Liberty Property Limited Partnership 10-Q 2 Nov 18 2018 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information Document - shares	9 Months Ended
	Sep. 30, 2018	Oct. 31, 2018
Entity Information [Line Items]
Entity Registrant Name	LIBERTY PROPERTY TRUST
Entity Central Index Key	921,112
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Document Type	10-Q
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3
Entity Emerging Growth Company	false
Entity Small Business	false
Amendment Flag	false
Entity Common Stock, Shares Outstanding		147,821,113
Liberty Property Limited Partnership [Member]
Entity Information [Line Items]
Entity Registrant Name	LIBERTY PROPERTY LIMITED PARTNERSHIP
Entity Central Index Key	921,113

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Real Estate [Abstract]
Land and land improvements	$ 1,174,720	$ 1,026,820
Building and improvements	4,459,247	4,135,150
Less accumulated depreciation	(957,292)	(866,355)
Operating real estate	4,676,675	4,295,615
Development in progress	440,503	333,437
Land held for development	357,120	330,748
Net real estate	5,474,298	4,959,800
Cash and cash equivalents	17,770	11,882
Restricted cash	13,371	13,803
Accounts receivable, net	13,406	10,389
Deferred rent receivable, net	121,820	107,150
Deferred financing and leasing costs, net of accumulated amortization (September 30, 2018, $165,054; December 31, 2017, $154,762)	153,970	147,837
Investments in and advances to unconsolidated joint ventures	357,970	288,456
Assets held for sale	294,191	610,611
Prepaid expenses and other assets	372,279	289,829
Total assets	6,819,075	6,439,757
LIABILITIES
Mortgage loans, net	233,682	267,093
Unsecured notes, net	2,285,565	2,283,513
Credit facilities	569,909	358,939
Accounts payable	72,553	76,313
Accrued interest	35,491	21,796
Distributions payable	60,415	60,330
Other liabilities	260,353	201,588
Liabilities held for sale	3,986	14,282
Total liabilities	3,521,954	3,283,854
Noncontrolling interest - operating partnership - 301,483 preferred units outstanding as of September 30, 2018 and December 31, 2017	7,537	7,537
EQUITY
Common shares of beneficial interest, $.001 par value, 283,987,000 shares authorized; 147,812,321 and 147,450,691 shares issued and outstanding as of September 30, 2018 and December 31, 2017, respectively	148	147
Additional paid-in capital	3,688,631	3,674,978
Accumulated other comprehensive loss	(46,322)	(37,797)
Distributions in excess of net income	(417,357)	(549,970)
Total Liberty Property Trust shareholders' equity	3,225,100	3,087,358
Noncontrolling interest - operating partnership
Noncontrolling interest – operating partnership - 3,520,205 common units outstanding as of September 30, 2018 and December 31, 2017	59,057	56,159
Noncontrolling interest - consolidated joint ventures	5,427	4,849
Total owners' equity	3,289,584	3,148,366
Total liabilities, noncontrolling interest - operating partnership and equity	6,819,075	6,439,757
Liberty Property Limited Partnership [Member]
Real Estate [Abstract]
Land and land improvements	1,174,720	1,026,820
Building and improvements	4,459,247	4,135,150
Less accumulated depreciation	(957,292)	(866,355)
Operating real estate	4,676,675	4,295,615
Development in progress	440,503	333,437
Land held for development	357,120	330,748
Net real estate	5,474,298	4,959,800
Cash and cash equivalents	17,770	11,882
Restricted cash	13,371	13,803
Accounts receivable, net	13,406	10,389
Deferred rent receivable, net	121,820	107,150
Deferred financing and leasing costs, net of accumulated amortization (September 30, 2018, $165,054; December 31, 2017, $154,762)	153,970	147,837
Investments in and advances to unconsolidated joint ventures	357,970	288,456
Assets held for sale	294,191	610,611
Prepaid expenses and other assets	372,279	289,829
Total assets	6,819,075	6,439,757
LIABILITIES
Mortgage loans, net	233,682	267,093
Unsecured notes, net	2,285,565	2,283,513
Credit facilities	569,909	358,939
Accounts payable	72,553	76,313
Accrued interest	35,491	21,796
Distributions payable	60,415	60,330
Other liabilities	260,353	201,588
Liabilities held for sale	3,986	14,282
Total liabilities	3,521,954	3,283,854
Noncontrolling interest - operating partnership - 301,483 preferred units outstanding as of September 30, 2018 and December 31, 2017	7,537	7,537
EQUITY
General partner’s equity - 147,812,321 and 147,450,691 common units outstanding as of September 30, 2018 and December 31, 2017, respectively	3,225,100	3,087,358
Limited partners’ equity – 3,520,205 common units outstanding as of September 30, 2018 and December 31, 2017	59,057	56,159
Noncontrolling interest - operating partnership
Noncontrolling interest - consolidated joint ventures	5,427	4,849
Total owners' equity	3,289,584	3,148,366
Total liabilities, noncontrolling interest - operating partnership and equity	$ 6,819,075	$ 6,439,757

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parentheticals) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Deferred financing and leasing costs, accumulated amortization	$ 165,054	$ 154,762
Limited Partners' common units outstanding	3,520,205	3,520,205
Common shares of beneficial interest, par value per share	$ 0.001	$ 0.001
Common shares of beneficial interest, shares authorized	283,987,000	283,987,000
Common shares of beneficial interest, shares issued	147,812,321	147,450,691
Common shares of beneficial interest, shares outstanding	147,812,321	147,450,691
Liberty Property Limited Partnership [Member]
Deferred financing and leasing costs, accumulated amortization	$ 165,054	$ 154,762
Limited Partners' common units outstanding	3,520,205	3,520,205
Common shares of beneficial interest, shares outstanding	147,812,321	147,450,691
Series I 2 [Member]
Noncontrolling interest - operating partnership, preferred units outstanding	301,483	301,483
Series I 2 [Member] | Liberty Property Limited Partnership [Member]
Noncontrolling interest - operating partnership, preferred units outstanding	301,483	301,483

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
OPERATING REVENUE
Rental	$ 120,775,000	$ 113,889,000	$ 357,185,000	$ 333,884,000
Operating expense reimbursement	37,453,000	33,522,000	110,729,000	100,161,000
Development service fee income	12,956,000	24,176,000	59,132,000	53,920,000
Total operating revenue	171,184,000	171,587,000	527,046,000	487,965,000
OPERATING EXPENSE
Rental property	14,457,000	14,833,000	44,023,000	42,938,000
Real estate taxes	23,193,000	20,427,000	67,452,000	61,326,000
General and administrative	13,121,000	11,873,000	43,406,000	44,069,000
Expensed pursuit costs	1,292,000	4,772,000	1,675,000	4,957,000
Systems implementation expense	1,177,000	142,000	3,352,000	157,000
Depreciation and amortization	43,375,000	41,754,000	127,280,000	123,924,000
Development service fee expense	12,924,000	23,665,000	120,799,000	52,497,000
Impairment charges - real estate assets	0	3,936,000	26,000,000	3,936,000
Total operating expenses	109,539,000	121,402,000	433,987,000	333,804,000
Operating income	61,645,000	50,185,000	93,059,000	154,161,000
OTHER INCOME (EXPENSE)
Interest and other income	3,433,000	1,734,000	8,568,000	5,487,000
Interest expense	(23,274,000)	(21,448,000)	(66,582,000)	(62,409,000)
Total other income (expense)	(19,841,000)	(19,714,000)	(58,014,000)	(56,922,000)
Income before gain on property dispositions, income taxes and equity in earnings of unconsolidated joint ventures	41,804,000	30,471,000	35,045,000	97,239,000
Gain on property dispositions	2,002,000	23,840,000	54,705,000	30,542,000
Income taxes	(472,000)	(582,000)	(1,967,000)	(1,528,000)
Equity in earnings of unconsolidated joint ventures	6,766,000	4,305,000	20,958,000	14,026,000
Income from continuing operations	50,100,000	58,034,000	108,741,000	140,279,000
Discontinued operations (including a net gain of $94.9 million and a net gain of $184.7 million on property dispositions for the three and nine months ended September 30, 2018, respectively)	103,971,000	3,125,000	209,945,000	17,942,000
Net income	154,071,000	61,159,000	318,686,000	158,221,000
Noncontrolling interest – operating partnership	(3,696,000)	(1,545,000)	(7,738,000)	(4,044,000)
Noncontrolling interest - consolidated joint ventures	(232,000)	(75,000)	(1,010,000)	(195,000)
Net income available to common shareholders or unitholders	150,143,000	59,539,000	309,938,000	153,982,000
Comprehensive income attributable to common shareholders or unitholders
Net income	154,071,000	61,159,000	318,686,000	158,221,000
Other comprehensive (loss) income - foreign currency translation	(3,015,000)	5,634,000	(9,221,000)	16,314,000
Other comprehensive income - derivative instruments	6,000	91,000	493,000	366,000
Other comprehensive (loss) income	(3,009,000)	5,725,000	(8,728,000)	16,680,000
Total comprehensive income	151,062,000	66,884,000	309,958,000	174,901,000
Less: comprehensive income attributable to noncontrolling interest	(3,858,000)	(1,754,000)	(8,545,000)	(4,629,000)
Comprehensive income attributable to common shareholders	$ 147,204,000	$ 65,130,000	$ 301,413,000	$ 170,272,000
Weighted average number of common shares or units outstanding
Basic	147,324	146,811	147,241	146,678
Diluted	148,271	147,596	148,160	147,430
Amounts attributable to common shareholders or unitholders
Income from continuing operations	$ 48,595,000	$ 56,487,000	$ 104,885,000	$ 136,460,000
Income from discontinued operations	101,548,000	3,052,000	205,053,000	17,522,000
Net income available to common shareholders or unitholders	150,143,000	59,539,000	309,938,000	153,982,000
Liberty Property Limited Partnership [Member]
OPERATING REVENUE
Rental	120,775,000	113,889,000	357,185,000	333,884,000
Operating expense reimbursement	37,453,000	33,522,000	110,729,000	100,161,000
Development service fee income	12,956,000	24,176,000	59,132,000	53,920,000
Total operating revenue	171,184,000	171,587,000	527,046,000	487,965,000
OPERATING EXPENSE
Rental property	14,457,000	14,833,000	44,023,000	42,938,000
Real estate taxes	23,193,000	20,427,000	67,452,000	61,326,000
General and administrative	13,121,000	11,873,000	43,406,000	44,069,000
Expensed pursuit costs	1,292,000	4,772,000	1,675,000	4,957,000
Systems implementation expense	1,177,000	142,000	3,352,000	157,000
Depreciation and amortization	43,375,000	41,754,000	127,280,000	123,924,000
Development service fee expense	12,924,000	23,665,000	120,799,000	52,497,000
Impairment charges - real estate assets	0	3,936,000	26,000,000	3,936,000
Total operating expenses	109,539,000	121,402,000	433,987,000	333,804,000
Operating income	61,645,000	50,185,000	93,059,000	154,161,000
OTHER INCOME (EXPENSE)
Interest and other income	3,433,000	1,734,000	8,568,000	5,487,000
Interest expense	(23,274,000)	(21,448,000)	(66,582,000)	(62,409,000)
Total other income (expense)	(19,841,000)	(19,714,000)	(58,014,000)	(56,922,000)
Income before gain on property dispositions, income taxes and equity in earnings of unconsolidated joint ventures	41,804,000	30,471,000	35,045,000	97,239,000
Gain on property dispositions	2,002,000	23,840,000	54,705,000	30,542,000
Income taxes	(472,000)	(582,000)	(1,967,000)	(1,528,000)
Equity in earnings of unconsolidated joint ventures	6,766,000	4,305,000	20,958,000	14,026,000
Income from continuing operations	50,100,000	58,034,000	108,741,000	140,279,000
Discontinued operations (including a net gain of $94.9 million and a net gain of $184.7 million on property dispositions for the three and nine months ended September 30, 2018, respectively)	103,971,000	3,125,000	209,945,000	17,942,000
Net income	154,071,000	61,159,000	318,686,000	158,221,000
Noncontrolling interest - consolidated joint ventures	(232,000)	(75,000)	(1,010,000)	(195,000)
Preferred unit distributions	(118,000)	(118,000)	(354,000)	(354,000)
Net income available to common shareholders or unitholders	153,721,000	60,966,000	317,322,000	157,672,000
Comprehensive income attributable to common shareholders or unitholders
Net income	154,071,000	61,159,000	318,686,000	158,221,000
Other comprehensive (loss) income - foreign currency translation	(3,015,000)	5,634,000	(9,221,000)	16,314,000
Other comprehensive income - derivative instruments	6,000	91,000	493,000	366,000
Other comprehensive (loss) income	(3,009,000)	5,725,000	(8,728,000)	16,680,000
Total comprehensive income	$ 151,062,000	$ 66,884,000	$ 309,958,000	$ 174,901,000
Weighted average number of common shares or units outstanding
Basic	150,844	150,339	150,761	150,206
Diluted	151,791	151,124	151,680	150,958
Net income allocated to general partners	$ 150,143,000	$ 59,539,000	$ 309,938,000	$ 153,982,000
Net income allocated to limited partners	3,696,000	1,545,000	7,738,000	4,044,000
Amounts attributable to common shareholders or unitholders
Income from continuing operations	49,868,000	57,959,000	107,731,000	140,084,000
Net income available to common shareholders or unitholders	$ 153,721,000	$ 60,966,000	$ 317,322,000	$ 157,672,000
Common Units [Member] | Liberty Property Limited Partnership [Member]
Basic:
Income from continuing operations	$ 0.33	$ 0.39	$ 0.71	$ 0.93
Income from discontinued operations	0.69	0.02	1.39	0.12
Income per common share or unit - basic	1.02	0.41	2.10	1.05
Diluted:
Income from continuing operations	0.33	0.38	0.71	0.92
Income from discontinued operations	0.68	0.02	1.38	0.12
Income per common share or unit - diluted	1.01	0.40	2.09	1.04
Distributions per common share or unit	0.40	0.40	1.20	1.20
Common shares [Member]
Basic:
Income from continuing operations	0.33	0.39	0.71	0.93
Income from discontinued operations	0.69	0.02	1.39	0.12
Income per common share or unit - basic	1.02	0.41	2.10	1.05
Diluted:
Income from continuing operations	0.33	0.38	0.71	0.92
Income from discontinued operations	0.68	0.02	1.38	0.12
Income per common share or unit - diluted	1.01	0.40	2.09	1.04
Distributions per common share or unit	$ 0.40	$ 0.40	$ 1.20	$ 1.20

