investee	210	39	(184)
Other comprehensive (loss) income	(2,570)	4,027	1,131
Comprehensive income	76,073	78,968	34,899
Comprehensive income attributable to noncontrolling interests	(6,453)	(6,325)	(4,902)
Comprehensive income attributable to COPT	69,620	72,643	29,997
Corporate Office Properties, L.P. [Member]
Net income	78,643	74,941	33,768
Other comprehensive (loss) income
Unrealized (loss) gain on interest rate derivatives	(2,373)	684	(2,915)
(Gain) loss on interest rate derivatives recognized in interest expense	(407)	3,304	4,230
Equity in other comprehensive income (loss) of equity method investee	210	39	(184)
Other comprehensive (loss) income	(2,570)	4,027	1,131
Comprehensive income	76,073	78,968	34,899
Comprehensive income attributable to noncontrolling interests	(3,940)	(3,646)	(3,715)
Comprehensive income attributable to COPT	$ 72,133	$ 75,322	$ 31,184

Consolidated Statements of Equi

Consolidated Statements of Equity - USD ($) $ in Thousands	Total	Forward Equity Sale Agreement [Member]	Common Stock Issued to Public Under At-the-Market Program [Member]	Common Shares [Member]Common Stock Issued to Public Under At-the-Market Program [Member]	Preferred Shares [Member]	Preferred Shares [Member]Preferred Shares [Member]	Common Shares [Member]	Common Shares [Member]Forward Equity Sale Agreement [Member]	Common Shares [Member]Common Stock Issued to Public Under At-the-Market Program [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member]Forward Equity Sale Agreement [Member]	Additional Paid-in Capital [Member]Common Stock Issued to Public Under At-the-Market Program [Member]	Additional Paid-in Capital [Member]Common Shares [Member]Common Stock Issued to Public Under At-the-Market Program [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]Forward Equity Sale Agreement [Member]	Corporate Office Properties, L.P. [Member]Common Stock Issued to Public Under At-the-Market Program [Member]	Corporate Office Properties, L.P. [Member]Common Shares [Member]	Corporate Office Properties, L.P. [Member]Common Shares [Member]Forward Equity Sale Agreement [Member]	Corporate Office Properties, L.P. [Member]Common Shares [Member]Common Stock Issued to Public Under At-the-Market Program [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, 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
Balance (in units/ shares) at Dec. 31, 2015	94,531,512																			98,208,903
Balance (preferred units) at Dec. 31, 2015																										352,000		7,431,667
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 (87,000, 339,513 and 1,904,615 shares in 2016, 2017 and 2018, respectively)	0						1			1,166						(1,167)
Common shares issued during the period (in units/shares)			3,721,227																			3,721,227
Common shares issued during the period				$ 109,053					$ 37				$ 109,016						$ 109,053			$ 109,053
Share-based compensation (158,912, 179,180 and 146,290 shares issued, net of redemptions in 2016, 2017 and 2018, 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	32,656													28,890	1,107	2,659	32,656			15,096			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															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		6,900,000
Balance (in units/ shares) at Dec. 31, 2016	98,498,651																			102,089,042
Balance at Dec. 31, 2016	$ 1,612,777				172,500		985			2,116,581				(747,825)	(1,731)	72,267	1,612,777			$ 1,419,710			(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 (87,000, 339,513 and 1,904,615 shares in 2016, 2017 and 2018, respectively)	$ 0						3			4,633						(4,636)
Common shares issued during the period (in units/shares)		1,678,913	591,042																	1,678,913		591,042
Common shares issued during the period		$ 49,944	$ 19,668					$ 17	6		$ 49,927	$ 19,662					49,944		19,668	$ 49,944		$ 19,668
Exercise of share options (in units/shares)	5,000																			5,000
Exercise of share options (5,000 shares in 2017)	$ 150						0			150							150			$ 150
Share-based compensation (158,912, 179,180 and 146,290 shares issued, net of redemptions in 2016, 2017 and 2018, 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	76,630													68,745	3,898	3,987	76,630			64,416			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
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	$ 1,468,307				0		1,013			2,201,047				(802,085)	2,167	66,165	1,468,307			$ 1,445,022			2,173	12,312		$ 8,800		$ 0
Increase (Decrease) in Shareholders' Equity
Cumulative effect of accounting change for adoption of hedge accounting guidance														(276)	276					(276)			276
Balance as adjusted	1,468,307				0		1,013			2,201,047				(802,361)	2,443	66,165	1,468,307			$ 1,444,746			2,449	12,312		8,800		0
Conversion of common units to common shares (87,000, 339,513 and 1,904,615 shares in 2016, 2017 and 2018, respectively)	0						19			27,394						(27,413)
Redemption of common units (in units)																				(13,377)
Redemption of common units	(339)															(339)	(339)			$ (339)
Common shares issued during the period (in units/shares)		5,907,000	991,664																		5,907,000	991,664
Common shares issued during the period		$ 172,294	$ 29,732					$ 59	$ 10		$ 172,235	$ 29,722						$ 172,294	$ 29,732		$ 172,294	$ 29,732
Share-based compensation (158,912, 179,180 and 146,290 shares issued, net of redemptions in 2016, 2017 and 2018, respectively)	$ 6,963						1			6,962							6,963			$ 6,963
Share-based compensation (in shares/units)	146,290																			146,290
Redemption of vested equity awards	$ (1,702)									(1,702)							(1,702)			$ (1,702)
Adjustments to noncontrolling interests resulting from changes in ownership of COPLP	0									(2,466)						2,466
Comprehensive income	73,550													72,301	(2,681)	3,930	73,550			74,043			(2,570)	1,417		660		0
Dividends/Distributions	(116,748)													(116,748)			(119,905)			(119,245)						$ (660)		$ 0
Distributions to owners of common and preferred units in COPLP	(3,157)															(3,157)
Distributions to noncontrolling interests in other consolidated entities	(15)															(15)	(15)							(15)
Adjustment to arrive at fair value of redeemable noncontrolling interests	$ (1,837)									(1,837)							(1,837)			$ (1,837)
Balance (preferred units) at Dec. 31, 2018																									352,000	352,000		0
Balance (in units/ shares) at Dec. 31, 2018	110,241,868																			111,574,754					1,332,886		110,241,868
Balance at Dec. 31, 2018	$ 1,627,048				$ 0		$ 1,102			$ 2,431,355				$ (846,808)	$ (238)	$ 41,637	$ 1,627,048			$ 1,604,655			$ (121)	$ 13,714		$ 8,800		$ 0

Consolidated Statements of Eq_2

Consolidated Statements of Equity (Parenthetical) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Balance (in units/ shares)	110,241,868	101,292,299	98,498,651	94,531,512
Conversion of common units to common shares (in shares/units)	1,904,615	339,513	87,000
Exercise of share options (in units/shares)		5,000
Restricted common units/shares net of redemptions	146,290	179,180	158,912
Forward Equity Sale Agreement [Member]
Shares issued to the public (in units/shares)	5,907,000	1,678,913
Common Stock Issued to Public Under At-the-Market Program [Member]
Shares issued to the public (in units/shares)	991,664	591,042	3,721,227

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities
Revenues from real estate operations received	$ 528,066	$ 510,551	$ 514,098
Construction contract and other service revenues received	33,579	102,531	76,824
Property operating expenses paid	(197,647)	(186,577)	(196,352)
Construction contract and other service expenses paid	(79,386)	(82,707)	(46,318)
General, administrative, leasing, business development and land carry costs paid	(27,006)	(32,673)	(34,877)
Interest expense paid	(72,460)	(73,079)	(77,982)
Lease incentives paid	(7,679)	(9,725)	(2,760)
Income taxes paid	(21)	(31)	(5)
Other	3,036	1,831	1,642
Net cash provided by operating activities	180,482	230,121	234,270
Cash flows from investing activities
Construction, development and redevelopment	(159,994)	(200,504)	(161,519)
Tenant improvements on operating properties	(35,098)	(33,409)	(34,275)
Other capital improvements on operating properties	(24,223)	(22,882)	(26,345)
Proceeds from dispositions of properties	0	180,839	262,866
Proceeds from partial sales of properties, net of related debt	0	0	43,089
Leasing costs paid	(10,926)	(14,581)	(10,296)
Other	(2,677)	1,174	(2,346)
Net cash (used in) provided by investing activities	(232,918)	(89,363)	71,174
Proceeds from debt
Revolving Credit Facility	381,000	352,000	495,500
Other debt proceeds	13,406	0	255,000
Repayments of debt
Revolving Credit Facility	(294,000)	(226,000)	(539,000)
Scheduled principal amortization	(4,240)	(4,062)	(5,595)
Other debt repayments	(100,000)	(200,000)	(322,907)
Deferred financing costs paid	(8,292)	(500)	(825)
Payments on capital lease obligations	(15,379)	0	0
Net proceeds from issuance of common shares	202,065	69,534	109,069
Redemption of preferred shares	0	(199,083)	0
Common share/unit dividends/distributions paid	(114,286)	(109,174)	(104,135)
Preferred share/unit dividends/distributions paid	0	(9,305)	(14,210)
Distributions paid to noncontrolling interests in COPLP	(3,699)	(4,426)	(4,619)
Distributions paid to redeemable noncontrolling interests	(1,382)	(8,215)	(15,206)
Redemption of vested equity awards	(1,702)	(1,973)	(2,466)
Other	(3,936)	2,658	(5,694)
Net cash provided by (used in) financing activities	49,555	(338,546)	(155,088)
Net (decrease) increase in cash and cash equivalents and restricted cash	(2,881)	(197,788)	150,356
Cash and cash equivalents and restricted cash
Beginning of year	14,831	212,619	62,263
End of year	11,950	14,831	212,619
Reconciliation of net income to net cash provided by operating activities:
Net income	78,643	74,941	33,768
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and other amortization	139,063	136,501	134,870
Impairment losses	2,367	15,116	101,341
Amortization of deferred financing costs and net debt discounts	3,393	4,307	5,885
Increase in deferred rent receivable	(4,621)	(2,651)	(145)
Gain on sales of real estate	(2,340)	(9,890)	(59,679)
Share-based compensation	6,376	5,615	6,843
Other	(2,733)	(4,216)	(2,605)
Changes in operating assets and liabilities:
Decrease (increase) in accounts receivable	5,673	2,783	(5,262)
(Increase) decrease in prepaid expenses and other assets, net	(987)	7,219	(16,559)
(Decrease) increase in accounts payable, accrued expenses and other liabilities	(49,179)	4,309	43,163
Increase (decrease) in rents received in advance and security deposits	4,827	(3,913)	(7,350)
Net cash provided by operating activities	180,482	230,121	234,270
Reconciliation of cash and cash equivalents and restricted cash:
Cash and cash equivalents and restricted cash	14,831	212,619	62,263
Supplemental schedule of non-cash investing and financing activities:
Increase (decrease) in accrued capital improvements, leasing and other investing activity costs	6,570	(10,654)	5,950
Increase in property in connection with capital lease obligation	0	16,127	0
Increase in property and redeemable noncontrolling interests in connection with property contributed into a joint venture	0	0	22,600
Non-cash changes from recognition of property sale previously accounted for as financing arrangement:
Decrease in assets held for sale, net	(42,226)	0	0
Decrease in deferred property sale	43,377	0	0
Non-cash changes from partial sale of properties, net of debt:
Decrease in properties, net	0	0	(114,597)
Increase in investment in unconsolidated real estate joint venture	0	0	44,373
Decrease in debt	0	0	59,534
Other net decreases in assets and liabilities	0	0	4,211
Increase in fair value of derivatives applied to accumulated other comprehensive income and noncontrolling interests	2,915	3,845	1,315
Decrease in redeemable noncontrolling interests and increase in other liabilities in connection with distribution payable to redeemable noncontrolling interests	0	0	6,675
Equity in other comprehensive income (loss) of an equity method investee	210	39	(184)
Reclassification of preferred shares to be redeemed to liability	0	0	26,583
Decrease in noncontrolling interests and increase in shareholders’ equity in connection with the conversion of common units into common shares	27,413	4,636	1,167
Adjustments to noncontrolling interests resulting from changes in COPLP ownership	2,466	1,486	2,158
Increase (decrease) in redeemable noncontrolling interests and decrease (increase) in equity to carry redeemable noncontrolling interests at fair value	1,837	(626)	621
Corporate Office Properties, L.P. [Member]
Cash flows from operating activities
Revenues from real estate operations received	528,066	510,551	514,098
Construction contract and other service revenues received	33,579	102,531	76,824
Property operating expenses paid	(197,647)	(186,577)	(196,352)
Construction contract and other service expenses paid	(79,386)	(82,707)	(46,318)
General, administrative, leasing, business development and land carry costs paid	(27,006)	(32,673)	(34,877)
Interest expense paid	(72,460)	(73,079)	(77,982)
Lease incentives paid	(7,679)	(9,725)	(2,760)
Income taxes paid	(21)	(31)	(5)
Other	3,036	1,831	1,642
Net cash provided by operating activities	180,482	230,121	234,270
Cash flows from investing activities
Construction, development and redevelopment	(159,994)	(200,504)	(161,519)
Tenant improvements on operating properties	(35,098)	(33,409)	(34,275)
Other capital improvements on operating properties	(24,223)	(22,882)	(26,345)
Proceeds from dispositions of properties	0	180,839	262,866
Proceeds from partial sales of properties, net of related debt	0	0	43,089
Leasing costs paid	(10,926)	(14,581)	(10,296)
Other	(2,677)	1,174	(2,346)
Net cash (used in) provided by investing activities	(232,918)	(89,363)	71,174
Proceeds from debt
Revolving Credit Facility	381,000	352,000	495,500
Other debt proceeds	13,406	0	255,000
Repayments of debt
Revolving Credit Facility	(294,000)	(226,000)	(539,000)
Scheduled principal amortization	(4,240)	(4,062)	(5,595)
Other debt repayments	(100,000)	(200,000)	(322,907)
Deferred financing costs paid	(8,292)	(500)	(825)
Payments on capital lease obligations	(15,379)	0	0
Net proceeds from issuance of common shares	202,065	69,534	109,069
Redemption of preferred shares	0	(199,083)	0
Common share/unit dividends/distributions paid	(117,325)	(112,940)	(108,094)
Preferred share/unit dividends/distributions paid	(660)	(9,965)	(14,870)
Distributions paid to redeemable noncontrolling interests	(1,382)	(8,215)	(15,206)
Redemption of vested equity awards	(1,702)	(1,973)	(2,466)
Other	(3,936)	2,658	(5,694)
Net cash provided by (used in) financing activities	49,555	(338,546)	(155,088)
Net (decrease) increase in cash and cash equivalents and restricted cash	(2,881)	(197,788)	150,356
Cash and cash equivalents and restricted cash
Beginning of year	14,831	212,619	62,263
End of year	11,950	14,831	212,619
Reconciliation of net income to net cash provided by operating activities:
Net income	78,643	74,941	33,768
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and other amortization	139,063	136,501	134,870
Impairment losses	2,367	15,116	101,341
Amortization of deferred financing costs and net debt discounts	3,393	4,307	5,885
Increase in deferred rent receivable	(4,621)	(2,651)	(145)
Gain on sales of real estate	(2,340)	(9,890)	(59,679)
Share-based compensation	6,376	5,615	6,843
Other	(2,733)	(4,216)	(2,605)
Changes in operating assets and liabilities:
Decrease (increase) in accounts receivable	5,673	2,783	(5,262)
(Increase) decrease in prepaid expenses and other assets, net	(1,735)	6,398	(16,885)
(Decrease) increase in accounts payable, accrued expenses and other liabilities	(48,431)	5,130	43,489
Increase (decrease) in rents received in advance and security deposits	4,827	(3,913)	(7,350)
Net cash provided by operating activities	180,482	230,121	234,270
Reconciliation of cash and cash equivalents and restricted cash:
Cash and cash equivalents and restricted cash	14,831	212,619	62,263
Supplemental schedule of non-cash investing and financing activities:
Increase (decrease) in accrued capital improvements, leasing and other investing activity costs	6,570	(10,654)	5,950
Increase in property in connection with capital lease obligation	0	16,127	0
Increase in property and redeemable noncontrolling interests in connection with property contributed into a joint venture	0	0	22,600
Non-cash changes from recognition of property sale previously accounted for as financing arrangement:
Decrease in assets held for sale, net	(42,226)	0	0
Decrease in deferred property sale	43,377	0	0
Non-cash changes from partial sale of properties, net of debt:
Decrease in properties, net	0	0	(114,597)
Increase in investment in unconsolidated real estate joint venture	0	0	44,373
Decrease in debt	0	0	59,534
Other net decreases in assets and liabilities	0	0	4,211
Increase in fair value of derivatives applied to accumulated other comprehensive income and noncontrolling interests	2,915	3,845	1,315
Decrease in redeemable noncontrolling interests and increase in other liabilities in connection with distribution payable to redeemable noncontrolling interests	0	0	6,675
Equity in other comprehensive income (loss) of an equity method investee	210	39	(184)
Reclassification of preferred shares to be redeemed to liability	0	0	26,583
Increase (decrease) in redeemable noncontrolling interests and decrease (increase) in equity to carry redeemable noncontrolling interests at fair value	$ 1,837	$ (626)	$ 621

Organization

Organization	12 Months Ended
	Dec. 31, 2018
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, 2018 , our properties included the following (all references to number of properties, square footage, acres and megawatts are unaudited): • 163 properties totaling 18.1 million square feet comprised of 15.1 million square feet in 145 office properties and 3.0 million square feet in 18 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 ( six office properties and four data center shells) that we estimate will total approximately 1.3 million square feet upon completion, including two partially-operational properties; and • approximately 900 acres of land controlled for future development that we believe could be developed into approximately 11.7 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, 2018 , COPT owned 98.8% 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
	Dec. 31, 2018
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. When we own an equity investment in an entity and cannot exert significant influence over its operations: • prior to January 1, 2018, we used the cost method of accounting; and • effective January 1, 2018, we measure the investment at fair value, with changes recognized through net income. For an investment without a readily determinable fair value, we measure the investment at cost, less any impairments, plus or minus changes resulting from observable price changes for an identical or similar investment of the same issuer. 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 (which is included in the line entitled “Prepaid expenses and other assets, net” on our consolidated balance sheets), 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 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. Deferred Financing Costs We defer costs of financing arrangements and recognize these costs as interest expense over the related debt terms on a straight-line basis, which approximates the amortization that would occur under the effective interest method of amortization. We expense any unamortized loan costs when loans are retired early. 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 Real Estate Operations Revenue 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. Construction Contract and Other Service Revenues We enter into construction contracts to complete various design and construction services primarily for our United States Government tenants. The revenues and expenses from these services consist primarily of subcontracted costs that are reimbursed to us by our customers along with a fee. These services are an ancillary component of our overall operations, with small operating margins relative to the revenue. We review each contract to determine the performance obligations and allocate the transaction price based on the standalone selling price, as discussed further below. We recognize revenue under these contracts as services are performed in an amount that reflects the consideration we expect to receive in exchange for those services. Our performance obligations are satisfied over time as work progresses. Revenue recognition is determined using the input method based on costs incurred as of point in time relative to the total estimated costs at completion to measure progress toward satisfying our performance obligations. We believe incurred costs of work performed best depicts the transfer of control of the services being transferred to the customer. In determining whether the performance obligations of each construction contract should be accounted for separately versus together, we consider numerous factors that may require significant judgment, including: whether the components contracted are substantially the same with the same pattern of transfer; whether the customer could contract with another party to perform construction based on our design project; and whether the customer can elect not to move forward after the design phase of the contract. Most of our contracts have a single performance obligation as the promise to transfer the services is not separately identifiable from other obligations in the contracts and, therefore, are not distinct. Some contracts have multiple performance obligations, most commonly due to having distinct project phases for design and construction for which our customer is making decisions and managing separately. In these cases, we allocate the transaction price between these performance obligations based on the relative standalone selling prices, which we determine by evaluating: the relative costs of each performance obligation; the expected operating margins (which typically do not vary significantly between obligations); and amounts set forth in the contracts for each obligation. Contract modifications, such as change orders, are routine for our construction contracts and are generally determined to be additions to the existing performance obligations because they would have been part of the initial performance obligations if they were identified at the initial contract date. We have three main types of compensation arrangements for our construction contracts: guaranteed maximum price (“GMP”); firm fixed price (“FFP”); and cost-plus fee. • GMP contracts provide for revenue equal to costs incurred plus a fee equal to a percentage of such costs, up to a maximum contract amount. We generally enter into GMP contracts for projects that are significant in nature based on the size of the project and total fees, and for which the full scope of the project has not been determined as of the contract date. GMP contracts are lower risk to us than FFP contracts since the costs and revenue move proportionately to one another. • FFP contracts provide for revenue equal to a fixed fee. These contracts are typically lower in value and scope relative to GMP contracts, and are generally entered into when the scope of the project is well defined. Typically, we assume more risk with FFP contracts than GMP contracts since the revenue is fixed and we could realize losses or less than expected profits if we incur more costs than originally estimated. However, these types of contracts offer the opportunity for additional profits when we complete the work for less than originally estimated. • Cost-plus fee contracts provide for revenue equal to costs incurred plus a fee equal to a percentage of such costs but, unlike GMP contracts, do not have a maximum contract amount. Similar to GMP contracts, cost-plus fee contracts are low risk to us since the costs and revenue move proportionately to one another. Construction contract cost estimates are based primarily on contracts in place with subcontractors to complete most of the work, but may also include assumptions, such as performance of subcontractors and cost and availability of materials, to project the outcome of future events over the course of the project. We review and update these estimates regularly as a significant change could affect the profitability of our construction contracts. We recognize adjustments in estimated profit on contracts under the cumulative catch-up method as the modification does not create a new performance obligation. Under this method, the impact of the adjustment on profit recorded to date on a contract is recognized in the period the adjustment is identified. Revenue and profit in future periods are recognized using the adjusted estimate. If at any time the estimate of contract profitability indicates an anticipated loss on the contract, we recognize the total loss in the quarter it is identified. Our timing of revenue recognition for construction contracts generally differs from the timing of invoicing to customers. We recognize such revenue as we satisfy our performance obligations. Payment terms and conditions vary by contract type. Under most of our contracts, we bill customers monthly, as work progresses, in accordance with the contract terms, with payment due in 30 days , although customers occasionally pay in advance of services being provided. We have determined that our contracts generally do not include a significant financing component. The primary purpose of the timing of our invoicing is for convenience, not to receive financing from our customers or to provide customers with financing. Additionally, the timing of transfer of the services is often at the discretion of the customer. Under most of our contracts, we bill customers one month subsequent to revenue recognition, resulting in contract assets representing unbilled construction revenue. Our contract liabilities consist of advance payments from our customers or billings in excess of construction contract revenue recognized. 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. Prior to our adoption of guidance issued by the FASB effective January 1, 2018, we: deferred only the effective portion of changes in fair value of the designated cash flow hedges to accumulated other comprehensive income (“AOCI”) or loss (“AOCL”), reclassifying such deferrals to interest expense as interest expense was recognized on the hedged forecasted transactions; and recognized the ineffective portion of the change in fair value of interest rate derivatives directly in interest expense. Effective January 1, 2018, we defer all changes in the fair value of designated cash flow hedges to AOCI or AOCL, reclassifying such deferrals to interest expense as interest expense is recognized on the hedged forecasted transactions. When an interest rate swap designated as a cash flow hedge no longer qualifies for hedge accounting and the hedged transactions are probable not to occur, 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. Refer to the section below entitled “Recent Accounting Pronouncements” for additional disclosure pertaining to the effect of the 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 account

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2018
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 sheets using quoted market prices, as are other marketable securities that we hold. The balance of the plan, which was fully funded, totaled $3.9 million as of December 31, 2018 and $4.6 million as of December 31, 2017 , and is included in the line entitled “prepaid expenses and other assets, net” on COPT’s consolidated balance sheets. 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 are classified in Level 1 of the fair value hierarchy, while the offsetting liability 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, 2018 and 2017 , 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. The fair values of our investing receivables, as disclosed in Note 8, were 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, 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 subsidiaries that are accounted for at fair value on a recurring basis as of December 31, 2018 and 2017 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 Inputs (Level 3) Total December 31, 2018: Assets: Marketable securities in deferred compensation plan (1) Mutual funds $ 3,819 $ — $ — $ 3,819 Other 49 — — 49 Interest rate derivatives — 5,617 — 5,617 Total assets $ 3,868 $ 5,617 $ — $ 9,485 Liabilities: Deferred compensation plan liability (2) $ — $ 3,868 $ — $ 3,868 Interest rate derivatives (2) — 5,459 — 5,459 Total liabilities $ — $ 9,327 $ — $ 9,327 December 31, 2017: Assets: Marketable securities in deferred compensation plan (1) Mutual funds $ 4,547 $ — $ — $ 4,547 Other 69 — — 69 Interest rate derivatives — 3,073 — 3,073 Total assets $ 4,616 $ 3,073 $ — $ 7,689 Liabilities: Deferred compensation plan liability (2) $ — $ 4,616 $ — $ 4,616 (1) Included in the line entitled “prepaid expenses and other assets, net” on COPT ’ s consolidated balance sheet. (2) Included in the line entitled “other liabilities” on COPT ’ s consolidated balance sheet. COPLP and Subsidiaries The tables below set forth financial assets and liabilities of COPLP and subsidiaries that are accounted for at fair value on a recurring basis as of December 31, 2018 and 2017 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 Inputs (Level 3) Total December 31, 2018: Assets: Interest rate derivatives $ — $ 5,617 $ — $ 5,617 Liabilities: Interest rate derivatives (1) $ — $ 5,459 $ — $ 5,459 December 31, 2017: Assets: Interest rate derivatives $ — $ 3,073 $ — $ 3,073 (1) Included in the line entitled “other liabilities” on COPLP ’ s consolidated balance sheet. 2017 Nonrecurring Fair Value Measurements As part of our closing process for each quarter 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 of 2017. 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 Description Quoted Prices in Significant Other Significant Inputs (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.

Concentration of Revenue

Concentration of Revenue	12 Months Ended
	Dec. 31, 2018
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 24% of our rental revenue in 2018 , 22% in 2017 and 21% in 2016 (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 Northern Virginia Defense/IT reportable sub-segments (see Note 17). No other individual tenants accounted for 10% or more of our revenue from real estate operations. We also derived 95% of our construction contract revenue from the United States Government in 2018 , 98% in 2017 and 87% in 2016 . We derived large concentrations of our revenue from real estate operations from certain business segments as set forth in Note 17.

Properties, Net

Properties, Net	12 Months Ended
	Dec. 31, 2018
Real Estate [Abstract]
Properties, Net	Properties, Net Operating properties, net consisted of the following (in thousands): December 31, 2018 2017 Land $ 503,274 $ 455,680 Buildings and improvements 3,241,894 3,068,124 Less: Accumulated depreciation (897,903 ) (786,193 ) Operating properties, net $ 2,847,265 $ 2,737,611 Properties we had in development or held for future development consisted of the following (in thousands): December 31, 2018 2017 Land $ 207,760 $ 240,825 Development in progress, excluding land 195,601 162,669 Projects in development or held for future development $ 403,361 $ 403,494 Our property held for sale as of December 31, 2017 was 11751 Meadowville Lane, an operating property totaling 193,000 square feet in Chester, Virginia (in our Data Center Shells sub-segment). We contractually closed on the sale of this property on October 27, 2017 for $44.0 million . 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; our financial guaranty to the buyer expired on October 1, 2018, resulting in no losses to us. We accounted for this transaction as a financing arrangement. Accordingly, we did not recognize the sale of this property for accounting purposes until the expiration of the guaranty on October 1, 2018, and we reported the sales proceeds as a liability on the consolidated balance sheets as of December 31, 2017 in the line entitled “deferred property sale.” In the fourth quarter of 2018, we recognized a gain on this sale of $1.5 million . The table below sets forth the components of this property’s assets as of December 31, 2017 (in thousands): Properties, net $ 38,670 Deferred rent receivable 3,237 Deferred leasing costs, net 319 Assets held for sale, net $ 42,226 2018 Construction Activities In 2018 , we placed into service 666,000 square feet in six newly-constructed properties (including two partially- operational properties), 22,000 square feet in one redeveloped property and land under a long-term contract. As of December 31, 2018 , we had nine properties under construction (including two partially-operational properties), or which we were contractually committed to construct, that we estimate will total 1.1 million square feet upon completion and one property under redevelopment that we estimate will total 106,000 square feet upon completion. In the fourth quarter of 2018, we abandoned plans to redevelop a property in our Fort Meade/BW Corridor sub-segment after we completed leasing on the property that did not require any redevelopment. Accordingly, we recognized an impairment loss of $2.4 million representing pre-development costs associated with the property. 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 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. 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.

Real Estate Joint Ventures

Real Estate Joint Ventures	12 Months Ended
	Dec. 31, 2018
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, 2018 (dollars in thousands): Nominal ownership % as of December 31, 2018 (1) Date Acquired Total Assets Encumbered Assets Total Liabilities 12/31/2018 Nature of Activity LW Redstone Company, LLC 3/23/2010 85% Development and operation of real estate (2) $ 169,533 $ 72,800 $ 50,530 M Square Associates, LLC 6/26/2007 50% Development and operation of real estate (3) 75,339 43,631 43,869 Stevens Investors, LLC 8/11/2015 95% Development of real estate (4) 83,118 82,618 16,017 $ 327,990 $ 199,049 $ 110,416 (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. We had advanced $40.0 million to the City through December 31, 2018 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 acquire 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 751,000 square feet of construction commencement through December 31, 2018 . Our partner has the right to receive some or all of the consideration for the acquisition of its interests in the form of common units in COPLP; • 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; and • 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 20. 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. Under the terms of the joint venture agreement, we and our partner receive returns in proportion to our investments in the 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 $39.8 million as of December 31, 2018 and $41.8 million as of December 31, 2017 .

Intangible Assets on Real Estat

Intangible Assets on Real Estate Acquisitions	12 Months Ended
	Dec. 31, 2018
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, 2018 December 31, 2017 Gross Carrying Amount Accumulated Amortization Net Carrying Amount Gross Carrying Amount Accumulated Amortization Net Carrying Amount In-place lease value $ 132,276 $ 117,520 $ 14,756 $ 132,276 $ 110,814 $ 21,462 Tenant relationship value 60,028 39,703 20,325 60,028 32,198 27,830 Below-market cost arrangements 8,880 1,507 7,373 15,102 7,507 7,595 Above-market leases 13,841 13,164 677 13,944 12,092 1,852 Other 1,333 994 339 1,333 980 353 $ 216,358 $ 172,888 $ 43,470 $ 222,683 $ 163,591 $ 59,092 Amortization of the intangible asset categories set forth above totaled $ 15.6 million in 2018 , $ 19.3 million in 2017 and $20.0 million in 2016 . The approximate weighted average amortization periods of the categories set forth above follow: in-place lease value: seven years; tenant relationship value: nine years; below-market cost arrangements: 33 years; above-market leases: six years; and other: 24 years. The approximate weighted average amortization period for all of the categories combined is 12 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: $8.3 million for 2019 ; $5.5 million for 2020 ; $5.3 million for 2021 ; $3.8 million for 2022 ; and $3.4 million for 2023 .

Investing Receivables

Investing Receivables	12 Months Ended
	Dec. 31, 2018
Receivables [Abstract]
Investing Receivables	Investing Receivables Investing receivables, including accrued interest thereon, consisted of the following (in thousands): December 31, 2018 2017 Notes receivable from City of Huntsville $ 53,961 $ 54,472 Other investing loans receivable 3,021 3,021 $ 56,982 $ 57,493 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 1, 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, 2018 or December 31, 2017 . The fair value of these receivables was approximately $58.2 million as of December 31, 2018 and $58.3 million as of December 31, 2017 .

Prepaid Expenses and Other Asse

Prepaid Expenses and Other Assets, Net	12 Months Ended
	Dec. 31, 2018
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, 2018 2017 Prepaid expenses $ 25,658 $ 24,670 Lease incentives, net 21,258 19,011 Furniture, fixtures and equipment, net 8,630 5,256 Non-real estate equity investments 5,940 5,056 Deferred financing costs, net (1) 4,733 1,202 Restricted cash 3,884 2,570 Construction contract costs incurred in excess of billings 3,189 4,884 Deferred tax asset, net 2,084 1,892 Other assets 6,337 2,177 Total for COPLP and subsidiaries 81,713 66,718 Marketable securities in deferred compensation plan 3,868 4,616 Total for COPT and subsidiaries $ 85,581 $ 71,334 (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, 2018 2017 Operating loss carry forward $ 4,354 $ 3,209 Share-based compensation 28 7 Accrued payroll 2 49 Property 427 43 Valuation allowance (2,727 ) (1,416 ) Deferred tax asset, net $ 2,084 $ 1,892 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 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, 2018 net deferred tax asset.

Debt, Net

Debt, Net	12 Months Ended
	Dec. 31, 2018
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, December 31, 2018 Stated Interest Rates Scheduled Maturity Mortgage and Other Secured Debt: Fixed rate mortgage debt (2) $ 147,141 $ 150,723 3.82% - 7.87% (3) 2019-2026 Variable rate secured loans (4) 23,282 13,115 LIBOR + 1.85% to 2.35% (5) 2020-2022 Total mortgage and other secured debt 170,423 163,838 Revolving Credit Facility (6) 213,000 126,000 LIBOR + 0.775% to 1.45% (7) March 2023 (6) Term Loan Facilities (8) 248,273 347,959 LIBOR + 0.85% to 1.65% (9) 2022 Unsecured Senior Notes (10) 3.600%, $350,000 aggregate principal 347,986 347,551 3.60% (11) May 2023 5.250%, $250,000 aggregate principal 247,136 246,645 5.25% (12) February 2024 3.700%, $300,000 aggregate principal 298,815 298,322 3.70% (13) June 2021 5.000%, $300,000 aggregate principal 297,109 296,731 5.00% (14) July 2025 Unsecured note payable 1,167 1,287 0% (15) May 2026 Total debt, net $ 1,823,909 $ 1,828,333 (1) The carrying values of our debt other than the Revolving Credit Facility reflect net deferred financing costs of $7.2 million as of December 31, 2018 and $5.0 million as of December 31, 2017 . (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 $281,000 as of December 31, 2018 and $349,000 as of December 31, 2017 . (3) The weighted average interest rate on our fixed rate mortgage debt was 4.17% as of December 31, 2018 . (4) Includes a construction loan with $98.4 million in remaining borrowing capacity as of December 31, 2018 . (5) The weighted average interest rate on our variable rate secured debt was 4.47% as of December 31, 2018 . (6) As discussed further below, we entered into a credit agreement on October 10, 2018 to replace our existing revolving credit facility with a new facility. (7) The weighted average interest rate on the Revolving Credit Facility was 3.49% as of December 31, 2018 . (8) As discussed below, we have the ability to borrow an additional $150.0 million in the aggregate under the remaining term loan facility, provided that there is no default under the facilities and subject to the approval of the lenders. (9) The interest rate on the remaining term loan facility was 3.60% as of December 31, 2018 . (10) Refer to the paragraphs below for further disclosure. (11) The carrying value of these notes reflects an unamortized discount totaling $1.4 million as of December 31, 2018 and $1.7 million as of December 31, 2017 . The effective interest rate under the notes, including amortization of the issuance costs, was 3.70% . (12) The carrying value of these notes reflects an unamortized discount totaling $2.6 million as of December 31, 2018 and $3.0 million as of December 31, 2017 . The effective interest rate under the notes, including amortization of the issuance costs, was 5.49% . (13) The carrying value of these notes reflects an unamortized discount totaling $943,000 as of December 31, 2018 and $1.3 million as of December 31, 2017 . The effective interest rate under the notes, including amortization of the issuance costs, was 3.85% . (14) The carrying value of these notes reflects an unamortized discount totaling $2.4 million as of December 31, 2018 and $2.7 million as of December 31, 2017 . The effective interest rate under the notes, including amortization of the issuance costs, was 5.15% (15) 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 $294,000 as of December 31, 2018 and $373,000 as of December 31, 2017 . All debt is owed by COPLP. While COPT is not directly obligated by any debt, it has guaranteed COPLP’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 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, 2018 , we were within the compliance requirements of these financial covenants. Our debt matures on the following schedule (in thousands): 2019 $ 4,387 2020 16,156 2021 303,875 2022 267,611 2023 629,590 Thereafter 616,885 Total $ 1,838,504 (1) (1) Represents scheduled principal amortization and maturities only and therefore excludes net discounts and deferred financing costs of $14.6 million . We capitalized interest costs of $5.9 million in 2018 , $5.2 million in 2017 and $5.7 million in 2016 . The following table sets forth information pertaining to the fair value of our debt (in thousands): December 31, 2018 December 31, 2017 Carrying Amount Estimated Fair Value Carrying Amount Estimated Fair Value Fixed-rate debt Unsecured Senior Notes $ 1,191,046 $ 1,219,603 $ 1,189,249 $ 1,229,398 Other fixed-rate debt 148,308 147,106 152,010 152,485 Variable-rate debt 484,555 486,497 487,074 485,694 $ 1,823,909 $ 1,853,206 $ 1,828,333 $ 1,867,577 Revolving Credit Facility On October 10, 2018, we entered into a credit agreement with a group of lenders to replace our existing unsecured revolving credit facility with a new facility (the prior facility and new facility are referred to collectively herein as our “Revolving Credit Facility”). The lenders’ aggregate commitment under the new facility is $800.0 million , with the ability for us to increase the lenders’ aggregate commitment to $1.25 billion , provided that there is no default under the facility and subject to the approval of the lenders. The new facility matures on March 10, 2023, 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 new facility is based on LIBOR plus 0.775% to 1.450% , as determined by the credit ratings assigned to COPLP by Standard & Poor’s Ratings Services, Moody’s Investors Service, Inc. or Fitch Ratings Ltd. (collectively, the “Ratings Agencies”). The new 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, 2018 , the maximum borrowing capacity under this facility totaled $800.0 million , of which $587.0 million was available. Weighted average borrowings under our Revolving Credit Facility totaled $188.1 million in 2018 and $97.8 million in 2017 . The weighted average interest rate on our Revolving Credit Facility was 3.08% in 2018 and 2.44% in 2017 . Term Loan Facilities 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 0.85% to 1.65% , as determined by the credit ratings assigned to COPLP by the Ratings Agencies. In addition to the term loan discussed above, we also had the following term loans that were repaid prior to December 31, 2018 : • for a term loan originating in 2012, we repaid $200.0 million in May 2017 and the remaining balance of $100.0 million in November 2018; and • for a term loan originating in 2012, we repaid the remaining balance of $120.0 million in 2016. In connection with our new Revolving Credit Facility discussed above, we have the ability to borrow up to $500.0 million under new term loans from the facility’s lender group provided that there is no default under the facility and subject to the approval of the lenders. Unsecured Senior Notes 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.