Consolidated Statements of Co_2

Consolidated Statements of Comprehensive Income (Parentheticals) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Gain on property dispositions in discontinued operations	$ 94,878	$ 0	$ 184,689	$ 0
Liberty Property Limited Partnership [Member]
Gain on property dispositions in discontinued operations	$ 94,878	$ 0	$ 184,689	$ 0

Consolidated Statement of Equit

Consolidated Statement of Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	AOCI Attributable to Parent [Member]	Accumulated Distributions in Excess of Net Income [Member]	Parent [Member]	Noncontrolling interest operating partnership Common [Member]	Noncontrolling interest-consolidated joint ventures [Member]	Liberty Property Limited Partnership [Member]	Liberty Property Limited Partnership [Member]General Partner [Member]	Liberty Property Limited Partnership [Member]Limited Partner [Member]	Liberty Property Limited Partnership [Member]Noncontrolling interest-consolidated joint ventures [Member]	Common shares [Member]	Common Units [Member]Liberty Property Limited Partnership [Member]General Partner [Member]	Common Units [Member]Liberty Property Limited Partnership [Member]Limited Partner [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Other comprehensive loss - foreign currency translation activity	$ 16,314								$ 16,314
Other comprehensive income - derivative instruments activity	$ 366								$ 366
Common shares outstanding - Beginning Balance (shares) at Dec. 31, 2017	147,450,691								147,450,691				147,450,691
Beginning Balance at Dec. 31, 2017	$ 3,148,366	$ 147	$ 3,674,978	$ (37,797)	$ (549,970)	$ 3,087,358	$ 56,159	$ 4,849	$ 3,148,366	$ 3,087,358	$ 56,159	$ 4,849
General Partner's units outstanding - Beginning Balance (units) at Dec. 31, 2017														147,450,691
Limited Partners' units outstanding - Beginning Balance (units) at Dec. 31, 2017	3,520,205								3,520,205						3,520,205
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net proceeds from the issuance of common shares	$ 5,255	1	5,254			5,255
Net proceeds from issuance of common shares (shares)													361,630
Net income	318,332				309,938	309,938	7,384	1,010	$ 318,332	309,938	7,384	1,010
Contributions from partners									13,654	13,654
Contributions from partners (units)														361,630
Distributions	(182,040)				(177,325)	(177,325)	(4,283)	(432)
Distributions to partners									(182,040)	(177,325)	(4,283)	(432)
Share-based compensation	8,399		8,399			8,399
Other comprehensive loss - foreign currency translation activity	(9,221)			(9,007)		(9,007)	(214)		(9,221)	(9,007)	(214)
Other comprehensive income - derivative instruments activity	$ 493			482		482	11		$ 493	482	11
Common shares outstanding - Ending Balance (shares) at Sep. 30, 2018	147,812,321								147,812,321				147,812,321
General Partner's units outstanding - Ending Balance (units) at Sep. 30, 2018														147,812,321
Limited Partners' units outstanding - Ending Balance (units) at Sep. 30, 2018	3,520,205								3,520,205						3,520,205
Ending Balance at Sep. 30, 2018	$ 3,289,584	$ 148	$ 3,688,631	$ (46,322)	$ (417,357)	$ 3,225,100	$ 59,057	$ 5,427	$ 3,289,584	$ 3,225,100	$ 59,057	$ 5,427
Noncontrolling Interest - Operating Partnership (Mezzanine) - Beginning Balance at Dec. 31, 2017	7,537								7,537
Increase (Decrease) in Temporary Equity [Roll Forward]
Net income (Mezzanine)	354								354
Distributions (Mezzanine)	(354)								(354)
Noncontrolling Interest - Operating Partnership (Mezzanine) - Ending Balance at Sep. 30, 2018	$ 7,537								$ 7,537

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
OPERATING ACTIVITIES
Net income	$ 318,686	$ 158,221
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	132,276	139,389
Amortization of deferred financing costs	2,876	2,811
Expensed pursuit costs	1,675	4,957
Impairment charges - real estate assets	26,000	9,650
Equity in earnings of unconsolidated joint ventures	(20,958)	(14,026)
Gain on property dispositions	(239,394)	(30,542)
Share-based compensation/Non-cash compensation	12,310	12,467
Other	(3,612)	(2,547)
Changes in operating assets and liabilities:
Accounts receivable	(1,543)	1,976
Deferred rent receivable	(14,782)	(15,360)
Prepaid expenses and other assets	29,981	(28,115)
Accounts payable	(6,435)	13,249
Accrued interest	13,695	12,829
Other liabilities	(16,035)	(42)
Other liabilities - development service fee	61,705	0
Net cash provided by operating activities	296,445	264,917
INVESTING ACTIVITIES
Investment in properties - acquisitions	(293,049)	(62,327)
Investment in properties - other	(34,331)	(39,719)
Investments in and advances to unconsolidated joint ventures	(79,036)	(41,154)
Distributions from unconsolidated joint ventures	30,055	17,089
Net proceeds from disposition of properties/land	574,542	56,474
Investment in development in progress	(165,197)	(189,282)
Investment in land held for development	(171,221)	(93,813)
Payment of deferred leasing costs	(18,452)	(28,424)
Escrow deposit	(167,892)	0
Release of escrows and other	38,982	24,096
Net cash used in investing activities	(285,599)	(357,060)
FINANCING ACTIVITIES
Net proceeds from issuance of common shares	5,255	7,356
Share repurchase including shares related to tax withholdings	(4,638)	(4,879)
Repayments of mortgage loans	(31,954)	(5,974)
Proceeds from credit facility	1,187,357	486,000
Repayments on credit facility	(975,456)	(191,000)
Payments of deferred financing costs	15	22
Distribution paid on common shares	(177,210)	(187,625)
Distributions to partners/noncontrolling interests	(5,130)	(5,450)
Net cash (used in) provided by financing activities	(1,791)	98,406
Net increase in cash, cash equivalents and restricted cash	9,055	6,263
(Decrease) increase in cash, cash equivalents and restricted cash related to foreign currency translation	(3,599)	2,202
Cash, cash equivalents and restricted cash at beginning of period	25,685	56,025
Cash, cash equivalents and restricted cash at end of period	31,141	64,490
Liberty Property Limited Partnership [Member]
OPERATING ACTIVITIES
Net income	318,686	158,221
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	132,276	139,389
Amortization of deferred financing costs	2,876	2,811
Expensed pursuit costs	1,675	4,957
Impairment charges - real estate assets	26,000	9,650
Equity in earnings of unconsolidated joint ventures	(20,958)	(14,026)
Gain on property dispositions	(239,394)	(30,542)
Share-based compensation/Non-cash compensation	12,310	12,467
Other	(3,612)	(2,547)
Changes in operating assets and liabilities:
Accounts receivable	(1,543)	1,976
Deferred rent receivable	(14,782)	(15,360)
Prepaid expenses and other assets	29,981	(28,115)
Accounts payable	(6,435)	13,249
Accrued interest	13,695	12,829
Other liabilities	(16,035)	(42)
Other liabilities - development service fee	61,705	0
Net cash provided by operating activities	296,445	264,917
INVESTING ACTIVITIES
Investment in properties - acquisitions	(293,049)	(62,327)
Investment in properties - other	(34,331)	(39,719)
Investments in and advances to unconsolidated joint ventures	(79,036)	(41,154)
Distributions from unconsolidated joint ventures	30,055	17,089
Net proceeds from disposition of properties/land	574,542	56,474
Investment in development in progress	(165,197)	(189,282)
Investment in land held for development	(171,221)	(93,813)
Payment of deferred leasing costs	(18,452)	(28,424)
Escrow deposit	(167,892)	0
Release of escrows and other	38,982	24,096
Net cash used in investing activities	(285,599)	(357,060)
FINANCING ACTIVITIES
Repayments of mortgage loans	(31,954)	(5,974)
Proceeds from credit facility	1,187,357	486,000
Repayments on credit facility	(975,456)	(191,000)
Payments of deferred financing costs	15	22
Capital contributions	5,255	7,356
Distributions to partners/noncontrolling interests	(186,978)	(197,954)
Net cash (used in) provided by financing activities	(1,791)	98,406
Net increase in cash, cash equivalents and restricted cash	9,055	6,263
(Decrease) increase in cash, cash equivalents and restricted cash related to foreign currency translation	(3,599)	2,202
Cash, cash equivalents and restricted cash at beginning of period	25,685	56,025
Cash, cash equivalents and restricted cash at end of period	$ 31,141	$ 64,490

Organization and Basis of Prese

Organization and Basis of Presentation	9 Months Ended
	Sep. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block]	Organization and Basis of Presentation Organization Liberty Property Trust (the “Trust”) is a self-administered and self-managed Maryland real estate investment trust (a “REIT”). Substantially all of the Trust’s assets are owned directly or indirectly, and substantially all of the Trust’s operations are conducted directly or indirectly, by its subsidiary, Liberty Property Limited Partnership, a Pennsylvania limited partnership (the “Operating Partnership” and, together with the Trust and their consolidated subsidiaries, the “Company”). The Trust is the sole general partner and also a limited partner of the Operating Partnership, owning 97.7% of the common equity of the Operating Partnership at September 30, 2018 . The Company owns and operates industrial properties nationally and currently owns and operates office properties in a focused group of office markets. Additionally, the Company owns certain assets in the United Kingdom. Unless otherwise indicated, the notes to the Consolidated Financial Statements apply to both the Trust and the Operating Partnership. The terms the “Company,” “we,” “our” and “us” mean the Trust and Operating Partnership collectively. The Operating Partnership is a variable interest entity (“VIE”) of the Trust as the limited partners do not have substantive kick-out or participating rights. The Trust is the primary beneficiary of the Operating Partnership as it has the power to direct the activities of the Operating Partnership and the rights to absorb 97.7% of the net income of the Operating Partnership. The Trust has no significant assets or liabilities other than its investment in the Operating Partnership. As the Operating Partnership is already consolidated in the balance sheets of the Trust, the identification of this entity as a VIE has no impact on the consolidated financial statements of the Trust. In addition, the Company holds a 20% interest in Liberty/Comcast 1701 JFK Boulevard, LP which was determined to be a VIE. The Company determined that it is not the primary beneficiary as the Company and its third party partner share control of the joint venture. The Company's maximum exposure to loss is equal to its equity investment in the joint venture which was $75.9 million and $17.3 million as of September 30, 2018 and December 31, 2017, respectively. See Note 12 for further discussion of Liberty/Comcast 1701 JFK Boulevard, LP. Basis of Presentation The accompanying unaudited consolidated financial statements of the Company have been prepared in accordance with United States generally accepted accounting principles (“U.S. GAAP”) for interim financial information and with the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, they do not include all of the information and footnotes required by U.S. GAAP for complete financial statements and should be read in conjunction with the consolidated financial statements and notes thereto included in the Annual Report on Form 10-K of the Company for the year ended December 31, 2017 . In the opinion of management, all adjustments (consisting solely of normal recurring adjustments) necessary for a fair presentation of the financial statements for these interim periods have been included. The results of interim periods are not necessarily indicative of the results to be obtained for a full fiscal year. Revenue Recognition In May 2014, the Financial Accounting Standards Board (“FASB”) issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASU 2014-09”), which supersedes nearly all existing revenue recognition guidance (except revenue in the scope of other accounting standards, including standards related to leasing). Subsequently, the FASB issued the following standards related to ASU 2014-09: ASU 2016-08, Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations; ASU 2016-10, Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing ; ASU 2016-12, Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients ; ASU 2017-05 , Gains and losses from the derecognition of nonfinancial assets (Topic 610-20) , and ASU 2017-13, Revenue from Contracts with Customers (Topic 606): Amendments to SEC Paragraphs Pursuant to the Staff Announcement at the July 20, 2017 EITF Meeting and Rescission of Prior SEC Staff Announcements and Observer Comments (collectively, the “New Revenue Standards”). The New Revenue Standards provide a unified model to determine how revenue is recognized. In accordance with the New Revenue Standards, the Company performs the following steps: (i) identify the contract with the customer, (ii) identify the performance obligations within the contract, (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations and (v) recognize revenue when (or as) a performance obligation is satisfied. The Company evaluated each of its revenue streams: lease agreement revenue, development service fee revenue, deferred land sale revenue and gain or loss on sale of nonfinancial assets and adopted the New Revenue Standards effective January 1, 2018 using the modified retrospective approach. The Company concluded that there are no revenue streams from its lease agreements that are covered by the New Revenue Standards with the possible exception of non-lease components as further discussed below. The New Revenue Standards did not have an impact on the amount and timing of recognizing the Company's development service fee income. The Company recognizes development service fee income on a variable basis as a percentage of costs incurred on third party development contracts. Property development services, which are a single performance obligation, continue to be satisfied and recognized over time. The Company measures its progress toward completing the performance obligation under each arrangement. The measurement of the transfer of value to the customer for these services utilizes the input method (actual costs incurred against anticipated project costs) since this method best depicts the actual transfer of value promised to the customer. Estimated expected losses on such contracts are accrued in the period in which they are determinable. The total amount of consideration to be received from these projects is assessed on a quarterly basis. Based on existing contracts, completion is anticipated by mid-2019. The Company recognizes revenues from improving land sites and selling the underlying land on behalf of its development partner to home builders in the United Kingdom. These agreements contain a pre-emption clause and a seller's call option. The Company recognizes revenue as the pre-emption period or seller's call option lapses utilizing the output method. There was $884,000 and $1.5 million in revenue recognized for such contracts during the three and nine months ended September 30, 2018, respectively. The New Revenue Standards did not have an impact on the gain or loss on sale of nonfinancial assets. The New Revenue Standards require the Company to derecognize nonfinancial assets once it transfers control of a distinct nonfinancial asset or distinct in-substance nonfinancial asset. Additionally, when the Company transfers its controlling interest in a nonfinancial asset, but retains a noncontrolling ownership interest, the Company is required to measure any noncontrolling interest it receives or retains at fair value. The guidance requires companies to recognize a full gain or loss on the transaction. See Notes 5 and 7 for further discussion of sales of nonfinancial assets during the three and nine months ended September 30, 2018. Estimated gross revenue related to the remaining performance obligations under existing contracts (before allocation of related costs and expenses) as of September 30, 2018 that will be recognized as revenue in future periods was approximately $54.2 million , which is expected to be recognized in 2018 through 2020 . The Company adopted the practical expedient to assess the recognition of revenue for open contracts during the transition period. There was no adjustment to the opening balance of retained earnings recorded at January 1, 2018. Development Fee Contracts From time to time, the Company enters into contracts to develop properties on a fee basis for joint ventures in which the Company holds an interest or for unrelated third parties. In these cases the Company typically agrees to be responsible for all aspects of the development of the project (and, in certain instances, related infrastructure) and to guarantee the timely lien-free completion of construction of the project and the payment, subject to certain exceptions, of cost overruns incurred in the development of the project. If the Company encounters construction delays or unexpected costs in the development of these projects or is otherwise unable to recover the costs it incurs, the resulting unrecovered costs and potential payments to customers could generate losses that would adversely affect the Company's cash flow and net income. On a quarterly basis, the Company applies reasonable estimates and judgments to assess whether or not it is necessary to accrue any estimated future losses with respect to such contracts. The Company recognized an aggregate net profit of $32,000 and a net loss $61.7 million on these contracts during the three and nine months ended September 30, 2018, respectively, and an aggregate net profit of $511,000 and $1.4 million during the three and nine months ended September 30, 2017, respectively. The total liability on the Company’s balance sheet as of September 30, 2018 related to the above-described development cost guarantees under these contracts is $67.3 million . Should external or internal circumstances change requiring the Company to adjust the estimated future cash flows from these development contracts or in a manner that indicates that such development contracts may result in additional losses, the Company could be required to record such additional losses in future periods. See Note 12 where certain fee development matters relating specifically to the Comcast Technology Center are discussed in further detail. Systems Implementation Expense The Company is incurring costs associated with its efforts to implement new financial and operating systems in certain cloud computing arrangements. The Company evaluated the arrangements in accordance with ASU 2015-05, “ Customer’s Accounting for Fees Paid in a Cloud Computing Arrangement” and concluded that such arrangements are service contracts under the standard. Accordingly, the Company expenses substantially all costs as incurred. Certain costs that relate to the software service agreements received over time are set up as prepaid and expensed over the applicable service period. These costs include license costs incurred during the implementation period as well as consulting, personnel and other direct costs related to the implementation. The Company incurred $1.2 million and $3.4 million of systems implementation expense for the three and nine months ended September 30, 2018, respectively. The Company incurred $142,000 and $157,000 of systems implementation expense for the three and nine months ended September 30, 2017, respectively. The FASB recently issued accounting guidance related to service contracts in a cloud computing arrangement. See ASU 2018-15 in Recently Issued Accounting Standards below. Expensed Pursuit Costs The Company capitalizes pre-development and pre-acquisition costs incurred in pursuit of new development, land or operating property opportunities for which the Company currently believes future development or asset acquisition is probable. Future development and the consummation of acquisitions is dependent upon various factors, including, as appropriate, due diligence, zoning and regulatory approval, rental market conditions and construction costs. Initial pre-development and pre-acquisition costs incurred on future development, land or operating property acquisitions that is not considered probable are expensed as incurred. In addition, if the status of a future development, land or operating property acquisitions by the Company is no longer probable, any capitalized pre-development or pre-acquisition costs are written off. The Company expensed costs related to pursuit costs of $1.3 million and $1.7 million for the three and nine months ended September 30, 2018, respectively, and $4.8 million and $5.0 million for the same periods in 2017, respectively. In the third quarter of 2017, the Company began to separately classify expensed pursuit costs in the Consolidated Statements of Comprehensive Income. These costs, which were reclassified retrospectively for all periods, were formerly classified as general and administrative expense. Recently Issued Accounting Standards In February 2016, the FASB issued ASU 2016-02, Leases (“ASU 2016-02”). Subsequently the FASB issued the following standards related to ASU 2016-02: ASU 2018-10, Codification Improvements to Topic 842, Leases and ASU 2018-11, Leases (Topic 842): Targeted Improvements (collectively, the "New Lease Standard"). The New Lease Standard amends the existing accounting standards for lease accounting, including requiring lessees to recognize most leases on their balance sheets and making targeted changes to lessor accounting. The New Lease Standard is effective for the Company beginning January 1, 2019. The standard requires a modified retrospective transition approach for all leases existing at, or entered into after, the date of initial application, with an option to use certain transition relief. Revenue related to the lease component of the contract will be recognized on a straight-line basis, while revenue related to the non-lease component will be recognized under the provisions of the New Revenue Standards. However, the New Lease Standard gives lessors a practical expedient to not separate non-lease components from the associated lease components if certain criteria are met. For lease agreements longer than one year in which the Company is the lessee, the Company will measure the present value of the future lease payments and recognize a right-of-use asset and corresponding lease liability on its balance sheet. In addition, the new standard states that only direct leasing costs may be capitalized. The Company has assembled a project team and developed a project plan. The project team is working to analyze and evaluate the impact of the guidance on its consolidated financial statements. The Company expects to elect the package of practical expedients, the land easement practical expedient and the practical expedient to not separate lease and non-lease components. The Company expects to adopt the new lease standard on January 1, 2019. In August 2016, the FASB issued ASU 2016-15, Classification of Certain Cash Receipts and Cash Payments (“ASU 2016-15”). ASU 2016-15 is designed to clarify how entities should classify cash receipts and cash payments in the statement of cash flows. ASU 2016-15 became effective for the Company beginning January 1, 2018. The standard requires retrospective application. The adoption of the ASU 2016-15 did not have a material impact on the Company's consolidated financial statements. In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows (Topic 230) - Restricted Cash (“ASU 2016-18”) , which requires that restricted cash and cash equivalents be included with cash and cash equivalents when reconciling beginning-of-period and end-of-period total amounts shown on the consolidated statements of cash flows. The new standard became effective for public entities for fiscal years beginning after December 15, 2017 and for interim periods therein. The Company adopted ASU 2016-18 as of December 31, 2017. The impact of the implementation of ASU 2016-18 was as follows (in thousands): Nine months ended September 30, 2017 Net cash provided by operating activities (prior to adoption of ASU 2016-18) $ 261,587 Impact of including restricted cash with cash and cash equivalents 3,330 Net cash provided by operating activities (after adoption of ASU 2016-18) $ 264,917 In February 2017, the FASB issued ASU 2017-05, Clarifying the Scope of Asset Derecognition Guidance and Accounting for Partial Sales of Nonfinancial Assets (“ASU 2017-05”). ASU 2017-05 is designed to provide guidance on how to recognize gain and losses on sales, including partial sale, of nonfinancial assets to noncustomers. The Company adopted ASU 2017-05 effective January 1, 2018 on a modified retrospective method and the adoption did not have an effect on the Company’s consolidated financial statements. In May 2017, the FASB issued ASU 2017-09, Scope of Modification Accounting (“ASU 2017-09”). ASU 2017-09 clarifies when changes to the terms or conditions of a share-based payment award must be accounted for as modifications. ASU 2017-09 became effective for the Company beginning January 1, 2018. The new guidance will be applied prospectively to awards modified on or after the adoption date. The adoption of the ASU 2017-09 did not have a material impact on the Company's consolidated financial statements. In August 2017, the FASB issued ASU 2017-12, Derivatives and Hedging: Targeted Improvements to Accounting for Hedging Activities (“ASU 2017-12”). ASU 2017-12 is designed to better align a company’s financial reporting for hedging activities with the economic objectives of those activities. ASU 2017-12 is effective for the Company beginning January 1, 2019. Early adoption is permitted using a modified retrospective transition method. This adoption method will require the Company to recognize the cumulative effect of initially applying ASU 2017-12 as an adjustment to accumulated other comprehensive income with a corresponding adjustment to the opening balance of retained earnings as of the beginning of the fiscal year that an entity adopts the update. The Company is evaluating the impact ASU 2017-12 will have on the Company's financial position and results of operations. In August 2018, the FASB issued ASU 2018-15, Intangibles - Goodwill and Other - Internal-Use Software: Customer's Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That is a Service Contract ("ASU 2018-15"). ASU 2018-15 amends the definition of a hosting arrangement and requires a customer in a hosting arrangement that is a service contract to capitalize certain implementation costs as if the arrangement was an internal-use software project. ASU 2018-15 is effective for the Company beginning January 1, 2020. Early adoption is permitted. The Company is evaluating the impact ASU 2018-15 will have on the Company's financial position and results of operations.