Interest Rate Derivatives

Interest Rate Derivatives	12 Months Ended
	Dec. 31, 2018
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 of interest rate risk (dollars in thousands): Notional Amount Effective Date Expiration Date Fair Value at December 31, Fixed Rate Floating Rate Index 2018 2017 $ 100,000 1.7300 % One-Month LIBOR 9/1/2015 8/1/2019 $ 472 $ 252 12,834 (1) 1.3900 % One-Month LIBOR 10/13/2015 10/1/2020 239 213 100,000 1.9013 % One-Month LIBOR 9/1/2016 12/1/2022 1,968 1,046 100,000 1.9050 % One-Month LIBOR 9/1/2016 12/1/2022 1,967 1,051 50,000 1.9079 % One-Month LIBOR 9/1/2016 12/1/2022 971 511 75,000 3.1760 % Three-Month LIBOR 6/30/2020 6/30/2030 (2,676 ) — 75,000 3.1920 % Three-Month LIBOR 6/30/2020 6/30/2030 (2,783 ) — $ 158 $ 3,073 (1) The notional amount of this instrument is scheduled to amortize to $12.1 million . 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 2018 2017 Interest rate swaps designated as cash flow hedges Interest rate derivatives $ 5,617 $ 3,073 Interest rate swaps designated as cash flow hedges Other liabilities (5,459 ) — The table below presents the effect of our interest rate derivatives on our consolidated statements of operations and comprehensive income (in thousands): Amount of (Loss) Gain Recognized in AOCI on Derivatives Amount of Gain (Loss) Reclassified from AOCI into Interest Expense on Statement of Operations For the Years Ended December 31, For the Years Ended December 31, Derivatives in Hedging Relationships 2018 2017 2016 2018 2017 2016 Interest rate derivatives $ (2,373 ) $ 684 $ (2,915 ) $ 407 $ (3,304 ) $ (4,230 ) Over the next 12 months, we estimate that approximately $2.1 million of gains will be reclassified from AOCI as a decrease 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, 2018 , the fair value of interest rate derivatives in a liability position related to these agreements was $5.5 million , excluding the effects of accrued interest and credit valuation adjustments. As of December 31, 2018 , we had not posted any collateral related to these agreements. We are not in default with any of these provisions. If we breached any of these provisions, we could be required to settle our obligations under the agreements at their termination value of $5.5 million .

Redeemable Noncontrolling Inter

Redeemable Noncontrolling Interests	12 Months Ended
	Dec. 31, 2018
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 for these redeemable noncontrolling interests (in thousands): For the Years Ended December 31, 2018 2017 2016 Beginning balance $ 23,125 $ 22,979 $ 19,218 Contributions from noncontrolling interests 186 — 22,779 Distributions to noncontrolling interests (1,411 ) (1,566 ) (21,881 ) Net income attributable to noncontrolling interests 2,523 2,338 2,242 Adjustment to arrive at fair value of interests 1,837 (626 ) 621 Ending balance $ 26,260 $ 23,125 $ 22,979 We determine the fair value of the interests based on unobservable inputs after considering the assumptions that market participants would make in pricing the interest. We apply a discount rate to the estimated future cash flows allocable to our partners from the properties underlying the respective joint ventures. Estimated cash flows used in such analyses are based on our plans for the properties and our views of market and economic conditions, and consider items such as current and future rental rates, occupancy projections and estimated operating and development expenditures.

Equity - COPT and Subsidiaries

Equity - COPT and Subsidiaries	12 Months Ended
	Dec. 31, 2018
Stockholders' Equity Note [Abstract]
Equity - COPT and Subsidiaries	Equity - COPT and Subsidiaries Preferred Shares As of December 31, 2018 , 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; 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 September 2016, COPT established an at-the-market (“ATM”) stock offering program under which it may, from time to time, offer and sell common shares in “at the market” stock offerings having an aggregate gross sales price of up to $200.0 million (the “2016 ATM Program”). COPT issued the following common shares under this ATM program: • 992,000 shares in 2018 at a weighted average price of $30.46 per share. Net proceeds from the shares issued totaled $29.8 million , after payment of $0.5 million in commissions to sales agents; • 591,000 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; and • 3.7 million 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. COPT contributed the net proceeds from these issuances to COPLP in exchange for an equal number of units in COPLP. In November 2018, COPT replaced its 2016 ATM Program with a new program under which it may offer and sell common shares in at-the-market stock offerings having an aggregate gross sales price of up to $300 million (the “2018 ATM Program”). Under the 2018 ATM Program, COPT may also, at its discretion, sell common shares under forward equity sales agreements. As of December 31, 2018 , COPT has not issued any shares under the 2018 ATM Program. 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. COPT issued the following common shares under these forward equity sale agreements: • 5.9 million shares in 2018 for net proceeds of $172.5 million ; and • 1.7 million shares in 2017 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. COPT’s remaining capacity under the forward equity sale agreements was 1.6 million common shares, with a settlement value of $46.4 million as of December 31, 2018 . Certain 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 1.9 million in 2018 , 339,513 in 2017 and 87,000 in 2016 . COPT declared dividends per common share of $1.10 in 2018 , 2017 and 2016 . COPT pays dividends at the discretion of its Board of Trustees. COPT’s ability to pay cash dividends will be dependent upon: (1) the cash flow generated from our operations; (2) cash generated or used by our financing and investing activities; and (3) the annual distribution requirements under the REIT provisions of the Code described in Note 2 and such other factors as the Board of Trustees deems relevant. COPT’s ability to make cash dividends will also be limited by the terms of COPLP’s Partnership Agreement, as well as by limitations imposed by state law. In addition, COPT is prohibited from paying cash dividends in excess of the amount necessary for it to qualify for taxation as a REIT if a default or event of default exists pursuant to the terms of our Revolving Credit Facility; this restriction does not currently limit COPT’s ability to pay dividends, and COPT does not believe that this restriction is reasonably likely to limit its ability to pay future dividends because it expects to comply with the terms of our Revolving Credit Facility. See Note 15 for disclosure of common share activity pertaining to our share-based compensation plans.

Equity - COPLP and Subsidiaries

Equity - COPLP and Subsidiaries	12 Months Ended
	Dec. 31, 2018
Class of Stock [Line Items]
Equity - COPLP and Subsidiaries	Equity - COPT and Subsidiaries Preferred Shares As of December 31, 2018 , 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; 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 September 2016, COPT established an at-the-market (“ATM”) stock offering program under which it may, from time to time, offer and sell common shares in “at the market” stock offerings having an aggregate gross sales price of up to $200.0 million (the “2016 ATM Program”). COPT issued the following common shares under this ATM program: • 992,000 shares in 2018 at a weighted average price of $30.46 per share. Net proceeds from the shares issued totaled $29.8 million , after payment of $0.5 million in commissions to sales agents; • 591,000 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; and • 3.7 million 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. COPT contributed the net proceeds from these issuances to COPLP in exchange for an equal number of units in COPLP. In November 2018, COPT replaced its 2016 ATM Program with a new program under which it may offer and sell common shares in at-the-market stock offerings having an aggregate gross sales price of up to $300 million (the “2018 ATM Program”). Under the 2018 ATM Program, COPT may also, at its discretion, sell common shares under forward equity sales agreements. As of December 31, 2018 , COPT has not issued any shares under the 2018 ATM Program. 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. COPT issued the following common shares under these forward equity sale agreements: • 5.9 million shares in 2018 for net proceeds of $172.5 million ; and • 1.7 million shares in 2017 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. COPT’s remaining capacity under the forward equity sale agreements was 1.6 million common shares, with a settlement value of $46.4 million as of December 31, 2018 . Certain 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 1.9 million in 2018 , 339,513 in 2017 and 87,000 in 2016 . COPT declared dividends per common share of $1.10 in 2018 , 2017 and 2016 . COPT pays dividends at the discretion of its Board of Trustees. COPT’s ability to pay cash dividends will be dependent upon: (1) the cash flow generated from our operations; (2) cash generated or used by our financing and investing activities; and (3) the annual distribution requirements under the REIT provisions of the Code described in Note 2 and such other factors as the Board of Trustees deems relevant. COPT’s ability to make cash dividends will also be limited by the terms of COPLP’s Partnership Agreement, as well as by limitations imposed by state law. In addition, COPT is prohibited from paying cash dividends in excess of the amount necessary for it to qualify for taxation as a REIT if a default or event of default exists pursuant to the terms of our Revolving Credit Facility; this restriction does not currently limit COPT’s ability to pay dividends, and COPT does not believe that this restriction is reasonably likely to limit its ability to pay future dividends because it expects to comply with the terms of our Revolving Credit Facility. 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. Following the completion of these redemptions in 2017, COPT held no preferred units in COPLP. 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 98.8% of COPLP’s common units as of December 31, 2018 and 96.9% as of December 31, 2017 . From 2016 through 2018, COPT acquired additional common units through the following common share issuances under its 2016 ATM Program: • 992,000 shares in 2018 at a weighted average price of $30.46 per share. Net proceeds from the shares issued totaled $29.8 million , after payment of $0.5 million in commissions to sales agents; • 591,000 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; and • 3.7 million shares 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. In 2018 and 2017, COPT also acquired additional common units through the following common share issuances under its forward equity sale agreements: • 5.9 million shares in 2018 for net proceeds of $172.5 million ; and • 1.7 million shares in 2017 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). Certain 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 1.9 million in 2018 , 339,513 in 2017 and 87,000 in 2016 . In addition, we redeemed 13,377 common units for cash payments totaling $339,000 in 2018. COPLP declared distributions per common unit of $1.10 in 2018 , 2017 and 2016 .

Share-Based Compensation and Ot

Share-Based Compensation and Other Compensation Matters	12 Months Ended
	Dec. 31, 2018
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 (the “2017 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. In November 2018, we amended the 2017 Plan to provide for the future grant of awards in the form of Profit Interest Units, which will be similar to restricted shares and PSUs, except that upon vesting recipients will receive common units in COPLP. 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, 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. 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, 2018 2017 2016 General, administrative and leasing expenses $ 5,415 $ 4,649 $ 5,816 Property operating expenses 961 966 1,027 Capitalized to development activities 587 480 610 Share-based compensation cost $ 6,963 $ 6,095 $ 7,453 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 6% for restricted shares. As of December 31, 2018 , unrecognized compensation costs related to unvested awards included: • $7.9 million on restricted shares expected to be recognized over a weighted average period of approximately two years; • $1.7 million on PSUs expected to be recognized over a weighted average performance period of approximately two years; and • $137,000 on deferred share awards expected to be recognized through May 2019. Our TRS is subject to Federal and state income taxes. We realized a windfall tax loss of $13,000 in 2017 and $331,000 in 2016 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 2016 , 2017 and 2018 : Shares Weighted Average Grant Date Fair Value 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 Granted 219,716 25.62 Forfeited (25,419 ) 30.02 Vested (181,238 ) 29.49 Unvested as of December 31, 2018 438,685 $ 28.38 Unvested shares as of December 31, 2018 that are expected to vest 413,273 $ 28.35 The aggregate intrinsic value of restricted shares that vested was $4.6 million in 2018 , $5.3 million in 2017 and $5.4 million in 2016 . PSUs We made the following grants of PSUs to executives from 2014 through 2018 (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, 2018 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 — 3/1/2016 26,299 1/1/2016 12/31/2018 $ 1,005 24,850 1/1/2017 39,351 1/1/2017 12/31/2019 $ 1,415 39,351 1/1/2018 59,110 1/1/2018 12/31/2020 $ 1,890 59,110 In 2017, we also 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 or greater 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 will settle the award by issuing fully-vested COPT shares equal to the number of earned PSUs in settlement of the award plan and either: • for awards granted January 1, 2017 and prior thereto, issuing fully-vested COPT shares equal to 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; or • for awards issued subsequent to January 1, 2017, paying cash equal to 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. If a performance period ends due to a sale event or qualified termination, the number of earned PSUs is prorated based on the portion of the three -year performance period that has elapsed. If employment is terminated by the employee or by us for cause, all PSUs are forfeited. PSUs do not carry voting rights. Based on COPT’s total shareholder return relative to its peer group of companies: • for the 2014 and 2015 PSUs issued to a former executive 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 a former executive 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 a former executive who departed on August 31, 2016, we issued 2,248 common shares on October 30, 2016 in settlement of such PSUs; • 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; and • for the 2015 PSUs issued to executives that vested on December 31, 2017, we issued 13,328 common shares in settlement of the PSUs on February 22, 2018. 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 2016 , 2017 and 2018 are set forth below: Grant Date Grant Date Fair Value Per Share Baseline Common Share Value Expected Volatility of Common Shares Risk-free Interest Rate 3/1/2016 $ 38.21 $ 23.90 20.4 % 0.96 % 1/1/2017 $ 38.43 $ 31.22 19.0 % 1.47 % 1/1/2018 $ 31.97 $ 29.20 17.0 % 2.04 % Deferred Share Awards We made the following grants of deferred share awards to nonemployee members of our Board of Trustees in 2016 , 2017 and 2018 (dollars in thousands, except per share data): Year of Grant Number of Deferred Share Awards Granted Aggregate Grant Date Fair Value Grant Date Fair Value Per Share 2016 24,944 $ 671 $ 26.89 2017 10,032 $ 326 $ 32.47 2018 13,832 $ 388 $ 28.08 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 2016 , 2017 and 2018 (dollars in thousands, except per share data): For the Years Ended December 31, 2018 2017 2016 Number of common shares issued 5,515 15,590 12,028 Grant date fair value per share $ 29.32 $ 26.89 $ 26.70 Aggregate intrinsic value $ 154 $ 508 $ 322 Options We have not issued options since 2009, and all of our options were vested and fully expensed prior to 2018. 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, 2015 425,347 $42.75 1 $ — December 31, 2016 201,100 $43.35 1 $ 31 December 31, 2017 60,000 $35.17 1 $ — December 31, 2018 30,000 $32.52 0.4 $ — The aggregate intrinsic value of options exercised was $18,000 in 2017. No options were exercised in 2018 or 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, our previous President and Chief Executive Officer, left the Company to pursue other interests, and he was not nominated for reelection as a Trustee. In addition, other executives departed the Company to pursue other interests effective March 31, 2016 and August 31, 2016. We recognized executive transition costs of approximately $6.5 million in 2016 primarily for termination benefits in connection with these departures.

Operating Leases

Operating Leases	12 Months Ended
	Dec. 31, 2018
Leases, Operating [Abstract]
Operating Leases	Operating Leases We lease our properties to tenants under operating leases with various expiration dates extending to the year 2063. Gross minimum future rentals on noncancelable leases in our properties as of December 31, 2018 were as follows (in thousands): Year Ending December 31, 2019 $ 400,617 2020 337,646 2021 280,369 2022 246,329 2023 194,888 Thereafter 523,932 $ 1,983,781

Information by Business Segment

Information by Business Segment	12 Months Ended
	Dec. 31, 2018
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, 2018 and 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; during 2017 and 2016, this segment also included suburban properties not meeting 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; 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, 2018 Revenues from real estate operations $ 248,927 $ 53,518 $ 46,286 $ 31,927 $ 14,745 $ 25,650 $ 421,053 $ 61,181 $ 31,892 $ 3,127 $ 517,253 Property operating expenses (82,975 ) (20,330 ) (26,888 ) (13,536 ) (6,050 ) (3,225 ) (153,004 ) (30,253 ) (16,342 ) (1,436 ) (201,035 ) UJV NOI allocable to COPT — — — — — 4,818 4,818 — — — 4,818 NOI from real estate operations $ 165,952 $ 33,188 $ 19,398 $ 18,391 $ 8,695 $ 27,243 $ 272,867 $ 30,928 $ 15,550 $ 1,691 $ 321,036 Additions to long-lived assets $ 38,612 $ 7,956 $ — $ 6,535 $ 573 $ — $ 53,676 $ 19,730 $ 856 $ 480 $ 74,742 Transfers from non-operating properties $ 35,648 $ 10,231 $ 14,718 $ (116 ) $ 4,167 $ 99,191 $ 163,839 $ — $ 2,304 $ — $ 166,143 Segment assets at December 31, 2018 $ 1,279,571 $ 399,339 $ 139,731 $ 188,911 $ 108,010 $ 353,165 $ 2,468,727 $ 395,380 $ 216,640 $ 4,115 $ 3,084,862 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 — — — — — 4,805 4,805 — — — 4,805 NOI from real estate operations $ 164,916 $ 30,180 $ 19,397 $ 16,921 $ 8,539 $ 26,416 $ 266,369 $ 39,280 $ 15,324 $ 2,848 $ 323,821 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 $ 301,996 $ 2,398,989 $ 400,512 $ 224,422 $ 4,082 $ 3,028,005 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 — — — — — 2,145 2,145 — — — 2,145 NOI from real estate operations $ 161,670 $ 31,140 $ 19,446 $ 15,507 $ 8,580 $ 23,307 $ 259,650 $ 51,710 $ 15,357 $ 3,862 $ 330,579 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 $ 227,796 $ 2,326,302 $ 442,811 $ 231,954 $ 21,293 $ 3,022,360 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, 2018 2017 2016 Segment revenues from real estate operations $ 517,253 $ 509,980 $ 525,964 Construction contract and other service revenues 60,859 102,840 48,364 Total revenues $ 578,112 $ 612,820 $ 574,328 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, 2018 2017 2016 UJV NOI allocable to COPT $ 4,818 $ 4,805 $ 2,145 Less: Income from UJV allocable to COPT attributable to depreciation and amortization expense and interest expense (3,314 ) (3,310 ) (1,413 ) Add: Equity in income (loss) of unconsolidated non-real estate entities 1,193 (5 ) 20 Equity in income of unconsolidated entities $ 2,697 $ 1,490 $ 752 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, 2018 2017 2016 Construction contract and other service revenues $ 60,859 $ 102,840 $ 48,364 Construction contract and other service expenses (58,326 ) (99,618 ) (45,481 ) NOI from service operations $ 2,533 $ 3,222 $ 2,883 The following table reconciles our NOI from real estate operations for reportable segments and NOI from service operations to net income as reported on our consolidated statements of operations (in thousands): For the Years Ended December 31, 2018 2017 2016 NOI from real estate operations $ 321,036 $ 323,821 $ 330,579 NOI from service operations 2,533 3,222 2,883 Interest and other income 4,358 6,318 5,444 Gain on sales of real estate 2,340 9,890 59,679 Equity in income of unconsolidated entities 2,697 1,490 752 Income tax benefit (expense) 363 (1,098 ) (244 ) Depreciation and other amortization associated with real estate operations (137,116 ) (134,228 ) (132,719 ) Impairment losses (2,367 ) (15,123 ) (101,391 ) General, administrative and leasing expenses (28,900 ) (30,837 ) (36,553 ) Business development expenses and land carry costs (5,840 ) (6,213 ) (8,244 ) Interest expense (75,385 ) (76,983 ) (83,163 ) Less: UJV NOI allocable to COPT included in equity in income of unconsolidated entities (4,818 ) (4,805 ) (2,145 ) Loss on early extinguishment of debt (258 ) (513 ) (1,110 ) Net income $ 78,643 $ 74,941 $ 33,768 The following table reconciles our segment assets to the consolidated total assets of COPT and subsidiaries (in thousands): As of December 31, 2018 2017 Segment assets $ 3,084,862 $ 3,028,005 Non-operating property assets 410,671 411,041 Other assets 160,472 156,159 Total COPT consolidated assets $ 3,656,005 $ 3,595,205 The accounting policies of the segments are the same as those used to prepare our consolidated financial statements. In the segment reporting presented above, we did not allocate interest expense, depreciation and amortization, impairment losses, gain on sales of real estate, loss on early extinguishment of debt 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.

Construction Contract and Other

Construction Contract and Other Service Revenues	12 Months Ended
	Dec. 31, 2018
Revenue from Contract with Customer [Abstract]
Construction Contract and Other Service Revenues	Construction Contract and Other Service Revenues We disaggregate our construction contract and other service revenues by compensation arrangement and by service type as we believe it best depicts the nature, timing and uncertainty of our revenue. The table below reports construction contract and other service revenues by compensation arrangement (in thousands): For the Years Ended December 31, 2018 2017 2016 Construction contract revenues: GMP $ 34,050 $ 78,401 $ 22,405 FFP 20,327 22,607 24,571 Cost-plus fee 5,540 801 464 Other 942 1,031 924 $ 60,859 $ 102,840 $ 48,364 The table below reports construction contract and other service revenues by service type (in thousands): For the Years Ended December 31, 2018 2017 2016 Construction contract revenues: Construction $ 57,986 $ 94,471 $ 46,989 Design 1,931 7,338 451 Other 942 1,031 924 $ 60,859 $ 102,840 $ 48,364 We recognized revenue of $349,000 , $586,000 and $483,000 in 2018 , 2017 and 2016 , respectively, from performance obligations satisfied (or partially satisfied) in previous periods. Accounts receivable related to our construction contract services is included in accounts receivable, net on our consolidated balance sheets. The beginning and ending balances of accounts receivable related to our construction contracts were as follows (in thousands): For the Years Ended December 31, 2018 2017 Beginning balance $ 4,577 $ 4,131 Ending balance $ 6,701 $ 4,577 Contract assets, which we refer to herein as construction costs in excess of billings, are included in prepaid expenses and other assets, net reported on our consolidated balance sheets. The beginning and ending balances of our contract assets were as follows (in thousands): For the Years Ended December 31, 2018 2017 Beginning balance $ 4,884 $ 10,350 Ending balance $ 3,189 $ 4,884 Contract liabilities are included in other liabilities reported on our consolidated balance sheets. Changes in contract liabilities were as follows (in thousands): For the Years Ended December 31, 2018 2017 Beginning balance $ 27,402 $ 32,650 Ending balance $ 568 $ 27,402 Portion of beginning balance recognized in revenue during the year $ 27,296 $ 32,650 The change in the contract liabilities balance reported above for 2018 was due primarily to our satisfaction of performance obligations during the period on a contract on which we previously received advance payments from a customer. Revenue allocated to the remaining performance obligations under existing contracts as of December 31, 2018 that will be recognized as revenue in future periods was $58.1 million , all of which we expect to recognize in 2019. We have no deferred incremental costs incurred to obtain or fulfill our construction contracts or other service revenues, and had no impairment losses on construction contracts receivable or unbilled construction revenue in 2018 , 2017 and 2016 .

Earnings Per Share ("EPS") and

Earnings Per Share ("EPS") and Earnings Per Unit (“EPU”)	12 Months Ended
	Dec. 31, 2018
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 COPT’s forward equity sale agreements and our 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, 2018 2017 2016 Numerator: Net income attributable to COPT $ 72,301 $ 68,745 $ 28,890 Preferred share dividends — (6,219 ) (14,297 ) Issuance costs associated with redeemed preferred shares — (6,847 ) (17 ) Income attributable to share-based compensation awards (462 ) (449 ) (419 ) Numerator for basic and diluted EPS on net income attributable to COPT common shareholders $ 71,839 $ 55,230 $ 14,157 Denominator (all weighted averages): Denominator for basic EPS (common shares) 103,946 98,969 94,502 Dilutive effect of share-based compensation awards 134 132 92 Dilutive effect of forward equity sale agreements 45 54 — Denominator for diluted EPS (common shares) 104,125 99,155 94,594 Basic EPS $ 0.69 $ 0.56 $ 0.15 Diluted EPS $ 0.69 $ 0.56 $ 0.15 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, 2018 2017 2016 Conversion of common units 2,468 3,362 3,633 Conversion of redeemable noncontrolling interests 936 689 809 Conversion of Series I preferred units 176 176 176 Conversion of Series K preferred shares — — 434 The following share-based compensation securities were also excluded from the computation of diluted EPS because their effect was antidilutive: • weighted average restricted shares and deferred share awards of 452,000 for 2018 , 433,000 for 2017 and 385,000 for 2016 ; and • weighted average options of 42,000 for 2018 , 70,000 for 2017 and 285,000 for 2016 . 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 COPT’s forward equity sale agreements and our 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, 2018 2017 2016 Numerator: Net income attributable to COPLP $ 74,703 $ 71,295 $ 30,053 Preferred unit distributions (660 ) (6,879 ) (14,957 ) Issuance costs associated with redeemed preferred units — (6,847 ) (17 ) Income attributable to share-based compensation awards (462 ) (449 ) (419 ) Numerator for basic and diluted EPU on net income attributable to COPLP common unitholders $ 73,581 $ 57,120 $ 14,660 Denominator (all weighted averages): Denominator for basic EPU (common units) 106,414 102,331 98,135 Dilutive effect of share-based compensation awards 134 132 92 Dilutive effect of forward equity sale agreements 45 54 — Denominator for diluted EPU (common units) 106,593 102,517 98,227 Basic EPU $ 0.69 $ 0.56 $ 0.15 Diluted EPU $ 0.69 $ 0.56 $ 0.15 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, 2018 2017 2016 Conversion of redeemable noncontrolling interests 936 689 809 Conversion of Series I preferred units 176 176 176 Conversion of Series K preferred units — — 434 The following share-based compensation securities were also excluded from the computation of diluted EPU because their effect was antidilutive: • weighted average restricted units and deferred share awards of 452,000 for 2018 , 433,000 for 2017 and 385,000 for 2016 ; and • weighted average options of 42,000 for 2018 , 70,000 for 2017 and 285,000 for 2016 .

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Litigation and Claims In the normal course of business, we are subject to legal actions and other claims. We record losses for specific legal proceedings and claims when we determine that a loss is probable and the amount of loss can be reasonably estimated. Management believes that it is reasonably possible that we could incur losses pursuant to such claims but do not believe such losses would materially affect our financial position, liquidity or results of operations. 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. 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. 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. While we are obligated to fund, through a special tax, any future shortfalls between debt service of the bonds and real estate taxes available to repay the bonds, as of December 31, 2018 , we do not expect any such future fundings will be required. 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, 2018 follow (in thousands): Year Ending December 31, 2019 $ 1,320 2020 1,294 2021 1,278 2022 1,164 2023 1,119 Thereafter 83,373 $ 89,548 Capital Lease On May 25, 2017, we entered into a ground lease on land under development in Washington, DC through 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 in 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 and have since paid down most of this liability. The remaining capital lease obligation as of December 31, 2018 was $660,000 , which is due in 2020. Contractual Obligations We had amounts remaining to be incurred under various contractual obligations as of December 31, 2018 that included the following (excluding amounts incurred and therefore reflected as liabilities reported on our consolidated balance sheets): • development and redevelopment obligations of $241.1 million ; • tenant and other capital improvements of $44.1 million ; • third party construction obligations of $47.9 million ; and • other obligations of $1.9 million .

Quarterly Data (Unaudited)

Quarterly Data (Unaudited)	12 Months Ended
	Dec. 31, 2018
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, 2018 and 2017 (in thousands, except per share/unit data). For the Year Ended December 31, 2018 For the Year Ended December 31, 2017 First Quarter Second Quarter Third Quarter Fourth Quarter First Quarter Second Quarter Third Quarter Fourth Quarter COPT and Subsidiaries Revenues $ 155,476 $ 146,743 $ 137,411 $ 138,482 $ 139,801 $ 151,435 $ 157,017 $ 164,567 Net income $ 18,780 $ 21,085 $ 20,322 $ 18,456 $ 22,740 $ 18,859 $ 22,334 $ 11,008 Net income attributable to noncontrolling interests (1,630 ) (1,651 ) (1,625 ) (1,436 ) (1,721 ) (1,333 ) (1,755 ) (1,387 ) Net income attributable to COPT 17,150 19,434 18,697 17,020 21,019 17,526 20,579 9,621 Preferred share dividends — — — — (3,180 ) (3,039 ) — — Issuance costs associated with redeemed preferred shares — — — — — (6,847 ) — — Net income attributable to COPT common shareholders $ 17,150 $ 19,434 $ 18,697 $ 17,020 $ 17,839 $ 7,640 $ 20,579 $ 9,621 Basic EPS $ 0.17 $ 0.19 $ 0.18 $ 0.16 $ 0.18 $ 0.08 $ 0.21 $ 0.10 Diluted EPS $ 0.17 $ 0.19 $ 0.18 $ 0.16 $ 0.18 $ 0.08 $ 0.21 $ 0.10 COPLP and Subsidiaries Revenues $ 155,476 $ 146,743 $ 137,411 $ 138,482 $ 139,801 $ 151,435 $ 157,017 $ 164,567 Net income $ 18,780 $ 21,085 $ 20,322 $ 18,456 $ 22,740 $ 18,859 $ 22,334 $ 11,008 Net income attributable to noncontrolling interests (921 ) (878 ) (1,080 ) (1,061 ) (934 ) (907 ) (897 ) (908 ) Net income attributable to COPLP 17,859 20,207 19,242 17,395 21,806 17,952 21,437 10,100 Preferred unit distributions (165 ) (165 ) (165 ) (165 ) (3,345 ) (3,204 ) (165 ) (165 ) Issuance costs associated with redeemed preferred units — — — — — (6,847 ) — — Net income attributable to COPLP common unitholders $ 17,694 $ 20,042 $ 19,077 $ 17,230 $ 18,461 $ 7,901 $ 21,272 $ 9,935 Basic EPU $ 0.17 $ 0.19 $ 0.18 $ 0.16 $ 0.18 $ 0.08 $ 0.21 $ 0.10 Diluted EPU $ 0.17 $ 0.19 $ 0.18 $ 0.16 $ 0.18 $ 0.08 $ 0.21 $ 0.10