Income per Common Share of the

Income per Common Share of the Trust	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Earnings Per Share [Text Block]	Income per Common Share of the Trust The following table sets forth the computation of basic and diluted income per common share of the Trust (in thousands except per share amounts): For the Three Months Ended For the Three Months Ended September 30, 2018 September 30, 2017 Income (Numerator) Weighted Average Shares (Denominator) Per Share Income (Numerator) Weighted Average Shares (Denominator) Per Share Income from continuing operations net of noncontrolling interest - basic $ 48,595 147,324 $ 0.33 $ 56,487 146,811 $ 0.39 Dilutive shares for long-term compensation plans — 947 — 785 Income from continuing operations net of noncontrolling interest - diluted $ 48,595 148,271 $ 0.33 $ 56,487 147,596 $ 0.38 Discontinued operations net of noncontrolling interest - basic $ 101,548 147,324 $ 0.69 $ 3,052 146,811 $ 0.02 Dilutive shares for long-term compensation plans — 947 — 785 Discontinued operations net of noncontrolling interest - diluted $ 101,548 148,271 $ 0.68 $ 3,052 147,596 $ 0.02 Net income available to common shareholders - basic $ 150,143 147,324 $ 1.02 $ 59,539 146,811 $ 0.41 Dilutive shares for long-term compensation plans — 947 — 785 Net income available to common shareholders - diluted $ 150,143 148,271 $ 1.01 $ 59,539 147,596 $ 0.40 For the Nine Months Ended For the Nine Months Ended September 30, 2018 September 30, 2017 Income Weighted Per Share Income Weighted Per Share Income from continuing operations net of noncontrolling interest - basic $ 104,885 147,241 $ 0.71 $ 136,460 146,678 $ 0.93 Dilutive shares for long-term compensation plans 919 752 Income from continuing operations net of noncontrolling interest - diluted $ 104,885 148,160 $ 0.71 $ 136,460 147,430 $ 0.92 Discontinued operations net of noncontrolling interest - basic $ 205,053 147,241 $ 1.39 $ 17,522 146,678 $ 0.12 Dilutive shares for long-term compensation plans 919 752 Discontinued operations net of noncontrolling interest - diluted $ 205,053 148,160 $ 1.38 $ 17,522 147,430 $ 0.12 Net income available to common shareholders - basic $ 309,938 147,241 $ 2.10 $ 153,982 146,678 $ 1.05 Dilutive shares for long-term compensation plans 919 752 Net income available to common shareholders - diluted $ 309,938 148,160 $ 2.09 $ 153,982 147,430 $ 1.04 Dilutive shares for long-term compensation plans represent the unvested common shares outstanding during the periods as well as the dilutive effect of outstanding options. There were no anti-dilutive options excluded from the computation of diluted income per common share for the three and nine months ended September 30, 2018 and the same periods in 2017 . During the three and nine months ended September 30, 2018 , 5,000 and 106,000 common shares, respectively, were issued upon the exercise of options. During the year ended December 31, 2017 , 193,000 common shares were issued upon the exercise of options. Share Repurchase The Company’s Board of Trustees has authorized a share repurchase plan under which the Company may purchase up to $250 million of the Company’s outstanding common shares through September 28, 2019. Purchases made pursuant to the program may be made in either the open market or in privately negotiated transactions from time to time as permitted by securities laws and other legal requirements. There were no purchases under the plan during the three and nine months ended September 30, 2018 .

Income per Common Unit of the O

Income per Common Unit of the Operating Partnership	9 Months Ended
	Sep. 30, 2018
Earnings Per Unit, Basic and Diluted [Line Items]
Earnings Per Share [Text Block]	Income per Common Share of the Trust The following table sets forth the computation of basic and diluted income per common share of the Trust (in thousands except per share amounts): For the Three Months Ended For the Three Months Ended September 30, 2018 September 30, 2017 Income (Numerator) Weighted Average Shares (Denominator) Per Share Income (Numerator) Weighted Average Shares (Denominator) Per Share Income from continuing operations net of noncontrolling interest - basic $ 48,595 147,324 $ 0.33 $ 56,487 146,811 $ 0.39 Dilutive shares for long-term compensation plans — 947 — 785 Income from continuing operations net of noncontrolling interest - diluted $ 48,595 148,271 $ 0.33 $ 56,487 147,596 $ 0.38 Discontinued operations net of noncontrolling interest - basic $ 101,548 147,324 $ 0.69 $ 3,052 146,811 $ 0.02 Dilutive shares for long-term compensation plans — 947 — 785 Discontinued operations net of noncontrolling interest - diluted $ 101,548 148,271 $ 0.68 $ 3,052 147,596 $ 0.02 Net income available to common shareholders - basic $ 150,143 147,324 $ 1.02 $ 59,539 146,811 $ 0.41 Dilutive shares for long-term compensation plans — 947 — 785 Net income available to common shareholders - diluted $ 150,143 148,271 $ 1.01 $ 59,539 147,596 $ 0.40 For the Nine Months Ended For the Nine Months Ended September 30, 2018 September 30, 2017 Income Weighted Per Share Income Weighted Per Share Income from continuing operations net of noncontrolling interest - basic $ 104,885 147,241 $ 0.71 $ 136,460 146,678 $ 0.93 Dilutive shares for long-term compensation plans 919 752 Income from continuing operations net of noncontrolling interest - diluted $ 104,885 148,160 $ 0.71 $ 136,460 147,430 $ 0.92 Discontinued operations net of noncontrolling interest - basic $ 205,053 147,241 $ 1.39 $ 17,522 146,678 $ 0.12 Dilutive shares for long-term compensation plans 919 752 Discontinued operations net of noncontrolling interest - diluted $ 205,053 148,160 $ 1.38 $ 17,522 147,430 $ 0.12 Net income available to common shareholders - basic $ 309,938 147,241 $ 2.10 $ 153,982 146,678 $ 1.05 Dilutive shares for long-term compensation plans 919 752 Net income available to common shareholders - diluted $ 309,938 148,160 $ 2.09 $ 153,982 147,430 $ 1.04 Dilutive shares for long-term compensation plans represent the unvested common shares outstanding during the periods as well as the dilutive effect of outstanding options. There were no anti-dilutive options excluded from the computation of diluted income per common share for the three and nine months ended September 30, 2018 and the same periods in 2017 . During the three and nine months ended September 30, 2018 , 5,000 and 106,000 common shares, respectively, were issued upon the exercise of options. During the year ended December 31, 2017 , 193,000 common shares were issued upon the exercise of options. Share Repurchase The Company’s Board of Trustees has authorized a share repurchase plan under which the Company may purchase up to $250 million of the Company’s outstanding common shares through September 28, 2019. Purchases made pursuant to the program may be made in either the open market or in privately negotiated transactions from time to time as permitted by securities laws and other legal requirements. There were no purchases under the plan during the three and nine months ended September 30, 2018 .
Liberty Property Limited Partnership [Member]
Earnings Per Unit, Basic and Diluted [Line Items]
Earnings Per Share [Text Block]	Income per Common Unit of the Operating Partnership The following table sets forth the computation of basic and diluted income per common unit of the Operating Partnership (in thousands, except per unit amounts): For the Three Months Ended For the Three Months Ended September 30, 2018 September 30, 2017 Income (Numerator) Weighted Average Units (Denominator) Per Unit Income (Numerator) Weighted Average Units (Denominator) Per Unit Income from continuing operations - net of noncontrolling interest - consolidated joint ventures $ 49,868 $ 57,959 Less: Preferred unit distributions (118 ) (118 ) Income from continuing operations available to common unitholders - basic $ 49,750 150,844 $ 0.33 $ 57,841 150,339 $ 0.39 Dilutive units for long-term compensation plans — 947 — 785 Income from continuing operations available to common unitholders - diluted $ 49,750 151,791 $ 0.33 $ 57,841 151,124 $ 0.38 Income from discontinued operations - basic $ 103,971 150,844 $ 0.69 $ 3,125 150,339 $ 0.02 Dilutive units for long-term compensation plans — 947 — 785 Income from discontinued operations - diluted $ 103,971 151,791 $ 0.68 $ 3,125 151,124 $ 0.02 Income available to common unitholders - basic $ 153,721 150,844 $ 1.02 $ 60,966 150,339 $ 0.41 Dilutive units for long-term compensation plans — 947 — 785 Income available to common unitholders - diluted $ 153,721 151,791 $ 1.01 $ 60,966 151,124 $ 0.40 For the Nine Months Ended For the Nine Months Ended September 30, 2018 September 30, 2017 Income (Numerator) Weighted Average Units (Denominator) Per Unit Income (Numerator) Weighted Average Units (Denominator) Per Unit Net income - net of noncontrolling interest - consolidated joint ventures $ 107,731 $ 140,084 Less: Preferred unit distributions (354 ) (354 ) Income from continuing operations available to common unitholders - basic 107,377 150,761 $ 0.71 139,730 150,206 $ 0.93 Dilutive units for long-term compensation plans — 919 — 752 Income from continuing operations available to common unitholders - diluted $ 107,377 151,680 $ 0.71 $ 139,730 150,958 $ 0.92 Income from discontinued operations - basic $ 209,945 150,761 $ 1.39 $ 17,942 150,206 $ 0.12 Dilutive units for long-term compensation plans — 919 — 752 Income from discontinued operations - diluted $ 209,945 151,680 $ 1.38 $ 17,942 150,958 $ 0.12 Net income available to common unitholders - basic $ 317,322 150,761 $ 2.10 $ 157,672 150,206 $ 1.05 Dilutive units for long-term compensation plans — 919 — 752 Net income available to common unitholders - diluted $ 317,322 151,680 $ 2.09 $ 157,672 150,958 $ 1.04 Dilutive units for long-term compensation plans represent the unvested common units outstanding during the periods as well as the dilutive effect of outstanding options. There were no anti-dilutive options excluded from the computation of diluted income per common unit for the three and nine months ended September 30, 2018 and the same periods in 2017 . During the three and nine months ended September 30, 2018 , 5,000 and 106,000 common units, respectively, were issued upon exercise of options. During the year ended December 31, 2017 , 193,000 common units were issued upon the exercise of options. Unit Repurchase The Company’s Board of Trustees has authorized a share repurchase plan under which the Company may purchase up to $250 million of the Company’s outstanding common units through September 28, 2019. Purchases made pursuant to the program may be made in either the open market or in privately negotiated transactions from time to time as permitted by securities laws and other legal requirements. There were no purchases under the plan during the three and nine months ended September 30, 2018 .

Accumulated Other Comprehensive

Accumulated Other Comprehensive Income	9 Months Ended
	Sep. 30, 2018
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Accumulated Other Comprehensive Income (Loss) Note [Text Block]	Accumulated Other Comprehensive Loss The following table sets forth the components of Accumulated Other Comprehensive Loss (in thousands): As of and for the nine months ended September 30, 2018 2017 Foreign Currency Translation: Beginning balance $ (38,701 ) $ (56,767 ) Translation adjustment (9,221 ) 16,314 Ending balance (47,922 ) (40,453 ) Derivative Instruments: Beginning balance 150 (455 ) Unrealized gain 620 (25 ) Reclassification adjustment (1) (127 ) 391 Ending balance 643 (89 ) Total accumulated other comprehensive loss (47,279 ) (40,542 ) Less: portion included in noncontrolling interest – operating partnership 957 801 Total accumulated other comprehensive loss included in shareholders' equity/owners' equity $ (46,322 ) $ (39,741 ) (1) Amounts reclassified out of Accumulated Other Comprehensive Loss/General & Limited Partner's Equity into contractual interest expense.

Real Estate

Real Estate	9 Months Ended
	Sep. 30, 2018
Property, Plant and Equipment [Abstract]
Disposal Groups, Including Discontinued Operations, Disclosure [Text Block]	Real Estate Information on the Operating Properties and land parcels the Company acquired during the three and nine months ended September 30, 2018 is as follows: Three Months Ended September 30, 2018 Nine Months Ended September 30, 2018 Number of Buildings Acres of Developable Land Leaseable Square Feet Purchase Price (in thousands) Number of Buildings Acres of Developable Land Leaseable Square Feet Purchase Price (in thousands) Florida — — — $ — — 28.4 — $ 14,728 Lehigh/Central PA — 16.4 — 3,000 — 170.2 — 34,043 United Kingdom — 1.6 — 709 — 8.7 — 4,949 Other: Dallas 1 — 900,043 52,491 1 — 900,043 52,491 New Jersey 2 7.5 412,553 61,375 3 7.5 584,569 84,875 Southern California — 12.9 — 33,125 3 36.0 1,002,471 231,061 3 38.4 1,312,596 $ 150,700 7 250.8 2,487,083 $ 422,147 Information on the Operating Properties and land parcels the Company sold during the three and nine months ended September 30, 2018 is as follows: Three Months Ended September 30, 2018 Nine Months Ended September 30, 2018 Number of Buildings Acres of Developable Land Leaseable Square Feet Gross Proceeds (in thousands) Number of Buildings Acres of Developable Land Leaseable Square Feet Gross Proceeds (in thousands) Carolinas/Richmond — — — $ — 1 1.5 80,000 $ 7,094 Chicago/Minneapolis — — — — — 8.3 — 2,714 Lehigh/Central PA — 65.8 — 9,700 — 65.8 — 9,700 Philadelphia — — — — 1 3.1 207,779 130,850 Southeastern PA — — — — 23 — 1,420,515 183,808 Other Arizona 5 — 805,746 $ 255,000 5 — 805,746 $ 255,000 5 65.8 805,746 $ 264,700 30 78.7 2,514,040 $ 589,166

Segment Information

Segment Information	9 Months Ended
	Sep. 30, 2018
Segment Reporting [Abstract]
Segment Information	Segment Information The Company owns and operates industrial properties nationally and owns and operates office properties in a focused group of office markets. Additionally, the Company owns certain assets in the United Kingdom. At September 30, 2018, the Company's reportable segments were based on the Company's method of internal reporting and were as follows: • Carolinas/Richmond; • Chicago/Minneapolis; • Florida; • Houston; • Lehigh/Central PA; • Philadelphia; • Southeastern PA; and • United Kingdom. Certain other segments are aggregated into an "Other" category which includes the reportable segments: Arizona; Atlanta; Cincinnati/Columbus/Indianapolis; Dallas; DC Metro; New Jersey; and Southern California. The Company evaluates the performance of its reportable segments based on segment net operating income (“SNOI”). SNOI is defined as net operating income (rental revenue and operating expense reimbursements less rental property and real estate tax expenses) less amortization of lease transaction costs and other operating expenses which relate directly to the management and operation of the assets within each reportable segment. The Company's accounting policies for the segments are the same as those used in the Company's consolidated financial statements. There are no material inter-segment transactions. The operating information by reportable segment is as follows (in thousands): Three Months Ended Nine Months Ended September 30, 2018 September 30, 2017 September 30, 2018 September 30, 2017 Operating revenue Carolinas/Richmond $ 19,417 $ 18,910 $ 58,723 $ 55,192 Chicago/Minneapolis 17,274 16,147 50,414 47,789 Florida 15,847 15,072 47,117 43,802 Houston 16,561 15,167 48,514 44,720 Lehigh/Central PA 39,188 40,698 115,799 122,048 Philadelphia 10,598 11,137 33,659 33,501 Southeastern PA 6,697 15,422 23,573 45,165 United Kingdom 3,523 3,773 11,579 10,204 Other 43,118 33,288 124,419 95,826 Segment-level operating revenue 172,223 169,614 513,797 498,247 Reconciliation to total operating revenues Development service fee income 12,956 24,176 59,132 53,920 Discontinued operations (14,013 ) (22,066 ) (45,511 ) (64,184 ) Other 18 (137 ) (372 ) (18 ) Total operating revenue $ 171,184 $ 171,587 $ 527,046 $ 487,965 SNOI Carolinas/Richmond $ 14,254 $ 13,621 $ 42,572 $ 39,698 Chicago/Minneapolis 10,292 9,554 30,417 28,913 Florida 11,148 10,176 32,734 29,686 Houston 9,514 8,701 28,225 23,263 Lehigh/Central PA 29,182 29,995 85,297 88,656 Philadelphia 8,678 8,743 27,398 25,987 Southeastern PA 5,316 9,246 16,721 25,866 United Kingdom 1,873 1,781 5,879 5,288 Other 30,000 21,901 85,082 64,089 SNOI 120,257 113,718 354,325 331,446 Reconciliation to income from continuing operations Interest expense (1) (24,670 ) (23,060 ) (71,331 ) (67,345 ) Development service fee income 12,956 24,176 59,132 53,920 Development service fee expense (12,924 ) (23,665 ) (120,799 ) (52,497 ) Depreciation/amortization expense (1) (2) (32,243 ) (33,521 ) (97,582 ) (99,829 ) Impairment charges - real estate assets (1) — (9,650 ) (26,000 ) (9,650 ) Gain on property dispositions 2,002 23,840 54,705 30,542 Equity in earnings of unconsolidated joint ventures 6,766 4,305 20,958 14,026 General and administrative expense (1) (2) (9,615 ) (7,962 ) (30,954 ) (31,073 ) Expensed pursuit costs (1,292 ) (4,772 ) (1,675 ) (4,957 ) Systems implementation expense (1,177 ) (142 ) (3,352 ) (157 ) Discontinued operations excluding (loss) gain on property dispositions (9,093 ) (3,125 ) (25,256 ) (17,942 ) Income taxes (2) (70 ) (64 ) (89 ) (240 ) Other (797 ) (2,044 ) (3,341 ) (5,965 ) Income from continuing operations $ 50,100 $ 58,034 $ 108,741 $ 140,279 (1) Includes activity in discontinued operations. (2) Excludes costs that are included in determining SNOI. The Company's total assets by reportable segment as of September 30, 2018 and December 31, 2017 is as follows (in thousands): September 30, 2018 December 31, 2017 Carolinas/Richmond $ 524,967 $ 543,922 Chicago/Minnesota 600,838 615,186 Florida 546,526 533,861 Houston 537,500 498,584 Lehigh/Central PA 1,220,160 1,210,746 Philadelphia 640,790 665,843 Southeastern PA 145,778 241,128 United Kingdom 400,321 251,824 Other 2,063,248 1,807,653 Segment-level total assets 6,680,128 6,368,747 Corporate Other 138,947 71,010 Total assets $ 6,819,075 $ 6,439,757