Schedule II - Valuation and Qua

Schedule II - Valuation and Qualifying Accounts (Notes)	12 Months Ended
	Dec. 31, 2018
SEC Schedule, 12-09, 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, 2018 , 2017 and 2016 (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, 2018 $ 607 $ 319 $ — $ (96 ) $ 830 Year ended December 31, 2017 $ 603 $ 368 $ (36 ) $ (328 ) $ 607 Year ended December 31, 2016 $ 1,525 $ (17 ) $ 235 $ (1,140 ) $ 603 Allowance for Deferred Rent Receivable Year ended December 31, 2018 $ 364 $ — $ (100 ) $ — $ 264 Year ended December 31, 2017 $ 373 $ — $ (9 ) $ — $ 364 Year ended December 31, 2016 $ 1,962 $ — $ (1,589 ) $ — $ 373 Allowance for Deferred Tax Asset Year ended December 31, 2018 $ 1,416 $ 668 $ — $ — $ 2,084 Year ended December 31, 2017 $ 2,062 $ (646 ) $ — $ — $ 1,416 Year ended December 31, 2016 $ 2,062 $ — $ — $ — $ 2,062 (1) Amounts charged to costs and expenses are net of recoveries. The change in the allowance for deferred tax asset was due primarily to: for 2018, additional losses reported for tax purposes during the year that we do not expect to realize; and for 2017, 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
	Dec. 31, 2018
SEC Schedule, 12-28, Real Estate Companies, Investment in 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, 2018 (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 $ 48,473 $ 26,715 $ 58,343 $ 8,065 $ 26,715 $ 66,408 $ 93,123 $ (13,232 ) 1973/2011 8/7/2015 1000 Redstone Gateway (O) Huntsville, AL 10,390 — 20,533 5 — 20,538 20,538 (2,978 ) 2013 3/23/2010 1100 Redstone Gateway (O) Huntsville, AL 10,917 — 19,593 6 — 19,599 19,599 (2,435 ) 2014 3/23/2010 114 National Business Parkway (O) Annapolis Junction, MD — 364 3,109 118 364 3,227 3,591 (1,401 ) 2002 6/30/2000 1200 Redstone Gateway (O) Huntsville, AL 12,616 — 22,389 — — 22,389 22,389 (2,824 ) 2013 3/23/2010 1201 M Street (O) Washington, DC — — 49,785 8,619 — 58,404 58,404 (14,892 ) 2001 9/28/2010 1201 Winterson Road (O) Linthicum, MD — 2,130 16,601 488 2,130 17,089 19,219 (4,665 ) 1985/2017 4/30/1998 1220 12th Street, SE (O) Washington, DC — — 42,464 7,355 — 49,819 49,819 (13,674 ) 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 3,962 1,906 11,585 13,491 (6,697 ) 1990 9/28/1998 132 National Business Parkway (O) Annapolis Junction, MD — 2,917 12,259 4,666 2,917 16,925 19,842 (9,325 ) 2000 5/28/1999 133 National Business Parkway (O) Annapolis Junction, MD — 2,517 10,068 5,584 2,517 15,652 18,169 (9,639 ) 1997 9/28/1998 134 National Business Parkway (O) Annapolis Junction, MD — 3,684 7,517 4,108 3,684 11,625 15,309 (6,036 ) 1999 11/13/1998 1340 Ashton Road (O) Hanover, MD — 905 3,620 1,516 905 5,136 6,041 (2,971 ) 1989 4/28/1999 13450 Sunrise Valley Road (O) Herndon, VA — 1,386 5,576 4,592 1,386 10,168 11,554 (5,295 ) 1998 7/25/2003 13454 Sunrise Valley Road (O) Herndon, VA — 2,899 11,986 8,191 2,899 20,177 23,076 (10,176 ) 1998 7/25/2003 135 National Business Parkway (O) Annapolis Junction, MD — 2,484 9,750 6,075 2,484 15,825 18,309 (8,783 ) 1998 12/30/1998 1362 Mellon Road (O) Hanover, MD — 950 3,864 206 950 4,070 5,020 (414 ) 2006 2/10/2006 13857 McLearen Road (O) Herndon, VA — 3,507 30,177 2,806 3,507 32,983 36,490 (10,850 ) 2007 7/11/2012 140 National Business Parkway (O) Annapolis Junction, MD — 3,407 24,167 1,487 3,407 25,654 29,061 (9,577 ) 2003 12/31/2003 141 National Business Parkway (O) Annapolis Junction, MD — 2,398 9,538 4,815 2,398 14,353 16,751 (8,006 ) 1990 9/28/1998 14280 Park Meadow Drive (O) Chantilly, VA — 3,731 15,953 3,039 3,731 18,992 22,723 (7,982 ) 1999 9/29/2004 1460 Dorsey Road (L) Hanover, MD — 1,577 67 — 1,577 67 1,644 — (7) 2/28/2006 14840 Conference Center Drive (O) Chantilly, VA — 1,572 8,175 3,433 1,572 11,608 13,180 (5,964 ) 2000 7/25/2003 14850 Conference Center Drive (O) Chantilly, VA — 1,615 8,358 3,108 1,615 11,466 13,081 (6,416 ) 2000 7/25/2003 14900 Conference Center Drive (O) Chantilly, VA — 3,436 14,402 7,817 3,436 22,219 25,655 (10,959 ) 1999 7/25/2003 1501 South Clinton Street (O) Baltimore, MD — 27,964 51,990 15,417 27,964 67,407 95,371 (22,475 ) 2006 10/27/2009 15049 Conference Center Drive (O) Chantilly, VA — 4,415 20,365 15,729 4,415 36,094 40,509 (13,957 ) 1997 8/14/2002 15059 Conference Center Drive (O) Chantilly, VA — 5,753 13,615 3,979 5,753 17,594 23,347 (8,680 ) 2000 8/14/2002 1550 West Nursery Road (O) Linthicum, MD — 14,071 16,930 — 14,071 16,930 31,001 (5,390 ) 2009 10/28/2009 1560 West Nursery Road (O) Linthicum, MD — 1,441 113 — 1,441 113 1,554 (13 ) 2014 10/28/2009 1610 West Nursery Road (O) Linthicum, MD — 259 246 — 259 246 505 (11 ) 2016 4/30/1998 1616 West Nursery Road (O) Linthicum, MD — 393 3,323 — 393 3,323 3,716 (98 ) 2017 4/30/1998 1622 West Nursery Road (O) Linthicum, MD — 393 2,542 — 393 2,542 2,935 (116 ) 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 2,582 894 613 3,476 4,089 (1,635 ) 2002 12/21/2004 16480 Commerce Drive (O) Dahlgren, VA — 1,856 7,154 2,170 1,856 9,324 11,180 (3,624 ) 2000 12/28/2004 16501 Commerce Drive (O) Dahlgren, VA — 522 2,090 976 522 3,066 3,588 (1,112 ) 2002 12/21/2004 16539 Commerce Drive (O) Dahlgren, VA — 688 2,860 2,188 688 5,048 5,736 (2,517 ) 1990 12/21/2004 16541 Commerce Drive (O) Dahlgren, VA — 773 3,094 1,794 773 4,888 5,661 (2,197 ) 1996 12/21/2004 16543 Commerce Drive (O) Dahlgren, VA — 436 1,742 716 436 2,458 2,894 (972 ) 2002 12/21/2004 1751 Pinnacle Drive (O) McLean, VA — 10,486 42,339 29,880 10,486 72,219 82,705 (32,435 ) 1989/1995 9/23/2004 1753 Pinnacle Drive (O) McLean, VA — 8,275 34,353 20,778 8,275 55,131 63,406 (20,770 ) 1976/2004 9/23/2004 206 Research Boulevard (O) Aberdeen, MD — — — — — — — — 2012 9/14/2007 209 Research Boulevard (O) Aberdeen, MD — 134 1,711 265 134 1,976 2,110 (381 ) 2010 9/14/2007 210 Research Boulevard (O) Aberdeen, MD — 113 1,402 204 113 1,606 1,719 (271 ) 2010 9/14/2007 2100 L Street (O) Washington, DC 13,577 56,351 19,446 — 56,351 19,446 75,797 — (8) 8/11/2015 2100 Rideout Road (O) Huntsville, AL — — 5,492 2,881 — 8,373 8,373 (866 ) 2016 3/23/2010 22289 Exploration Drive (O) Lexington Park, MD — 1,422 5,594 1,867 1,422 7,461 8,883 (3,598 ) 2000 3/24/2004 22299 Exploration Drive (O) Lexington Park, MD — 1,362 5,685 2,550 1,362 8,235 9,597 (4,040 ) 1998 3/24/2004 22300 Exploration Drive (O) Lexington Park, MD — 1,094 4,929 2,522 1,094 7,451 8,545 (2,658 ) 1997 11/9/2004 22309 Exploration Drive (O) Lexington Park, MD — 2,243 10,419 7,983 2,243 18,402 20,645 (6,799 ) 1984/1997 3/24/2004 23535 Cottonwood Parkway (O) California, MD — 692 3,051 571 692 3,622 4,314 (1,695 ) 1984 3/24/2004 250 W Pratt St (O) Baltimore, MD — 8,057 34,588 10,557 8,057 45,145 53,202 (10,084 ) 1985 3/19/2015 2500 Riva Road (O) Annapolis, MD — 2,791 12,193 1 2,791 12,194 14,985 (6,467 ) 2000 3/4/2003 2600 Park Tower Drive (O) Vienna, VA — 20,304 34,443 711 20,304 35,154 55,458 (4,939 ) 1999 4/15/2015 2691 Technology Drive (O) Annapolis Junction, MD — 2,098 17,334 5,557 2,098 22,891 24,989 (10,541 ) 2005 5/26/2000 2701 Technology Drive (O) Annapolis Junction, MD — 1,737 15,266 5,034 1,737 20,300 22,037 (10,559 ) 2001 5/26/2000 2711 Technology Drive (O) Annapolis Junction, MD — 2,251 21,611 2,282 2,251 23,893 26,144 (12,299 ) 2002 11/13/2000 2720 Technology Drive (O) Annapolis Junction, MD — 3,863 29,272 1,218 3,863 30,490 34,353 (11,349 ) 2004 1/31/2002 2721 Technology Drive (O) Annapolis Junction, MD — 4,611 14,597 2,801 4,611 17,398 22,009 (8,280 ) 2000 10/21/1999 2730 Hercules Road (O) Annapolis Junction, MD — 8,737 31,612 8,697 8,737 40,309 49,046 (20,174 ) 1990 9/28/1998 30 Light Street (O) Baltimore, MD 4,078 — 12,101 867 — 12,968 12,968 (1,113 ) 2009 8/7/2015 300 Sentinel Drive (O) Annapolis Junction, MD — 1,517 59,165 1,201 1,517 60,366 61,883 (13,166 ) 2009 11/14/2003 302 Sentinel Drive (O) Annapolis Junction, MD — 2,648 29,687 777 2,648 30,464 33,112 (8,381 ) 2007 11/14/2003 304 Sentinel Drive (O) Annapolis Junction, MD — 3,411 24,917 1,814 3,411 26,731 30,142 (8,441 ) 2005 11/14/2003 306 Sentinel Drive (O) Annapolis Junction, MD — 3,260 22,592 1,239 3,260 23,831 27,091 (7,357 ) 2006 11/14/2003 308 Sentinel Drive (O) Annapolis Junction, MD — 1,422 26,208 2,330 1,422 28,538 29,960 (5,245 ) 2010 11/14/2003 310 Sentinel Way (O) Annapolis Junction, MD — 2,372 39,990 — 2,372 39,990 42,362 (2,956 ) 2016 11/14/2003 310 The Bridge Street (O) Huntsville, AL — 261 26,531 4,088 261 30,619 30,880 (8,677 ) 2009 8/9/2011 312 Sentinel Way (O) Annapolis Junction, MD — 3,138 27,797 — 3,138 27,797 30,935 (2,999 ) 2014 11/14/2003 314 Sentinel Way (O) Annapolis Junction, MD — 1,254 7,741 — 1,254 7,741 8,995 (781 ) 2008 11/14/2003 316 Sentinel Way (O) Annapolis Junction, MD — 2,748 38,156 145 2,748 38,301 41,049 (6,475 ) 2011 11/14/2003 318 Sentinel Way (O) Annapolis Junction, MD — 2,185 28,426 560 2,185 28,986 31,171 (9,182 ) 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 65 2,067 21,688 23,755 (5,935 ) 2007 11/14/2003 322 Sentinel Way (O) Annapolis Junction, MD — 2,605 22,827 1,900 2,605 24,727 27,332 (7,047 ) 2006 11/14/2003 324 Sentinel Way (O) Annapolis Junction, MD — 1,656 23,018 — 1,656 23,018 24,674 (4,805 ) 2010 6/29/2006 4000 Market Street (O) Huntsville, AL — — 6,820 — — 6,820 6,820 (7 ) 2018 (8) 3/23/2010 4100 Market Street (O) Huntsville, AL — — 4,612 — — 4,612 4,612 — (8) 3/23/2010 410 National Business Parkway (O) Annapolis Junction, MD — 1,831 23,257 119 1,831 23,376 25,207 (3,497 ) 2012 6/29/2006 420 National Business Parkway (O) Annapolis Junction, MD — 2,370 27,751 108 2,370 27,859 30,229 (3,340 ) 2013 6/29/2006 430 National Business Parkway (O) Annapolis Junction, MD — 1,852 21,563 127 1,852 21,690 23,542 (3,680 ) 2011 6/29/2006 44408 Pecan Court (O) California, MD — 817 1,464 1,691 817 3,155 3,972 (1,079 ) 1986 3/24/2004 44414 Pecan Court (O) California, MD — 405 1,619 1,065 405 2,684 3,089 (1,137 ) 1986 3/24/2004 44417 Pecan Court (O) California, MD — 434 3,794 208 434 4,002 4,436 (1,624 ) 1989/2015 3/24/2004 44420 Pecan Court (O) California, MD — 344 880 270 344 1,150 1,494 (423 ) 1989 11/9/2004 44425 Pecan Court (O) California, MD — 1,309 3,506 2,123 1,309 5,629 6,938 (2,750 ) 1997 5/5/2004 45310 Abell House Lane (O) California, MD — 2,272 13,808 471 2,272 14,279 16,551 (2,455 ) 2011 8/30/2010 46579 Expedition Drive (O) Lexington Park, MD — 1,406 5,796 1,831 1,406 7,627 9,033 (3,664 ) 2002 3/24/2004 46591 Expedition Drive (O) Lexington Park, MD — 1,200 7,199 1,977 1,200 9,176 10,376 (3,052 ) 2005 3/24/2004 4851 Stonecroft Boulevard (O) Chantilly, VA — 1,878 11,558 34 1,878 11,592 13,470 (4,116 ) 2004 8/14/2002 540 National Business Parkway (O) Annapolis Junction, MD — 2,035 29,608 — 2,035 29,608 31,643 (934 ) 2017 6/29/2006 5520 Research Park Drive (O) Catonsville, MD — — 20,072 1,327 — 21,399 21,399 (4,886 ) 2009 4/4/2006 5522 Research Park Drive (O) Catonsville, MD — — 4,550 210 — 4,760 4,760 (1,313 ) 2007 3/8/2006 5801 University Research Court (O) College Park, MD — — 15,936 — — 15,936 15,936 (240 ) 2018 (8) 11/9/2016 5825 University Research Court (O) College Park, MD 20,875 — 22,771 783 — 23,554 23,554 (5,680 ) 2008 1/29/2008 5850 University Research Court (O) College Park, MD 22,085 — 31,906 405 — 32,311 32,311 (7,165 ) 2008 1/29/2008 6700 Alexander Bell Drive (O) Columbia, MD — 1,755 7,019 7,509 1,755 14,528 16,283 (7,484 ) 1988 5/14/2001 6708 Alexander Bell Drive (O) Columbia, MD — 897 12,631 1,622 897 14,253 15,150 (4,111 ) 1988/2016 5/14/2001 6711 Columbia Gateway Drive (O) Columbia, MD — 2,683 23,239 1,550 2,683 24,789 27,472 (7,598 ) 2006-2007 9/28/2000 6716 Alexander Bell Drive (O) Columbia, MD — 1,242 4,969 4,285 1,242 9,254 10,496 (5,506 ) 1990 12/31/1998 6721 Columbia Gateway Drive (O) Columbia, MD — 1,753 34,090 122 1,753 34,212 35,965 (8,373 ) 2009 9/28/2000 6724 Alexander Bell Drive (O) Columbia, MD — 449 5,039 1,535 449 6,574 7,023 (2,946 ) 2001 5/14/2001 6731 Columbia Gateway Drive (O) Columbia, MD — 2,807 19,098 4,921 2,807 24,019 26,826 (11,156 ) 2002 3/29/2000 6740 Alexander Bell Drive (O) Columbia, MD — 1,424 5,696 3,441 1,424 9,137 10,561 (5,875 ) 1992 12/31/1998 6741 Columbia Gateway Drive (O) Columbia, MD — 675 1,711 154 675 1,865 2,540 (521 ) 2008 9/28/2000 6750 Alexander Bell Drive (O) Columbia, MD — 1,263 12,461 4,981 1,263 17,442 18,705 (9,484 ) 2001 12/31/1998 6760 Alexander Bell Drive (O) Columbia, MD — 890 3,561 3,830 890 7,391 8,281 (4,272 ) 1991 12/31/1998 6940 Columbia Gateway Drive (O) Columbia, MD — 3,545 9,916 7,330 3,545 17,246 20,791 (9,035 ) 1999 11/13/1998 6950 Columbia Gateway Drive (O) Columbia, MD — 3,596 15,738 3,220 3,596 18,958 22,554 (10,912 ) 1998 (8) 10/22/1998 7000 Columbia Gateway Drive (O) Columbia, MD — 3,131 12,103 6,479 3,131 18,582 21,713 (7,126 ) 1999 5/31/2002 7005 Columbia Gateway Drive (L) Columbia, MD — 3,036 747 — 3,036 747 3,783 — (7) 6/26/2014 7015 Albert Einstein Drive (O) Columbia, MD 412 2,058 6,093 3,319 2,058 9,412 11,470 (3,746 ) 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 (2,747 ) 2000 8/30/2001 7063 Columbia Gateway Drive (O) Columbia, MD — 902 3,684 3,416 902 7,100 8,002 (3,553 ) 2000 8/30/2001 7065 Columbia Gateway Drive (O) Columbia, MD — 919 3,763 3,095 919 6,858 7,777 (4,070 ) 2000 8/30/2001 7067 Columbia Gateway Drive (O) Columbia, MD — 1,829 11,823 4,480 1,829 16,303 18,132 (7,496 ) 2001 8/30/2001 7125 Columbia Gateway Drive (O) Columbia, MD — 20,487 46,994 18,044 20,487 65,038 85,525 (23,141 ) 1973/1999 6/29/2006 7130 Columbia Gateway Drive (O) Columbia, MD — 1,350 4,359 2,559 1,350 6,918 8,268 (3,396 ) 1989 9/19/2005 7134 Columbia Gateway Drive (O) Columbia, MD — 704 4,700 353 704 5,053 5,757 (1,540 ) 1990/2016 9/19/2005 7138 Columbia Gateway Drive (O) Columbia, MD — 1,104 3,518 2,843 1,104 6,361 7,465 (3,658 ) 1990 9/19/2005 7142 Columbia Gateway Drive (O) Columbia, MD — 1,342 7,158 2,608 1,342 9,766 11,108 (3,137 ) 1994/2018 9/19/2005 7150 Columbia Gateway Drive (O) Columbia, MD — 1,032 3,429 813 1,032 4,242 5,274 (1,557 ) 1991 9/19/2005 7150 Riverwood Drive (O) Columbia, MD — 1,821 4,388 1,824 1,821 6,212 8,033 (2,570 ) 2000 1/10/2007 7160 Riverwood Drive (O) Columbia, MD — 2,732 7,006 2,490 2,732 9,496 12,228 (4,050 ) 2000 1/10/2007 7170 Riverwood Drive (O) Columbia, MD — 1,283 3,096 1,704 1,283 4,800 6,083 (2,026 ) 2000 1/10/2007 7175 Riverwood Drive (O) Columbia, MD — 1,788 7,269 — 1,788 7,269 9,057 (934 ) 1996/2013 7/27/2005 7200 Redstone Gateway (O) Huntsville, AL 6,121 — 8,348 5 — 8,353 8,353 (963 ) 2013 3/23/2010 7200 Riverwood Road (O) Columbia, MD — 4,089 22,630 4,538 4,089 27,168 31,257 (11,024 ) 1986 10/13/1998 7205 Riverwood Drive (O) Columbia, MD — 1,367 21,419 — 1,367 21,419 22,786 (2,916 ) 2013 7/27/2005 7272 Park Circle Drive (O) Hanover, MD — 1,479 6,300 4,578 1,479 10,878 12,357 (4,427 ) 1991/1996 1/10/2007 7318 Parkway Drive (O) Hanover, MD — 972 3,888 1,250 972 5,138 6,110 (2,566 ) 1984 4/16/1999 7400 Redstone Gateway (O) Huntsville, AL 6,713 — 9,223 — — 9,223 9,223 (813 ) 2015 3/23/2010 7467 Ridge Road (O) Hanover, MD — 1,565 3,116 4,428 1,565 7,544 9,109 (2,699 ) 1990 4/28/1999 7740 Milestone Parkway (O) Hanover, MD 17,786 3,825 34,176 567 3,825 34,743 38,568 (7,395 ) 2009 7/2/2007 7770 Backlick Road (O) Springfield, VA — 6,387 76,593 300 6,387 76,893 83,280 (11,016 ) 2012 3/10/2010 7880 Milestone Parkway (O) Hanover, MD — 4,857 25,925 200 4,857 26,125 30,982 (2,023 ) 2015 9/17/2013 8621 Robert Fulton Drive (O) Columbia, MD — 2,317 12,642 6,437 2,317 19,079 21,396 (4,777 ) 2005-2006 6/10/2005 8661 Robert Fulton Drive (O) Columbia, MD — 1,510 3,764 2,453 1,510 6,217 7,727 (2,926 ) 2002 12/30/2003 8671 Robert Fulton Drive (O) Columbia, MD — 1,718 4,280 4,233 1,718 8,513 10,231 (4,021 ) 2002 12/30/2003 870 Elkridge Landing Road (O) Linthicum, MD — 2,003 9,442 9,301 2,003 18,743 20,746 (9,936 ) 1981 8/3/2001 8800 Redstone Gateway (O) Huntsville, AL — — 992 — — 992 992 — (8) 3/23/2010 891 Elkridge Landing Road (O) Linthicum, MD — 1,165 4,772 3,450 1,165 8,222 9,387 (4,582 ) 1984 7/2/2001 901 Elkridge Landing Road (O) Linthicum, MD — 1,156 4,437 3,383 1,156 7,820 8,976 (3,955 ) 1984 7/2/2001 911 Elkridge Landing Road (O) Linthicum, MD — 1,215 4,861 2,901 1,215 7,762 8,977 (4,170 ) 1985 4/30/1998 938 Elkridge Landing Road (O) Linthicum, MD — 922 4,748 1,446 922 6,194 7,116 (2,828 ) 1984 7/2/2001 939 Elkridge Landing Road (O) Linthicum, MD — 939 3,756 4,438 939 8,194 9,133 (4,642 ) 1983 4/30/1998 9651 Hornbaker Road (D) Manassas, VA — 6,050 251,367 4,689 6,050 256,056 262,106 (50,191 ) 2010 9/14/2010 Arundel Preserve (L) Hanover, MD — 13,401 9,583 — 13,401 9,583 22,984 — (7) 7/2/2007 Bethlehem Tech. Park - DC 18 (O) Manassas, VA — 3,599 26,636 — 3,599 26,636 30,235 (964 ) 2017 6/17/2016 Bethlehem Tech. Park - DC 19 (O) Manassas, VA — 3,708 16,606 — 3,708 16,606 20,314 (865 ) 2016 6/9/2016 Bethlehem Tech. Park - DC 20 (O) Manassas, VA — 3,599 24,062 — 3,599 24,062 27,661 (965 ) 2017 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 23 (O) Manassas, VA — — 4,883 — — 4,883 4,883 — (8) 6/9/2016 BLC 1 (O) Northern Virginia — 12,034 17,917 — 12,034 17,917 29,951 (245 ) 2018 12/28/2017 BLC 2 (O) Northern Virginia — 12,034 17,659 — 12,034 17,659 29,693 (210 ) 2018 12/28/2017 Canton Crossing Land (L) Baltimore, MD — 17,285 8,179 — 17,285 8,179 25,464 — (7) 10/27/2009 Canton Crossing Util Distr Ctr (O) Baltimore, MD — 6,100 10,450 1,192 6,100 11,642 17,742 (5,053 ) 2006 10/27/2009 Columbia Gateway - Southridge (L) Columbia, MD — 6,387 3,722 — 6,387 3,722 10,109 — (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 — 705 730 — 705 730 1,435 — (7) 3/24/2004 IN 1 (O) Northern Virginia — 1,815 11,532 — 1,815 11,532 13,347 — (8) 8/31/2016 IN 2 (O) Northern Virginia — 2,627 4,655 — 2,627 4,655 7,282 — (8) 8/31/2016 M Square Research Park (L) College Park, MD — — 3,202 — — 3,202 3,202 — (7) 1/29/2008 MP 1 (O) Northern Virginia — 9,426 15,865 — 9,426 15,865 25,291 — (8) 11/20/2017 MP 2 (O) Northern Virginia — 9,426 25,191 — 9,426 25,191 34,617 — 2018 11/20/2017 MR Land (L) Northern Virginia — 9,038 55 — 9,038 55 9,093 — (7) 11/8/2018 National Business Park North (L) Annapolis Junction, MD — 28,060 47,371 — 28,060 47,371 75,431 — (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) (9) Chantilly, VA — 2,096 46,840 — 2,096 46,840 48,936 (4,577 ) 2015 7/31/2002 NOVA Office B (O) (9) Chantilly, VA — 739 33,881 — 739 33,881 34,620 (1,817 ) 2016 7/31/2002 NOVA Office D (O) (9) Chantilly, VA — 6,587 40,525 — 6,587 40,525 47,112 (1,417 ) 2017 7/31/2002 Old Annapolis Road (O) Columbia, MD — 1,637 5,500 6,531 1,637 12,031 13,668 (3,898 ) 1974/1985 12/14/2000 Paragon Park - DC 21 (O) Sterling, VA — 7,828 19,999 — 7,828 19,999 27,827 (560 ) 2017 5/8/2017 Paragon Park - DC 22 (O) Sterling, VA — 7,828 18,755 — 7,828 18,755 26,583 (515 ) 2017 5/8/2017 Patriot Point - DC 15 (O) Ashburn, VA — 12,156 17,175 — 12,156 17,175 29,331 (1,180 ) 2016 10/15/2015 Patriot Point - DC 16 (O) Ashburn, VA — 12,156 17,043 — 12,156 17,043 29,199 (1,135 ) 2016 10/15/2015 Patriot Point - DC 17 (O) Ashburn, VA — 6,078 16,408 — 6,078 16,408 22,486 (930 ) 2016 10/15/2015 Patriot Ridge (L) Springfield, VA — 18,517 14,505 — 18,517 14,505 33,022 — (7) 3/10/2010 Project EX (O) (10) Confidential-USA — 8,958 5,744 — 8,958 5,744 14,702 — 2018 7/16/2008 Redstone Gateway (L) Huntsville, AL — — 21,807 — — 21,807 21,807 — (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 — 4,052 1,833 — 4,052 1,833 5,885 — (7) 3/30/2005 Sentry Gateway - T (O) San Antonio, TX — 14,020 38,804 13 14,020 38,817 52,837 (11,532 ) 1982/2008 3/30/2005 Sentry Gateway - V (O) San Antonio, TX — — 1,066 — — 1,066 1,066 (268 ) 2007 3/30/2005 Sentry Gateway - W (O) San Antonio, TX — — 1,884 71 — 1,955 1,955 (444 ) 2009 3/30/2005 Sentry Gateway - X (O) San Antonio, TX — 1,964 21,178 — 1,964 21,178 23,142 (4,316 ) 2010 1/20/2006 Sentry Gateway - Y (O) San Antonio, TX — 1,964 21,298 — 1,964 21,298 23,262 (4,342 ) 2010 1/20/2006 Sentry Gateway - Z (O) San Antonio, TX — 1,964 30,573 — 1,964 30,573 32,537 (2,908 ) 2015 6/14/2005 Westfields - Park Center (L) Chantilly, VA — 16,418 12,097 — 16,418 12,097 28,515 — (7) 7/18/2002 Westfields Corporate Center (L) Chantilly, VA — 7,141 1,576 — 7,141 1,576 8,717 — (7) 7/31/2002 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) Other Developments, including intercompany eliminations (V) Various — 8 121 373 8 494 502 (77 ) Various Various $ 174,043 $ 711,034 $ 3,002,155 $ 435,340 $ 711,034 $ 3,437,495 $ 4,148,529 $ (897,903 ) (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 Revolving Credit Facility of $213.0 million , term loan facilities of $248.3 million , unsecured senior notes of $1.2 billion , unsecured notes payable of $1.2 million , and deferred financing costs, net of premiums, on the remaining loans of $3.6 million . (3) The aggregate cost of these assets for Federal income tax purposes was approximately $3.5 billion as of December 31, 2018 . (4) As discussed in Note 5 to our Consolidated Financial Statements, we recognized an impairment loss of $2.4 million in connection with an operating property still owned as of December 31, 2018 . (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 as of December 31, 2018 . (8) Under construction or redevelopment as of December 31, 2018 . (9) The carrying amounts of these properties exclude allocated costs of the garage being constructed to support the properties. (10) This property represents land under a long-term contract. The following table summarizes our changes in cost of properties for the years ended December 31, 2018, 2017 and 2016 (in thousands): 2018 2017 2016 Beginning balance $ 3,980,813 $ 3,874,715 $ 4,158,616 Improvements and other additions 224,524 259,548 251,960 Sales (53,547 ) (138,216 ) (268,038 ) Impairments (2,493 ) (15,116 ) (143,502 ) Other dispositions (768 ) (118 ) (124,321 ) Ending balance $ 4,148,529 $ 3,980,813 $ 3,874,715 The following table summarizes our changes in accumulated depreciation for the same time periods (in thousands): 2018 2017 2016 Beginning balance $ 801,038 $ 715,951 $ 718,680 Depreciation expense 112,610 107,772 105,763 Sales (14,845 ) (22,567 ) (56,607 ) Impairments (132 ) — (42,161 ) Other dispositions (768 ) (118 ) (9,724 ) Ending balance $ 897,903 $ 801,038 $ 715,951