Impairment and Disposal of Long

Impairment and Disposal of Long-Lived Assets	9 Months Ended
	Sep. 30, 2018
Property, Plant and Equipment [Abstract]
Disposal Groups, Including Discontinued Operations, Disclosure [Text Block]	7 : Impairment or Disposal of Long-Lived Assets In 2017, the Company initiated a strategic shift whereby it plans to divest of its remaining suburban office properties. In the third quarter of 2018, the Company updated its strategy whereby it plans to divest of all of its remaining office properties. The Company determined that the strategic shift would have a major effect on its operations and financial results. As such, properties sold or those that meet the criteria to be classified as held for sale within the new corporate strategy were classified within discontinued operations. Consistent with the held for sale criteria these properties are expected to be sold within one year . As the result of the classification within discontinued operations, the in-service assets and liabilities of this portfolio are required to be presented as held for sale for all prior periods presented in our Consolidated Balance Sheets. Operating results pertaining to these properties were reclassified to discontinued operations for all prior periods presented in our Consolidated Statements of Comprehensive Income. The following table illustrates the number of sold or held for sale properties included in, or excluded from, discontinued operations: Held for Sale as of September 30, 2018 Sold during the nine months ended September 30, 2018 Sold during the year ended December 31, 2017 Total Properties included in discontinued operations 13 28 2 43 Properties included in continuing operations 1 2 8 11 Properties sold or classified as held for sale 14 30 10 54 The properties held for sale with operating results in discontinued operations as of September 30, 2018 were located in the following reportable segments: Eight properties in Southeastern PA, three properties in Florida and two properties in DC Metro. A summary of the results of operations for the properties classified as discontinued operations is as follows (in thousands): Three Months Ended Nine Months Ended September 30, 2018 September 30, 2017 September 30, 2018 September 30, 2017 Revenues $ 14,013 $ 22,066 $ 45,511 $ 64,184 Operating expenses (3,239 ) (6,834 ) (11,099 ) (21,783 ) Depreciation and amortization (295 ) (4,828 ) (4,444 ) (13,907 ) Impairment expense — (5,714 ) — (5,714 ) Interest and other income 10 47 37 98 Interest expense (1,396 ) (1,612 ) (4,749 ) (4,936 ) Income from discontinued operations before gain on property dispositions 9,093 3,125 25,256 17,942 Gain on property dispositions 94,878 — 184,689 — Income from discontinued operations 103,971 3,125 209,945 17,942 Noncontrolling interest - operating partnership (2,423 ) (73 ) (4,892 ) (420 ) Income from discontinued operations available to common shareholders $ 101,548 $ 3,052 $ 205,053 $ 17,522 Interest expense has been allocated to discontinued operations. The allocation of interest expense to discontinued operations was based on the ratio of net assets sold and held for sale included in discontinued operations to the sum of total net assets plus consolidated debt. Capital expenditures on a cash basis related to properties within discontinued operations for the three and nine months ended September 30, 2018 were $759,000 and $5.4 million , respectively, and $14.0 million and $46.4 million , respectively, for the same periods in 2017. Assets Held for Sale As of September 30, 2018 , 14 properties were classified as held for sale, of which 13 properties met the criteria to be classified within discontinued operations and one property was classified within continuing operations. The following table illustrates aggregate balance sheet information for all held for sale properties (in thousands): September 30, 2018 December 31, 2017 Included in Continuing Operations Included in Discontinued Operations Total Included in Continuing Operations Included in Discontinued Operations Total Land and land improvements $ 1,301 $ 111,517 $ 112,818 $ 3,476 $ 152,708 $ 156,184 Buildings and improvements 5,638 222,983 228,621 80,738 471,628 552,366 Development in progress — — — — 45,035 45,035 Land held for development 10,227 — 10,227 863 — 863 Accumulated depreciation (1,546 ) (67,821 ) (69,367 ) (11,785 ) (172,604 ) (184,389 ) Deferred financing and leasing costs, net 58 4,433 4,491 2,210 14,914 17,124 Other assets 165 7,236 7,401 5,137 18,291 23,428 Total assets held for sale $ 15,843 $ 278,348 $ 294,191 $ 80,639 $ 529,972 $ 610,611 Total liabilities held for sale $ 191 $ 3,795 $ 3,986 $ 1,153 $ 13,129 $ 14,282 Impairment Charges - Real Estate Assets The Company recorded $26.0 million of impairment charges during the nine months ended September 30, 2018 . There were no such charges during the three months ended September 30, 2018. This charge was related to the Camden Waterfront project located in Camden, New Jersey and reported in the Company's Philadelphia reportable segment. We continually evaluate under generally accepted accounting principles the recoverability of the carrying value of our total investment in the Camden Waterfront project. Factors considered in evaluating the carrying value of this project include probability weighted projections of future cash flows, which are influenced by management’s judgments regarding the site configuration, absorption rates and timing of future fee development projects, the amount, timing and sunset provisions of government incentives aimed at inducing office users to relocate to Camden, and general market conditions affecting demand for office space in Camden. As a result of changes to management’s estimates of probability weighted future cash flow impacted by the above-described factors during the second quarter of 2018, the Company concluded that an indicator of impairment exists and the Company may not recover the carrying value of its investment in the Camden Waterfront project. As such, the Company recorded the impairment charge in the second quarter of 2018. The impairment charge is equal to the amount by which we estimate the carrying value of our total investment in the Camden Waterfront project exceeds the current estimated fair value of our investment. The Company recorded $9.7 million in impairment charges for the three and nine months ended September 30, 2017. These charges were related to an operating property and land located in the Company's Houston reporting segment. The Company determined these impairments based on third party offer prices and quoted offer prices for comparable transactions which are Level 2 and Level 3 inputs, respectively, according to the fair value hierarchy established in ASC 820. The Company has applied reasonable estimates and judgments in evaluating each of its properties and land held for development and has determined that there are no additional valuation adjustments necessary at September 30, 2018 . Should external or internal circumstances change requiring the need to shorten the holding periods or adjust the estimated future cash flows of the Company’s assets, the Company could be required to record additional impairment charges in the future.

Noncontrolling Interests of the

Noncontrolling Interests of the Trust	9 Months Ended
	Sep. 30, 2018
Stockholders' Equity Attributable to Noncontrolling Interest [Abstract]
Noncontrolling Interests	Noncontrolling Interests of the Trust Noncontrolling interests in the accompanying financial statements represent the interests of the common and preferred units in the Operating Partnership not held by the Trust. In addition, noncontrolling interests include third-party ownership interests in consolidated joint venture investments. Common units The common units of the Operating Partnership not held by the Trust outstanding as of September 30, 2018 have the same economic characteristics as common shares of the Trust. The 3.5 million outstanding common units of the Operating Partnership not held by the Trust share proportionately in the net income or loss and in any distributions of the Operating Partnership. The common units of the Operating Partnership not held by the Trust are redeemable at any time at the option of the holder. The Trust, as the sole general partner of the Operating Partnership, may at its option elect to settle the redemption in cash or through the exchange on a one-for-one basis with unregistered common shares of the Trust. The market value of the 3.5 million outstanding common units based on the closing price of the common shares of the Trust at September 30, 2018 was $ 148.7 million .

Limited Partners' Equity of the

Limited Partners' Equity of the Operating Partnership	9 Months Ended
	Sep. 30, 2018
Preferred Units [Line Items]
Noncontrolling Interest Disclosure [Text Block]	Noncontrolling Interests of the Trust Noncontrolling interests in the accompanying financial statements represent the interests of the common and preferred units in the Operating Partnership not held by the Trust. In addition, noncontrolling interests include third-party ownership interests in consolidated joint venture investments. Common units The common units of the Operating Partnership not held by the Trust outstanding as of September 30, 2018 have the same economic characteristics as common shares of the Trust. The 3.5 million outstanding common units of the Operating Partnership not held by the Trust share proportionately in the net income or loss and in any distributions of the Operating Partnership. The common units of the Operating Partnership not held by the Trust are redeemable at any time at the option of the holder. The Trust, as the sole general partner of the Operating Partnership, may at its option elect to settle the redemption in cash or through the exchange on a one-for-one basis with unregistered common shares of the Trust. The market value of the 3.5 million outstanding common units based on the closing price of the common shares of the Trust at September 30, 2018 was $ 148.7 million .
Liberty Property Limited Partnership [Member]
Preferred Units [Line Items]
Noncontrolling Interest Disclosure [Text Block]	Limited Partners' Equity and Noncontrolling Interest of the Operating Partnership Limited partners' equity in the accompanying financial statements represents the interests of the common and preferred units in the Operating Partnership not held by the Trust. The Operating Partnership's noncontrolling interest includes third-party ownership interests in consolidated joint venture investments. Common units The common units outstanding have the same economic characteristics as common shares of the Trust. The 3.5 million outstanding common units as of September 30, 2018 not held by the Trust are the limited partners' equity - common units held by persons and entities other than the Trust. The common units of the Operating Partnership not held by the Trust are redeemable at any time at the option of the holder. The Trust, as the sole general partner of the Operating Partnership, may at its option elect to settle the redemption in cash or through the exchange on a one-for-one basis with unregistered common shares of the Trust. The market value of the 3.5 million outstanding common units based on the closing price of the common shares of the Trust at September 30, 2018 was $ 148.7 million .

Noncontrolling Interest - Opera

Noncontrolling Interest - Operating Partnership/Limited Partners' Equity - Preferred Units	9 Months Ended
	Sep. 30, 2018
Temporary Equity [Line Items]
Stockholders' Equity Note Disclosure [Text Block]	Noncontrolling Interest - Operating Partnership/Limited Partners' Equity - Preferred Units As of September 30, 2018 , the Company had outstanding the following cumulative preferred units of the Operating Partnership: ISSUE AMOUNT UNITS LIQUIDATION PREFERENCE DIVIDEND RATE (in 000’s) Series I-2 $ 7,537 301 $25 6.25 % The preferred units are putable at the holder's option at any time and are callable at the Operating Partnership's option after a stated period of time for cash.

Disclosure of Fair Value of Fin

Disclosure of Fair Value of Financial Instruments	9 Months Ended
	Sep. 30, 2018
Debt Instrument, Fair Value Disclosure [Abstract]
Disclosure of Fair Value of Financial Instruments	Fair Value of Financial Instruments ASC 820, Fair Value Measurements and Disclosures, provides guidance on the fair value measurement of a financial asset or liability. Inputs used to develop fair value are classified in one of three categories: Level 1 inputs (quoted prices (unadjusted) in active markets for identical assets or liabilities), Level 2 inputs (inputs other than quoted prices that are observable for the asset or liability, either directly or indirectly) and Level 3 inputs (unobservable inputs for the asset or liability). The following disclosure of estimated fair value was determined by management using available market information and appropriate valuation methodologies. However, considerable judgment is necessary to interpret market data and develop estimated fair value. Accordingly, the following estimates are not necessarily indicative of the amounts the Company could have realized on disposition of the financial instruments at September 30, 2018 and December 31, 2017 . The use of different market assumptions and/or estimation methodologies may have a material effect on the estimated fair value amounts. The carrying value of cash and cash equivalents, restricted cash, accounts receivable, accounts payable, accrued interest, dividend and distributions payable and other liabilities are reasonable estimates of fair value because of the short-term nature of these instruments. The carrying value of the outstanding amounts under the Company's credit facilities is a reasonable estimate of fair value because interest rates float at a rate based on LIBOR. The Company determines the fair value of its interest rate swaps by using the standard methodology of netting discounted future fixed cash payments with the discounted expected variable cash receipts. These variable cash receipts of interest rate swaps are based on expectations of future LIBOR interest rates (forward curves) estimated by observing market LIBOR interest rate curves. This is a Level 2 fair value calculation. Also, credit valuation adjustments are factored into the fair value calculations to account for potential nonperformance risk. These credit valuation adjustments were concluded to be not significant inputs for the fair value calculations for the periods presented. See Note 13 - Derivative Instruments. The Company used a discounted cash flow model to determine the estimated fair value of its debt as of September 30, 2018 and December 31, 2017 . This is a Level 3 fair value calculation. The inputs used in preparing the discounted cash flow model include actual maturity dates and scheduled cash flows as well as estimates for market value discount rates. The Company updates the discounted cash flow model on a quarterly basis to reflect any changes in the Company's debt holdings and changes to discount rate assumptions. The following summarizes the fair value of the Company's mortgage loans and unsecured notes as of December 31, 2017 and September 30, 2018 (in thousands): Mortgage Loans Unsecured Notes Carrying Value Fair Value Carrying Value Fair Value As of December 31, 2017 $ 267,093 $ 267,298 $ 2,283,513 $ 2,359,998 As of September 30, 2018 $ 233,682 $ 230,987 $ 2,285,565 $ 2,272,500

Unconsolidated Joint Ventures

Unconsolidated Joint Ventures	9 Months Ended
	Sep. 30, 2018
Equity Method Investments and Joint Ventures [Abstract]
Equity Method Investments and Joint Ventures Disclosure [Text Block]	Unconsolidated Joint Ventures Liberty Property 18th & Arch LP and Liberty Property 18th & Arch Hotel, LP On June 30, 2014, the Company entered into two joint ventures for the purpose of developing and owning the Comcast Technology Center (the “Project”) located in Philadelphia, Pennsylvania as part of a mixed-use development. The 60 -story building includes 1.3 million square feet of leasable office space (the “Office”) and a 217 -room Four Seasons Hotel (the “Hotel”) (collectively, “Liberty Property 18th and Arch”). Project costs for the development of the Project, exclusive of tenant-funded interior improvements, are anticipated to be approximately $960 million . As of September 30, 2018, the Company's investment in these joint ventures was $190.5 million and is reflected in investments in and advances to unconsolidated joint ventures in the Company's consolidated balance sheet. These joint ventures are part of the Company's Philadelphia reportable segment. During the nine months ended September 30, 2018, the joint ventures for Liberty Property 18th & Arch brought into service 250,000 square feet of office space representing a Total Investment by the Office joint venture of $138.3 million . The office portion of the Project is now substantially complete. The 217 -room Four Seasons hotel representing an aggregate Total Investment by the Hotel joint venture of $223.8 million when completed continued to be developed by the Hotel joint venture as of September 30, 2018. The two joint ventures have engaged the Company as the developer of the Project pursuant to a Development Agreement by which the Company agrees, in consideration for a development fee, to be responsible for all aspects of the development of the Project and to guarantee the timely lien-free completion of construction of the Project as well as the payment, subject to certain exceptions, of any cost overruns incurred in the development of the Project. To mitigate its risk, the Company entered into guaranteed maximum price contracts with a third-party contractor (the “GMP Contracts”) to construct the Project. The Company has been notified by its third-party contractor that the contractor has incurred cost overruns and expects to incur additional construction costs in connection with completing the Project in excess of the guaranteed maximum price payable to the contractor under the GMP Contracts (as that guaranteed maximum price has been adjusted pursuant to change orders accepted to date). The Company intends to vigorously pursue all remedies to recover from the third-party contractor any amounts expended by the Company or the joint ventures in excess of their contractual obligations. However, based upon information we have received from the third-party contractor, the Company has determined that it is probable that the Company may be required to initially fund cost overruns in compliance with its obligations under its development cost guarantee to the joint ventures. The Company has accrued such amounts as additional development service fee expense in its consolidated statements of comprehensive income for the nine months ended September 30, 2018. As of September 30, 2018 and December 31, 2017, the Company had accrued $67.3 million and $5.6 million , respectively, relating to the above-described development cost guarantees which are included in other liabilities in the accompanying consolidated balance sheets. There can be no assurances that amounts incurred, including as a result of claims that have been or may be asserted by the third-party contractor, will not exceed these estimates. The Company is not able to reasonably estimate the amount of additional expenses, if any, that it may incur as a result of claims that have been or may be asserted by the third-party contractor. If the Company were to incur additional expenses in connection with its development cost guarantee, such amounts would be accrued when they are determined to be probable of being incurred and are reasonably estimable, and could be material to the Company’s results of operations in future periods. If the Company were to subsequently recover any of the cost overruns initially funded by the Company, such recoveries would be recorded when and if realized in future periods. Liberty/Comcast 1701 JFK Boulevard, LP During the nine months ended September 30, 2018, Liberty/Comcast 1701 JFK Boulevard LP (a joint venture in which the Company holds a 20% interest) paid in full its $305.2 million mortgage loan. The payment was funded through loans from the joint venture partners in proportion to their ownership interests. The Company's portion of the loan to the joint venture is included in investments in, and advances to, unconsolidated joint ventures in the Company's consolidated balance sheets. Cambridge Medipark Ltd During the three and nine months ended September 30, 2018 , Cambridge Medipark, Ltd (a joint venture in which the Company holds a 50% interest) recognized gains on the sale of land leasehold interests. The Company's share of these gains was $1.2 million and $5.0 million for the three and nine months ended September 30, 2018 , respectively, compared to $1.3 million and $5.8 million , respectively, for the same periods in 2017.

Derivative Instruments

Derivative Instruments	9 Months Ended
	Sep. 30, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Interest rate swaps [Text Block]	Derivative Instruments The Company borrows funds at a combination of fixed and variable rates. Borrowings under the Company's revolving credit facility and certain bank mortgage loans bear interest at variable rates. Our long-term debt typically bears interest at fixed rates. The Company's interest rate risk management objectives are to limit generally the impact of interest rate changes on earnings and cash flows and to lower the Company's overall borrowing costs. To achieve these objectives, from time to time, the Company enters into interest rate hedge contracts such as collars, swaps, caps and treasury lock agreements in order to mitigate our interest rate risk with respect to various debt instruments. The Company generally does not hold or issue these derivative contracts for trading or speculative purposes. Interest rate swaps involve the receipt of variable-rate amounts from a counterparty in exchange for making fixed-rate payments over the life of the agreements without exchange of the underlying notional amount. The effective portion of changes in the fair value of derivatives designated and that qualify as cash flow hedges is recorded in accumulated other comprehensive loss (for the Trust) and general partner's equity and limited partners ’ equity - common units (for the Operating Partnership) and is subsequently reclassified into interest expense in the period that the hedged forecasted transaction affects earnings. The Company determines the fair value of its interest rate swaps by using the standard methodology of netting discounted future fixed cash payments with the discounted expected variable cash receipts. These variable cash receipts of interest rate swaps are based on expectations of future LIBOR interest rates (forward curves) estimated by observing market LIBOR interest rate curves. This is a Level 2 fair value calculation. Also, credit valuation adjustments are factored into the fair value calculations to account for potential nonperformance risk. These credit valuation adjustments were concluded to be not significant inputs for the fair value calculations for the periods presented. The Company holds an interest in two interest rate swap contracts (“Swaps”) that eliminate the impact of changes in interest rates on the payments required under variable rate mortgages. The Swaps had aggregate notional amounts of $67.4 million and $96.2 million at September 30, 2018 and December 31, 2017 , respectively, and expire in 2020 . One interest rate swap contract and related mortgage loan were repaid in April 2018 on the maturity date. The Company accounts for the effective portion of changes in the fair value of a derivative in accumulated other comprehensive loss and subsequently reclassifies the effective portion to earnings over the term that the hedged transaction affects earnings. The Company accounts for the ineffective portion of changes in the fair value of a derivative directly in earnings. The following table presents the location in the financial statements of the gains or losses recognized related to the Company’s cash flow hedges for the three and nine months ended September 30, 2018 and 2017 (in thousands): Three Months Ended Nine Months Ended September 30, 2018 September 30, 2017 September 30, 2018 September 30, 2017 Amount of gain (loss) related to the effective portion recognized in other comprehensive (loss) income $ 69 $ 19 $ 619 $ (7 ) Amount of loss related to the effective portion subsequently reclassified to interest expense $ (66 ) $ (75 ) $ (127 ) $ (391 ) Amount of (loss) gain related to the ineffective portion recognized in interest expense $ — $ (8 ) $ (27 ) $ 5 The fair value of the Swaps in the amounts of $0.7 million and $2.2 million as of September 30, 2018 and December 31, 2017 , respectively, is included in other liabilities in the accompanying consolidated balance sheets. The Company estimates that $0.6 million will be reclassified from accumulated other comprehensive loss as a decrease to interest expense over the next 12 months. The Company has agreements with its derivative counterparties that contain a provision whereby if the Company defaults on any of its indebtedness, including defaults where repayment of the indebtedness has not been accelerated by the lender, then the Company could also be declared in default on its derivative obligations. If the Company were to breach any of the contractual provisions of the derivative contracts, it would be required to settle its obligations under the agreements at their termination value including accrued interest, which totaled approximately $0.7 million as of September 30, 2018 .

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Environmental Matters Substantially all of the Company's properties and land were subject to Phase I Environmental Assessments and when appropriate Phase II Environmental Assessments (collectively, the “Environmental Assessments”) obtained in contemplation of their acquisition by the Company or obtained by predecessor owners prior to the sale of the property or land to the Company. The Environmental Assessments did not reveal, nor is the Company aware of, any non-compliance with environmental laws, environmental liability or other environmental claim that the Company believes would likely have a material adverse effect on the Company. Operating Ground Lease Agreements Future minimum rental payments under the terms of all non-cancelable operating ground leases under which the Company is the lessee, as of September 30, 2018 , were as follows (in thousands): Year Amount 2018 (remaining) $ 170 2019 934 2020 934 2021 934 2022 934 2023 and thereafter 32,388 Total $ 36,294 Operating ground lease expense for the three and nine months ended September 30, 2018 was $250,000 and $1.2 million, respectively, as compared to $313,000 and $936,000 , respectively, for the same periods in 2017 . Legal Matters From time to time, the Company is a party to a variety of legal proceedings, claims and assessments arising in the normal course of business. As of September 30, 2018 there were no legal proceedings, claims or assessments that the Company expects to have a material adverse effect on the Company. However, see Note 12 for discussion of Comcast Technology Center. Other As of September 30, 2018 , the Company had letter of credit obligations of $5.8 million . As of September 30, 2018 , the Company had 27 buildings under development. These buildings are expected to contain, when completed, a total of 8.2 million square feet of leasable space and represent an anticipated aggregate investment of $698.8 million . At September 30, 2018 , development in progress totaled $440.5 million . In addition, as of September 30, 2018 , the Company had invested $13.2 million in deferred leasing costs related to these development buildings. As of September 30, 2018 , the Company was committed to $15.3 million in improvements on certain buildings and land parcels. As of September 30, 2018 , the Company was committed to $53.4 million in future land acquisitions. The Company expects to complete these purchases during the years ending December 31, 2018 and 2019. As of September 30, 2018 , the Company was obligated to pay for tenant improvements not yet completed for a maximum of $17.1 million . The Company is currently developing three properties for its unconsolidated joint ventures which represent an anticipated aggregate investment by the joint ventures of $255.3 million . As of September 30, 2018 , the Company was also committed to approximately $172.9 million in costs related to its agreement to develop, on a fee basis, an office building for American Water Works and related infrastructure improvements in Camden, New Jersey. As of September 30, 2018 , $151.5 million of these costs had been incurred. The Company maintains cash and cash equivalents at financial institutions. The combined account balances at each institution typically exceed FDIC insurance coverage and, as a result, there is a concentration of credit risk related to amounts on deposit in excess of FDIC insurance coverage. The Company believes the risk is not significant.