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
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. When we own an equity investment in an entity and cannot exert significant influence over its operations: • prior to January 1, 2018, we used the cost method of accounting; and • effective January 1, 2018, we measure the investment at fair value, with changes recognized through net income. For an investment without a readily determinable fair value, we measure the investment at cost, less any impairments, plus or minus changes resulting from observable price changes for an identical or similar investment of the same issuer.
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 (which is included in the line entitled “Prepaid expenses and other assets, net” on our consolidated balance sheets), 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 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. Deferred Financing Costs We defer costs of financing arrangements and recognize these costs as interest expense over the related debt terms on a straight-line basis, which approximates the amortization that would occur under the effective interest method of amortization. We expense any unamortized loan costs when loans are retired early. 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 Real Estate Operations Revenue 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. Construction Contract and Other Service Revenues We enter into construction contracts to complete various design and construction services primarily for our United States Government tenants. The revenues and expenses from these services consist primarily of subcontracted costs that are reimbursed to us by our customers along with a fee. These services are an ancillary component of our overall operations, with small operating margins relative to the revenue. We review each contract to determine the performance obligations and allocate the transaction price based on the standalone selling price, as discussed further below. We recognize revenue under these contracts as services are performed in an amount that reflects the consideration we expect to receive in exchange for those services. Our performance obligations are satisfied over time as work progresses. Revenue recognition is determined using the input method based on costs incurred as of point in time relative to the total estimated costs at completion to measure progress toward satisfying our performance obligations. We believe incurred costs of work performed best depicts the transfer of control of the services being transferred to the customer. In determining whether the performance obligations of each construction contract should be accounted for separately versus together, we consider numerous factors that may require significant judgment, including: whether the components contracted are substantially the same with the same pattern of transfer; whether the customer could contract with another party to perform construction based on our design project; and whether the customer can elect not to move forward after the design phase of the contract. Most of our contracts have a single performance obligation as the promise to transfer the services is not separately identifiable from other obligations in the contracts and, therefore, are not distinct. Some contracts have multiple performance obligations, most commonly due to having distinct project phases for design and construction for which our customer is making decisions and managing separately. In these cases, we allocate the transaction price between these performance obligations based on the relative standalone selling prices, which we determine by evaluating: the relative costs of each performance obligation; the expected operating margins (which typically do not vary significantly between obligations); and amounts set forth in the contracts for each obligation. Contract modifications, such as change orders, are routine for our construction contracts and are generally determined to be additions to the existing performance obligations because they would have been part of the initial performance obligations if they were identified at the initial contract date. We have three main types of compensation arrangements for our construction contracts: guaranteed maximum price (“GMP”); firm fixed price (“FFP”); and cost-plus fee. • GMP contracts provide for revenue equal to costs incurred plus a fee equal to a percentage of such costs, up to a maximum contract amount. We generally enter into GMP contracts for projects that are significant in nature based on the size of the project and total fees, and for which the full scope of the project has not been determined as of the contract date. GMP contracts are lower risk to us than FFP contracts since the costs and revenue move proportionately to one another. • FFP contracts provide for revenue equal to a fixed fee. These contracts are typically lower in value and scope relative to GMP contracts, and are generally entered into when the scope of the project is well defined. Typically, we assume more risk with FFP contracts than GMP contracts since the revenue is fixed and we could realize losses or less than expected profits if we incur more costs than originally estimated. However, these types of contracts offer the opportunity for additional profits when we complete the work for less than originally estimated. • Cost-plus fee contracts provide for revenue equal to costs incurred plus a fee equal to a percentage of such costs but, unlike GMP contracts, do not have a maximum contract amount. Similar to GMP contracts, cost-plus fee contracts are low risk to us since the costs and revenue move proportionately to one another. Construction contract cost estimates are based primarily on contracts in place with subcontractors to complete most of the work, but may also include assumptions, such as performance of subcontractors and cost and availability of materials, to project the outcome of future events over the course of the project. We review and update these estimates regularly as a significant change could affect the profitability of our construction contracts. We recognize adjustments in estimated profit on contracts under the cumulative catch-up method as the modification does not create a new performance obligation. Under this method, the impact of the adjustment on profit recorded to date on a contract is recognized in the period the adjustment is identified. Revenue and profit in future periods are recognized using the adjusted estimate. If at any time the estimate of contract profitability indicates an anticipated loss on the contract, we recognize the total loss in the quarter it is identified. Our timing of revenue recognition for construction contracts generally differs from the timing of invoicing to customers. We recognize such revenue as we satisfy our performance obligations. Payment terms and conditions vary by contract type. Under most of our contracts, we bill customers monthly, as work progresses, in accordance with the contract terms, with payment due in 30 days , although customers occasionally pay in advance of services being provided. We have determined that our contracts generally do not include a significant financing component. The primary purpose of the timing of our invoicing is for convenience, not to receive financing from our customers or to provide customers with financing. Additionally, the timing of transfer of the services is often at the discretion of the customer. Under most of our contracts, we bill customers one month subsequent to revenue recognition, resulting in contract assets representing unbilled construction revenue. Our contract liabilities consist of advance payments from our customers or billings in excess of construction contract revenue recognized.
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. Prior to our adoption of guidance issued by the FASB effective January 1, 2018, we: deferred only the effective portion of changes in fair value of the designated cash flow hedges to accumulated other comprehensive income (“AOCI”) or loss (“AOCL”), reclassifying such deferrals to interest expense as interest expense was recognized on the hedged forecasted transactions; and recognized the ineffective portion of the change in fair value of interest rate derivatives directly in interest expense. Effective January 1, 2018, we defer all changes in the fair value of designated cash flow hedges to AOCI or AOCL, reclassifying such deferrals to interest expense as interest expense is recognized on the hedged forecasted transactions. When an interest rate swap designated as a cash flow hedge no longer qualifies for hedge accounting and the hedged transactions are probable not to occur, 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. Refer to the section below entitled “Recent Accounting Pronouncements” for additional disclosure pertaining to the effect of the 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, 2018 2017 2016 2018 2017 2016 Ordinary income 83.1 % 86.5 % 48.0 % N/A 100.0 % 100.0 % Return of capital 16.9 % 13.5 % 52.0 % N/A 0.0 % 0.0 % We distributed all of COPT’s REIT taxable income in 2018 , 2017 and 2016 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 $47 million lower than the amount reported on our consolidated balance sheet as of December 31, 2018 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, including restricted cash and marketable securities that were reclassified to the line entitled “prepaid expenses and other assets, net” on our consolidated balance sheets after having been reported on a separate line in our Quarterly Reports on Form 10-Q filed in prior years and previous Annual Reports on Form 10-K.
Recent Accounting Pronouncements	Recent Accounting Pronouncements We adopted guidance issued by the Financial Accounting Standards Board (“FASB”) effective January 1, 2018 regarding the recognition of revenue from contracts with customers (“Topic 606”). Under this guidance, an entity recognizes 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 determined that Topic 606 is applicable to our construction contract and other service revenues, which includes predominantly construction and design projects performed primarily for tenants of our properties. We used the modified retrospective method for contracts that were not completed as of January 1, 2018. Under this method, the cumulative effect of initially applying the guidance is recognized as an adjustment to the opening balance of retained earnings as of the date of initial application. Our adoption of Topic 606 effective January 1, 2018 did not affect our consolidated financial statements other than additional disclosure provided in accordance with the guidance. We did not elect to use any of the practical expedients provided for under the guidance. As discussed further below, Topic 606 will also apply to lease revenue deemed to be non-lease components (such as common area maintenance and provision of utilities) once the new guidance setting forth principles for the recognition, measurement, presentation and disclosure of leases goes into effect on January 1, 2019. We adopted, prospectively effective January 1, 2018, guidance issued by the FASB 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. Our adoption of this guidance had no effect on our consolidated financial statements. We adopted, retrospectively effective January 1, 2018, guidance issued by the FASB pertaining to reporting on the statement of cash flows 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; and • 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. As a result of our adoption of this guidance, the change in restricted cash is no longer reported as either operating or investing activities on our statements of cash flows. Our restricted cash primarily consists of cash escrowed under mortgage debt for capital improvements and real estate taxes and certain tenant security deposits. Our adoption of this guidance had the following effects on our consolidated statements of cash flows for the years ended December 31, 2017 and December 31, 2016 (in thousands): For the Year Ended December 31, 2017 For the Year Ended December 31, 2016 As Previously Reported Impact of Adoption As As Previously Reported Impact of Adoption As Net cash provided by operating activities $ 230,654 $ (533 ) $ 230,121 $ 232,538 $ 1,732 $ 234,270 Net cash (used in) provided by investing activities $ (89,710 ) $ 347 $ (89,363 ) $ 71,449 $ (275 ) $ 71,174 Net cash used in financing activities $ (338,546 ) $ — $ (338,546 ) $ (154,434 ) $ (654 ) $ (155,088 ) Net (decrease) increase in cash and cash equivalents and restricted cash $ (197,602 ) $ (186 ) $ (197,788 ) $ 149,553 $ 803 $ 150,356 Beginning of period cash and cash equivalents and restricted cash $ 209,863 $ 2,756 $ 212,619 $ 60,310 $ 1,953 $ 62,263 End of period cash and cash equivalents and restricted cash $ 12,261 $ 2,570 $ 14,831 $ 209,863 $ 2,756 $ 212,619 We adopted guidance issued by the FASB that clarifies the scope of provisions and accounting for nonfinancial asset derecognition, including partial sales of real estate assets, effective January 1, 2018 using the full retrospective method. 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 are now 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 the previous guidance that meets the criteria for immediate full gain recognition under the new guidance; as a result, we retrospectively recognized an additional $18 million in income in 2016 that was being amortized into income in subsequent periods under the previous guidance. The recognition pattern for our other sales of real estate were not changed by this new guidance. The full retrospective method requires adjustment of each reporting period presented at the time of adoption. The tables below set forth the impact of the adoption of this guidance for amounts previously reported on the consolidated financial statements of COPT and subsidiaries (in thousands, except per share data): As of December 31, 2017 As of December 31, 2016 Consolidated Balance Sheets As Previously Reported Impact of Adoption As As Previously Reported Impact of Adoption As Investment in unconsolidated real estate joint venture $ 25,066 $ 16,721 $ 41,787 $ 25,548 $ 18,113 $ 43,661 Cumulative distributions in excess of net income $ (818,190 ) $ 16,105 $ (802,085 ) $ (765,276 ) $ 17,451 $ (747,825 ) Noncontrolling interests in subsidiaries $ 65,549 $ 616 $ 66,165 $ 71,605 $ 662 $ 72,267 For the Year Ended December 31, 2017 For the Year Ended December 31, 2016 Consolidated Statements of Operations and Comprehensive Income As Previously Reported Impact of Adoption As As Previously Reported Impact of Adoption As Gain on sales of real estate $ 9,890 $ — $ 9,890 $ 40,986 $ 18,693 $ 59,679 Income before equity in income of unconsolidated entities and income taxes $ 74,549 $ — $ 74,549 $ 14,567 $ 18,693 $ 33,260 Equity in income of unconsolidated entities $ 2,882 $ (1,392 ) $ 1,490 $ 1,332 $ (580 ) $ 752 Net income $ 76,333 $ (1,392 ) $ 74,941 $ 15,655 $ 18,113 $ 33,768 Net (income) loss attributable to noncontrolling interests - Common units in COPLP $ (1,936 ) $ 46 $ (1,890 ) $ 155 $ (662 ) $ (507 ) Net income attributable to COPT $ 70,091 $ (1,346 ) $ 68,745 $ 11,439 $ 17,451 $ 28,890 Net income (loss) attributable to COPT common shareholders $ 57,025 $ (1,346 ) $ 55,679 $ (2,875 ) $ 17,451 $ 14,576 Earnings per common share - basic and diluted $ 0.57 $ (0.01 ) $ 0.56 $ (0.03 ) $ 0.18 $ 0.15 Comprehensive income $ 80,360 $ (1,392 ) $ 78,968 $ 16,786 $ 18,113 $ 34,899 Comprehensive income attributable to COPT $ 73,989 $ (1,346 ) $ 72,643 $ 12,546 $ 17,451 $ 29,997 The tables below set forth the impact of the adoption of this guidance for amounts previously reported on the consolidated financial statements of COPLP and subsidiaries (in thousands, except per unit data): As of December 31, 2017 As of December 31, 2016 Consolidated Balance Sheets As Previously Reported Impact of Adoption As As Previously Reported Impact of Adoption As Investment in unconsolid. real estate joint venture $ 25,066 $ 16,721 $ 41,787 $ 25,548 $ 18,113 $ 43,661 Common units $ 1,428,301 $ 16,721 $ 1,445,022 $ 1,401,597 $ 18,113 $ 1,419,710 For the Year Ended December 31, 2017 For the Year Ended December 31, 2016 Consolidated Statements of Operations and Comprehensive Income As Previously Reported Impact of Adoption As As Previously Reported Impact of Adoption As Gain on sales of real estate $ 9,890 $ — $ 9,890 $ 40,986 $ 18,693 $ 59,679 Income before equity in income of unconsolidated entities and income taxes $ 74,549 $ — $ 74,549 $ 14,567 $ 18,693 $ 33,260 Equity in income of unconsolidated entities $ 2,882 $ (1,392 ) $ 1,490 $ 1,332 $ (580 ) $ 752 Net income $ 76,333 $ (1,392 ) $ 74,941 $ 15,655 $ 18,113 $ 33,768 Net income attributable to COPLP $ 72,687 $ (1,392 ) $ 71,295 $ 11,940 $ 18,113 $ 30,053 Net income (loss) attributable to COPLP common unitholders $ 58,961 $ (1,392 ) $ 57,569 $ (3,034 ) $ 18,113 $ 15,079 Earnings per common unit - basic and diluted $ 0.57 $ (0.01 ) $ 0.56 $ (0.04 ) $ 0.19 $ 0.15 Comprehensive income $ 80,360 $ (1,392 ) $ 78,968 $ 16,786 $ 18,113 $ 34,899 Comprehensive income attributable to COPLP $ 76,714 $ (1,392 ) $ 75,322 $ 13,071 $ 18,113 $ 31,184 Adoption of this guidance had no impact to cash provided by or used in operating, financing or investing activities on our consolidated statements of cash flows for the years ended December 31, 2017 and December 31, 2016. We early adopted guidance issued by the FASB effective January 1, 2018 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. We adopted this guidance using the modified retrospective transition method under which we eliminated $276,000 in previously-recorded cumulative hedge ineffectiveness as of January 1, 2018 by means of a cumulative-effect adjustment to our beginning balance of accumulated other comprehensive income (“AOCI”), with a corresponding adjustment to the beginning balance of: cumulative distributions in excess of net income for COPT and subsidiaries; and common units for COPLP and subsidiaries. We adopted amendments by the Securities and Exchange Commission to its rules effective November 5, 2018 to simplify or eliminate outdated, duplicative or overlapping disclosure requirements. The amendments also expanded certain disclosure requirements, such as requiring (effective January 1, 2019) current and comparative quarter and year-to-date reporting of changes in shareholders’ equity in interim periods. The resulting changes in disclosure were not material to the consolidated financial statements included herein. 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. We adopted this guidance on January 1, 2019 using a modified retrospective transition approach under which we apply the guidance effective January 1, 2019, with a cumulative-effect adjustment as of such date, and not adjust prior comparative reporting periods. 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. The guidance also permits lessors to elect a practical expedient (by class of underlying asset) to avoid separating non-lease components that otherwise would need to be accounted for under the recently-adopted revenue accounting guidance (such as common area maintenance and provision of utilities) from the associated lease component if (1) the non-lease components have the same timing and pattern of transfer as the associated lease component and (2) the lease component, if accounted for separately, would be classified as an operating lease. Once this practical expedient is adopted, the lessor would be able to account for the combination of the lease component and non-lease components as an operating lease as long as the lease component is the predominant component of the combined components. We elected each of these practical expedients. Below is a summary of the effects of this guidance on our accounting and reporting. • Real estate leases in which we are the lessor: ◦ Balance sheet reporting: 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 be expensing future non-incremental costs in connection with new or extended tenant leases the recognition of which would have been deferred under current accounting; these deferrals totaled $1.2 million in 2018 and $1.1 million in each of 2017 and 2016. • Leases in which we are the lessee: ◦ Our most significant leases as lessee are ground leases. We will be required to recognize right-of-use assets and lease liabilities for the present value of these minimum lease payments. These types of leases will 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, we elected to apply the package of practical expedients under which we will continue to account for our existing ground leases as operating leases upon adoption of the guidance. Upon adoption of this guidance on January 1, 2019, in connection with our ground leases, we recognized right-of-use assets and offsetting lease liabilities totaling approximately $14 million to $19 million , and also reclassified amounts previously presented elsewhere on our balance sheet in connection with these leases to the right-of-use assets. 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 2018, the FASB issued guidance that modifies disclosure requirements for fair value measurements. This guidance is effective for us beginning January 1, 2020. Early adoption is permitted for this guidance, and entities are permitted to early adopt with respect to any removed or modified disclosures while delaying adoption of additional disclosure requirements until the effective date. We do not expect the adoption of this guidance to have a material impact on our consolidated financial statements. In August 2018, the FASB issued guidance that aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software. FASB guidance did not previously address the accounting for such implementation costs. The guidance is effective for us beginning January 1, 2020, with early adoption permitted. We do not expect the adoption of this guidance to have a material impact on our consolidated financial statements.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 2017 2016 2018 2017 2016 Ordinary income 83.1 % 86.5 % 48.0 % N/A 100.0 % 100.0 % Return of capital 16.9 % 13.5 % 52.0 % N/A 0.0 % 0.0 %
Schedule of new accounting pronouncements and changes in accounting principles	Our adoption of this guidance had the following effects on our consolidated statements of cash flows for the years ended December 31, 2017 and December 31, 2016 (in thousands): For the Year Ended December 31, 2017 For the Year Ended December 31, 2016 As Previously Reported Impact of Adoption As As Previously Reported Impact of Adoption As Net cash provided by operating activities $ 230,654 $ (533 ) $ 230,121 $ 232,538 $ 1,732 $ 234,270 Net cash (used in) provided by investing activities $ (89,710 ) $ 347 $ (89,363 ) $ 71,449 $ (275 ) $ 71,174 Net cash used in financing activities $ (338,546 ) $ — $ (338,546 ) $ (154,434 ) $ (654 ) $ (155,088 ) Net (decrease) increase in cash and cash equivalents and restricted cash $ (197,602 ) $ (186 ) $ (197,788 ) $ 149,553 $ 803 $ 150,356 Beginning of period cash and cash equivalents and restricted cash $ 209,863 $ 2,756 $ 212,619 $ 60,310 $ 1,953 $ 62,263 End of period cash and cash equivalents and restricted cash $ 12,261 $ 2,570 $ 14,831 $ 209,863 $ 2,756 $ 212,619 The tables below set forth the impact of the adoption of this guidance for amounts previously reported on the consolidated financial statements of COPT and subsidiaries (in thousands, except per share data): As of December 31, 2017 As of December 31, 2016 Consolidated Balance Sheets As Previously Reported Impact of Adoption As As Previously Reported Impact of Adoption As Investment in unconsolidated real estate joint venture $ 25,066 $ 16,721 $ 41,787 $ 25,548 $ 18,113 $ 43,661 Cumulative distributions in excess of net income $ (818,190 ) $ 16,105 $ (802,085 ) $ (765,276 ) $ 17,451 $ (747,825 ) Noncontrolling interests in subsidiaries $ 65,549 $ 616 $ 66,165 $ 71,605 $ 662 $ 72,267 For the Year Ended December 31, 2017 For the Year Ended December 31, 2016 Consolidated Statements of Operations and Comprehensive Income As Previously Reported Impact of Adoption As As Previously Reported Impact of Adoption As Gain on sales of real estate $ 9,890 $ — $ 9,890 $ 40,986 $ 18,693 $ 59,679 Income before equity in income of unconsolidated entities and income taxes $ 74,549 $ — $ 74,549 $ 14,567 $ 18,693 $ 33,260 Equity in income of unconsolidated entities $ 2,882 $ (1,392 ) $ 1,490 $ 1,332 $ (580 ) $ 752 Net income $ 76,333 $ (1,392 ) $ 74,941 $ 15,655 $ 18,113 $ 33,768 Net (income) loss attributable to noncontrolling interests - Common units in COPLP $ (1,936 ) $ 46 $ (1,890 ) $ 155 $ (662 ) $ (507 ) Net income attributable to COPT $ 70,091 $ (1,346 ) $ 68,745 $ 11,439 $ 17,451 $ 28,890 Net income (loss) attributable to COPT common shareholders $ 57,025 $ (1,346 ) $ 55,679 $ (2,875 ) $ 17,451 $ 14,576 Earnings per common share - basic and diluted $ 0.57 $ (0.01 ) $ 0.56 $ (0.03 ) $ 0.18 $ 0.15 Comprehensive income $ 80,360 $ (1,392 ) $ 78,968 $ 16,786 $ 18,113 $ 34,899 Comprehensive income attributable to COPT $ 73,989 $ (1,346 ) $ 72,643 $ 12,546 $ 17,451 $ 29,997 The tables below set forth the impact of the adoption of this guidance for amounts previously reported on the consolidated financial statements of COPLP and subsidiaries (in thousands, except per unit data): As of December 31, 2017 As of December 31, 2016 Consolidated Balance Sheets As Previously Reported Impact of Adoption As As Previously Reported Impact of Adoption As Investment in unconsolid. real estate joint venture $ 25,066 $ 16,721 $ 41,787 $ 25,548 $ 18,113 $ 43,661 Common units $ 1,428,301 $ 16,721 $ 1,445,022 $ 1,401,597 $ 18,113 $ 1,419,710 For the Year Ended December 31, 2017 For the Year Ended December 31, 2016 Consolidated Statements of Operations and Comprehensive Income As Previously Reported Impact of Adoption As As Previously Reported Impact of Adoption As Gain on sales of real estate $ 9,890 $ — $ 9,890 $ 40,986 $ 18,693 $ 59,679 Income before equity in income of unconsolidated entities and income taxes $ 74,549 $ — $ 74,549 $ 14,567 $ 18,693 $ 33,260 Equity in income of unconsolidated entities $ 2,882 $ (1,392 ) $ 1,490 $ 1,332 $ (580 ) $ 752 Net income $ 76,333 $ (1,392 ) $ 74,941 $ 15,655 $ 18,113 $ 33,768 Net income attributable to COPLP $ 72,687 $ (1,392 ) $ 71,295 $ 11,940 $ 18,113 $ 30,053 Net income (loss) attributable to COPLP common unitholders $ 58,961 $ (1,392 ) $ 57,569 $ (3,034 ) $ 18,113 $ 15,079 Earnings per common unit - basic and diluted $ 0.57 $ (0.01 ) $ 0.56 $ (0.04 ) $ 0.19 $ 0.15 Comprehensive income $ 80,360 $ (1,392 ) $ 78,968 $ 16,786 $ 18,113 $ 34,899 Comprehensive income attributable to COPLP $ 76,714 $ (1,392 ) $ 75,322 $ 13,071 $ 18,113 $ 31,184

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2018
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 subsidiaries that are accounted for at fair value on a recurring basis as of December 31, 2018 and 2017 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 Inputs (Level 3) Total December 31, 2018: Assets: Marketable securities in deferred compensation plan (1) Mutual funds $ 3,819 $ — $ — $ 3,819 Other 49 — — 49 Interest rate derivatives — 5,617 — 5,617 Total assets $ 3,868 $ 5,617 $ — $ 9,485 Liabilities: Deferred compensation plan liability (2) $ — $ 3,868 $ — $ 3,868 Interest rate derivatives (2) — 5,459 — 5,459 Total liabilities $ — $ 9,327 $ — $ 9,327 December 31, 2017: Assets: Marketable securities in deferred compensation plan (1) Mutual funds $ 4,547 $ — $ — $ 4,547 Other 69 — — 69 Interest rate derivatives — 3,073 — 3,073 Total assets $ 4,616 $ 3,073 $ — $ 7,689 Liabilities: Deferred compensation plan liability (2) $ — $ 4,616 $ — $ 4,616 (1) Included in the line entitled “prepaid expenses and other assets, net” on COPT ’ s consolidated balance sheet. (2) Included in the line entitled “other liabilities” on COPT ’ s consolidated balance sheet. COPLP and Subsidiaries The tables below set forth financial assets and liabilities of COPLP and subsidiaries that are accounted for at fair value on a recurring basis as of December 31, 2018 and 2017 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 Inputs (Level 3) Total December 31, 2018: Assets: Interest rate derivatives $ — $ 5,617 $ — $ 5,617 Liabilities: Interest rate derivatives (1) $ — $ 5,459 $ — $ 5,459 December 31, 2017: Assets: Interest rate derivatives $ — $ 3,073 $ — $ 3,073
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 Description Quoted Prices in Significant Other Significant Inputs (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
	Dec. 31, 2018
Real Estate [Abstract]
Schedule of operating properties, net	Operating properties, net consisted of the following (in thousands): December 31, 2018 2017 Land $ 503,274 $ 455,680 Buildings and improvements 3,241,894 3,068,124 Less: Accumulated depreciation (897,903 ) (786,193 ) Operating properties, net $ 2,847,265 $ 2,737,611
Schedule of projects in development or held for future development	Properties we had in development or held for future development consisted of the following (in thousands): December 31, 2018 2017 Land $ 207,760 $ 240,825 Development in progress, excluding land 195,601 162,669 Projects in development or held for future development $ 403,361 $ 403,494
Components of assets held for sale	The table below sets forth the components of this property’s assets as of December 31, 2017 (in thousands): Properties, net $ 38,670 Deferred rent receivable 3,237 Deferred leasing costs, net 319 Assets held for sale, net $ 42,226
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 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.

Real Estate Joint Ventures (Tab

Real Estate Joint Ventures (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 (dollars in thousands): Nominal ownership % as of December 31, 2018 (1) Date Acquired Total Assets Encumbered Assets Total Liabilities 12/31/2018 Nature of Activity LW Redstone Company, LLC 3/23/2010 85% Development and operation of real estate (2) $ 169,533 $ 72,800 $ 50,530 M Square Associates, LLC 6/26/2007 50% Development and operation of real estate (3) 75,339 43,631 43,869 Stevens Investors, LLC 8/11/2015 95% Development of real estate (4) 83,118 82,618 16,017 $ 327,990 $ 199,049 $ 110,416 (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 Est_2

Intangible Assets on Real Estate Acquisitions (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 December 31, 2017 Gross Carrying Amount Accumulated Amortization Net Carrying Amount Gross Carrying Amount Accumulated Amortization Net Carrying Amount In-place lease value $ 132,276 $ 117,520 $ 14,756 $ 132,276 $ 110,814 $ 21,462 Tenant relationship value 60,028 39,703 20,325 60,028 32,198 27,830 Below-market cost arrangements 8,880 1,507 7,373 15,102 7,507 7,595 Above-market leases 13,841 13,164 677 13,944 12,092 1,852 Other 1,333 994 339 1,333 980 353 $ 216,358 $ 172,888 $ 43,470 $ 222,683 $ 163,591 $ 59,092

Investing Receivables (Tables)

Investing Receivables (Tables)	12 Months Ended
	Dec. 31, 2018
Receivables [Abstract]
Schedule of investing receivables	Investing receivables, including accrued interest thereon, consisted of the following (in thousands): December 31, 2018 2017 Notes receivable from City of Huntsville $ 53,961 $ 54,472 Other investing loans receivable 3,021 3,021 $ 56,982 $ 57,493

Prepaid Expenses and Other As_2

Prepaid Expenses and Other Assets, Net (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 2017 Prepaid expenses $ 25,658 $ 24,670 Lease incentives, net 21,258 19,011 Furniture, fixtures and equipment, net 8,630 5,256 Non-real estate equity investments 5,940 5,056 Deferred financing costs, net (1) 4,733 1,202 Restricted cash 3,884 2,570 Construction contract costs incurred in excess of billings 3,189 4,884 Deferred tax asset, net 2,084 1,892 Other assets 6,337 2,177 Total for COPLP and subsidiaries 81,713 66,718 Marketable securities in deferred compensation plan 3,868 4,616 Total for COPT and subsidiaries $ 85,581 $ 71,334 (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, 2018 2017 Operating loss carry forward $ 4,354 $ 3,209 Share-based compensation 28 7 Accrued payroll 2 49 Property 427 43 Valuation allowance (2,727 ) (1,416 ) Deferred tax asset, net $ 2,084 $ 1,892

Debt, Net (Tables)

Debt, Net (Tables)	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Schedule of debt	Our debt consisted of the following (dollars in thousands): Carrying Value (1) as of December 31, December 31, December 31, 2018 Stated Interest Rates Scheduled Maturity Mortgage and Other Secured Debt: Fixed rate mortgage debt (2) $ 147,141 $ 150,723 3.82% - 7.87% (3) 2019-2026 Variable rate secured loans (4) 23,282 13,115 LIBOR + 1.85% to 2.35% (5) 2020-2022 Total mortgage and other secured debt 170,423 163,838 Revolving Credit Facility (6) 213,000 126,000 LIBOR + 0.775% to 1.45% (7) March 2023 (6) Term Loan Facilities (8) 248,273 347,959 LIBOR + 0.85% to 1.65% (9) 2022 Unsecured Senior Notes (10) 3.600%, $350,000 aggregate principal 347,986 347,551 3.60% (11) May 2023 5.250%, $250,000 aggregate principal 247,136 246,645 5.25% (12) February 2024 3.700%, $300,000 aggregate principal 298,815 298,322 3.70% (13) June 2021 5.000%, $300,000 aggregate principal 297,109 296,731 5.00% (14) July 2025 Unsecured note payable 1,167 1,287 0% (15) May 2026 Total debt, net $ 1,823,909 $ 1,828,333 (1) The carrying values of our debt other than the Revolving Credit Facility reflect net deferred financing costs of $7.2 million as of December 31, 2018 and $5.0 million as of December 31, 2017 . (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 $281,000 as of December 31, 2018 and $349,000 as of December 31, 2017 . (3) The weighted average interest rate on our fixed rate mortgage debt was 4.17% as of December 31, 2018 . (4) Includes a construction loan with $98.4 million in remaining borrowing capacity as of December 31, 2018 . (5) The weighted average interest rate on our variable rate secured debt was 4.47% as of December 31, 2018 . (6) As discussed further below, we entered into a credit agreement on October 10, 2018 to replace our existing revolving credit facility with a new facility. (7) The weighted average interest rate on the Revolving Credit Facility was 3.49% as of December 31, 2018 . (8) As discussed below, we have the ability to borrow an additional $150.0 million in the aggregate under the remaining term loan facility, provided that there is no default under the facilities and subject to the approval of the lenders. (9) The interest rate on the remaining term loan facility was 3.60% as of December 31, 2018 . (10) Refer to the paragraphs below for further disclosure. (11) The carrying value of these notes reflects an unamortized discount totaling $1.4 million as of December 31, 2018 and $1.7 million as of December 31, 2017 . The effective interest rate under the notes, including amortization of the issuance costs, was 3.70% . (12) The carrying value of these notes reflects an unamortized discount totaling $2.6 million as of December 31, 2018 and $3.0 million as of December 31, 2017 . The effective interest rate under the notes, including amortization of the issuance costs, was 5.49% . (13) The carrying value of these notes reflects an unamortized discount totaling $943,000 as of December 31, 2018 and $1.3 million as of December 31, 2017 . The effective interest rate under the notes, including amortization of the issuance costs, was 3.85% . (14) The carrying value of these notes reflects an unamortized discount totaling $2.4 million as of December 31, 2018 and $2.7 million as of December 31, 2017 . The effective interest rate under the notes, including amortization of the issuance costs, was 5.15% (15) 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 $294,000 as of December 31, 2018 and $373,000 as of December 31, 2017 .
Schedule of debt maturities	Our debt matures on the following schedule (in thousands): 2019 $ 4,387 2020 16,156 2021 303,875 2022 267,611 2023 629,590 Thereafter 616,885 Total $ 1,838,504 (1) (1) Represents scheduled principal amortization and maturities only and therefore excludes net discounts and deferred financing costs of $14.6 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, 2018 December 31, 2017 Carrying Amount Estimated Fair Value Carrying Amount Estimated Fair Value Fixed-rate debt Unsecured Senior Notes $ 1,191,046 $ 1,219,603 $ 1,189,249 $ 1,229,398 Other fixed-rate debt 148,308 147,106 152,010 152,485 Variable-rate debt 484,555 486,497 487,074 485,694 $ 1,823,909 $ 1,853,206 $ 1,828,333 $ 1,867,577

Interest Rate Derivatives (Tabl

Interest Rate Derivatives (Tables)	12 Months Ended
	Dec. 31, 2018
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 of interest rate risk (dollars in thousands): Notional Amount Effective Date Expiration Date Fair Value at December 31, Fixed Rate Floating Rate Index 2018 2017 $ 100,000 1.7300 % One-Month LIBOR 9/1/2015 8/1/2019 $ 472 $ 252 12,834 (1) 1.3900 % One-Month LIBOR 10/13/2015 10/1/2020 239 213 100,000 1.9013 % One-Month LIBOR 9/1/2016 12/1/2022 1,968 1,046 100,000 1.9050 % One-Month LIBOR 9/1/2016 12/1/2022 1,967 1,051 50,000 1.9079 % One-Month LIBOR 9/1/2016 12/1/2022 971 511 75,000 3.1760 % Three-Month LIBOR 6/30/2020 6/30/2030 (2,676 ) — 75,000 3.1920 % Three-Month LIBOR 6/30/2020 6/30/2030 (2,783 ) — $ 158 $ 3,073 (1) The notional amount of this instrument is scheduled to amortize to $12.1 million .
Schedule of fair value and balance sheet classification of interest rate derivatives	The table below sets forth the fair value of our interest rate derivatives as well as their classification on our consolidated balance sheets (in thousands): Fair Value at December 31, Derivatives Balance Sheet Location 2018 2017 Interest rate swaps designated as cash flow hedges Interest rate derivatives $ 5,617 $ 3,073 Interest rate swaps designated as cash flow hedges Other liabilities (5,459 ) —
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): Amount of (Loss) Gain Recognized in AOCI on Derivatives Amount of Gain (Loss) Reclassified from AOCI into Interest Expense on Statement of Operations For the Years Ended December 31, For the Years Ended December 31, Derivatives in Hedging Relationships 2018 2017 2016 2018 2017 2016 Interest rate derivatives $ (2,373 ) $ 684 $ (2,915 ) $ 407 $ (3,304 ) $ (4,230 )

Redeemable Noncontrolling Int_2

Redeemable Noncontrolling Interests (Tables)	12 Months Ended
	Dec. 31, 2018
Noncontrolling Interest [Abstract]
Schedule of activity for redeemable noncontrolling interest	The table below sets forth the activity for these redeemable noncontrolling interests (in thousands): For the Years Ended December 31, 2018 2017 2016 Beginning balance $ 23,125 $ 22,979 $ 19,218 Contributions from noncontrolling interests 186 — 22,779 Distributions to noncontrolling interests (1,411 ) (1,566 ) (21,881 ) Net income attributable to noncontrolling interests 2,523 2,338 2,242 Adjustment to arrive at fair value of interests 1,837 (626 ) 621 Ending balance $ 26,260 $ 23,125 $ 22,979

Share-Based Compensation and _2

Share-Based Compensation and Other Compensation Matters (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 2017 2016 General, administrative and leasing expenses $ 5,415 $ 4,649 $ 5,816 Property operating expenses 961 966 1,027 Capitalized to development activities 587 480 610 Share-based compensation cost $ 6,963 $ 6,095 $ 7,453
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 2016 , 2017 and 2018 : Shares Weighted Average Grant Date Fair Value 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 Granted 219,716 25.62 Forfeited (25,419 ) 30.02 Vested (181,238 ) 29.49 Unvested as of December 31, 2018 438,685 $ 28.38 Unvested shares as of December 31, 2018 that are expected to vest 413,273 $ 28.35
Schedule of PSU's Granted	We made the following grants of PSUs to executives from 2014 through 2018 (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, 2018 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 — 3/1/2016 26,299 1/1/2016 12/31/2018 $ 1,005 24,850 1/1/2017 39,351 1/1/2017 12/31/2019 $ 1,415 39,351 1/1/2018 59,110 1/1/2018 12/31/2020 $ 1,890 59,110
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 or greater 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 2016 , 2017 and 2018 are set forth below: Grant Date Grant Date Fair Value Per Share Baseline Common Share Value Expected Volatility of Common Shares Risk-free Interest Rate 3/1/2016 $ 38.21 $ 23.90 20.4 % 0.96 % 1/1/2017 $ 38.43 $ 31.22 19.0 % 1.47 % 1/1/2018 $ 31.97 $ 29.20 17.0 % 2.04 %
Deferred share awards	We made the following grants of deferred share awards to nonemployee members of our Board of Trustees in 2016 , 2017 and 2018 (dollars in thousands, except per share data): Year of Grant Number of Deferred Share Awards Granted Aggregate Grant Date Fair Value Grant Date Fair Value Per Share 2016 24,944 $ 671 $ 26.89 2017 10,032 $ 326 $ 32.47 2018 13,832 $ 388 $ 28.08
Deferred share awards vested activity	We issued the following common shares in settlement of deferred shares in 2016 , 2017 and 2018 (dollars in thousands, except per share data): For the Years Ended December 31, 2018 2017 2016 Number of common shares issued 5,515 15,590 12,028 Grant date fair value per share $ 29.32 $ 26.89 $ 26.70 Aggregate intrinsic value $ 154 $ 508 $ 322
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, 2015 425,347 $42.75 1 $ — December 31, 2016 201,100 $43.35 1 $ 31 December 31, 2017 60,000 $35.17 1 $ — December 31, 2018 30,000 $32.52 0.4 $ —

Operating Leases (Tables)

Operating Leases (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 were as follows (in thousands): Year Ending December 31, 2019 $ 400,617 2020 337,646 2021 280,369 2022 246,329 2023 194,888 Thereafter 523,932 $ 1,983,781

Information by Business Segme_2

Information by Business Segment (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 Revenues from real estate operations $ 248,927 $ 53,518 $ 46,286 $ 31,927 $ 14,745 $ 25,650 $ 421,053 $ 61,181 $ 31,892 $ 3,127 $ 517,253 Property operating expenses (82,975 ) (20,330 ) (26,888 ) (13,536 ) (6,050 ) (3,225 ) (153,004 ) (30,253 ) (16,342 ) (1,436 ) (201,035 ) UJV NOI allocable to COPT — — — — — 4,818 4,818 — — — 4,818 NOI from real estate operations $ 165,952 $ 33,188 $ 19,398 $ 18,391 $ 8,695 $ 27,243 $ 272,867 $ 30,928 $ 15,550 $ 1,691 $ 321,036 Additions to long-lived assets $ 38,612 $ 7,956 $ — $ 6,535 $ 573 $ — $ 53,676 $ 19,730 $ 856 $ 480 $ 74,742 Transfers from non-operating properties $ 35,648 $ 10,231 $ 14,718 $ (116 ) $ 4,167 $ 99,191 $ 163,839 $ — $ 2,304 $ — $ 166,143 Segment assets at December 31, 2018 $ 1,279,571 $ 399,339 $ 139,731 $ 188,911 $ 108,010 $ 353,165 $ 2,468,727 $ 395,380 $ 216,640 $ 4,115 $ 3,084,862 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 — — — — — 4,805 4,805 — — — 4,805 NOI from real estate operations $ 164,916 $ 30,180 $ 19,397 $ 16,921 $ 8,539 $ 26,416 $ 266,369 $ 39,280 $ 15,324 $ 2,848 $ 323,821 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 $ 301,996 $ 2,398,989 $ 400,512 $ 224,422 $ 4,082 $ 3,028,005 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 — — — — — 2,145 2,145 — — — 2,145 NOI from real estate operations $ 161,670 $ 31,140 $ 19,446 $ 15,507 $ 8,580 $ 23,307 $ 259,650 $ 51,710 $ 15,357 $ 3,862 $ 330,579 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 $ 227,796 $ 2,326,302 $ 442,811 $ 231,954 $ 21,293 $ 3,022,360
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, 2018 2017 2016 Segment revenues from real estate operations $ 517,253 $ 509,980 $ 525,964 Construction contract and other service revenues 60,859 102,840 48,364 Total revenues $ 578,112 $ 612,820 $ 574,328
Reconciliation of UJV NOI allocable to COPT 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, 2018 2017 2016 UJV NOI allocable to COPT $ 4,818 $ 4,805 $ 2,145 Less: Income from UJV allocable to COPT attributable to depreciation and amortization expense and interest expense (3,314 ) (3,310 ) (1,413 ) Add: Equity in income (loss) of unconsolidated non-real estate entities 1,193 (5 ) 20 Equity in income of unconsolidated entities $ 2,697 $ 1,490 $ 752
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, 2018 2017 2016 Construction contract and other service revenues $ 60,859 $ 102,840 $ 48,364 Construction contract and other service expenses (58,326 ) (99,618 ) (45,481 ) NOI from service operations $ 2,533 $ 3,222 $ 2,883
Schedule of reconciliation of net operating income from real estate operations and service operations to net income	The following table reconciles our NOI from real estate operations for reportable segments and NOI from service operations to net income as reported on our consolidated statements of operations (in thousands): For the Years Ended December 31, 2018 2017 2016 NOI from real estate operations $ 321,036 $ 323,821 $ 330,579 NOI from service operations 2,533 3,222 2,883 Interest and other income 4,358 6,318 5,444 Gain on sales of real estate 2,340 9,890 59,679 Equity in income of unconsolidated entities 2,697 1,490 752 Income tax benefit (expense) 363 (1,098 ) (244 ) Depreciation and other amortization associated with real estate operations (137,116 ) (134,228 ) (132,719 ) Impairment losses (2,367 ) (15,123 ) (101,391 ) General, administrative and leasing expenses (28,900 ) (30,837 ) (36,553 ) Business development expenses and land carry costs (5,840 ) (6,213 ) (8,244 ) Interest expense (75,385 ) (76,983 ) (83,163 ) Less: UJV NOI allocable to COPT included in equity in income of unconsolidated entities (4,818 ) (4,805 ) (2,145 ) Loss on early extinguishment of debt (258 ) (513 ) (1,110 ) Net income $ 78,643 $ 74,941 $ 33,768
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, 2018 2017 Segment assets $ 3,084,862 $ 3,028,005 Non-operating property assets 410,671 411,041 Other assets 160,472 156,159 Total COPT consolidated assets $ 3,656,005 $ 3,595,205

Construction Contract and Oth_2

Construction Contract and Other Service Revenues (Tables)	12 Months Ended
	Dec. 31, 2018
Revenue from Contract with Customer [Abstract]
Disaggregation of Revenue	The table below reports construction contract and other service revenues by compensation arrangement (in thousands): For the Years Ended December 31, 2018 2017 2016 Construction contract revenues: GMP $ 34,050 $ 78,401 $ 22,405 FFP 20,327 22,607 24,571 Cost-plus fee 5,540 801 464 Other 942 1,031 924 $ 60,859 $ 102,840 $ 48,364 The table below reports construction contract and other service revenues by service type (in thousands): For the Years Ended December 31, 2018 2017 2016 Construction contract revenues: Construction $ 57,986 $ 94,471 $ 46,989 Design 1,931 7,338 451 Other 942 1,031 924 $ 60,859 $ 102,840 $ 48,364
Schedule of Accounts Receivable	The beginning and ending balances of accounts receivable related to our construction contracts were as follows (in thousands): For the Years Ended December 31, 2018 2017 Beginning balance $ 4,577 $ 4,131 Ending balance $ 6,701 $ 4,577
Contract with Customer, Asset and Liability	Contract assets, which we refer to herein as construction costs in excess of billings, are included in prepaid expenses and other assets, net reported on our consolidated balance sheets. The beginning and ending balances of our contract assets were as follows (in thousands): For the Years Ended December 31, 2018 2017 Beginning balance $ 4,884 $ 10,350 Ending balance $ 3,189 $ 4,884 Contract liabilities are included in other liabilities reported on our consolidated balance sheets. Changes in contract liabilities were as follows (in thousands): For the Years Ended December 31, 2018 2017 Beginning balance $ 27,402 $ 32,650 Ending balance $ 568 $ 27,402 Portion of beginning balance recognized in revenue during the year $ 27,296 $ 32,650

Earnings Per Share ("EPS") an_2

Earnings Per Share ("EPS") and Earnings Per Unit (“EPU”) (Tables)	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Line Items]
Summary of calculation of numerator and denominator in basic and diluted earnings per share	Summaries of the numerator and denominator for purposes of basic and diluted EPS calculations are set forth below (in thousands, except per share data): For the Years Ended December 31, 2018 2017 2016 Numerator: Net income attributable to COPT $ 72,301 $ 68,745 $ 28,890 Preferred share dividends — (6,219 ) (14,297 ) Issuance costs associated with redeemed preferred shares — (6,847 ) (17 ) Income attributable to share-based compensation awards (462 ) (449 ) (419 ) Numerator for basic and diluted EPS on net income attributable to COPT common shareholders $ 71,839 $ 55,230 $ 14,157 Denominator (all weighted averages): Denominator for basic EPS (common shares) 103,946 98,969 94,502 Dilutive effect of share-based compensation awards 134 132 92 Dilutive effect of forward equity sale agreements 45 54 — Denominator for diluted EPS (common shares) 104,125 99,155 94,594 Basic EPS $ 0.69 $ 0.56 $ 0.15 Diluted EPS $ 0.69 $ 0.56 $ 0.15
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, 2018 2017 2016 Conversion of common units 2,468 3,362 3,633 Conversion of redeemable noncontrolling interests 936 689 809 Conversion of Series I preferred units 176 176 176 Conversion of Series K preferred shares — — 434
Corporate Office Properties, L.P. [Member]
Earnings Per Share [Line Items]
Summary of calculation of numerator and denominator in basic and diluted earnings per share	Summaries of the numerator and denominator for purposes of basic and diluted EPU calculations are set forth below (in thousands, except per unit data): For the Years Ended December 31, 2018 2017 2016 Numerator: Net income attributable to COPLP $ 74,703 $ 71,295 $ 30,053 Preferred unit distributions (660 ) (6,879 ) (14,957 ) Issuance costs associated with redeemed preferred units — (6,847 ) (17 ) Income attributable to share-based compensation awards (462 ) (449 ) (419 ) Numerator for basic and diluted EPU on net income attributable to COPLP common unitholders $ 73,581 $ 57,120 $ 14,660 Denominator (all weighted averages): Denominator for basic EPU (common units) 106,414 102,331 98,135 Dilutive effect of share-based compensation awards 134 132 92 Dilutive effect of forward equity sale agreements 45 54 — Denominator for diluted EPU (common units) 106,593 102,517 98,227 Basic EPU $ 0.69 $ 0.56 $ 0.15 Diluted EPU $ 0.69 $ 0.56 $ 0.15
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, 2018 2017 2016 Conversion of redeemable noncontrolling interests 936 689 809 Conversion of Series I preferred units 176 176 176 Conversion of Series K preferred units — — 434

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 follow (in thousands): Year Ending December 31, 2019 $ 1,320 2020 1,294 2021 1,278 2022 1,164 2023 1,119 Thereafter 83,373 $ 89,548

Quarterly Data (Unaudited) (Tab

Quarterly Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2018
Quarterly Data [Line Items]
Schedule of selected quarterly information	The tables below set forth selected quarterly information for the years ended December 31, 2018 and 2017 (in thousands, except per share/unit data). For the Year Ended December 31, 2018 For the Year Ended December 31, 2017 First Quarter Second Quarter Third Quarter Fourth Quarter First Quarter Second Quarter Third Quarter Fourth Quarter COPT and Subsidiaries Revenues $ 155,476 $ 146,743 $ 137,411 $ 138,482 $ 139,801 $ 151,435 $ 157,017 $ 164,567 Net income $ 18,780 $ 21,085 $ 20,322 $ 18,456 $ 22,740 $ 18,859 $ 22,334 $ 11,008 Net income attributable to noncontrolling interests (1,630 ) (1,651 ) (1,625 ) (1,436 ) (1,721 ) (1,333 ) (1,755 ) (1,387 ) Net income attributable to COPT 17,150 19,434 18,697 17,020 21,019 17,526 20,579 9,621 Preferred share dividends — — — — (3,180 ) (3,039 ) — — Issuance costs associated with redeemed preferred shares — — — — — (6,847 ) — — Net income attributable to COPT common shareholders $ 17,150 $ 19,434 $ 18,697 $ 17,020 $ 17,839 $ 7,640 $ 20,579 $ 9,621 Basic EPS $ 0.17 $ 0.19 $ 0.18 $ 0.16 $ 0.18 $ 0.08 $ 0.21 $ 0.10 Diluted EPS $ 0.17 $ 0.19 $ 0.18 $ 0.16 $ 0.18 $ 0.08 $ 0.21 $ 0.10
Corporate Office Properties, L.P. [Member]
Quarterly Data [Line Items]
Schedule of selected quarterly information	COPLP and Subsidiaries Revenues $ 155,476 $ 146,743 $ 137,411 $ 138,482 $ 139,801 $ 151,435 $ 157,017 $ 164,567 Net income $ 18,780 $ 21,085 $ 20,322 $ 18,456 $ 22,740 $ 18,859 $ 22,334 $ 11,008 Net income attributable to noncontrolling interests (921 ) (878 ) (1,080 ) (1,061 ) (934 ) (907 ) (897 ) (908 ) Net income attributable to COPLP 17,859 20,207 19,242 17,395 21,806 17,952 21,437 10,100 Preferred unit distributions (165 ) (165 ) (165 ) (165 ) (3,345 ) (3,204 ) (165 ) (165 ) Issuance costs associated with redeemed preferred units — — — — — (6,847 ) — — Net income attributable to COPLP common unitholders $ 17,694 $ 20,042 $ 19,077 $ 17,230 $ 18,461 $ 7,901 $ 21,272 $ 9,935 Basic EPU $ 0.17 $ 0.19 $ 0.18 $ 0.16 $ 0.18 $ 0.08 $ 0.21 $ 0.10 Diluted EPU $ 0.17 $ 0.19 $ 0.18 $ 0.16 $ 0.18 $ 0.08 $ 0.21 $ 0.10

Organization (Details)

Organization (Details) - Dec. 31, 2018 ft² in Millions	a	MW	ft²	Property
Operating properties [Member]
Investments in real estate
Number of real estate properties				163
Area of real estate property (in sqft or acres) | ft²			18.1
Operating properties [Member] | Office Properties [Member]
Investments in real estate
Number of real estate properties				145
Area of real estate property (in sqft or acres) | ft²			15.1
Operating properties [Member] | Single-tenant data centers [Member]
Investments in real estate
Number of real estate properties				18
Area of real estate property (in sqft or acres) | ft²			3
Operating wholesale data centers [Member]
Investments in real estate
Critical load (in megawatts) | MW		19.25
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			1.3
Properties under, or contractually committed for, construction or approved for redevelopment [Member] | Office Properties [Member]
Investments in real estate
Number of real estate properties				6
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				4
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
Land controlled for future development [Member]
Investments in real estate
Area of real estate property (in sqft or acres)	900		11.7
Other land [Member]
Investments in real estate
Area of real estate property (in sqft or acres) | a	150
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
	Dec. 31, 2018	Dec. 31, 2017
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	98.80%	96.90%

Summary of Significant Accoun_4

Summary of Significant Accounting Policies (Details)	12 Months Ended
	Dec. 31, 2018
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 Accoun_5

Summary of Significant Accounting Policies (Details 1)	12 Months Ended
	Dec. 31, 2018compensation_arrangementForm
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 | Form	3
Corporate Office Properties, L.P. [Member]
Noncontrolling Interest [Abstract]
Interests in several real estate entities owned directly by COPT	1.00%
Construction contract revenue [Member]
Noncontrolling Interest [Abstract]
Number of compensation arrangements | compensation_arrangement	3
Typical period from billing for payment to be due (in days)	30 days

Summary of Significant Accoun_6

Summary of Significant Accounting Policies (Details 2) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Change in Accounting Estimate [Line Items]
Approximate amount by which basis of assets and liabilities for tax reporting purposes is lower than amount reported on consolidated balance sheet	$ 47
Common Shares [Member]
Change in Accounting Estimate [Line Items]
Ordinary income (as a percent)	83.10%	86.50%	48.00%
Return of capital (as a percent)	16.90%	13.50%	52.00%
Preferred Shares [Member]
Change in Accounting Estimate [Line Items]
Ordinary income (as a percent)		100.00%	100.00%
Return of capital (as a percent)		0.00%	0.00%

Summary of Significant Accoun_7

Summary of Significant Accounting Policies (Details 3) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net cash provided by operating activities	$ 180,482	$ 230,121	$ 234,270
Net cash used in investing activities	(232,918)	(89,363)	71,174
Net cash used in financing activities	49,555	(338,546)	(155,088)
Net decrease in cash and cash equivalents and restricted cash	(2,881)	(197,788)	150,356
Beginning of year	14,831	212,619	62,263
End of year	11,950	14,831	212,619
Accounting Standards Update 2016-18 and 2016-15 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net cash provided by operating activities		(533)	1,732
Net cash used in investing activities		347	(275)
Net cash used in financing activities		0	(654)
Net decrease in cash and cash equivalents and restricted cash		(186)	803
Beginning of year	2,570	2,756	1,953
End of year		2,570	2,756
Accounting Standards Update 2017-05 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Effect of change on net income			18,000
As Previously Reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net cash provided by operating activities		230,654	232,538
Net cash used in investing activities		(89,710)	71,449
Net cash used in financing activities		(338,546)	(154,434)
Net decrease in cash and cash equivalents and restricted cash		(197,602)	149,553
Beginning of year	$ 12,261	209,863	60,310
End of year		$ 12,261	$ 209,863

Summary of Significant Accoun_8

Summary of Significant Accounting Policies (Details 4) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Investment in unconsolidated real estate joint venture	$ 39,845				$ 41,787				$ 39,845	$ 41,787	$ 43,661
Cumulative distributions in excess of net income	(846,808)				(802,085)				(846,808)	(802,085)	(747,825)
Noncontrolling interests in subsidiaries	41,637				66,165				41,637	66,165	72,267
Gain on sales of real estate									2,340	9,890	59,679
Income before equity in income of unconsolidated entities and income taxes									75,583	74,549	33,260
Equity in income of unconsolidated entities									2,697	1,490	752
Net income	18,456	$ 20,322	$ 21,085	$ 18,780	11,008	$ 22,334	$ 18,859	$ 22,740	78,643	74,941	33,768
Net (income) loss attributable to noncontrolling interests - Common units in COPLP									(1,742)	(1,890)	(507)
Net income attributable to COPT/COPLP	17,020	18,697	19,434	17,150	9,621	20,579	17,526	21,019	72,301	68,745	28,890
Net income (loss) attributable to COPT/COPLP common share/unit holders	17,020	18,697	19,434	17,150	9,621	20,579	7,640	17,839	72,301	$ 55,679	$ 14,576
Earnings per common share/unit - basic and diluted (in dollars per share)										$ 0.56	$ 0.15
Comprehensive income									76,073	$ 78,968	$ 34,899
Comprehensive income attributable to COPT/ COPLP									69,620	72,643	29,997
Common units	1,102				1,013				1,102	1,013
Corporate Office Properties, L.P. [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Investment in unconsolidated real estate joint venture	39,845				41,787				39,845	41,787	43,661
Noncontrolling interests in subsidiaries	13,714				12,312				13,714	12,312
Gain on sales of real estate									2,340	9,890	59,679
Income before equity in income of unconsolidated entities and income taxes									75,583	74,549	33,260
Equity in income of unconsolidated entities									2,697	1,490	752
Net income	18,456	20,322	21,085	18,780	11,008	22,334	18,859	22,740	78,643	74,941	33,768
Net income attributable to COPT/COPLP	17,395	19,242	20,207	17,859	10,100	21,437	17,952	21,806	74,703	71,295	30,053
Net income (loss) attributable to COPT/COPLP common share/unit holders	17,230	$ 19,077	$ 20,042	$ 17,694	9,935	$ 21,272	$ 7,901	$ 18,461	74,043	$ 57,569	$ 15,079
Earnings per common share/unit - basic and diluted (in dollars per share)										$ 0.56	$ 0.15
Comprehensive income									76,073	$ 78,968	$ 34,899
Comprehensive income attributable to COPT/ COPLP									72,133	75,322	31,184
Common units	$ 1,604,655				1,445,022				$ 1,604,655	1,445,022	1,419,710
As Previously Reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Investment in unconsolidated real estate joint venture					25,066					25,066	25,548
Cumulative distributions in excess of net income					(818,190)					(818,190)	(765,276)
Noncontrolling interests in subsidiaries					65,549					65,549	71,605
Gain on sales of real estate										9,890	40,986
Income before equity in income of unconsolidated entities and income taxes										74,549	14,567
Equity in income of unconsolidated entities										2,882	1,332
Net income										76,333	15,655
Net (income) loss attributable to noncontrolling interests - Common units in COPLP										(1,936)	155
Net income attributable to COPT/COPLP										70,091	11,439
Net income (loss) attributable to COPT/COPLP common share/unit holders										$ 57,025	$ (2,875)
Earnings per common share/unit - basic and diluted (in dollars per share)										$ 0.57	$ (0.03)
Comprehensive income										$ 80,360	$ 16,786
Comprehensive income attributable to COPT/ COPLP										73,989	12,546
As Previously Reported [Member] | Corporate Office Properties, L.P. [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Investment in unconsolidated real estate joint venture					25,066					25,066	25,548
Gain on sales of real estate										9,890	40,986
Income before equity in income of unconsolidated entities and income taxes										74,549	14,567
Equity in income of unconsolidated entities										2,882	1,332
Net income										76,333	15,655
Net income attributable to COPT/COPLP										72,687	11,940
Net income (loss) attributable to COPT/COPLP common share/unit holders										$ 58,961	$ (3,034)
Earnings per common share/unit - basic and diluted (in dollars per share)										$ 0.57	$ (0.04)
Comprehensive income										$ 80,360	$ 16,786
Comprehensive income attributable to COPT/ COPLP										76,714	13,071
Common units					1,428,301					1,428,301	1,401,597
Accounting Standards Update 2017-05 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Investment in unconsolidated real estate joint venture					16,721					16,721	18,113
Cumulative distributions in excess of net income					16,105					16,105	17,451
Noncontrolling interests in subsidiaries					616					616	662
Gain on sales of real estate										0	18,693
Income before equity in income of unconsolidated entities and income taxes										0	18,693
Equity in income of unconsolidated entities										(1,392)	(580)
Net income										(1,392)	18,113
Net (income) loss attributable to noncontrolling interests - Common units in COPLP										46	(662)
Net income attributable to COPT/COPLP										(1,346)	17,451
Net income (loss) attributable to COPT/COPLP common share/unit holders										$ (1,346)	$ 17,451
Earnings per common share/unit - basic and diluted (in dollars per share)										$ (0.01)	$ 0.18
Comprehensive income										$ (1,392)	$ 18,113
Comprehensive income attributable to COPT/ COPLP										(1,346)	17,451
Accounting Standards Update 2017-05 [Member] | Corporate Office Properties, L.P. [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Investment in unconsolidated real estate joint venture					16,721					16,721	18,113
Gain on sales of real estate										0	18,693
Income before equity in income of unconsolidated entities and income taxes										0	18,693
Equity in income of unconsolidated entities										(1,392)	(580)
Net income										(1,392)	18,113
Net income attributable to COPT/COPLP										(1,392)	18,113
Net income (loss) attributable to COPT/COPLP common share/unit holders										$ (1,392)	$ 18,113
Earnings per common share/unit - basic and diluted (in dollars per share)										$ (0.01)	$ 0.19
Comprehensive income										$ (1,392)	$ 18,113
Comprehensive income attributable to COPT/ COPLP										(1,392)	18,113
Common units					$ 16,721					$ 16,721	$ 18,113

Summary of Significant Accoun_9

Summary of Significant Accounting Policies (Details 5) - USD ($) $ in Thousands	Jan. 01, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred non-incremental lease cost		$ 1,200	$ 1,100	$ 1,100
Cumulative Distributions in Excess of Net Income [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting change for adoption of hedge accounting guidance			(276)
Cumulative Distributions in Excess of Net Income [Member] | Accounting Standards Update 2017-12 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting change for adoption of hedge accounting guidance					$ (276)
Accumulated Other Comprehensive Income (Loss) [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting change for adoption of hedge accounting guidance			276
Accumulated Other Comprehensive Income (Loss) [Member] | Accounting Standards Update 2017-12 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting change for adoption of hedge accounting guidance					276
Accumulated Other Comprehensive Income (Loss) [Member] | Corporate Office Properties, L.P. [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting change for adoption of hedge accounting guidance			276
Accumulated Other Comprehensive Income (Loss) [Member] | Corporate Office Properties, L.P. [Member] | Accounting Standards Update 2017-12 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting change for adoption of hedge accounting guidance					276
Common Shares [Member] | Corporate Office Properties, L.P. [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting change for adoption of hedge accounting guidance			$ (276)
Common Shares [Member] | Corporate Office Properties, L.P. [Member] | Accounting Standards Update 2017-12 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting change for adoption of hedge accounting guidance					$ (276)
Scenario, Forecast [Member] | Right-of-use Assets [Member] | Accounting Standards Update 2016-02 [Member] | Minimum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Effect of new accounting principle	$ 14,000
Scenario, Forecast [Member] | Right-of-use Assets [Member] | Accounting Standards Update 2016-02 [Member] | Maximum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Effect of new accounting principle	19,000
Scenario, Forecast [Member] | Lease Liability [Member] | Accounting Standards Update 2016-02 [Member] | Minimum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Effect of new accounting principle	14,000
Scenario, Forecast [Member] | Lease Liability [Member] | Accounting Standards Update 2016-02 [Member] | Maximum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Effect of new accounting principle	$ 19,000

Fair Value Measurements (Detail

Fair Value Measurements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Assets and liabilities measured at fair value on a recurring basis
Marketable securities in deferred compensation plan	$ 3,868	$ 4,616
Deferred Compensation, Excluding Share-based Payments and Retirement Benefits [Member] | Trustees and Management [Member]
Assets and liabilities measured at fair value on a recurring basis
Maximum percentage of participants' compensation which is deferrable (as a percent)	100.00%
Marketable securities in deferred compensation plan	$ 3,900	$ 4,600

Fair Value Measurements (Deta_2

Fair Value Measurements (Details 2) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets:
Marketable securities in deferred compensation plan	$ 3,868	$ 4,616
Fair value measurement on a recurring basis [Member]
Assets:
Interest rate derivatives	5,617	3,073
Total assets	9,485	7,689
Liabilities:
Deferred compensation plan liability	3,868	4,616
Interest rate derivatives	5,459
Total liabilities	9,327
Fair value measurement on a recurring basis [Member] | Mutual funds [Member]
Assets:
Marketable securities in deferred compensation plan	3,819	4,547
Fair value measurement on a recurring basis [Member] | Other [Member]
Assets:
Marketable securities in deferred compensation plan	49	69
Fair value measurement on a recurring basis [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets:
Total assets	3,868	4,616
Fair value measurement on a recurring basis [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Mutual funds [Member]
Assets:
Marketable securities in deferred compensation plan	3,819	4,547
Fair value measurement on a recurring basis [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Other [Member]
Assets:
Marketable securities in deferred compensation plan	49	69
Fair value measurement on a recurring basis [Member] | Significant Other Observable Inputs (Level 2) [Member]
Assets:
Interest rate derivatives	5,617	3,073
Total assets	5,617	3,073
Liabilities:
Deferred compensation plan liability	3,868	4,616
Interest rate derivatives	5,459
Total liabilities	9,327
Corporate Office Properties, L.P. [Member] | Fair value measurement on a recurring basis [Member]
Assets:
Interest rate derivatives	5,617	3,073
Liabilities:
Interest rate derivatives	5,459
Corporate Office Properties, L.P. [Member] | Fair value measurement on a recurring basis [Member] | Significant Other Observable Inputs (Level 2) [Member]
Assets:
Interest rate derivatives	5,617	$ 3,073
Liabilities:
Interest rate derivatives	$ 5,459

Fair Value Measurements (Deta_3

Fair Value Measurements (Details 3) $ in Thousands

3 Months Ended

12 Months Ended

Dec. 31, 2018USD ($)

Sep. 30, 2018USD ($)

Jun. 30, 2018USD ($)

Mar. 31, 2018USD ($)

Dec. 31, 2017USD ($)

Sep. 30, 2017USD ($)

Jun. 30, 2017USD ($)

Mar. 31, 2017USD ($)

Dec. 31, 2016USD ($)

Sep. 30, 2016USD ($)

Jun. 30, 2016USD ($)

Mar. 31, 2016USD ($)Property

Dec. 31, 2018USD ($)

Dec. 31, 2017USD ($)

Dec. 31, 2016USD ($)

Dec. 31, 2015USD ($)

Fair value of impaired properties and other assets

Assets held for sale, net

$ 0

$ 42,226

$ 0

$ 42,226

Impairment losses

2,367

15,123

$ 101,391

Revenues from real estate operations

$ 138,482

$ 137,411

$ 146,743

$ 155,476

164,567

$ 157,017

$ 151,435

$ 139,801

$ 578,112

612,820

574,328

Held-for-sale [Member]

Fair value of impaired properties and other assets

Assets held for sale, net

42,226

42,226

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

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

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

$ 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] | White Marsh, Maryland [Member] | Held-for-sale [Member]

Fair value of impaired properties and other assets

Impairment of long-lived assets

$ 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

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

Property Sales [Member] | Fair value measurement on a nonrecurring basis [Member] | Disposed of by sale [Member]

Fair value of impaired properties and other assets

Revenues from real estate operations

$ 54,100

$ 210,700

Fair Value Measurements (Deta_4

Fair Value Measurements (Details 4) - Fair value measurement on a nonrecurring basis [Member] - Fair Value, Inputs, Level 3 [Member] $ in Thousands	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] | Discount Rate [Member] | Minimum [Member]
Assets and liabilities measured at fair value on a recurring basis
Measurement inputs	0.14
Discounted cash flow [Member] | Discount Rate [Member] | Maximum [Member]
Assets and liabilities measured at fair value on a recurring basis
Measurement inputs	0.16
Discounted cash flow [Member] | Discount Rate [Member] | Weighted Average [Member]
Assets and liabilities measured at fair value on a recurring basis
Measurement inputs	0.14
Discounted cash flow [Member] | Terminal Capitalization Rate [Member] | Weighted Average [Member]
Assets and liabilities measured at fair value on a recurring basis
Measurement inputs	0.12
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, 2018	Dec. 31, 2017	Dec. 31, 2016
Rental revenue [Member]
Concentration Risk
Percentage of revenue	24.00%	22.00%	21.00%
Construction contract revenue [Member]
Concentration Risk
Percentage of revenue	95.00%	98.00%	87.00%

Properties, Net (Details)

Properties, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Properties
Gross	$ 4,148,529	$ 3,980,813	$ 3,874,715	$ 4,158,616
Operating properties, net	2,847,265	2,737,611
Operating properties, net [Member]
Properties
Less: accumulated depreciation	(897,903)	(786,193)
Operating properties, net	2,847,265	2,737,611
Operating properties, net [Member] | Land [Member]
Properties
Gross	503,274	455,680
Operating properties, net [Member] | Buildings and improvements [Member]
Properties
Gross	$ 3,241,894	$ 3,068,124

Properties, Net (Details 2)

Properties, Net (Details 2) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Properties
Projects in development or held for future development	$ 403,361	$ 403,494
Projects in development or held for future development [Member] | Land [Member]
Properties
Projects in development or held for future development	207,760	240,825
Projects in development or held for future development [Member] | Development in progress, excluding land [Member]
Properties
Projects in development or held for future development	$ 195,601	$ 162,669

Properties, Net (Details 3)

Properties, Net (Details 3) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets held for sale, net	$ 0	$ 42,226
Held-for-sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Properties, net		38,670
Deferred rent receivable		3,237
Deferred leasing costs, net		319
Assets held for sale, net		$ 42,226

Properties, Net (Details 4)

Properties, Net (Details 4) ft² in Thousands, $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2018USD ($)ft²Property	Dec. 31, 2018USD ($)ft²Property	Dec. 31, 2017USD ($)ft²Property	Dec. 31, 2016USD ($)ft²builldingProperty
Construction Activities [Line Items]
Impairment losses | $		$ 2,367	$ 15,123	$ 101,391
Fort Meade/BW Corridor [Member]
Construction Activities [Line Items]
Impairment losses | $	$ 2,400
Newly-constructed properties placed in service [Member]
Construction Activities [Line Items]
Square footage of real estate properties (in square feet) | ft²	666	666	1,100	639
Number of real estate properties	6	6	8	6
Newly-constructed properties placed in service, partially operational [Member]
Construction Activities [Line Items]
Number of real estate properties	2	2
Newly redeveloped properties placed In service [Member]
Construction Activities [Line Items]
Square footage of real estate properties (in square feet) | ft²	22	22	94	61
Number of real estate properties	1	1		3
Properties Under Construction or Contractually Committed, Partially Operational [Member]
Construction Activities [Line Items]
Number of real estate properties	2	2
Properties under Construction or Contractually Commited for Construction [Member]
Construction Activities [Line Items]
Square footage of real estate properties (in square feet) | ft²	1,100	1,100
Number of real estate properties	9	9
Properties under redevelopment [Member]
Construction Activities [Line Items]
Square footage of real estate properties (in square feet) | ft²	106	106
Number of real estate properties	1	1

Properties, Net (Details 5)

Properties, Net (Details 5) ft² in Thousands, $ in Thousands	Jul. 21, 2016USD ($)Property	Dec. 31, 2018USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)ft²buillding	Dec. 31, 2016USD ($)ft²buillding	Oct. 27, 2017USD ($)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 contributed into a joint venture			$ 0	$ 0	$ 22,600
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 contributed into a joint venture	$ 147,600
Initial amount borrowed					60,000
Ownership percentage	50.00%
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
Gains on sale of other land				4,200	7,200
Operating properties [Member] | Single-tenant data centers [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
Operating properties [Member] | Held-for-sale [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]
Undiscounted indemnification amount						$ 20,000
Gain on Sale		$ 1,500
Total Rentable Square Feet | ft²						193
Transaction Value						$ 44,000
Operating properties [Member] | Disposed of by sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on Sale				$ 5,636	$ 15,871
Number of Buildings | buillding				12	21
Total Rentable Square Feet | ft²				799	1,577
Transaction Value				$ 125,829	$ 248,738
Operating properties [Member] | Disposed of by sale [Member] | Regional Office [Member] | Arborcrest Corporate Campus [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on Sale					$ 4,742
Number of Buildings | buillding					4
Total Rentable Square Feet | ft²					654
Transaction Value					$ 142,800
Operating properties [Member] | Disposed of by sale [Member] | Regional Office [Member] | 8003 Corporate Drive [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on Sale					$ 0
Number of Buildings | buillding					1
Total Rentable Square Feet | ft²					18
Transaction Value					$ 2,400
Operating properties [Member] | Disposed of by sale [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]
Gain on Sale					$ 1,906
Number of Buildings | buillding					5
Total Rentable Square Feet | ft²					130
Transaction Value					$ 14,513
Operating properties [Member] | Disposed of by sale [Member] | Regional Office [Member] | 4940 Campbell Drive [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on Sale					$ 0
Number of Buildings | buillding					1
Total Rentable Square Feet | ft²					50
Transaction Value					$ 5,200
Operating properties [Member] | Disposed of by sale [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]
Gain on Sale					$ 848
Number of Buildings | buillding					2
Total Rentable Square Feet | ft²					25
Transaction Value					$ 2,900
Operating properties [Member] | Disposed of by sale [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]
Gain on Sale					$ 8,375
Number of Buildings | buillding					3
Total Rentable Square Feet | ft²					299
Transaction Value					$ 48,100
Operating properties [Member] | Disposed of by sale [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]
Gain on Sale					$ 0
Number of Buildings | buillding					1
Total Rentable Square Feet | ft²					29
Transaction Value					$ 2,625
Operating properties [Member] | Disposed of by sale [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]
Gain on Sale					$ 0
Number of Buildings | buillding					1
Total Rentable Square Feet | ft²					101
Transaction Value					$ 7,800
Operating properties [Member] | Disposed of by sale [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]
Gain on Sale				$ 0
Number of Buildings | buillding				1
Total Rentable Square Feet | ft²				190
Transaction Value				$ 39,000
Operating properties [Member] | Disposed of by sale [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]
Gain on Sale					$ 0
Number of Buildings | buillding					1
Total Rentable Square Feet | ft²					151
Transaction Value					$ 12,100
Operating properties [Member] | Disposed of by sale [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]
Gain on Sale				$ 0
Number of Buildings | buillding				1
Total Rentable Square Feet | ft²				37
Transaction Value				$ 2,300
Operating properties [Member] | Disposed of by sale [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]
Gain on Sale				$ 831
Number of Buildings | buillding				1
Total Rentable Square Feet | ft²				57
Transaction Value				$ 7,529
Operating properties [Member] | Disposed of by sale [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]
Gain on Sale				$ 3,625
Number of Buildings | buillding				1
Total Rentable Square Feet | ft²				103
Transaction Value				$ 29,500
Operating properties [Member] | Disposed of by sale [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]
Gain on Sale					$ 0
Number of Buildings | buillding					2
Total Rentable Square Feet | ft²					120
Transaction Value					$ 10,300
Operating properties [Member] | Disposed of by sale [Member] | White Marsh, Maryland [Member] | Regional Office and Other [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on Sale				$ 1,180
Number of Buildings | buillding				8
Total Rentable Square Feet | ft²				412
Transaction Value				$ 47,500

Real Estate Joint Ventures (Det

Real Estate Joint Ventures (Details) $ in Thousands	Jul. 21, 2016USD ($)Property	Jan. 31, 2016USD ($)	Dec. 31, 2018USD ($)ft²	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Investments in consolidated real estate joint ventures
Increase in property and redeemable noncontrolling interests in connection with property contributed into a joint venture			$ 0	$ 0	$ 22,600
Distributions paid to redeemable noncontrolling interests			1,382	8,215	15,206
Consolidated real estate joint ventures [Member]
Investments in consolidated real estate joint ventures
Total Assets			327,990
Encumbered Assets			199,049
Total Liabilities			$ 110,416
Single-tenant data centers [Member]
Investments in consolidated real estate joint ventures
Increase in property and redeemable noncontrolling interests in connection with property contributed into 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]
Investments in consolidated real estate joint ventures
Number of properties contributed | Property	6
Stevens Investors, LLC [Member] | Minimum [Member]
Investments in consolidated real estate joint ventures
Percentage of residual distributable cash flows in excess of unpaid cumulative preferred returns and return of invested capital entitled to the company			60.00%
LW Redstone Company, LLC [Member]
Investments in consolidated real estate joint ventures
Partner's capital account upon formation			$ 9,000
Percentage of residual distributable cash flows in excess of unpaid cumulative preferred returns and return of invested capital entitled to the company			85.00%
Percentage of residual distributable cash flows in excess of unpaid cumulative preferred returns and return of invested capital entitled to the entity's partners			15.00%
Number of years following construction commencement threshold achievement before partner's interest can be purchased at fair value			5 years
Construction commencement threshold (in square feet) | ft²			4,400,000
Construction commencement completed (in sqft) | ft²			751,000
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%
LW Redstone Company, LLC [Member] | Notes receivable from City of Huntsville [Member]
Investments in consolidated real estate joint ventures
Infrastructure costs funded to date for reimbursement by the City of Huntsville			$ 40,000
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
Real estate equity method investments			$ 39,800	41,800
LW Redstone Company, LLC [Member] | Primary Beneficiary [Member]
Investments in consolidated real estate joint ventures
Ownership (as a percent)			85.00%
Total Assets			$ 169,533
Encumbered Assets			72,800
Total Liabilities			$ 50,530
M Square Associates, LLC [Member] | Primary Beneficiary [Member]
Investments in consolidated real estate joint ventures
Ownership (as a percent)			50.00%
Total Assets			$ 75,339
Encumbered Assets			43,631
Total Liabilities			$ 43,869
Stevens Investors, LLC [Member] | Primary Beneficiary [Member]
Investments in consolidated real estate joint ventures
Ownership (as a percent)			95.00%
Total Assets			$ 83,118
Encumbered Assets			82,618
Total Liabilities			$ 16,017
Increase in property and redeemable noncontrolling interests in connection with property contributed into a joint venture		$ 22,600
Distributions paid to redeemable noncontrolling interests				$ 6,700	$ 13,400

Intangible Assets on Real Est_3

Intangible Assets on Real Estate Acquisitions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Intangible assets on real estate acquisitions
Gross Carrying Amount	$ 216,358	$ 222,683
Accumulated Amortization	172,888	163,591
Net Carrying Amount	43,470	59,092
Amortization of the intangible assets	$ 15,600	19,300	$ 20,000
Weighted average amortization period of intangible assets	12 years
Estimated future amortization expense associated with the intangible asset categories for the next five years
2,019	$ 8,300
2,020	5,500
2,021	5,300
2,022	3,800
2,023	3,400
In-place lease value [Member]
Intangible assets on real estate acquisitions
Gross Carrying Amount	132,276	132,276
Accumulated Amortization	117,520	110,814
Net Carrying Amount	$ 14,756	21,462
Weighted average amortization period of intangible assets	7 years
Tenant relationship value [Member]
Intangible assets on real estate acquisitions
Gross Carrying Amount	$ 60,028	60,028
Accumulated Amortization	39,703	32,198
Net Carrying Amount	$ 20,325	27,830
Weighted average amortization period of intangible assets	9 years
Below-market cost arrangements [Member]
Intangible assets on real estate acquisitions
Gross Carrying Amount	$ 8,880	15,102
Accumulated Amortization	1,507	7,507
Net Carrying Amount	$ 7,373	7,595
Weighted average amortization period of intangible assets	33 years
Above-market leases [Member]
Intangible assets on real estate acquisitions
Gross Carrying Amount	$ 13,841	13,944
Accumulated Amortization	13,164	12,092
Net Carrying Amount	$ 677	1,852
Weighted average amortization period of intangible assets	6 years
Other [Member]
Intangible assets on real estate acquisitions
Gross Carrying Amount	$ 1,333	1,333
Accumulated Amortization	994	980
Net Carrying Amount	$ 339	$ 353
Weighted average amortization period of intangible assets	24 years

Investing Receivables (Details)

Investing Receivables (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Investing receivables	$ 56,982	$ 57,493
Investing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fair value of investing receivables	58,200	58,300
Notes receivable from City of Huntsville [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Investing receivables	$ 53,961	54,472
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 As_3

Prepaid Expenses and Other Assets, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Restricted cash	$ 3,884	$ 2,570	$ 2,756	$ 1,953
Marketable securities in deferred compensation plan	3,868	4,616
Total for COPT and subsidiaries	85,581	71,334
Corporate Office Properties, L.P. [Member]
Property, Plant and Equipment [Line Items]
Prepaid expenses	25,658	24,670
Lease incentives, net	21,258	19,011
Furniture, fixtures and equipment, net	8,630	5,256
Non-real estate equity investments	5,940	5,056
Deferred financing costs, net	4,733	1,202
Restricted cash	3,884	2,570	$ 2,756	$ 1,953
Construction contract costs incurred in excess of billings	3,189	4,884
Deferred tax asset, net	2,084	1,892
Other assets	6,337	2,177
Total for COPT and subsidiaries	$ 81,713	$ 66,718

Prepaid Expenses and Other As_4

Prepaid Expenses and Other Assets, Net (Details 2) - TRS [Member] - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Tax Credit Carryforward [Line Items]
Operating loss carry forward	$ 4,354	$ 3,209
Share-based compensation	28	7
Accrued payroll	2	49
Property	427	43
Valuation allowance	(2,727)	(1,416)
Deferred tax asset, net	$ 2,084	$ 1,892

Debt, Net (Details)

Debt, Net (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Debt
Carrying Value	$ 1,823,909,000	$ 1,828,333,000
Revolving Credit Facility [Member]
Debt
Carrying Value	$ 213,000,000	126,000,000
Weighted average interest rate (as a percent)	3.49%
Term Loan Facilities [Member]
Debt
Carrying Value	$ 248,273,000	347,959,000
Interest rate (as a percent)	3.60%
Aggregate additional borrowing capacity	$ 150,000,000
Unsecured notes payable [Member]
Debt
Carrying Value	$ 1,167,000	1,287,000
Interest rate (as a percent)	0.00%
Unamortized discount included in carrying value	$ 294,000	373,000
London Interbank Offered Rate (LIBOR) [Member] | Revolving Credit Facility [Member] | Minimum [Member]
Debt
Variable rate, spread (as a percent)	0.775%
London Interbank Offered Rate (LIBOR) [Member] | Revolving Credit Facility [Member] | Maximum [Member]
Debt
Variable rate, spread (as a percent)	1.45%
London Interbank Offered Rate (LIBOR) [Member] | Term Loan Facilities [Member] | Minimum [Member]
Debt
Variable rate, spread (as a percent)	0.85%
London Interbank Offered Rate (LIBOR) [Member] | Term Loan Facilities [Member] | Maximum [Member]
Debt
Variable rate, spread (as a percent)	1.65%
Mortgage and other secured debt [Member]
Debt
Carrying Value	$ 170,423,000	163,838,000
Mortgage and other secured debt [Member] | Fixed rate mortgage debt [Member]
Debt
Carrying Value	147,141,000	150,723,000
Unamortized premium included in carrying value	$ 281,000	349,000
Weighted average interest rate (as a percent)	4.17%
Mortgage and other secured debt [Member] | Fixed rate mortgage debt [Member] | Minimum [Member]
Debt
Interest rate (as a percent)	3.82%
Mortgage and other secured debt [Member] | Fixed rate mortgage debt [Member] | Maximum [Member]
Debt
Interest rate (as a percent)	7.87%
Mortgage and other secured debt [Member] | Variable rate secured loans [Member]
Debt
Carrying Value	$ 23,282,000	13,115,000
Weighted average interest rate (as a percent)	4.47%
Mortgage and other secured debt [Member] | London Interbank Offered Rate (LIBOR) [Member] | Variable rate secured loans [Member] | Minimum [Member]
Debt
Variable rate, spread (as a percent)	1.85%
Mortgage and other secured debt [Member] | London Interbank Offered Rate (LIBOR) [Member] | Variable rate secured loans [Member] | Maximum [Member]
Debt
Variable rate, spread (as a percent)	2.35%
Mortgage and other secured debt [Member] | London Interbank Offered Rate (LIBOR) [Member] | Construction Loan Payable [Member]
Debt
Remaining borrowing capacity	$ 98,400,000
Senior Notes [Member] | 3.60% Senior Notes [Member]
Debt
Carrying Value	347,986,000	347,551,000
Debt instrument, face amount	$ 350,000,000
Interest rate (as a percent)	3.60%
Unamortized discount included in carrying value	$ 1,400,000	1,700,000
Effective interest rate on debt (as a percent)	3.70%
Senior Notes [Member] | 5.250% Senior Notes [Member]
Debt
Carrying Value	$ 247,136,000	246,645,000
Debt instrument, face amount	$ 250,000,000
Interest rate (as a percent)	5.25%
Unamortized discount included in carrying value	$ 2,600,000	3,000,000
Effective interest rate on debt (as a percent)	5.49%
Senior Notes [Member] | 3.70% Senior Notes [Member]
Debt
Carrying Value	$ 298,815,000	298,322,000
Debt instrument, face amount	$ 300,000,000
Interest rate (as a percent)	3.70%
Unamortized discount included in carrying value	$ 900,000	1,300,000
Effective interest rate on debt (as a percent)	3.85%
Senior Notes [Member] | 5.0% Senior Notes [Member]
Debt
Carrying Value	$ 297,109,000	296,731,000
Debt instrument, face amount	$ 300,000,000
Interest rate (as a percent)	5.00%
Unamortized discount included in carrying value	$ 2,400,000	2,700,000
Effective interest rate on debt (as a percent)	5.15%
Loans Payable [Member]
Debt
Deferred financing costs, net	$ 7,200,000	$ 5,000,000

Debt, Net (Details 2)

Debt, Net (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule on basis of which debt matures
2,019	$ 4,387
2,020	16,156
2,021	303,875
2,022	267,611
2,023	629,590
Thereafter	616,885
Total	1,838,504
Net discounts and deferred financing costs	14,600
Capitalized interest costs	$ 5,900	$ 5,200	$ 5,700

Debt, Net (Details 3)

Debt, Net (Details 3) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Carrying Amount [Member]
Debt Instrument [Line Items]
Long-term debt	$ 1,823,909	$ 1,828,333
Carrying Amount [Member] | Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Long-term debt	1,191,046	1,189,249
Carrying Amount [Member] | Other fixed-rate debt [Member]
Debt Instrument [Line Items]
Long-term debt	148,308	152,010
Carrying Amount [Member] | Variable-rate debt [Member]
Debt Instrument [Line Items]
Long-term debt	484,555	487,074
Estimated Fair Value [Member]
Debt Instrument [Line Items]
Long-term debt	1,853,206	1,867,577
Estimated Fair Value [Member] | Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Long-term debt	1,219,603	1,229,398
Estimated Fair Value [Member] | Other fixed-rate debt [Member]
Debt Instrument [Line Items]
Long-term debt	147,106	152,485
Estimated Fair Value [Member] | Variable-rate debt [Member]
Debt Instrument [Line Items]
Long-term debt	$ 486,497	$ 485,694

Debt, Net (Details 4)

Debt, Net (Details 4)	Oct. 10, 2018USD ($)extension	Dec. 28, 2016USD ($)	Dec. 17, 2015USD ($)	Nov. 30, 2018USD ($)	May 31, 2017USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Term Loan Facilities Maturing 2020 [Member]
Debt Instrument [Line Items]
Repayments of term loan				$ 100,000,000	$ 200,000,000
Term Credit Facility Effective August 2012 [Member]
Debt Instrument [Line Items]
Repayments of term loan								$ 120,000,000
Term Loan Facilities [Member]
Debt Instrument [Line Items]
Stated interest rate (as a percent)						3.60%
Minimum [Member] | Term Loan Facilities [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Variable rate, spread (as a percent)						0.85%
Maximum [Member] | Term Loan Facilities [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Variable rate, spread (as a percent)						1.65%
Revolving Credit Facility [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Weighted average borrowings						$ 188,100,000	$ 97,800,000
Weighted average interest rate (as a percent)						3.08%	2.44%
Line of Credit [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Maximum availability	$ 800,000,000					$ 800,000,000
Aggregate maximum borrowing capacity	$ 1,250,000,000
Number of extensions | extension	2
Extension option period (in years)	6 months
Line of credit facility, extension fee percentage	0.075%
Remaining borrowing capacity						587,000,000
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)	0.775%
Commitment fee percentage	0.125%
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)	1.45%
Commitment fee percentage	0.30%
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] | Term Loan Facilities [Member]
Debt Instrument [Line Items]
Maximum availability	$ 500,000,000
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)			0.85%
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)			1.65%
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%
Unamortized discount included in carrying value						$ 2,400,000	$ 2,700,000
Interest rate on debt (as a percent)						5.15%
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]
Loan amount						$ 300,000,000
Stated interest rate (as a percent)						3.70%
Unamortized discount included in carrying value						$ 900,000	1,300,000
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]
Loan amount						$ 350,000,000
Stated interest rate (as a percent)						3.60%
Unamortized discount included in carrying value						$ 1,400,000	1,700,000
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]
Loan amount						$ 250,000,000
Stated interest rate (as a percent)						5.25%
Unamortized discount included in carrying value						$ 2,600,000	$ 3,000,000
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%

Interest Rate Derivatives (Deta

Interest Rate Derivatives (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivatives in Hedging Relationships
Amount of (Loss) Gain Recognized in AOCI on Derivatives	$ (2,373)	$ 684	$ (2,915)
Interest rate swaps | Interest expense [Member]
Derivatives in Hedging Relationships
Amount of Gain (Loss) Reclassified from AOCI into Interest Expense on Statement of Operations	407	(3,304)	$ (4,230)
Interest rate swaps | Interest rate derivatives [Member]
Fair value of interest rate derivatives and balance sheet classification
Interest rate swaps designated as cash flow hedges	5,617	3,073
Interest rate swaps | Other liabilities [Member]
Fair value of interest rate derivatives and balance sheet classification
Interest rate swaps designated as cash flow hedges	(5,459)	0
Designated [Member]
Fair values of interest rate swap derivatives
Fair value of interest rate swaps	158	3,073
Designated [Member] | Interest rate swaps
Derivatives in Hedging Relationships
Approximate amount of gains to be reclassified from AOCL to interest expense over the next 12 months	2,100
Fair value of derivative liability	5,500
Settlement amount of derivative obligation at termination value	5,500
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	$ 472	252
Designated [Member] | Interest rate swap, effective October 13, 2015 [Member]
Fair values of interest rate swap derivatives
Notional Amount	$ 12,834
Fixed Rate (as a percent)	1.39%
Fair value of interest rate swaps	$ 239	213
Notional amount of interest rate derivatives	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	$ 1,968	1,046
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	$ 1,967	1,051
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	$ 971	511
Designated [Member] | Interest rate swap, effective June 30, 2020, swap one [Member]
Fair values of interest rate swap derivatives
Notional Amount	$ 75,000
Fixed Rate (as a percent)	3.176%
Fair value of interest rate swaps	$ (2,676)	0
Designated [Member] | Interest rate swap, effective June 30, 2020, swap two [Member]
Fair values of interest rate swap derivatives
Notional Amount	$ 75,000
Fixed Rate (as a percent)	3.192%
Fair value of interest rate swaps	$ (2,783)	$ 0

Redeemable Noncontrolling Int_3

Redeemable Noncontrolling Interests (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)joint_venture	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Noncontrolling Interest [Abstract]
Number of joint ventures with redeemable noncontrolling interests | joint_venture	2
Redeemable Noncontrolling Interest [Roll Forward]
Beginning balance	$ 23,125	$ 22,979	$ 19,218
Contributions from noncontrolling interests	186	0	22,779
Distributions to noncontrolling interests	(1,411)	(1,566)	(21,881)
Net income attributable to noncontrolling interests	2,523	2,338	2,242
Adjustment to arrive at fair value of interests	1,837	(626)	621
Ending balance	$ 26,260	$ 23,125	$ 22,979

Equity - COPT and Subsidiaries

Equity - COPT and Subsidiaries (Details) - USD ($)	Nov. 02, 2017	Jun. 27, 2017	Jan. 21, 2017	Nov. 30, 2018	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Preferred Stock
Stock redeemed or called during period value								$ 172,500,000	$ 26,583,000
Common Shares
Number of common shares for each converted common unit (in shares)							1
Number of operating partnerships units converted into common shares (in units)							1,904,615	339,513	87,000
Dividends declared per common share (in dollars per share)							$ 1.1	$ 1.10	$ 1.10
Preferred Shares [Member]
Preferred Stock
Number of preferred shares authorized							25,000,000
Number of preferred shares of beneficial interest authorized (in dollars per share)							$ 0.01
Series 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
2016 ATM Program [Member]
Common Shares
Shares issued to the public							992,000	591,000	3,700,000
Issuance of stock, weighted average price per share (in dollars per share)						$ 29.56	$ 30.46	$ 33.84	$ 29.56
Payments of stock issuance costs							$ 500,000	$ 300,000	$ 900,000
Net proceeds from issuance of shares after underwriting discounts and commissions but before offering expenses							$ 29,800,000	$ 19,700,000	$ 109,100,000
2016 ATM Program [Member] | Common Shares [Member]
Common Shares
At-market-stock, offering program established, aggregate value					$ 200,000,000
Shares issued to the public						3,700,000	992,000	591,000
Issuance of stock, weighted average price per share (in dollars per share)						$ 29.56	$ 30.46	$ 33.84	$ 29.56
Payments of stock issuance costs						$ 900,000	$ 500,000	$ 300,000
Net proceeds from issuance of shares after underwriting discounts and commissions but before offering expenses						109,100,000	$ 29,800,000	$ 19,700,000
2018 ATM Program [Member] | Common Shares [Member]
Common Shares
At-market-stock, offering program established, aggregate value				$ 300,000,000
Forward Equity Sale Agreement [Member] | Common Shares [Member]
Common Shares
Forward Equity Sale Agreement, Offering Program Established, Aggregate Shares	9,200,000
Shares issued to the public							5,900,000	1,700,000
Net proceeds from issuance of shares before underwriting discounts, commission and offering expenses	$ 285,200,000
Initial gross offering price per share (in dollars per share)	$ 31
Net proceeds from issuance of shares after underwriting discounts and commissions but before offering expenses							$ 172,500,000	$ 50,000,000
Remaining shares under agreements							1,600,000
Settlement value of remaining capacity							$ 46,400,000
Corporate Office Properties, L.P. [Member]
Preferred Stock
Stock redeemed or called during period value								$ 172,500,000	$ 26,583,000
Common Shares
Number of common shares for each converted common unit (in shares)							1
Number of operating partnerships units converted into common shares (in units)							1,900,000	339,513	87,000
Dividends declared per common share (in dollars per share)							$ 1.1	$ 1.10	$ 1.10
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			$ 26,600,000
Issuance costs associated with redeemed preferred shares						$ 17,000
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		$ 172,500,000
Issuance costs associated with redeemed preferred shares								$ 6,800,000

Equity - COPLP and Subsidiari_2

Equity - COPLP and Subsidiaries (Details) - USD ($) $ / shares in Units, $ in Thousands	Jun. 27, 2017	Jan. 21, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Preferred Stock
Stock redeemed or called during period value					$ 172,500	$ 26,583
Preferred units in COPLP				$ 8,800	$ 8,800
Common Shares
Number of common shares for each converted common unit (in shares)				1
Number of operating partnerships units converted into common shares (in units)				1,904,615	339,513	87,000
Redemption of preferred shares				$ 339
Dividends declared per common share (in dollars per share)				$ 1.1	$ 1.10	$ 1.10
2016 ATM Program [Member]
Common Shares
Shares issued to the public				992,000	591,000	3,700,000
Issuance of stock, weighted average price per share (in dollars per share)			$ 29.56	$ 30.46	$ 33.84	$ 29.56
Payments of stock issuance costs				$ 500	$ 300	$ 900
Net proceeds from issuance of shares after underwriting discounts and commissions but before offering expenses				$ 29,800	$ 19,700	$ 109,100
Common Shares [Member] | 2016 ATM Program [Member]
Common Shares
Shares issued to the public			3,700,000	992,000	591,000
Issuance of stock, weighted average price per share (in dollars per share)			$ 29.56	$ 30.46	$ 33.84	$ 29.56
Payments of stock issuance costs			$ 900	$ 500	$ 300
Net proceeds from issuance of shares after underwriting discounts and commissions but before offering expenses			109,100	$ 29,800	$ 19,700
Common Shares [Member] | Forward Equity Sale Agreement [Member]
Common Shares
Shares issued to the public				5,900,000	1,700,000
Net proceeds from issuance of shares after underwriting discounts and commissions but before offering expenses				$ 172,500	$ 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
Number of common shares for each converted common unit (in shares)				1
Number of operating partnerships units converted into common shares (in units)				1,900,000	339,513	87,000
Redemption of preferred shares				$ 339
Dividends declared per common share (in dollars per share)				$ 1.1	$ 1.10	$ 1.10
Corporate Office Properties, L.P. [Member] | Common Shares [Member]
Common Shares
Stock redeemed or called during period (in units/ shares)				13,377
Redemption of preferred shares				$ 339
Corporate Office Properties, L.P. [Member] | Common Units [Member]
Common Shares
Percentage ownership in operating partnership				98.80%	96.90%
Stock redeemed or called during period (in units/ shares)				13,377
Redemption of preferred shares				$ 339
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
Preferred units in COPLP				$ 8,800
Preferred stock, liquidation preference per share				$ 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 _3

Share-Based Compensation and Other Compensation Matters (Details 1) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-Based Compensation
Share-based compensation cost	$ 6,963	$ 6,095	$ 7,453
General, adminstrative and leasing expenses [Member]
Share-Based Compensation
Share-based compensation cost	5,415	4,649	5,816
Property operating expenses [Member]
Share-Based Compensation
Share-based compensation cost	961	966	1,027
Capitalized to development activities [Member]
Share-Based Compensation
Share-based compensation cost	$ 587	$ 480	$ 610

Share-Based Compensation and _4

Share-Based Compensation and Other Compensation Matters (Details 2) - Restricted shares [Member] - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Shares
Unvested at the beginning of the period (in shares)	425,626	371,247	378,200
Stock awards granted (in shares or units)	219,716	239,479	231,937
Forfeited (in shares)	(25,419)	(27,056)	(22,907)
Vested (in shares)	(181,238)	(158,044)	(215,983)
Unvested at the end of the period (in shares)	438,685	425,626	371,247
Restricted shares expected to vest (in shares)	413,273
Weighted Average Grant Date Fair Value
Unvested at the beginning of the period (in dollars per share)	$ 30.37	$ 26.20	$ 27.58
Grant date fair value (in dollars per share)	25.62	33.84	24.77
Forfeited (in dollars per share)	30.02	27.80	25.31
Vested (in dollars per share)	29.49	26.27	27.19
Unvested at the end of the period (in dollars per share)	28.38	$ 30.37	$ 26.20
Restricted shares expected to vest (in dollars per share)	$ 28.35

Share-Based Compensation and _5

Share-Based Compensation and Other Compensation Matters (Details 3) - Performance share units [Member] - USD ($) $ / shares in Units, $ in Thousands	Jan. 01, 2018	Jan. 01, 2017	Mar. 01, 2016	Mar. 05, 2015	Mar. 06, 2014	Dec. 31, 2018
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)						0
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			$ 1,005
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		$ 1,415
Unvested at the end of the period (in shares)						39,351
Grant date fair value (in dollars per share)		$ 38.43
2018 PSU Grants [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock awards granted (in shares or units)	59,110
Grant date fair value	$ 1,890
Unvested at the end of the period (in shares)						59,110
Grant date fair value (in dollars per share)	$ 31.97

Share-Based Compensation and _6

Share-Based Compensation and Other Compensation Matters (Details 4) - Performance share units [Member] - $ / shares	Jan. 01, 2018	Jan. 01, 2017	Mar. 01, 2016	Dec. 31, 2018
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%
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%
2018 PSU Grants [Member]
Assumptions used to value stock awards
Grant date fair value (in dollars per share)	$ 31.97
Baseline Common Share Value (in dollars per share)	$ 29.20
Expected Volatility of Common Shares	17.00%
Risk-free Interest Rate	2.04%

Share-Based Compensation and _7

Share-Based Compensation and Other Compensation Matters (Details 5) - Deferred Share Awards [Member] - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Deferred Share Awards Granted (in shares or units)	13,832	10,032	24,944
Aggregate Grant Date Fair Value	$ 388	$ 326	$ 671
Grant Date Fair Value Per Share (in dollars per share)	$ 28.08	$ 32.47	$ 26.89
Number of common shares issued (in shares)	5,515	15,590	12,028
Grant date fair value (in dollars per share)	$ 29.32	$ 26.89	$ 26.70
Aggregate intrinsic value	$ 154	$ 508	$ 322

Share-Based Compensation and _8

Share-Based Compensation and Other Compensation Matters (Details 6) - Options [Member] - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options Outstanding and Exercisable (in shares)	30,000	60,000	201,100	425,347
Weighted Average Exercise Price Per Share (in dollars per share)	$ 32.52	$ 35.17	$ 43.35	$ 42.75
Weighted Average Remaining Contractual Term (years)	4 months 29 days	1 year	1 year	1 year
Aggregate Intrinsic Value	$ 0	$ 0	$ 31	$ 0

Share-Based Compensation and _9

Share-Based Compensation and Other Compensation Matters (Details 7) $ / shares in Units, $ in Thousands	Feb. 22, 2018shares	Jan. 01, 2018$ / shares	Feb. 07, 2017shares	Jan. 01, 2017$ / shares	Oct. 30, 2016shares	Jul. 12, 2016shares	May 30, 2016shares	Dec. 31, 2018USD ($)Percentile_Rank$ / shares	Dec. 31, 2017USD ($)$ / shares	Dec. 31, 2016USD ($)$ / 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
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								$ 1,700
Expected weighted average period during which unrecognized compensation cost will be recognized								2 years
Performance period of the award								3 years
Grant date fair value									236
Deferred Share Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Estimated pre-vesting forfeitures								0.00%
Unrecognized compensation cost								$ 137
Aggregate intrinsic value								$ 154	$ 508	$ 322
Grant date fair value (in dollars per share) | $ / shares								$ 28.08	$ 32.47	$ 26.89
Restricted shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost								$ 7,900
Expected weighted average period during which unrecognized compensation cost will be recognized								2 years
Aggregate intrinsic value								$ 4,600	$ 5,300	$ 5,400
Grant date fair value (in dollars per share) | $ / shares								$ 25.62	$ 33.84	$ 24.77
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								6.00%
Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate intrinsic value of options exercised									$ 18
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
2018 PSU Grants [Member] | Performance share units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
The number of percentile ranks to fall between to earn interpolated PSUs between such percentile ranks, conditioned on the percentile rank exceeding 25% | Percentile_Rank								2
Grant date fair value (in dollars per share) | $ / shares		$ 31.97
2014 and 2015 Performance Share Unit Grants [Member] | Performance share units [Member] | 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	10,326
2014, 2015 and 2016 Performance Share Unit Grants [Member] | Performance share units [Member] | 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					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
2015 PSU Grants [Member] | Performance share units [Member] | 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	13,328

Operating Leases (Details)

Operating Leases (Details) $ in Thousands	Dec. 31, 2018USD ($)
Gross minimum future rentals
2,019	$ 400,617
2,020	337,646
2,021	280,369
2,022	246,329
2,023	194,888
Thereafter	523,932
Total	$ 1,983,781

Information by Business Segme_3

Information by Business Segment (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment financial information for real estate operations
Revenues from real estate operations	$ 138,482	$ 137,411	$ 146,743	$ 155,476	$ 164,567	$ 157,017	$ 151,435	$ 139,801	$ 578,112	$ 612,820	$ 574,328
Property operating expenses									(201,035)	(190,964)	(197,530)
UJV NOI allocable to COPT									4,818	4,805	2,145
Segment assets	3,656,005				3,595,205				3,656,005	3,595,205
Segment assets [Member]
Segment financial information for real estate operations
Segment assets	3,084,862				3,028,005				3,084,862	3,028,005
Real estate operations [Member]
Segment financial information for real estate operations
Revenues from real estate operations									517,253	509,980	525,964
Property operating expenses									(201,035)	(190,964)	(197,530)
UJV NOI allocable to COPT									4,818	4,805	2,145
NOI from real estate operations									321,036	323,821	330,579
Additions to long-lived assets									74,742	73,341	70,324
Transfers from non-operating properties									166,143	202,211	184,640
Real estate operations [Member] | Defense/Information Technology Sector [Member]
Segment financial information for real estate operations
Revenues from real estate operations									421,053	408,122	406,210
Property operating expenses									(153,004)	(146,558)	(148,705)
UJV NOI allocable to COPT									4,818	4,805	2,145
NOI from real estate operations									272,867	266,369	259,650
Additions to long-lived assets									53,676	44,352	57,131
Transfers from non-operating properties									163,839	202,185	184,943
Real estate operations [Member] | Defense/Information Technology Sector [Member] | Fort Meade/BW Corridor [Member]
Segment financial information for real estate operations
Revenues from real estate operations									248,927	245,613	245,354
Property operating expenses									(82,975)	(80,697)	(83,684)
UJV NOI allocable to COPT									0	0	0
NOI from real estate operations									165,952	164,916	161,670
Additions to long-lived assets									38,612	26,659	26,267
Transfers from non-operating properties									35,648	43,370	49,937
Real estate operations [Member] | Defense/Information Technology Sector [Member] | Northern Virginia Defense/IT [Member]
Segment financial information for real estate operations
Revenues from real estate operations									53,518	47,118	48,964
Property operating expenses									(20,330)	(16,938)	(17,824)
UJV NOI allocable to COPT									0	0	0
NOI from real estate operations									33,188	30,180	31,140
Additions to long-lived assets									7,956	8,115	17,344
Transfers from non-operating properties									10,231	48,328	28,230
Real estate operations [Member] | Defense/Information Technology Sector [Member] | Lackland Air Force Base [Member]
Segment financial information for real estate operations
Revenues from real estate operations									46,286	47,209	46,803
Property operating expenses									(26,888)	(27,812)	(27,357)
UJV NOI allocable to COPT									0	0	0
NOI from real estate operations									19,398	19,397	19,446
Additions to long-lived assets									0	71	0
Transfers from non-operating properties									14,718	0	240
Real estate operations [Member] | Defense/Information Technology Sector [Member] | Navy Support Locations [Member]
Segment financial information for real estate operations
Revenues from real estate operations									31,927	29,540	28,197
Property operating expenses									(13,536)	(12,619)	(12,690)
UJV NOI allocable to COPT									0	0	0
NOI from real estate operations									18,391	16,921	15,507
Additions to long-lived assets									6,535	8,451	9,168
Transfers from non-operating properties									(116)	474	0
Real estate operations [Member] | Defense/Information Technology Sector [Member] | Redstone Arsenal [Member]
Segment financial information for real estate operations
Revenues from real estate operations									14,745	14,322	13,056
Property operating expenses									(6,050)	(5,783)	(4,476)
UJV NOI allocable to COPT									0	0	0
NOI from real estate operations									8,695	8,539	8,580
Additions to long-lived assets									573	1,056	4,352
Transfers from non-operating properties									4,167	2,159	3,169
Real estate operations [Member] | Defense/Information Technology Sector [Member] | Data Center Shells [Member]
Segment financial information for real estate operations
Revenues from real estate operations									25,650	24,320	23,836
Property operating expenses									(3,225)	(2,709)	(2,674)
UJV NOI allocable to COPT									4,818	4,805	2,145
NOI from real estate operations									27,243	26,416	23,307
Additions to long-lived assets									0	0	0
Transfers from non-operating properties									99,191	107,854	103,367
Real estate operations [Member] | Regional Office [Member]
Segment financial information for real estate operations
Revenues from real estate operations									61,181	68,262	85,805
Property operating expenses									(30,253)	(28,982)	(34,095)
UJV NOI allocable to COPT									0	0	0
NOI from real estate operations									30,928	39,280	51,710
Additions to long-lived assets									19,730	25,299	12,559
Transfers from non-operating properties									0	0	82
Real estate operations [Member] | Operating wholesale data centers [Member]
Segment financial information for real estate operations
Revenues from real estate operations									31,892	28,875	26,869
Property operating expenses									(16,342)	(13,551)	(11,512)
UJV NOI allocable to COPT									0	0	0
NOI from real estate operations									15,550	15,324	15,357
Additions to long-lived assets									856	3,580	299
Transfers from non-operating properties									2,304	8	(377)
Real estate operations [Member] | Other [Member]
Segment financial information for real estate operations
Revenues from real estate operations									3,127	4,721	7,080
Property operating expenses									(1,436)	(1,873)	(3,218)
UJV NOI allocable to COPT									0	0	0
NOI from real estate operations									1,691	2,848	3,862
Additions to long-lived assets									480	110	335
Transfers from non-operating properties									0	18	(8)
Real estate operations [Member] | Segment assets [Member]
Segment financial information for real estate operations
Segment assets	3,084,862				3,028,005				3,084,862	3,028,005	3,022,360
Real estate operations [Member] | Segment assets [Member] | Defense/Information Technology Sector [Member]
Segment financial information for real estate operations
Segment assets	2,468,727				2,398,989				2,468,727	2,398,989	2,326,302
Real estate operations [Member] | Segment assets [Member] | Defense/Information Technology Sector [Member] | Fort Meade/BW Corridor [Member]
Segment financial information for real estate operations
Segment assets	1,279,571				1,263,567				1,279,571	1,263,567	1,255,230
Real estate operations [Member] | Segment assets [Member] | Defense/Information Technology Sector [Member] | Northern Virginia Defense/IT [Member]
Segment financial information for real estate operations
Segment assets	399,339				402,076				399,339	402,076	404,438
Real estate operations [Member] | Segment assets [Member] | Defense/Information Technology Sector [Member] | Lackland Air Force Base [Member]
Segment financial information for real estate operations
Segment assets	139,731				128,755				139,731	128,755	131,957
Real estate operations [Member] | Segment assets [Member] | Defense/Information Technology Sector [Member] | Navy Support Locations [Member]
Segment financial information for real estate operations
Segment assets	188,911				194,476				188,911	194,476	196,486
Real estate operations [Member] | Segment assets [Member] | Defense/Information Technology Sector [Member] | Redstone Arsenal [Member]
Segment financial information for real estate operations
Segment assets	108,010				108,119				108,010	108,119	110,395
Real estate operations [Member] | Segment assets [Member] | Defense/Information Technology Sector [Member] | Data Center Shells [Member]
Segment financial information for real estate operations
Segment assets	353,165				301,996				353,165	301,996	227,796
Real estate operations [Member] | Segment assets [Member] | Regional Office [Member]
Segment financial information for real estate operations
Segment assets	395,380				400,512				395,380	400,512	442,811
Real estate operations [Member] | Segment assets [Member] | Operating wholesale data centers [Member]
Segment financial information for real estate operations
Segment assets	216,640				224,422				216,640	224,422	231,954
Real estate operations [Member] | Segment assets [Member] | Other [Member]
Segment financial information for real estate operations
Segment assets	$ 4,115				$ 4,082				$ 4,115	$ 4,082	$ 21,293

Information by Business Segme_4

Information by Business Segment (Details 2) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of segment revenues to total revenues
Construction contract and other service revenues									$ 60,859	$ 102,840	$ 48,364
Total revenues	$ 138,482	$ 137,411	$ 146,743	$ 155,476	$ 164,567	$ 157,017	$ 151,435	$ 139,801	578,112	612,820	574,328
Reconciliation of Operatiing Income from Unconsolidated Joint Ventures to Equity Method Investments [Abstract]
UJV NOI allocable to COPT									4,818	4,805	2,145
Less: Income from UJV allocable to COPT attributable to depreciation and amortization expense and interest expense									(3,314)	(3,310)	(1,413)
Add: Equity in income (loss) of unconsolidated non-real estate entities									1,193	(5)	20
Equity in income of unconsolidated entities									2,697	1,490	752
Computation of net operating income from service operations
Construction contract and other service revenues									60,859	102,840	48,364
Construction contract and other service expenses									(58,326)	(99,618)	(45,481)
Real estate operations [Member]
Reconciliation of segment revenues to total revenues
Total revenues									517,253	509,980	525,964
Reconciliation of Operatiing Income from Unconsolidated Joint Ventures to Equity Method Investments [Abstract]
UJV NOI allocable to COPT									4,818	4,805	2,145
Service operations [Member]
Computation of net operating income from service operations
NOI from service operations									$ 2,533	$ 3,222	$ 2,883

Information by Business Segme_5

Information by Business Segment (Details 3) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment financial information for real estate operations
Interest and other income	$ 4,358	$ 6,318	$ 5,444
Gain on sales of real estate	2,340	9,890	59,679
Equity in income of unconsolidated entities	2,697	1,490	752
Income tax benefit (expense)	363	(1,098)	(244)
Depreciation and other amortization associated with real estate operations	(137,116)	(134,228)	(132,719)
Impairment losses	(2,367)	(15,123)	(101,391)
General, administrative and leasing expenses	(28,900)	(30,837)	(36,553)
Business development expenses and land carry costs	(5,840)	(6,213)	(8,244)
Interest expense	(75,385)	(76,983)	(83,163)
UJV NOI allocable to COPT	(4,818)	(4,805)	(2,145)
Loss on early extinguishment of debt	(258)	(513)	(1,110)
Net income	78,643	74,941	33,768
Real estate operations [Member]
Segment financial information for real estate operations
NOI from real estate operations	321,036	323,821	330,579
UJV NOI allocable to COPT	(4,818)	(4,805)	(2,145)
Service operations [Member]
Segment financial information for real estate operations
NOI from service operations	$ 2,533	$ 3,222	$ 2,883

Information by Business Segme_6

Information by Business Segment (Details 4) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of segment assets to total assets
Total COPT consolidated assets	$ 3,656,005	$ 3,595,205
Segment assets [Member]
Reconciliation of segment assets to total assets
Total COPT consolidated assets	3,084,862	3,028,005
Non-operating property assets [Member]
Reconciliation of segment assets to total assets
Total COPT consolidated assets	410,671	411,041
Other assets [Member]
Reconciliation of segment assets to total assets
Total COPT consolidated assets	$ 160,472	$ 156,159

Construction Contract and Oth_3

Construction Contract and Other Service Revenues (Details) - Construction contract revenue [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Performance obligations satisfied or already satisfied	$ 349	$ 586	$ 483
Remaining performance obligations	$ 58,100

Construction Contract and Oth_4

Construction Contract and Other Service Revenues (Details 1) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Construction contract and other service revenues	$ 60,859	$ 102,840	$ 48,364
Construction
Disaggregation of Revenue [Line Items]
Construction contract and other service revenues	57,986	94,471	46,989
Design
Disaggregation of Revenue [Line Items]
Construction contract and other service revenues	1,931	7,338	451
Other
Disaggregation of Revenue [Line Items]
Construction contract and other service revenues	942	1,031	924
GMP
Disaggregation of Revenue [Line Items]
Construction contract and other service revenues	34,050	78,401	22,405
FFP
Disaggregation of Revenue [Line Items]
Construction contract and other service revenues	20,327	22,607	24,571
Cost-plus fee
Disaggregation of Revenue [Line Items]
Construction contract and other service revenues	5,540	801	464
Other
Disaggregation of Revenue [Line Items]
Construction contract and other service revenues	$ 942	$ 1,031	$ 924

Construction Contract and Oth_5

Construction Contract and Other Service Revenues (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Change in Accounts Receivable
Beginning balance	$ 26,277	$ 31,802
Ending balance	26,277	31,802
Construction contract revenue [Member]
Change in Accounts Receivable
Beginning balance	6,701	4,577	$ 4,131
Ending balance	6,701	4,577	4,131
Change in Contract with Customer, Asset
Beginning balance	3,189	4,884	10,350
Ending balance	3,189	4,884	10,350
Change in Contract with Customer, Liability
Beginning balance	568	27,402	32,650
Ending balance	568	27,402	$ 32,650
Revenue recognized included in beginning balance	$ 27,296	$ 32,650

Earnings Per Share ("EPS") an_3

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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Numerator:
Net income attributable to COPT/COPLP	$ 17,020	$ 18,697	$ 19,434	$ 17,150	$ 9,621	$ 20,579	$ 17,526	$ 21,019	$ 72,301		$ 68,745		$ 28,890
Preferred share/ unit dividends/ distributions	0	0	0	0	0	0	(3,039)	(3,180)	0		(6,219)		(14,297)
Issuance costs associated with redeemed preferred shares	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ (6,847)	$ 0	0		(6,847)		(17)
Income attributable to share-based compensation awards									(462)		(449)		(419)
Numerator for basic and diluted EPS/EPU on net income attributable to COPT/COPLP common shareholders/unitholders									$ 71,839		$ 55,230		$ 14,157
Denominator (all weighted averages):
Denominator for basic EPS (common shares)									103,946		98,969		94,502
Dilutive effect of share-based compensation awards (common shares)									134		132		92
Dilutive effect of forward equity sale agreements (common shares)									45		54		0
Denominator for diluted EPS (common shares)									104,125		99,155		94,594
Basic earnings per common share/unit (in dollars per share/unit)	$ 0.16	$ 0.18	$ 0.19	$ 0.17	$ 0.10	$ 0.21	$ 0.08	$ 0.18	$ 0.69		$ 0.56		$ 0.15
Diluted earnings per common share/unit (in dollars per share/unit)	$ 0.16	$ 0.18	$ 0.19	$ 0.17	$ 0.10	$ 0.21	$ 0.08	$ 0.18	$ 0.69		$ 0.56		$ 0.15
Corporate Office Properties, L.P. [Member]
Numerator:
Net income attributable to COPT/COPLP	$ 17,395	$ 19,242	$ 20,207	$ 17,859	$ 10,100	$ 21,437	$ 17,952	$ 21,806	$ 74,703		$ 71,295		$ 30,053
Preferred share/ unit dividends/ distributions	(165)	(165)	(165)	(165)	(165)	(165)	(3,204)	(3,345)	(660)		(6,879)		(14,957)
Issuance costs associated with redeemed preferred shares	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ (6,847)	$ 0	0		(6,847)		(17)
Income attributable to share-based compensation awards									(462)		(449)		(419)
Numerator for basic and diluted EPS/EPU on net income attributable to COPT/COPLP common shareholders/unitholders									$ 73,581		$ 57,120		$ 14,660
Denominator (all weighted averages):
Denominator for basic EPS (common shares)									106,414		102,331		98,135
Dilutive effect of share-based compensation awards (common shares)									134		132		92
Dilutive effect of forward equity sale agreements (common shares)									45		54		0
Denominator for diluted EPS (common shares)									106,593		102,517		98,227
Basic earnings per common share/unit (in dollars per share/unit)	$ 0.16	$ 0.18	$ 0.19	$ 0.17	$ 0.10	$ 0.21	$ 0.08	$ 0.18	$ 0.69	[1]	$ 0.56	[1]	$ 0.15	[1]
Diluted earnings per common share/unit (in dollars per share/unit)	$ 0.16	$ 0.18	$ 0.19	$ 0.17	$ 0.10	$ 0.21	$ 0.08	$ 0.18	$ 0.69	[1]	$ 0.56	[1]	$ 0.15	[1]
[1]	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") an_4

Earnings Per Share ("EPS") and Earnings Per Unit (“EPU”) (Details 2) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Conversion of common units [Member]
Antidilutive securities
Weighted average antidilutive securities excluded from computation of diluted earnings per share (in shares)	2,468	3,362	3,633
Conversion of redeemable noncontrolling interests [Member]
Antidilutive securities
Weighted average antidilutive securities excluded from computation of diluted earnings per share (in shares)	936	689	809
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	0	434
Weighted average restricted shares/units and deferred share awards [Member]
Antidilutive securities
Weighted average antidilutive securities excluded from computation of diluted earnings per share (in shares)	452	433	385
Weighted average stock options [Member]
Antidilutive securities
Weighted average antidilutive securities excluded from computation of diluted earnings per share (in shares)	42	70	285
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	0	434
Corporate Office Properties, L.P. [Member] | Weighted average restricted shares/units and deferred share awards [Member]
Antidilutive securities
Weighted average antidilutive securities excluded from computation of diluted earnings per share (in shares)	452	433	385
Corporate Office Properties, L.P. [Member] | Weighted average stock options [Member]
Antidilutive securities
Weighted average antidilutive securities excluded from computation of diluted earnings per share (in shares)	42	70	285

Commitments and Contingencies -

Commitments and Contingencies - Narrative (Details)	May 25, 2017USD ($)	Dec. 31, 2018USD ($)Property	Dec. 31, 2017USD ($)	Aug. 31, 2010USD ($)
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
Capital Lease
Capital lease term	99 years
Purchase option	$ 1
Capital lease obligation	$ 16,100,000	660,000	$ 15,853,000
Capital lease obligations due in 2020		660,000
Development and Redevelopment Obligations [Member]
Capital Lease
Contractual obligation		241,100,000
Tenant and Other Capital Improvements [Member]
Capital Lease
Contractual obligation		44,100,000
Third Party Construction [Member]
Capital Lease
Contractual obligation		47,900,000
Other Obligations [Member]
Capital Lease
Contractual obligation		$ 1,900,000
Tax Incremental Financing Bond [Member] | Anne Arundel County, Maryland [Member] | Specialty Tax Guarantee [Member]
Environmental Indemnity Agreement
Loan amount				$ 30,000,000

Commitments and Contingencies_2

Commitments and Contingencies - Operating Leases Future Minimum Payments (Details) $ in Thousands	Dec. 31, 2018USD ($)
Commitments and Contingencies Disclosure [Abstract]
2,019	$ 1,320
2,020	1,294
2,021	1,278
2,022	1,164
2,023	1,119
Thereafter	83,373
Total	$ 89,548

Quarterly Data (Unaudited) (Det

Quarterly Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Quarterly Data [Line Items]
Total revenues	$ 138,482	$ 137,411	$ 146,743	$ 155,476	$ 164,567	$ 157,017	$ 151,435	$ 139,801	$ 578,112		$ 612,820		$ 574,328
Net income	18,456	20,322	21,085	18,780	11,008	22,334	18,859	22,740	78,643		74,941		33,768
Net income attributable to noncontrolling interests	(1,436)	(1,625)	(1,651)	(1,630)	(1,387)	(1,755)	(1,333)	(1,721)
Net income attributable to COPT	17,020	18,697	19,434	17,150	9,621	20,579	17,526	21,019	72,301		68,745		28,890
Preferred share/ unit dividends/ distributions	0	0	0	0	0	0	(3,039)	(3,180)	0		(6,219)		(14,297)
Issuance costs associated with redeemed preferred shares	0	0	0	0	0	0	(6,847)	0	0		(6,847)		(17)
Net income attributable to COPT common shareholders	$ 17,020	$ 18,697	$ 19,434	$ 17,150	$ 9,621	$ 20,579	$ 7,640	$ 17,839	$ 72,301		$ 55,679		$ 14,576
Basic earnings per common share/unit (in dollars per share/unit)	$ 0.16	$ 0.18	$ 0.19	$ 0.17	$ 0.10	$ 0.21	$ 0.08	$ 0.18	$ 0.69		$ 0.56		$ 0.15
Diluted earnings per common share/unit (in dollars per share/unit)	$ 0.16	$ 0.18	$ 0.19	$ 0.17	$ 0.10	$ 0.21	$ 0.08	$ 0.18	$ 0.69		$ 0.56		$ 0.15
Corporate Office Properties, L.P. [Member]
Quarterly Data [Line Items]
Total revenues	$ 138,482	$ 137,411	$ 146,743	$ 155,476	$ 164,567	$ 157,017	$ 151,435	$ 139,801	$ 578,112		$ 612,820		$ 574,328
Net income	18,456	20,322	21,085	18,780	11,008	22,334	18,859	22,740	78,643		74,941		33,768
Net income attributable to noncontrolling interests	(1,061)	(1,080)	(878)	(921)	(908)	(897)	(907)	(934)	(3,940)		(3,646)		(3,715)
Net income attributable to COPT	17,395	19,242	20,207	17,859	10,100	21,437	17,952	21,806	74,703		71,295		30,053
Preferred share/ unit dividends/ distributions	(165)	(165)	(165)	(165)	(165)	(165)	(3,204)	(3,345)	(660)		(6,879)		(14,957)
Issuance costs associated with redeemed preferred shares	0	0	0	0	0	0	(6,847)	0	0		(6,847)		(17)
Net income attributable to COPT common shareholders	$ 17,230	$ 19,077	$ 20,042	$ 17,694	$ 9,935	$ 21,272	$ 7,901	$ 18,461	$ 74,043		$ 57,569		$ 15,079
Basic earnings per common share/unit (in dollars per share/unit)	$ 0.16	$ 0.18	$ 0.19	$ 0.17	$ 0.10	$ 0.21	$ 0.08	$ 0.18	$ 0.69	[1]	$ 0.56	[1]	$ 0.15	[1]
Diluted earnings per common share/unit (in dollars per share/unit)	$ 0.16	$ 0.18	$ 0.19	$ 0.17	$ 0.10	$ 0.21	$ 0.08	$ 0.18	$ 0.69	[1]	$ 0.56	[1]	$ 0.15	[1]
[1]	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 Q_2

Schedule II - Valuation and Qualifying Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounts Receivables-Allowance for doubtful accounts [Member]
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	$ 607	$ 603	$ 1,525
Charged to Costs and Expenses	319	368	(17)
Charged to Other Accounts	0	(36)	235
Deductions	(96)	(328)	(1,140)
Balance at End of Year	830	607	603
Allowance for Deferred Rent Receivable [Member]
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	364	373	1,962
Charged to Costs and Expenses	0	0	0
Charged to Other Accounts	(100)	(9)	(1,589)
Deductions	0	0	0
Balance at End of Year	264	364	373
Allowance for Deferred Tax Asset [Member]
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	1,416	2,062	2,062
Charged to Costs and Expenses	668	(646)	0
Charged to Other Accounts	0	0	0
Deductions	0	0	0
Balance at End of Year	$ 2,084	$ 1,416	$ 2,062

Schedule III - Real Estate an_2

Schedule III - Real Estate and Accumulated Depreciation (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Real Estate and Accumulated Depreciation
Encumbrances	$ 174,043	$ 174,043
Initial Cost
Land	711,034	711,034
Building and Land Improvements	3,002,155	3,002,155
Costs Capitalized Subsequent to Acquisition	435,340	435,340
Gross Amounts Carried At Close of Period
Land	711,034	711,034
Building and Land Improvements	3,437,495	3,437,495
Total	4,148,529	4,148,529	$ 3,980,813	$ 3,874,715	$ 4,158,616
Accumulated Depreciation	(897,903)	(897,903)	(801,038)	(715,951)	$ (718,680)
Additional information
Debt excluded from encumbrances	1,823,909	1,823,909	1,828,333
Net discounts and deferred financing costs	14,600	14,600
Aggregate cost of assets for federal income tax purposes	3,500,000	3,500,000
Impairment losses		$ 2,367	15,123	$ 101,391
Buildings improvements [Member] | Minimum [Member]
Additional information
Estimated lives over which depreciation is recognized		10 years
Buildings improvements [Member] | Maximum [Member]
Additional information
Estimated lives over which depreciation is recognized		40 years
Revolving Credit Facility [Member]
Additional information
Debt excluded from encumbrances	213,000	$ 213,000	126,000
Term Loan Facilities [Member]
Additional information
Debt excluded from encumbrances	248,273	248,273	$ 347,959
Unsecured Senior Notes [Member]
Additional information
Debt excluded from encumbrances	1,200,000	1,200,000
Unsecured notes payable [Member]
Additional information
Debt excluded from encumbrances	1,200	1,200
Fixed rate mortgage loans [Member]
Additional information
Net discounts and deferred financing costs	3,600	3,600
100 Light Street [Member]
Real Estate and Accumulated Depreciation
Encumbrances	48,473	48,473
Initial Cost
Land	26,715	26,715
Building and Land Improvements	58,343	58,343
Costs Capitalized Subsequent to Acquisition	8,065	8,065
Gross Amounts Carried At Close of Period
Land	26,715	26,715
Building and Land Improvements	66,408	66,408
Total	93,123	93,123
Accumulated Depreciation	(13,232)	(13,232)
1000 Redstone Gateway [Member]
Real Estate and Accumulated Depreciation
Encumbrances	10,390	10,390
Initial Cost
Land	0	0
Building and Land Improvements	20,533	20,533
Costs Capitalized Subsequent to Acquisition	5	5
Gross Amounts Carried At Close of Period
Land	0	0
Building and Land Improvements	20,538	20,538
Total	20,538	20,538
Accumulated Depreciation	(2,978)	(2,978)
1100 Redstone Gateway [Member]
Real Estate and Accumulated Depreciation
Encumbrances	10,917	10,917
Initial Cost
Land	0	0
Building and Land Improvements	19,593	19,593
Costs Capitalized Subsequent to Acquisition	6	6
Gross Amounts Carried At Close of Period
Land	0	0
Building and Land Improvements	19,599	19,599
Total	19,599	19,599
Accumulated Depreciation	(2,435)	(2,435)
114 National Business Parkway [Member]
Initial Cost
Land	364	364
Building and Land Improvements	3,109	3,109
Costs Capitalized Subsequent to Acquisition	118	118
Gross Amounts Carried At Close of Period
Land	364	364
Building and Land Improvements	3,227	3,227
Total	3,591	3,591
Accumulated Depreciation	(1,401)	(1,401)
1200 Redstone Gateway [Member]
Real Estate and Accumulated Depreciation
Encumbrances	12,616	12,616
Initial Cost
Land	0	0
Building and Land Improvements	22,389	22,389
Gross Amounts Carried At Close of Period
Land	0	0
Building and Land Improvements	22,389	22,389
Total	22,389	22,389
Accumulated Depreciation	(2,824)	(2,824)
1201 M Street [Member]
Initial Cost
Land	0	0
Building and Land Improvements	49,785	49,785
Costs Capitalized Subsequent to Acquisition	8,619	8,619
Gross Amounts Carried At Close of Period
Land	0	0
Building and Land Improvements	58,404	58,404
Total	58,404	58,404
Accumulated Depreciation	(14,892)	(14,892)
1201 Winterson Road [Member]
Initial Cost
Land	2,130	2,130
Building and Land Improvements	16,601	16,601
Costs Capitalized Subsequent to Acquisition	488	488
Gross Amounts Carried At Close of Period
Land	2,130	2,130
Building and Land Improvements	17,089	17,089
Total	19,219	19,219
Accumulated Depreciation	(4,665)	(4,665)
1220 12th Street, SE [Member]
Initial Cost
Land	0	0
Building and Land Improvements	42,464	42,464
Costs Capitalized Subsequent to Acquisition	7,355	7,355
Gross Amounts Carried At Close of Period
Land	0	0
Building and Land Improvements	49,819	49,819
Total	49,819	49,819
Accumulated Depreciation	(13,674)	(13,674)
1243 Winterson Road [Member]
Initial Cost
Land	630	630
Building and Land Improvements	0	0
Gross Amounts Carried At Close of Period
Land	630	630
Building and Land Improvements	0	0
Total	630	630
131 National Business Parkway [Member]
Initial Cost
Land	1,906	1,906
Building and Land Improvements	7,623	7,623
Costs Capitalized Subsequent to Acquisition	3,962	3,962
Gross Amounts Carried At Close of Period
Land	1,906	1,906
Building and Land Improvements	11,585	11,585
Total	13,491	13,491
Accumulated Depreciation	(6,697)	(6,697)
132 National Business Parkway [Member]
Initial Cost
Land	2,917	2,917
Building and Land Improvements	12,259	12,259
Costs Capitalized Subsequent to Acquisition	4,666	4,666
Gross Amounts Carried At Close of Period
Land	2,917	2,917
Building and Land Improvements	16,925	16,925
Total	19,842	19,842
Accumulated Depreciation	(9,325)	(9,325)
133 National Business Parkway [Member]
Initial Cost
Land	2,517	2,517
Building and Land Improvements	10,068	10,068
Costs Capitalized Subsequent to Acquisition	5,584	5,584
Gross Amounts Carried At Close of Period
Land	2,517	2,517
Building and Land Improvements	15,652	15,652
Total	18,169	18,169
Accumulated Depreciation	(9,639)	(9,639)
134 National Business Parkway [Member]
Initial Cost
Land	3,684	3,684
Building and Land Improvements	7,517	7,517
Costs Capitalized Subsequent to Acquisition	4,108	4,108
Gross Amounts Carried At Close of Period
Land	3,684	3,684
Building and Land Improvements	11,625	11,625
Total	15,309	15,309
Accumulated Depreciation	(6,036)	(6,036)
1340 Ashton Road [Member]
Initial Cost
Land	905	905
Building and Land Improvements	3,620	3,620
Costs Capitalized Subsequent to Acquisition	1,516	1,516
Gross Amounts Carried At Close of Period
Land	905	905
Building and Land Improvements	5,136	5,136
Total	6,041	6,041
Accumulated Depreciation	(2,971)	(2,971)
13450 Sunrise Valley Road [Member]
Initial Cost
Land	1,386	1,386
Building and Land Improvements	5,576	5,576
Costs Capitalized Subsequent to Acquisition	4,592	4,592
Gross Amounts Carried At Close of Period
Land	1,386	1,386
Building and Land Improvements	10,168	10,168
Total	11,554	11,554
Accumulated Depreciation	(5,295)	(5,295)
13454 Sunrise Valley Road [Member]
Initial Cost
Land	2,899	2,899
Building and Land Improvements	11,986	11,986
Costs Capitalized Subsequent to Acquisition	8,191	8,191
Gross Amounts Carried At Close of Period
Land	2,899	2,899
Building and Land Improvements	20,177	20,177
Total	23,076	23,076
Accumulated Depreciation	(10,176)	(10,176)
135 National Business Parkway [Member]
Initial Cost
Land	2,484	2,484
Building and Land Improvements	9,750	9,750
Costs Capitalized Subsequent to Acquisition	6,075	6,075
Gross Amounts Carried At Close of Period
Land	2,484	2,484
Building and Land Improvements	15,825	15,825
Total	18,309	18,309
Accumulated Depreciation	(8,783)	(8,783)
1362 Mellon Road [Member]
Initial Cost
Land	950	950
Building and Land Improvements	3,864	3,864
Costs Capitalized Subsequent to Acquisition	206	206
Gross Amounts Carried At Close of Period
Land	950	950
Building and Land Improvements	4,070	4,070
Total	5,020	5,020
Accumulated Depreciation	(414)	(414)
13857 McLearen Road [Member]
Initial Cost
Land	3,507	3,507
Building and Land Improvements	30,177	30,177
Costs Capitalized Subsequent to Acquisition	2,806	2,806
Gross Amounts Carried At Close of Period
Land	3,507	3,507
Building and Land Improvements	32,983	32,983
Total	36,490	36,490
Accumulated Depreciation	(10,850)	(10,850)
140 National Business Parkway [Member]
Initial Cost
Land	3,407	3,407
Building and Land Improvements	24,167	24,167
Costs Capitalized Subsequent to Acquisition	1,487	1,487
Gross Amounts Carried At Close of Period
Land	3,407	3,407
Building and Land Improvements	25,654	25,654
Total	29,061	29,061
Accumulated Depreciation	(9,577)	(9,577)
141 National Business Parkway [Member]
Initial Cost
Land	2,398	2,398
Building and Land Improvements	9,538	9,538
Costs Capitalized Subsequent to Acquisition	4,815	4,815
Gross Amounts Carried At Close of Period
Land	2,398	2,398
Building and Land Improvements	14,353	14,353
Total	16,751	16,751
Accumulated Depreciation	(8,006)	(8,006)
14280 Park Meadow Drive [Member]
Initial Cost
Land	3,731	3,731
Building and Land Improvements	15,953	15,953
Costs Capitalized Subsequent to Acquisition	3,039	3,039
Gross Amounts Carried At Close of Period
Land	3,731	3,731
Building and Land Improvements	18,992	18,992
Total	22,723	22,723
Accumulated Depreciation	(7,982)	(7,982)
1460 Dorsey Road [Member]
Initial Cost
Land	1,577	1,577
Building and Land Improvements	67	67
Gross Amounts Carried At Close of Period
Land	1,577	1,577
Building and Land Improvements	67	67
Total	1,644	1,644
14840 Conference Center Drive [Member]
Initial Cost
Land	1,572	1,572
Building and Land Improvements	8,175	8,175
Costs Capitalized Subsequent to Acquisition	3,433	3,433
Gross Amounts Carried At Close of Period
Land	1,572	1,572
Building and Land Improvements	11,608	11,608
Total	13,180	13,180
Accumulated Depreciation	(5,964)	(5,964)
14850 Conference Center Drive [Member]
Initial Cost
Land	1,615	1,615
Building and Land Improvements	8,358	8,358
Costs Capitalized Subsequent to Acquisition	3,108	3,108
Gross Amounts Carried At Close of Period
Land	1,615	1,615
Building and Land Improvements	11,466	11,466
Total	13,081	13,081
Accumulated Depreciation	(6,416)	(6,416)
14900 Conference Center Drive [Member]
Initial Cost
Land	3,436	3,436
Building and Land Improvements	14,402	14,402
Costs Capitalized Subsequent to Acquisition	7,817	7,817
Gross Amounts Carried At Close of Period
Land	3,436	3,436
Building and Land Improvements	22,219	22,219
Total	25,655	25,655
Accumulated Depreciation	(10,959)	(10,959)
1501 South Clinton Street [Member]
Initial Cost
Land	27,964	27,964
Building and Land Improvements	51,990	51,990
Costs Capitalized Subsequent to Acquisition	15,417	15,417
Gross Amounts Carried At Close of Period
Land	27,964	27,964
Building and Land Improvements	67,407	67,407
Total	95,371	95,371
Accumulated Depreciation	(22,475)	(22,475)
15049 Conference Center Drive [Member]
Initial Cost
Land	4,415	4,415
Building and Land Improvements	20,365	20,365
Costs Capitalized Subsequent to Acquisition	15,729	15,729
Gross Amounts Carried At Close of Period
Land	4,415	4,415
Building and Land Improvements	36,094	36,094
Total	40,509	40,509
Accumulated Depreciation	(13,957)	(13,957)
15059 Conference Center Drive [Member]
Initial Cost
Land	5,753	5,753
Building and Land Improvements	13,615	13,615
Costs Capitalized Subsequent to Acquisition	3,979	3,979
Gross Amounts Carried At Close of Period
Land	5,753	5,753
Building and Land Improvements	17,594	17,594
Total	23,347	23,347
Accumulated Depreciation	(8,680)	(8,680)
1550 West Nursery Road [Member]
Initial Cost
Land	14,071	14,071
Building and Land Improvements	16,930	16,930
Gross Amounts Carried At Close of Period
Land	14,071	14,071
Building and Land Improvements	16,930	16,930
Total	31,001	31,001
Accumulated Depreciation	(5,390)	(5,390)
1560 West Nursery Road [Member]
Initial Cost
Land	1,441	1,441
Building and Land Improvements	113	113
Gross Amounts Carried At Close of Period
Land	1,441	1,441
Building and Land Improvements	113	113
Total	1,554	1,554
Accumulated Depreciation	(13)	(13)
1610 West Nursery Road [Member]
Initial Cost
Land	259	259
Building and Land Improvements	246	246
Gross Amounts Carried At Close of Period
Land	259	259
Building and Land Improvements	246	246
Total	505	505
Accumulated Depreciation	(11)	(11)
1616 West Nursery Road [Member]
Initial Cost
Land	393	393
Building and Land Improvements	3,323	3,323
Gross Amounts Carried At Close of Period
Land	393	393
Building and Land Improvements	3,323	3,323
Total	3,716	3,716
Accumulated Depreciation	(98)	(98)
1622 West Nursery Road [Member]
Initial Cost
Land	393	393
Building and Land Improvements	2,542	2,542
Gross Amounts Carried At Close of Period
Land	393	393
Building and Land Improvements	2,542	2,542
Total	2,935	2,935
Accumulated Depreciation	(116)	(116)
16442 Commerce Drive [Member]
Initial Cost
Land	613	613
Building and Land Improvements	2,582	2,582
Costs Capitalized Subsequent to Acquisition	894	894
Gross Amounts Carried At Close of Period
Land	613	613
Building and Land Improvements	3,476	3,476
Total	4,089	4,089
Accumulated Depreciation	(1,635)	(1,635)
16480 Commerce Drive [Member]
Initial Cost
Land	1,856	1,856
Building and Land Improvements	7,154	7,154
Costs Capitalized Subsequent to Acquisition	2,170	2,170
Gross Amounts Carried At Close of Period
Land	1,856	1,856
Building and Land Improvements	9,324	9,324
Total	11,180	11,180
Accumulated Depreciation	(3,624)	(3,624)
16501 Commerce Drive [Member]
Initial Cost
Land	522	522
Building and Land Improvements	2,090	2,090
Costs Capitalized Subsequent to Acquisition	976	976
Gross Amounts Carried At Close of Period
Land	522	522
Building and Land Improvements	3,066	3,066
Total	3,588	3,588
Accumulated Depreciation	(1,112)	(1,112)
16539 Commerce Drive [Member]
Initial Cost
Land	688	688
Building and Land Improvements	2,860	2,860
Costs Capitalized Subsequent to Acquisition	2,188	2,188
Gross Amounts Carried At Close of Period
Land	688	688
Building and Land Improvements	5,048	5,048
Total	5,736	5,736
Accumulated Depreciation	(2,517)	(2,517)
16541 Commerce Drive [Member]
Initial Cost
Land	773	773
Building and Land Improvements	3,094	3,094
Costs Capitalized Subsequent to Acquisition	1,794	1,794
Gross Amounts Carried At Close of Period
Land	773	773
Building and Land Improvements	4,888	4,888
Total	5,661	5,661
Accumulated Depreciation	(2,197)	(2,197)
16543 Commerce Drive [Member]
Initial Cost
Land	436	436
Building and Land Improvements	1,742	1,742
Costs Capitalized Subsequent to Acquisition	716	716
Gross Amounts Carried At Close of Period
Land	436	436
Building and Land Improvements	2,458	2,458
Total	2,894	2,894
Accumulated Depreciation	(972)	(972)
1751 Pinnacle Drive [Member]
Initial Cost
Land	10,486	10,486
Building and Land Improvements	42,339	42,339
Costs Capitalized Subsequent to Acquisition	29,880	29,880
Gross Amounts Carried At Close of Period
Land	10,486	10,486
Building and Land Improvements	72,219	72,219
Total	82,705	82,705
Accumulated Depreciation	(32,435)	(32,435)
1753 Pinnacle Drive [Member]
Initial Cost
Land	8,275	8,275
Building and Land Improvements	34,353	34,353
Costs Capitalized Subsequent to Acquisition	20,778	20,778
Gross Amounts Carried At Close of Period
Land	8,275	8,275
Building and Land Improvements	55,131	55,131
Total	63,406	63,406
Accumulated Depreciation	(20,770)	(20,770)
206 Research Boulevard [Member]
Initial Cost
Land	0	0
Building and Land Improvements	0	0
Costs Capitalized Subsequent to Acquisition	0	0
Gross Amounts Carried At Close of Period
Land	0	0
Building and Land Improvements	0	0
Total	0	0
Accumulated Depreciation	0	0
209 Research Boulevard [Member]
Initial Cost
Land	134	134
Building and Land Improvements	1,711	1,711
Costs Capitalized Subsequent to Acquisition	265	265
Gross Amounts Carried At Close of Period
Land	134	134
Building and Land Improvements	1,976	1,976
Total	2,110	2,110
Accumulated Depreciation	(381)	(381)
210 Research Boulevard [Member]
Initial Cost
Land	113	113
Building and Land Improvements	1,402	1,402
Costs Capitalized Subsequent to Acquisition	204	204
Gross Amounts Carried At Close of Period
Land	113	113
Building and Land Improvements	1,606	1,606
Total	1,719	1,719
Accumulated Depreciation	(271)	(271)
2100 L Street [Member]
Real Estate and Accumulated Depreciation
Encumbrances	13,577	13,577
Initial Cost
Land	56,351	56,351
Building and Land Improvements	19,446	19,446
Gross Amounts Carried At Close of Period
Land	56,351	56,351
Building and Land Improvements	19,446	19,446
Total	75,797	75,797
2100 Rideout Road [Member]
Initial Cost
Land	0	0
Building and Land Improvements	5,492	5,492
Costs Capitalized Subsequent to Acquisition	2,881	2,881
Gross Amounts Carried At Close of Period
Land	0	0
Building and Land Improvements	8,373	8,373
Total	8,373	8,373
Accumulated Depreciation	(866)	(866)
22289 Exploration Drive [Member]
Initial Cost
Land	1,422	1,422
Building and Land Improvements	5,594	5,594
Costs Capitalized Subsequent to Acquisition	1,867	1,867
Gross Amounts Carried At Close of Period
Land	1,422	1,422
Building and Land Improvements	7,461	7,461
Total	8,883	8,883
Accumulated Depreciation	(3,598)	(3,598)
22299 Exploration Drive [Member]
Initial Cost
Land	1,362	1,362
Building and Land Improvements	5,685	5,685
Costs Capitalized Subsequent to Acquisition	2,550	2,550
Gross Amounts Carried At Close of Period
Land	1,362	1,362
Building and Land Improvements	8,235	8,235
Total	9,597	9,597
Accumulated Depreciation	(4,040)	(4,040)
22300 Exploration Drive [Member]
Initial Cost
Land	1,094	1,094
Building and Land Improvements	4,929	4,929
Costs Capitalized Subsequent to Acquisition	2,522	2,522
Gross Amounts Carried At Close of Period
Land	1,094	1,094
Building and Land Improvements	7,451	7,451
Total	8,545	8,545
Accumulated Depreciation	(2,658)	(2,658)
22309 Exploration Drive [Member]
Initial Cost
Land	2,243	2,243
Building and Land Improvements	10,419	10,419
Costs Capitalized Subsequent to Acquisition	7,983	7,983
Gross Amounts Carried At Close of Period
Land	2,243	2,243
Building and Land Improvements	18,402	18,402
Total	20,645	20,645
Accumulated Depreciation	(6,799)	(6,799)
23535 Cottonwood Parkway [Member]
Initial Cost
Land	692	692
Building and Land Improvements	3,051	3,051
Costs Capitalized Subsequent to Acquisition	571	571
Gross Amounts Carried At Close of Period
Land	692	692
Building and Land Improvements	3,622	3,622
Total	4,314	4,314
Accumulated Depreciation	(1,695)	(1,695)
250 W Pratt St [Member]
Initial Cost
Land	8,057	8,057
Building and Land Improvements	34,588	34,588
Costs Capitalized Subsequent to Acquisition	10,557	10,557
Gross Amounts Carried At Close of Period
Land	8,057	8,057
Building and Land Improvements	45,145	45,145
Total	53,202	53,202
Accumulated Depreciation	(10,084)	(10,084)
2500 Riva Road [Member]
Initial Cost
Land	2,791	2,791
Building and Land Improvements	12,193	12,193
Costs Capitalized Subsequent to Acquisition	1	1
Gross Amounts Carried At Close of Period
Land	2,791	2,791
Building and Land Improvements	12,194	12,194
Total	14,985	14,985
Accumulated Depreciation	(6,467)	(6,467)
2600 Park Tower Drive [Member]
Initial Cost
Land	20,304	20,304
Building and Land Improvements	34,443	34,443
Costs Capitalized Subsequent to Acquisition	711	711
Gross Amounts Carried At Close of Period
Land	20,304	20,304
Building and Land Improvements	35,154	35,154
Total	55,458	55,458
Accumulated Depreciation	(4,939)	(4,939)
2691 Technology Drive [Member]
Initial Cost
Land	2,098	2,098
Building and Land Improvements	17,334	17,334
Costs Capitalized Subsequent to Acquisition	5,557	5,557
Gross Amounts Carried At Close of Period
Land	2,098	2,098
Building and Land Improvements	22,891	22,891
Total	24,989	24,989
Accumulated Depreciation	(10,541)	(10,541)
2701 Technology Drive [Member]
Initial Cost
Land	1,737	1,737
Building and Land Improvements	15,266	15,266
Costs Capitalized Subsequent to Acquisition	5,034	5,034
Gross Amounts Carried At Close of Period
Land	1,737	1,737
Building and Land Improvements	20,300	20,300
Total	22,037	22,037
Accumulated Depreciation	(10,559)	(10,559)
2711 Technology Drive [Member]
Initial Cost
Land	2,251	2,251
Building and Land Improvements	21,611	21,611
Costs Capitalized Subsequent to Acquisition	2,282	2,282
Gross Amounts Carried At Close of Period
Land	2,251	2,251
Building and Land Improvements	23,893	23,893
Total	26,144	26,144
Accumulated Depreciation	(12,299)	(12,299)
2720 Technology Drive [Member]
Initial Cost
Land	3,863	3,863
Building and Land Improvements	29,272	29,272
Costs Capitalized Subsequent to Acquisition	1,218	1,218
Gross Amounts Carried At Close of Period
Land	3,863	3,863
Building and Land Improvements	30,490	30,490
Total	34,353	34,353
Accumulated Depreciation	(11,349)	(11,349)
2721 Technology Drive [Member]
Initial Cost
Land	4,611	4,611
Building and Land Improvements	14,597	14,597
Costs Capitalized Subsequent to Acquisition	2,801	2,801
Gross Amounts Carried At Close of Period
Land	4,611	4,611
Building and Land Improvements	17,398	17,398
Total	22,009	22,009
Accumulated Depreciation	(8,280)	(8,280)
2730 Hercules Road [Member]
Initial Cost
Land	8,737	8,737
Building and Land Improvements	31,612	31,612
Costs Capitalized Subsequent to Acquisition	8,697	8,697
Gross Amounts Carried At Close of Period
Land	8,737	8,737
Building and Land Improvements	40,309	40,309
Total	49,046	49,046
Accumulated Depreciation	(20,174)	(20,174)
30 Light Street [Member]
Real Estate and Accumulated Depreciation
Encumbrances	4,078	4,078
Initial Cost
Land	0	0
Building and Land Improvements	12,101	12,101
Costs Capitalized Subsequent to Acquisition	867	867
Gross Amounts Carried At Close of Period
Land	0	0
Building and Land Improvements	12,968	12,968
Total	12,968	12,968
Accumulated Depreciation	(1,113)	(1,113)
300 Sentinel Drive [Member]
Initial Cost
Land	1,517	1,517
Building and Land Improvements	59,165	59,165
Costs Capitalized Subsequent to Acquisition	1,201	1,201
Gross Amounts Carried At Close of Period
Land	1,517	1,517
Building and Land Improvements	60,366	60,366
Total	61,883	61,883
Accumulated Depreciation	(13,166)	(13,166)
302 Sentinel Drive [Member]
Initial Cost
Land	2,648	2,648
Building and Land Improvements	29,687	29,687
Costs Capitalized Subsequent to Acquisition	777	777
Gross Amounts Carried At Close of Period
Land	2,648	2,648
Building and Land Improvements	30,464	30,464
Total	33,112	33,112
Accumulated Depreciation	(8,381)	(8,381)
304 Sentinel Drive [Member]
Initial Cost
Land	3,411	3,411
Building and Land Improvements	24,917	24,917
Costs Capitalized Subsequent to Acquisition	1,814	1,814
Gross Amounts Carried At Close of Period
Land	3,411	3,411
Building and Land Improvements	26,731	26,731
Total	30,142	30,142
Accumulated Depreciation	(8,441)	(8,441)
306 Sentinel Drive [Member]
Initial Cost
Land	3,260	3,260
Building and Land Improvements	22,592	22,592
Costs Capitalized Subsequent to Acquisition	1,239	1,239
Gross Amounts Carried At Close of Period
Land	3,260	3,260
Building and Land Improvements	23,831	23,831
Total	27,091	27,091
Accumulated Depreciation	(7,357)	(7,357)
308 Sentinel Drive [Member]
Initial Cost
Land	1,422	1,422
Building and Land Improvements	26,208	26,208
Costs Capitalized Subsequent to Acquisition	2,330	2,330
Gross Amounts Carried At Close of Period
Land	1,422	1,422
Building and Land Improvements	28,538	28,538
Total	29,960	29,960
Accumulated Depreciation	(5,245)	(5,245)
310 Sentinel Way [Member]
Initial Cost
Land	2,372	2,372
Building and Land Improvements	39,990	39,990
Gross Amounts Carried At Close of Period
Land	2,372	2,372
Building and Land Improvements	39,990	39,990
Total	42,362	42,362
Accumulated Depreciation	(2,956)	(2,956)
310 The Bridge Street [Member]
Initial Cost
Land	261	261
Building and Land Improvements	26,531	26,531
Costs Capitalized Subsequent to Acquisition	4,088	4,088
Gross Amounts Carried At Close of Period
Land	261	261
Building and Land Improvements	30,619	30,619
Total	30,880	30,880
Accumulated Depreciation	(8,677)	(8,677)
312 Sentinel Way [Member]
Initial Cost
Land	3,138	3,138
Building and Land Improvements	27,797	27,797
Gross Amounts Carried At Close of Period
Land	3,138	3,138
Building and Land Improvements	27,797	27,797
Total	30,935	30,935
Accumulated Depreciation	(2,999)	(2,999)
314 Sentinel Way [Member]
Initial Cost
Land	1,254	1,254
Building and Land Improvements	7,741	7,741
Gross Amounts Carried At Close of Period
Land	1,254	1,254
Building and Land Improvements	7,741	7,741
Total	8,995	8,995
Accumulated Depreciation	(781)	(781)
316 Sentinel Way [Member]
Initial Cost
Land	2,748	2,748
Building and Land Improvements	38,156	38,156
Costs Capitalized Subsequent to Acquisition	145	145
Gross Amounts Carried At Close of Period
Land	2,748	2,748
Building and Land Improvements	38,301	38,301
Total	41,049	41,049
Accumulated Depreciation	(6,475)	(6,475)
318 Sentinel Way [Member]
Initial Cost
Land	2,185	2,185
Building and Land Improvements	28,426	28,426
Costs Capitalized Subsequent to Acquisition	560	560
Gross Amounts Carried At Close of Period
Land	2,185	2,185
Building and Land Improvements	28,986	28,986
Total	31,171	31,171
Accumulated Depreciation	(9,182)	(9,182)
320 Sentinel Way [Member]
Initial Cost
Land	2,067	2,067
Building and Land Improvements	21,623	21,623
Costs Capitalized Subsequent to Acquisition	65	65
Gross Amounts Carried At Close of Period
Land	2,067	2,067
Building and Land Improvements	21,688	21,688
Total	23,755	23,755
Accumulated Depreciation	(5,935)	(5,935)
322 Sentinel Way [Member]
Initial Cost
Land	2,605	2,605
Building and Land Improvements	22,827	22,827
Costs Capitalized Subsequent to Acquisition	1,900	1,900
Gross Amounts Carried At Close of Period
Land	2,605	2,605
Building and Land Improvements	24,727	24,727
Total	27,332	27,332
Accumulated Depreciation	(7,047)	(7,047)
324 Sentinel Way [Member]
Initial Cost
Land	1,656	1,656
Building and Land Improvements	23,018	23,018
Gross Amounts Carried At Close of Period
Land	1,656	1,656
Building and Land Improvements	23,018	23,018
Total	24,674	24,674
Accumulated Depreciation	(4,805)	(4,805)
4000 Market Street [Member]
Initial Cost
Land	0	0
Building and Land Improvements	6,820	6,820
Gross Amounts Carried At Close of Period
Land	0	0
Building and Land Improvements	6,820	6,820
Total	6,820	6,820
Accumulated Depreciation	(7)	(7)
4100 Market Street [Member]
Initial Cost
Land	0	0
Building and Land Improvements	4,612	4,612
Gross Amounts Carried At Close of Period
Land	0	0
Building and Land Improvements	4,612	4,612
Total	4,612	4,612
410 National Business Parkway [Member]
Initial Cost
Land	1,831	1,831
Building and Land Improvements	23,257	23,257
Costs Capitalized Subsequent to Acquisition	119	119
Gross Amounts Carried At Close of Period
Land	1,831	1,831
Building and Land Improvements	23,376	23,376
Total	25,207	25,207
Accumulated Depreciation	(3,497)	(3,497)
420 National Business Parkway [Member]
Initial Cost
Land	2,370	2,370
Building and Land Improvements	27,751	27,751
Costs Capitalized Subsequent to Acquisition	108	108
Gross Amounts Carried At Close of Period
Land	2,370	2,370
Building and Land Improvements	27,859	27,859
Total	30,229	30,229
Accumulated Depreciation	(3,340)	(3,340)
430 National Business Parkway [Member]
Initial Cost
Land	1,852	1,852
Building and Land Improvements	21,563	21,563
Costs Capitalized Subsequent to Acquisition	127	127
Gross Amounts Carried At Close of Period
Land	1,852	1,852
Building and Land Improvements	21,690	21,690
Total	23,542	23,542
Accumulated Depreciation	(3,680)	(3,680)
44408 Pecan Court [Member]
Initial Cost
Land	817	817
Building and Land Improvements	1,464	1,464
Costs Capitalized Subsequent to Acquisition	1,691	1,691
Gross Amounts Carried At Close of Period
Land	817	817
Building and Land Improvements	3,155	3,155
Total	3,972	3,972
Accumulated Depreciation	(1,079)	(1,079)
44414 Pecan Court [Member]
Initial Cost
Land	405	405
Building and Land Improvements	1,619	1,619
Costs Capitalized Subsequent to Acquisition	1,065	1,065
Gross Amounts Carried At Close of Period
Land	405	405
Building and Land Improvements	2,684	2,684
Total	3,089	3,089
Accumulated Depreciation	(1,137)	(1,137)
44417 Pecan Court [Member]
Initial Cost
Land	434	434
Building and Land Improvements	3,794	3,794
Costs Capitalized Subsequent to Acquisition	208	208
Gross Amounts Carried At Close of Period
Land	434	434
Building and Land Improvements	4,002	4,002
Total	4,436	4,436
Accumulated Depreciation	(1,624)	(1,624)
44420 Pecan Court [Member]
Initial Cost
Land	344	344
Building and Land Improvements	880	880
Costs Capitalized Subsequent to Acquisition	270	270
Gross Amounts Carried At Close of Period
Land	344	344
Building and Land Improvements	1,150	1,150
Total	1,494	1,494
Accumulated Depreciation	(423)	(423)
44425 Pecan Court [Member]
Initial Cost
Land	1,309	1,309
Building and Land Improvements	3,506	3,506
Costs Capitalized Subsequent to Acquisition	2,123	2,123
Gross Amounts Carried At Close of Period
Land	1,309	1,309
Building and Land Improvements	5,629	5,629
Total	6,938	6,938
Accumulated Depreciation	(2,750)	(2,750)
45310 Abell House Lane [Member]
Initial Cost
Land	2,272	2,272
Building and Land Improvements	13,808	13,808
Costs Capitalized Subsequent to Acquisition	471	471
Gross Amounts Carried At Close of Period
Land	2,272	2,272
Building and Land Improvements	14,279	14,279
Total	16,551	16,551
Accumulated Depreciation	(2,455)	(2,455)
46579 Expedition Drive [Member]
Initial Cost
Land	1,406	1,406
Building and Land Improvements	5,796	5,796
Costs Capitalized Subsequent to Acquisition	1,831	1,831
Gross Amounts Carried At Close of Period
Land	1,406	1,406
Building and Land Improvements	7,627	7,627
Total	9,033	9,033
Accumulated Depreciation	(3,664)	(3,664)
46591 Expedition Drive [Member]
Initial Cost
Land	1,200	1,200
Building and Land Improvements	7,199	7,199
Costs Capitalized Subsequent to Acquisition	1,977	1,977
Gross Amounts Carried At Close of Period
Land	1,200	1,200
Building and Land Improvements	9,176	9,176
Total	10,376	10,376
Accumulated Depreciation	(3,052)	(3,052)
4851 Stonecroft Boulevard [Member]
Initial Cost
Land	1,878	1,878
Building and Land Improvements	11,558	11,558
Costs Capitalized Subsequent to Acquisition	34	34
Gross Amounts Carried At Close of Period
Land	1,878	1,878
Building and Land Improvements	11,592	11,592
Total	13,470	13,470
Accumulated Depreciation	(4,116)	(4,116)
540 National Business Parkway [Member]
Initial Cost
Land	2,035	2,035
Building and Land Improvements	29,608	29,608
Gross Amounts Carried At Close of Period
Land	2,035	2,035
Building and Land Improvements	29,608	29,608
Total	31,643	31,643
Accumulated Depreciation	(934)	(934)
5520 Research Park Drive [Member]
Initial Cost
Land	0	0
Building and Land Improvements	20,072	20,072
Costs Capitalized Subsequent to Acquisition	1,327	1,327
Gross Amounts Carried At Close of Period
Land	0	0
Building and Land Improvements	21,399	21,399
Total	21,399	21,399
Accumulated Depreciation	(4,886)	(4,886)
5522 Research Park Drive [Member]
Initial Cost
Land	0	0
Building and Land Improvements	4,550	4,550
Costs Capitalized Subsequent to Acquisition	210	210
Gross Amounts Carried At Close of Period
Land	0	0
Building and Land Improvements	4,760	4,760
Total	4,760	4,760
Accumulated Depreciation	(1,313)	(1,313)
5801 University Research Court [Member]
Initial Cost
Land	0	0
Building and Land Improvements	15,936	15,936
Gross Amounts Carried At Close of Period
Land	0	0
Building and Land Improvements	15,936	15,936
Total	15,936	15,936
Accumulated Depreciation	(240)	(240)
5825 University Research Court [Member]
Real Estate and Accumulated Depreciation
Encumbrances	20,875	20,875
Initial Cost
Land	0	0
Building and Land Improvements	22,771	22,771
Costs Capitalized Subsequent to Acquisition	783	783
Gross Amounts Carried At Close of Period
Land	0	0
Building and Land Improvements	23,554	23,554
Total	23,554	23,554
Accumulated Depreciation	(5,680)	(5,680)
5850 University Research Court [Member]
Real Estate and Accumulated Depreciation
Encumbrances	22,085	22,085
Initial Cost
Land	0	0
Building and Land Improvements	31,906	31,906
Costs Capitalized Subsequent to Acquisition	405	405
Gross Amounts Carried At Close of Period
Land	0	0
Building and Land Improvements	32,311	32,311
Total	32,311	32,311
Accumulated Depreciation	(7,165)	(7,165)
6700 Alexander Bell Drive [Member]
Initial Cost
Land	1,755	1,755
Building and Land Improvements	7,019	7,019
Costs Capitalized Subsequent to Acquisition	7,509	7,509
Gross Amounts Carried At Close of Period
Land	1,755	1,755
Building and Land Improvements	14,528	14,528
Total	16,283	16,283
Accumulated Depreciation	(7,484)	(7,484)
6708 Alexander Bell Drive [Member]
Initial Cost
Land	897	897
Building and Land Improvements	12,631	12,631
Costs Capitalized Subsequent to Acquisition	1,622	1,622
Gross Amounts Carried At Close of Period
Land	897	897
Building and Land Improvements	14,253	14,253
Total	15,150	15,150
Accumulated Depreciation	(4,111)	(4,111)
6711 Columbia Gateway Drive [Member]
Initial Cost
Land	2,683	2,683
Building and Land Improvements	23,239	23,239
Costs Capitalized Subsequent to Acquisition	1,550	1,550
Gross Amounts Carried At Close of Period
Land	2,683	2,683
Building and Land Improvements	24,789	24,789
Total	27,472	27,472
Accumulated Depreciation	(7,598)	(7,598)
6716 Alexander Bell Drive [Member]
Initial Cost
Land	1,242	1,242
Building and Land Improvements	4,969	4,969
Costs Capitalized Subsequent to Acquisition	4,285	4,285
Gross Amounts Carried At Close of Period
Land	1,242	1,242
Building and Land Improvements	9,254	9,254
Total	10,496	10,496
Accumulated Depreciation	(5,506)	(5,506)
6721 Columbia Gateway Drive [Member]
Initial Cost
Land	1,753	1,753
Building and Land Improvements	34,090	34,090
Costs Capitalized Subsequent to Acquisition	122	122
Gross Amounts Carried At Close of Period
Land	1,753	1,753
Building and Land Improvements	34,212	34,212
Total	35,965	35,965
Accumulated Depreciation	(8,373)	(8,373)
6724 Alexander Bell Drive [Member]
Initial Cost
Land	449	449
Building and Land Improvements	5,039	5,039
Costs Capitalized Subsequent to Acquisition	1,535	1,535
Gross Amounts Carried At Close of Period
Land	449	449
Building and Land Improvements	6,574	6,574
Total	7,023	7,023
Accumulated Depreciation	(2,946)	(2,946)
6731 Columbia Gateway Drive [Member]
Initial Cost
Land	2,807	2,807
Building and Land Improvements	19,098	19,098
Costs Capitalized Subsequent to Acquisition	4,921	4,921
Gross Amounts Carried At Close of Period
Land	2,807	2,807
Building and Land Improvements	24,019	24,019
Total	26,826	26,826
Accumulated Depreciation	(11,156)	(11,156)
6740 Alexander Bell Drive [Member]
Initial Cost
Land	1,424	1,424
Building and Land Improvements	5,696	5,696
Costs Capitalized Subsequent to Acquisition	3,441	3,441
Gross Amounts Carried At Close of Period
Land	1,424	1,424
Building and Land Improvements	9,137	9,137
Total	10,561	10,561
Accumulated Depreciation	(5,875)	(5,875)
6741 Columbia Gateway Drive [Member]
Initial Cost
Land	675	675
Building and Land Improvements	1,711	1,711
Costs Capitalized Subsequent to Acquisition	154	154
Gross Amounts Carried At Close of Period
Land	675	675
Building and Land Improvements	1,865	1,865
Total	2,540	2,540
Accumulated Depreciation	(521)	(521)
6750 Alexander Bell Drive [Member]
Initial Cost
Land	1,263	1,263
Building and Land Improvements	12,461	12,461
Costs Capitalized Subsequent to Acquisition	4,981	4,981
Gross Amounts Carried At Close of Period
Land	1,263	1,263
Building and Land Improvements	17,442	17,442
Total	18,705	18,705
Accumulated Depreciation	(9,484)	(9,484)
6760 Alexander Bell Drive [Member]
Initial Cost
Land	890	890
Building and Land Improvements	3,561	3,561
Costs Capitalized Subsequent to Acquisition	3,830	3,830
Gross Amounts Carried At Close of Period
Land	890	890
Building and Land Improvements	7,391	7,391
Total	8,281	8,281
Accumulated Depreciation	(4,272)	(4,272)
6940 Columbia Gateway Drive [Member]
Initial Cost
Land	3,545	3,545
Building and Land Improvements	9,916	9,916
Costs Capitalized Subsequent to Acquisition	7,330	7,330
Gross Amounts Carried At Close of Period
Land	3,545	3,545
Building and Land Improvements	17,246	17,246
Total	20,791	20,791
Accumulated Depreciation	(9,035)	(9,035)
6950 Columbia Gateway Drive [Member]
Initial Cost
Land	3,596	3,596
Building and Land Improvements	15,738	15,738
Costs Capitalized Subsequent to Acquisition	3,220	3,220
Gross Amounts Carried At Close of Period
Land	3,596	3,596
Building and Land Improvements	18,958	18,958
Total	22,554	22,554
Accumulated Depreciation	(10,912)	(10,912)
7000 Columbia Gateway Drive [Member]
Initial Cost
Land	3,131	3,131
Building and Land Improvements	12,103	12,103
Costs Capitalized Subsequent to Acquisition	6,479	6,479
Gross Amounts Carried At Close of Period
Land	3,131	3,131
Building and Land Improvements	18,582	18,582
Total	21,713	21,713
Accumulated Depreciation	(7,126)	(7,126)
7005 Columbia Gateway Drive [Member]
Initial Cost
Land	3,036	3,036
Building and Land Improvements	747	747
Gross Amounts Carried At Close of Period
Land	3,036	3,036
Building and Land Improvements	747	747
Total	3,783	3,783
7015 Albert Einstein Drive [Member]
Real Estate and Accumulated Depreciation
Encumbrances	412	412
Initial Cost
Land	2,058	2,058
Building and Land Improvements	6,093	6,093
Costs Capitalized Subsequent to Acquisition	3,319	3,319
Gross Amounts Carried At Close of Period
Land	2,058	2,058
Building and Land Improvements	9,412	9,412
Total	11,470	11,470
Accumulated Depreciation	(3,746)	(3,746)
7061 Columbia Gateway Drive [Member]
Initial Cost
Land	729	729
Building and Land Improvements	3,094	3,094
Costs Capitalized Subsequent to Acquisition	2,018	2,018
Gross Amounts Carried At Close of Period
Land	729	729
Building and Land Improvements	5,112	5,112
Total	5,841	5,841
Accumulated Depreciation	(2,747)	(2,747)
7063 Columbia Gateway Drive [Member]
Initial Cost
Land	902	902
Building and Land Improvements	3,684	3,684
Costs Capitalized Subsequent to Acquisition	3,416	3,416
Gross Amounts Carried At Close of Period
Land	902	902
Building and Land Improvements	7,100	7,100
Total	8,002	8,002
Accumulated Depreciation	(3,553)	(3,553)
7065 Columbia Gateway Drive [Member]
Initial Cost
Land	919	919
Building and Land Improvements	3,763	3,763
Costs Capitalized Subsequent to Acquisition	3,095	3,095
Gross Amounts Carried At Close of Period
Land	919	919
Building and Land Improvements	6,858	6,858
Total	7,777	7,777
Accumulated Depreciation	(4,070)	(4,070)
7067 Columbia Gateway Drive [Member]
Initial Cost
Land	1,829	1,829
Building and Land Improvements	11,823	11,823
Costs Capitalized Subsequent to Acquisition	4,480	4,480
Gross Amounts Carried At Close of Period
Land	1,829	1,829
Building and Land Improvements	16,303	16,303
Total	18,132	18,132
Accumulated Depreciation	(7,496)	(7,496)
7125 Columbia Gateway Drive [Member]
Initial Cost
Land	20,487	20,487
Building and Land Improvements	46,994	46,994
Costs Capitalized Subsequent to Acquisition	18,044	18,044
Gross Amounts Carried At Close of Period
Land	20,487	20,487
Building and Land Improvements	65,038	65,038
Total	85,525	85,525
Accumulated Depreciation	(23,141)	(23,141)
7130 Columbia Gateway Drive [Member]
Initial Cost
Land	1,350	1,350
Building and Land Improvements	4,359	4,359
Costs Capitalized Subsequent to Acquisition	2,559	2,559
Gross Amounts Carried At Close of Period
Land	1,350	1,350
Building and Land Improvements	6,918	6,918
Total	8,268	8,268
Accumulated Depreciation	(3,396)	(3,396)
7134 Columbia Gateway Drive [Member]
Initial Cost
Land	704	704
Building and Land Improvements	4,700	4,700
Costs Capitalized Subsequent to Acquisition	353	353
Gross Amounts Carried At Close of Period
Land	704	704
Building and Land Improvements	5,053	5,053
Total	5,757	5,757
Accumulated Depreciation	(1,540)	(1,540)
7138 Columbia Gateway Drive [Member]
Initial Cost
Land	1,104	1,104
Building and Land Improvements	3,518	3,518
Costs Capitalized Subsequent to Acquisition	2,843	2,843
Gross Amounts Carried At Close of Period
Land	1,104	1,104
Building and Land Improvements	6,361	6,361
Total	7,465	7,465
Accumulated Depreciation	(3,658)	(3,658)
7142 Columbia Gateway Drive [Member]
Initial Cost
Land	1,342	1,342
Building and Land Improvements	7,158	7,158
Costs Capitalized Subsequent to Acquisition	2,608	2,608
Gross Amounts Carried At Close of Period
Land	1,342	1,342
Building and Land Improvements	9,766	9,766
Total	11,108	11,108
Accumulated Depreciation	(3,137)	(3,137)
7150 Columbia Gateway Drive [Member]
Initial Cost
Land	1,032	1,032
Building and Land Improvements	3,429	3,429
Costs Capitalized Subsequent to Acquisition	813	813
Gross Amounts Carried At Close of Period
Land	1,032	1,032
Building and Land Improvements	4,242	4,242
Total	5,274	5,274
Accumulated Depreciation	(1,557)	(1,557)
7150 Riverwood Drive [Member]
Initial Cost
Land	1,821	1,821
Building and Land Improvements	4,388	4,388
Costs Capitalized Subsequent to Acquisition	1,824	1,824
Gross Amounts Carried At Close of Period
Land	1,821	1,821
Building and Land Improvements	6,212	6,212
Total	8,033	8,033
Accumulated Depreciation	(2,570)	(2,570)
7160 Riverwood Drive [Member]
Initial Cost
Land	2,732	2,732
Building and Land Improvements	7,006	7,006
Costs Capitalized Subsequent to Acquisition	2,490	2,490
Gross Amounts Carried At Close of Period
Land	2,732	2,732
Building and Land Improvements	9,496	9,496
Total	12,228	12,228
Accumulated Depreciation	(4,050)	(4,050)
7170 Riverwood Drive [Member]
Initial Cost
Land	1,283	1,283
Building and Land Improvements	3,096	3,096
Costs Capitalized Subsequent to Acquisition	1,704	1,704
Gross Amounts Carried At Close of Period
Land	1,283	1,283
Building and Land Improvements	4,800	4,800
Total	6,083	6,083
Accumulated Depreciation	(2,026)	(2,026)
7175 Riverwood Drive [Member]
Initial Cost
Land	1,788	1,788
Building and Land Improvements	7,269	7,269
Gross Amounts Carried At Close of Period
Land	1,788	1,788
Building and Land Improvements	7,269	7,269
Total	9,057	9,057
Accumulated Depreciation	(934)	(934)
7200 Redstone Gateway [Member]
Real Estate and Accumulated Depreciation
Encumbrances	6,121	6,121
Initial Cost
Land	0	0
Building and Land Improvements	8,348	8,348
Costs Capitalized Subsequent to Acquisition	5	5
Gross Amounts Carried At Close of Period
Land	0	0
Building and Land Improvements	8,353	8,353
Total	8,353	8,353
Accumulated Depreciation	(963)	(963)
7200 Riverwood Road [Member]
Initial Cost
Land	4,089	4,089
Building and Land Improvements	22,630	22,630
Costs Capitalized Subsequent to Acquisition	4,538	4,538
Gross Amounts Carried At Close of Period
Land	4,089	4,089
Building and Land Improvements	27,168	27,168
Total	31,257	31,257
Accumulated Depreciation	(11,024)	(11,024)
7205 Riverwood Drive [Member]
Initial Cost
Land	1,367	1,367
Building and Land Improvements	21,419	21,419
Gross Amounts Carried At Close of Period
Land	1,367	1,367
Building and Land Improvements	21,419	21,419
Total	22,786	22,786
Accumulated Depreciation	(2,916)	(2,916)
7272 Park Circle Drive [Member]
Initial Cost
Land	1,479	1,479
Building and Land Improvements	6,300	6,300
Costs Capitalized Subsequent to Acquisition	4,578	4,578
Gross Amounts Carried At Close of Period
Land	1,479	1,479
Building and Land Improvements	10,878	10,878
Total	12,357	12,357
Accumulated Depreciation	(4,427)	(4,427)
7318 Parkway Drive [Member]
Initial Cost
Land	972	972
Building and Land Improvements	3,888	3,888
Costs Capitalized Subsequent to Acquisition	1,250	1,250
Gross Amounts Carried At Close of Period
Land	972	972
Building and Land Improvements	5,138	5,138
Total	6,110	6,110
Accumulated Depreciation	(2,566)	(2,566)
7400 Redstone Gateway [Member]
Real Estate and Accumulated Depreciation
Encumbrances	6,713	6,713
Initial Cost
Land	0	0
Building and Land Improvements	9,223	9,223
Gross Amounts Carried At Close of Period
Land	0	0
Building and Land Improvements	9,223	9,223
Total	9,223	9,223
Accumulated Depreciation	(813)	(813)
7467 Ridge Road [Member]
Initial Cost
Land	1,565	1,565
Building and Land Improvements	3,116	3,116
Costs Capitalized Subsequent to Acquisition	4,428	4,428
Gross Amounts Carried At Close of Period
Land	1,565	1,565
Building and Land Improvements	7,544	7,544
Total	9,109	9,109
Accumulated Depreciation	(2,699)	(2,699)
7740 Milestone Parkway [Member]
Real Estate and Accumulated Depreciation
Encumbrances	17,786	17,786
Initial Cost
Land	3,825	3,825
Building and Land Improvements	34,176	34,176
Costs Capitalized Subsequent to Acquisition	567	567
Gross Amounts Carried At Close of Period
Land	3,825	3,825
Building and Land Improvements	34,743	34,743
Total	38,568	38,568
Accumulated Depreciation	(7,395)	(7,395)
7770 Backlick Road [Member]
Initial Cost
Land	6,387	6,387
Building and Land Improvements	76,593	76,593
Costs Capitalized Subsequent to Acquisition	300	300
Gross Amounts Carried At Close of Period
Land	6,387	6,387
Building and Land Improvements	76,893	76,893
Total	83,280	83,280
Accumulated Depreciation	(11,016)	(11,016)
7880 Milestone Parkway [Member]
Initial Cost
Land	4,857	4,857
Building and Land Improvements	25,925	25,925
Costs Capitalized Subsequent to Acquisition	200	200
Gross Amounts Carried At Close of Period
Land	4,857	4,857
Building and Land Improvements	26,125	26,125
Total	30,982	30,982
Accumulated Depreciation	(2,023)	(2,023)
8621 Robert Fulton Drive [Member]
Initial Cost
Land	2,317	2,317
Building and Land Improvements	12,642	12,642
Costs Capitalized Subsequent to Acquisition	6,437	6,437
Gross Amounts Carried At Close of Period
Land	2,317	2,317
Building and Land Improvements	19,079	19,079
Total	21,396	21,396
Accumulated Depreciation	(4,777)	(4,777)
8661 Robert Fulton Drive [Member]
Initial Cost
Land	1,510	1,510
Building and Land Improvements	3,764	3,764
Costs Capitalized Subsequent to Acquisition	2,453	2,453
Gross Amounts Carried At Close of Period
Land	1,510	1,510
Building and Land Improvements	6,217	6,217
Total	7,727	7,727
Accumulated Depreciation	(2,926)	(2,926)
8671 Robert Fulton Drive [Member]
Initial Cost
Land	1,718	1,718
Building and Land Improvements	4,280	4,280
Costs Capitalized Subsequent to Acquisition	4,233	4,233
Gross Amounts Carried At Close of Period
Land	1,718	1,718
Building and Land Improvements	8,513	8,513
Total	10,231	10,231
Accumulated Depreciation	(4,021)	(4,021)
870 Elkridge Landing Road [Member]
Initial Cost
Land	2,003	2,003
Building and Land Improvements	9,442	9,442
Costs Capitalized Subsequent to Acquisition	9,301	9,301
Gross Amounts Carried At Close of Period
Land	2,003	2,003
Building and Land Improvements	18,743	18,743
Total	20,746	20,746
Accumulated Depreciation	(9,936)	(9,936)
8800 Redstone Gateway [Member]
Initial Cost
Land	0	0
Building and Land Improvements	992	992
Costs Capitalized Subsequent to Acquisition	0	0
Gross Amounts Carried At Close of Period
Land	0	0
Building and Land Improvements	992	992
Total	992	992
Accumulated Depreciation	0	0
891 Elkridge Landing Road [Member]
Initial Cost
Land	1,165	1,165
Building and Land Improvements	4,772	4,772
Costs Capitalized Subsequent to Acquisition	3,450	3,450
Gross Amounts Carried At Close of Period
Land	1,165	1,165
Building and Land Improvements	8,222	8,222
Total	9,387	9,387
Accumulated Depreciation	(4,582)	(4,582)
901 Elkridge Landing Road [Member]
Initial Cost
Land	1,156	1,156
Building and Land Improvements	4,437	4,437
Costs Capitalized Subsequent to Acquisition	3,383	3,383
Gross Amounts Carried At Close of Period
Land	1,156	1,156
Building and Land Improvements	7,820	7,820
Total	8,976	8,976
Accumulated Depreciation	(3,955)	(3,955)
911 Elkridge Landing Road [Member]
Initial Cost
Land	1,215	1,215
Building and Land Improvements	4,861	4,861
Costs Capitalized Subsequent to Acquisition	2,901	2,901
Gross Amounts Carried At Close of Period
Land	1,215	1,215
Building and Land Improvements	7,762	7,762
Total	8,977	8,977
Accumulated Depreciation	(4,170)	(4,170)
938 Elkridge Landing Road [Member]
Initial Cost
Land	922	922
Building and Land Improvements	4,748	4,748
Costs Capitalized Subsequent to Acquisition	1,446	1,446
Gross Amounts Carried At Close of Period
Land	922	922
Building and Land Improvements	6,194	6,194
Total	7,116	7,116
Accumulated Depreciation	(2,828)	(2,828)
939 Elkridge Landing Road [Member]
Initial Cost
Land	939	939
Building and Land Improvements	3,756	3,756
Costs Capitalized Subsequent to Acquisition	4,438	4,438
Gross Amounts Carried At Close of Period
Land	939	939
Building and Land Improvements	8,194	8,194
Total	9,133	9,133
Accumulated Depreciation	(4,642)	(4,642)
9651 Hornbaker Road [Member]
Initial Cost
Land	6,050	6,050
Building and Land Improvements	251,367	251,367
Costs Capitalized Subsequent to Acquisition	4,689	4,689
Gross Amounts Carried At Close of Period
Land	6,050	6,050
Building and Land Improvements	256,056	256,056
Total	262,106	262,106
Accumulated Depreciation	(50,191)	(50,191)
Arundel Preserve [Member]
Initial Cost
Land	13,401	13,401
Building and Land Improvements	9,583	9,583
Gross Amounts Carried At Close of Period
Land	13,401	13,401
Building and Land Improvements	9,583	9,583
Total	22,984	22,984
Bethlehem Tech Park-DC18 [Member]
Initial Cost
Land	3,599	3,599
Building and Land Improvements	26,636	26,636
Gross Amounts Carried At Close of Period
Land	3,599	3,599
Building and Land Improvements	26,636	26,636
Total	30,235	30,235
Accumulated Depreciation	(964)	(964)
Bethlehem Tech Park-DC19 [Member]
Initial Cost
Land	3,708	3,708
Building and Land Improvements	16,606	16,606
Gross Amounts Carried At Close of Period
Land	3,708	3,708
Building and Land Improvements	16,606	16,606
Total	20,314	20,314
Accumulated Depreciation	(865)	(865)
Bethlehem Tech Park-DC20 [Member]
Initial Cost
Land	3,599	3,599
Building and Land Improvements	24,062	24,062
Gross Amounts Carried At Close of Period
Land	3,599	3,599
Building and Land Improvements	24,062	24,062
Total	27,661	27,661
Accumulated Depreciation	(965)	(965)
Bethlehem Tech Park - DC 23 [Member]
Initial Cost
Land	0	0
Building and Land Improvements	4,883	4,883
Gross Amounts Carried At Close of Period
Land	0	0
Building and Land Improvements	4,883	4,883
Total	4,883	4,883
BLC 1 [Member]
Initial Cost
Land	12,034	12,034
Building and Land Improvements	17,917	17,917
Gross Amounts Carried At Close of Period
Land	12,034	12,034
Building and Land Improvements	17,917	17,917
Total	29,951	29,951
Accumulated Depreciation	(245)	(245)
BLC 2 [Member]
Initial Cost
Land	12,034	12,034
Building and Land Improvements	17,659	17,659
Gross Amounts Carried At Close of Period
Land	12,034	12,034
Building and Land Improvements	17,659	17,659
Total	29,693	29,693
Accumulated Depreciation	(210)	(210)
Canton Crossing Land [Member]
Initial Cost
Land	17,285	17,285
Building and Land Improvements	8,179	8,179
Gross Amounts Carried At Close of Period
Land	17,285	17,285
Building and Land Improvements	8,179	8,179
Total	25,464	25,464
Canton Crossing Util Distr Ctr [Member]
Initial Cost
Land	6,100	6,100
Building and Land Improvements	10,450	10,450
Costs Capitalized Subsequent to Acquisition	1,192	1,192
Gross Amounts Carried At Close of Period
Land	6,100	6,100
Building and Land Improvements	11,642	11,642
Total	17,742	17,742
Accumulated Depreciation	(5,053)	(5,053)
Columbia Gateway - Southridge [Member]
Initial Cost
Land	6,387	6,387
Building and Land Improvements	3,722	3,722
Gross Amounts Carried At Close of Period
Land	6,387	6,387
Building and Land Improvements	3,722	3,722
Total	10,109	10,109
Dahlgren Technology Center [Member]
Initial Cost
Land	978	978
Building and Land Improvements	178	178
Gross Amounts Carried At Close of Period
Land	978	978
Building and Land Improvements	178	178
Total	1,156	1,156
Expedition VII [Member]
Initial Cost
Land	705	705
Building and Land Improvements	730	730
Gross Amounts Carried At Close of Period
Land	705	705
Building and Land Improvements	730	730
Total	1,435	1,435
IN 1 [Member]
Initial Cost
Land	1,815	1,815
Building and Land Improvements	11,532	11,532
Gross Amounts Carried At Close of Period
Land	1,815	1,815
Building and Land Improvements	11,532	11,532
Total	13,347	13,347
IN 2 [Member]
Initial Cost
Land	2,627	2,627
Building and Land Improvements	4,655	4,655
Gross Amounts Carried At Close of Period
Land	2,627	2,627
Building and Land Improvements	4,655	4,655
Total	7,282	7,282
M Square Research Park [Member]
Initial Cost
Land	0	0
Building and Land Improvements	3,202	3,202
Gross Amounts Carried At Close of Period
Land	0	0
Building and Land Improvements	3,202	3,202
Total	3,202	3,202
MP 1 [Member]
Initial Cost
Land	9,426	9,426
Building and Land Improvements	15,865	15,865
Gross Amounts Carried At Close of Period
Land	9,426	9,426
Building and Land Improvements	15,865	15,865
Total	25,291	25,291
MP 2 [Member]
Initial Cost
Land	9,426	9,426
Building and Land Improvements	25,191	25,191
Gross Amounts Carried At Close of Period
Land	9,426	9,426
Building and Land Improvements	25,191	25,191
Total	34,617	34,617
Accumulated Depreciation	0	0
MR Land [Member]
Initial Cost
Land	9,038	9,038
Building and Land Improvements	55	55
Gross Amounts Carried At Close of Period
Land	9,038	9,038
Building and Land Improvements	55	55
Total	9,093	9,093
National Business Park North [Member]
Initial Cost
Land	28,060	28,060
Building and Land Improvements	47,371	47,371
Gross Amounts Carried At Close of Period
Land	28,060	28,060
Building and Land Improvements	47,371	47,371
Total	75,431	75,431
North Gate Business Park [Member]
Initial Cost
Land	1,755	1,755
Building and Land Improvements	0	0
Gross Amounts Carried At Close of Period
Land	1,755	1,755
Building and Land Improvements	0	0
Total	1,755	1,755
Northwest Crossroads [Member]
Initial Cost
Land	7,430	7,430
Building and Land Improvements	847	847
Gross Amounts Carried At Close of Period
Land	7,430	7,430
Building and Land Improvements	847	847
Total	8,277	8,277
NOVA Office A [Member]
Initial Cost
Land	2,096	2,096
Building and Land Improvements	46,840	46,840
Gross Amounts Carried At Close of Period
Land	2,096	2,096
Building and Land Improvements	46,840	46,840
Total	48,936	48,936
Accumulated Depreciation	(4,577)	(4,577)
NOVA Office B [Member]
Initial Cost
Land	739	739
Building and Land Improvements	33,881	33,881
Gross Amounts Carried At Close of Period
Land	739	739
Building and Land Improvements	33,881	33,881
Total	34,620	34,620
Accumulated Depreciation	(1,817)	(1,817)
NOVA Office D [Member]
Initial Cost
Land	6,587	6,587
Building and Land Improvements	40,525	40,525
Gross Amounts Carried At Close of Period
Land	6,587	6,587
Building and Land Improvements	40,525	40,525
Total	47,112	47,112
Accumulated Depreciation	(1,417)	(1,417)
Old Annapolis Road [Member]
Initial Cost
Land	1,637	1,637
Building and Land Improvements	5,500	5,500
Costs Capitalized Subsequent to Acquisition	6,531	6,531
Gross Amounts Carried At Close of Period
Land	1,637	1,637
Building and Land Improvements	12,031	12,031
Total	13,668	13,668
Accumulated Depreciation	(3,898)	(3,898)
Paragon Park - DC 21 [Member]
Initial Cost
Land	7,828	7,828
Building and Land Improvements	19,999	19,999
Gross Amounts Carried At Close of Period
Land	7,828	7,828
Building and Land Improvements	19,999	19,999
Total	27,827	27,827
Accumulated Depreciation	(560)	(560)
Paragon Park - DC 22 [Member]
Initial Cost
Land	7,828	7,828
Building and Land Improvements	18,755	18,755
Gross Amounts Carried At Close of Period
Land	7,828	7,828
Building and Land Improvements	18,755	18,755
Total	26,583	26,583
Accumulated Depreciation	(515)	(515)
Patriot Point - DC15 [Member]
Initial Cost
Land	12,156	12,156
Building and Land Improvements	17,175	17,175
Gross Amounts Carried At Close of Period
Land	12,156	12,156
Building and Land Improvements	17,175	17,175
Total	29,331	29,331
Accumulated Depreciation	(1,180)	(1,180)
Patriot Point - DC16 [Member]
Initial Cost
Land	12,156	12,156
Building and Land Improvements	17,043	17,043
Gross Amounts Carried At Close of Period
Land	12,156	12,156
Building and Land Improvements	17,043	17,043
Total	29,199	29,199
Accumulated Depreciation	(1,135)	(1,135)
Patriot Point - DC17 [Member]
Initial Cost
Land	6,078	6,078
Building and Land Improvements	16,408	16,408
Gross Amounts Carried At Close of Period
Land	6,078	6,078
Building and Land Improvements	16,408	16,408
Total	22,486	22,486
Accumulated Depreciation	(930)	(930)
Patriot Ridge [Member]
Initial Cost
Land	18,517	18,517
Building and Land Improvements	14,505	14,505
Gross Amounts Carried At Close of Period
Land	18,517	18,517
Building and Land Improvements	14,505	14,505
Total	33,022	33,022
Project EX [Member]
Initial Cost
Land	8,958	8,958
Building and Land Improvements	5,744	5,744
Gross Amounts Carried At Close of Period
Land	8,958	8,958
Building and Land Improvements	5,744	5,744
Total	14,702	14,702
Accumulated Depreciation	0	0
Redstone Gateway [Member]
Initial Cost
Land	0	0
Building and Land Improvements	21,807	21,807
Gross Amounts Carried At Close of Period
Land	0	0
Building and Land Improvements	21,807	21,807
Total	21,807	21,807
Route 15/Biggs Ford Road [Member]
Initial Cost
Land	1,129	1,129
Building and Land Improvements	0	0
Gross Amounts Carried At Close of Period
Land	1,129	1,129
Building and Land Improvements	0	0
Total	1,129	1,129
Sentry Gateway [Member]
Initial Cost
Land	4,052	4,052
Building and Land Improvements	1,833	1,833
Gross Amounts Carried At Close of Period
Land	4,052	4,052
Building and Land Improvements	1,833	1,833
Total	5,885	5,885
Sentry Gateway - T [Member]
Initial Cost
Land	14,020	14,020
Building and Land Improvements	38,804	38,804
Costs Capitalized Subsequent to Acquisition	13	13
Gross Amounts Carried At Close of Period
Land	14,020	14,020
Building and Land Improvements	38,817	38,817
Total	52,837	52,837
Accumulated Depreciation	(11,532)	(11,532)
Sentry Gateway - V [Member]
Initial Cost
Land	0	0
Building and Land Improvements	1,066	1,066
Gross Amounts Carried At Close of Period
Land	0	0
Building and Land Improvements	1,066	1,066
Total	1,066	1,066
Accumulated Depreciation	(268)	(268)
Sentry Gateway - W [Member]
Initial Cost
Land	0	0
Building and Land Improvements	1,884	1,884
Costs Capitalized Subsequent to Acquisition	71	71
Gross Amounts Carried At Close of Period
Land	0	0
Building and Land Improvements	1,955	1,955
Total	1,955	1,955
Accumulated Depreciation	(444)	(444)
Sentry Gateway - X [Member]
Initial Cost
Land	1,964	1,964
Building and Land Improvements	21,178	21,178
Gross Amounts Carried At Close of Period
Land	1,964	1,964
Building and Land Improvements	21,178	21,178
Total	23,142	23,142
Accumulated Depreciation	(4,316)	(4,316)
Sentry Gateway - Y [Member]
Initial Cost
Land	1,964	1,964
Building and Land Improvements	21,298	21,298
Gross Amounts Carried At Close of Period
Land	1,964	1,964
Building and Land Improvements	21,298	21,298
Total	23,262	23,262
Accumulated Depreciation	(4,342)	(4,342)
Sentry Gateway - Z [Member]
Initial Cost
Land	1,964	1,964
Building and Land Improvements	30,573	30,573
Gross Amounts Carried At Close of Period
Land	1,964	1,964
Building and Land Improvements	30,573	30,573
Total	32,537	32,537
Accumulated Depreciation	(2,908)	(2,908)
Westfields - Park Center [Member]
Initial Cost
Land	16,418	16,418
Building and Land Improvements	12,097	12,097
Gross Amounts Carried At Close of Period
Land	16,418	16,418
Building and Land Improvements	12,097	12,097
Total	28,515	28,515
Westfields Corporate Center [Member]
Initial Cost
Land	7,141	7,141
Building and Land Improvements	1,576	1,576
Gross Amounts Carried At Close of Period
Land	7,141	7,141
Building and Land Improvements	1,576	1,576
Total	8,717	8,717
Other Developments, including intercompany eliminations [Member]
Initial Cost
Land	8	8
Building and Land Improvements	121	121
Costs Capitalized Subsequent to Acquisition	373	373
Gross Amounts Carried At Close of Period
Land	8	8
Building and Land Improvements	494	494
Total	502	502
Accumulated Depreciation	(77)	$ (77)
Fort Meade/BW Corridor [Member]
Additional information
Impairment losses	$ 2,400

Schedule III - Real Estate an_3

Schedule III - Real Estate and Accumulated Depreciation (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Changes in cost of properties
Beginning balance	$ 3,980,813	$ 3,874,715	$ 4,158,616
Improvements and other additions	224,524	259,548	251,960
Sales	(53,547)	(138,216)	(268,038)
Impairments	(2,493)	(15,116)	(143,502)
Other dispositions	(768)	(118)	(124,321)
Ending balance	4,148,529	3,980,813	3,874,715
Changes in accumulated depreciation
Beginning balance	801,038	715,951	718,680
Depreciation expense	112,610	107,772	105,763
Sales	(14,845)	(22,567)	(56,607)
Impairments	(132)	0	(42,161)
Other dispositions	(768)	(118)	(9,724)
Ending balance	$ 897,903	$ 801,038	$ 715,951