Supplemental Disclosure to Stat

Supplemental Disclosure to Statements of Cash Flows	9 Months Ended
	Sep. 30, 2018
Supplemental Cash Flow Information [Abstract]
Supplemental Disclosure to Statements of Cash Flows	Supplemental Disclosure to Consolidated Statements of Cash Flows The following are supplemental disclosures to the consolidated statements of cash flows for the nine months ended September 30, 2018 and 2017 (amounts in thousands): 2018 2017 Write-off of fully depreciated/amortized property and deferred costs - properties included in continuing operations $ 18,507 $ 17,672 Write-off of fully depreciated/amortized property and deferred costs - properties included in discontinued operations $ 479 $ 8,937 Write-off of depreciated property and deferred costs due to sale/demolition - properties included in continuing operations $ 20,273 $ 13,127 Write-off of depreciated property and deferred costs due to sale - properties included in discontinued operations $ 99,072 $ — Changes in accrued development capital expenditures - properties included in continuing operations $ 9,636 $ 17,744 Changes in accrued development capital expenditures - properties included in discontinued operations $ (1,903 ) $ 511 Unrealized gain on cash flow hedge $ 493 $ 366 Capitalized equity-based compensation $ 727 $ 877 Redemption of noncontrolling interests - common units $ — $ 27 Amounts paid in cash for deferred leasing costs incurred in connection with signed leases with tenants are paid in conjunction with improving (acquiring) property, plant and equipment. Such costs are not contained within net real estate. However, they are integral to the completion of a tenant lease and ultimately are related to the improvement and thus the value of the Company’s property, plant and equipment. They are therefore included in investing activities in the Company’s consolidated statements of cash flows. The following is a reconciliation of the Company's cash and cash equivalents and restricted cash at the beginning and end of the nine months ended September 30, 2018 and 2017 (amounts in thousands): 2018 2017 Cash and cash equivalents at beginning of period $ 11,882 $ 43,642 Restricted cash at beginning of period 13,803 12,383 Cash and cash equivalents and restricted cash at beginning of period $ 25,685 $ 56,025 Cash and cash equivalents at end of period $ 17,770 $ 47,666 Restricted cash at end of period 13,371 16,824 Cash and cash equivalents and restricted cash at end of period $ 31,141 $ 64,490 Restricted cash includes tenant security deposits and escrow funds that the Company maintains pursuant to certain mortgage loans. Restricted cash also includes the undistributed proceeds from the sale of land in Kent County, United Kingdom.

Subsequent Events (Notes)

Subsequent Events (Notes)	9 Months Ended
	Sep. 30, 2018
Subsequent Events [Abstract]
Subsequent Events [Text Block]	On October 1, 2018 the Company purchased seven industrial properties totaling 1.1 million square feet in the Company's United Kingdom reportable segment for $144.7 million . This purchase was financed from borrowings on the Company's credit facility as of September 30, 2018, which were held on deposit pending closing and recorded as prepaid expenses and other assets on the accompanying consolidated balance sheet as of September 30, 2018.

Organization and Basis of Pre_2

Organization and Basis of Presentation (Policy)	9 Months Ended
	Sep. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Selling, General and Administrative Expenses, Policy [Policy Text Block]	The Company capitalizes pre-development and pre-acquisition costs incurred in pursuit of new development, land or operating property opportunities for which the Company currently believes future development or asset acquisition is probable. Future development and the consummation of acquisitions is dependent upon various factors, including, as appropriate, due diligence, zoning and regulatory approval, rental market conditions and construction costs. Initial pre-development and pre-acquisition costs incurred on future development, land or operating property acquisitions that is not considered probable are expensed as incurred. In addition, if the status of a future development, land or operating property acquisitions by the Company is no longer probable, any capitalized pre-development or pre-acquisition costs are written off.
New Accounting Pronouncements, Policy [Policy Text Block]	Revenue Recognition In May 2014, the Financial Accounting Standards Board (“FASB”) issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASU 2014-09”), which supersedes nearly all existing revenue recognition guidance (except revenue in the scope of other accounting standards, including standards related to leasing). Subsequently, the FASB issued the following standards related to ASU 2014-09: ASU 2016-08, Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations; ASU 2016-10, Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing ; ASU 2016-12, Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients ; ASU 2017-05 , Gains and losses from the derecognition of nonfinancial assets (Topic 610-20) , and ASU 2017-13, Revenue from Contracts with Customers (Topic 606): Amendments to SEC Paragraphs Pursuant to the Staff Announcement at the July 20, 2017 EITF Meeting and Rescission of Prior SEC Staff Announcements and Observer Comments (collectively, the “New Revenue Standards”). The New Revenue Standards provide a unified model to determine how revenue is recognized. In accordance with the New Revenue Standards, the Company performs the following steps: (i) identify the contract with the customer, (ii) identify the performance obligations within the contract, (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations and (v) recognize revenue when (or as) a performance obligation is satisfied. The Company evaluated each of its revenue streams: lease agreement revenue, development service fee revenue, deferred land sale revenue and gain or loss on sale of nonfinancial assets and adopted the New Revenue Standards effective January 1, 2018 using the modified retrospective approach. The Company concluded that there are no revenue streams from its lease agreements that are covered by the New Revenue Standards with the possible exception of non-lease components as further discussed below. The New Revenue Standards did not have an impact on the amount and timing of recognizing the Company's development service fee income. The Company recognizes development service fee income on a variable basis as a percentage of costs incurred on third party development contracts. Property development services, which are a single performance obligation, continue to be satisfied and recognized over time. The Company measures its progress toward completing the performance obligation under each arrangement. The measurement of the transfer of value to the customer for these services utilizes the input method (actual costs incurred against anticipated project costs) since this method best depicts the actual transfer of value promised to the customer. Estimated expected losses on such contracts are accrued in the period in which they are determinable. The total amount of consideration to be received from these projects is assessed on a quarterly basis. Based on existing contracts, completion is anticipated by mid-2019. The Company recognizes revenues from improving land sites and selling the underlying land on behalf of its development partner to home builders in the United Kingdom. These agreements contain a pre-emption clause and a seller's call option. The Company recognizes revenue as the pre-emption period or seller's call option lapses utilizing the output method. There was $884,000 and $1.5 million in revenue recognized for such contracts during the three and nine months ended September 30, 2018, respectively. The New Revenue Standards did not have an impact on the gain or loss on sale of nonfinancial assets. The New Revenue Standards require the Company to derecognize nonfinancial assets once it transfers control of a distinct nonfinancial asset or distinct in-substance nonfinancial asset. Additionally, when the Company transfers its controlling interest in a nonfinancial asset, but retains a noncontrolling ownership interest, the Company is required to measure any noncontrolling interest it receives or retains at fair value. The guidance requires companies to recognize a full gain or loss on the transaction. See Notes 5 and 7 for further discussion of sales of nonfinancial assets during the three and nine months ended September 30, 2018. Estimated gross revenue related to the remaining performance obligations under existing contracts (before allocation of related costs and expenses) as of September 30, 2018 that will be recognized as revenue in future periods was approximately $54.2 million , which is expected to be recognized in 2018 through 2020 . The Company adopted the practical expedient to assess the recognition of revenue for open contracts during the transition period. There was no adjustment to the opening balance of retained earnings recorded at January 1, 2018. Development Fee Contracts From time to time, the Company enters into contracts to develop properties on a fee basis for joint ventures in which the Company holds an interest or for unrelated third parties. In these cases the Company typically agrees to be responsible for all aspects of the development of the project (and, in certain instances, related infrastructure) and to guarantee the timely lien-free completion of construction of the project and the payment, subject to certain exceptions, of cost overruns incurred in the development of the project. If the Company encounters construction delays or unexpected costs in the development of these projects or is otherwise unable to recover the costs it incurs, the resulting unrecovered costs and potential payments to customers could generate losses that would adversely affect the Company's cash flow and net income. On a quarterly basis, the Company applies reasonable estimates and judgments to assess whether or not it is necessary to accrue any estimated future losses with respect to such contracts. The Company recognized an aggregate net profit of $32,000 and a net loss $61.7 million on these contracts during the three and nine months ended September 30, 2018, respectively, and an aggregate net profit of $511,000 and $1.4 million during the three and nine months ended September 30, 2017, respectively. The total liability on the Company’s balance sheet as of September 30, 2018 related to the above-described development cost guarantees under these contracts is $67.3 million . Should external or internal circumstances change requiring the Company to adjust the estimated future cash flows from these development contracts or in a manner that indicates that such development contracts may result in additional losses, the Company could be required to record such additional losses in future periods. See Note 12 where certain fee development matters relating specifically to the Comcast Technology Center are discussed in further detail. Systems Implementation Expense The Company is incurring costs associated with its efforts to implement new financial and operating systems in certain cloud computing arrangements. The Company evaluated the arrangements in accordance with ASU 2015-05, “ Customer’s Accounting for Fees Paid in a Cloud Computing Arrangement” and concluded that such arrangements are service contracts under the standard. Accordingly, the Company expenses substantially all costs as incurred. Certain costs that relate to the software service agreements received over time are set up as prepaid and expensed over the applicable service period. These costs include license costs incurred during the implementation period as well as consulting, personnel and other direct costs related to the implementation. The Company incurred $1.2 million and $3.4 million of systems implementation expense for the three and nine months ended September 30, 2018, respectively. The Company incurred $142,000 and $157,000 of systems implementation expense for the three and nine months ended September 30, 2017, respectively. The FASB recently issued accounting guidance related to service contracts in a cloud computing arrangement. See ASU 2018-15 in Recently Issued Accounting Standards below. Expensed Pursuit Costs The Company capitalizes pre-development and pre-acquisition costs incurred in pursuit of new development, land or operating property opportunities for which the Company currently believes future development or asset acquisition is probable. Future development and the consummation of acquisitions is dependent upon various factors, including, as appropriate, due diligence, zoning and regulatory approval, rental market conditions and construction costs. Initial pre-development and pre-acquisition costs incurred on future development, land or operating property acquisitions that is not considered probable are expensed as incurred. In addition, if the status of a future development, land or operating property acquisitions by the Company is no longer probable, any capitalized pre-development or pre-acquisition costs are written off. The Company expensed costs related to pursuit costs of $1.3 million and $1.7 million for the three and nine months ended September 30, 2018, respectively, and $4.8 million and $5.0 million for the same periods in 2017, respectively. In the third quarter of 2017, the Company began to separately classify expensed pursuit costs in the Consolidated Statements of Comprehensive Income. These costs, which were reclassified retrospectively for all periods, were formerly classified as general and administrative expense. Recently Issued Accounting Standards In February 2016, the FASB issued ASU 2016-02, Leases (“ASU 2016-02”). Subsequently the FASB issued the following standards related to ASU 2016-02: ASU 2018-10, Codification Improvements to Topic 842, Leases and ASU 2018-11, Leases (Topic 842): Targeted Improvements (collectively, the "New Lease Standard"). The New Lease Standard amends the existing accounting standards for lease accounting, including requiring lessees to recognize most leases on their balance sheets and making targeted changes to lessor accounting. The New Lease Standard is effective for the Company beginning January 1, 2019. The standard requires a modified retrospective transition approach for all leases existing at, or entered into after, the date of initial application, with an option to use certain transition relief. Revenue related to the lease component of the contract will be recognized on a straight-line basis, while revenue related to the non-lease component will be recognized under the provisions of the New Revenue Standards. However, the New Lease Standard gives lessors a practical expedient to not separate non-lease components from the associated lease components if certain criteria are met. For lease agreements longer than one year in which the Company is the lessee, the Company will measure the present value of the future lease payments and recognize a right-of-use asset and corresponding lease liability on its balance sheet. In addition, the new standard states that only direct leasing costs may be capitalized. The Company has assembled a project team and developed a project plan. The project team is working to analyze and evaluate the impact of the guidance on its consolidated financial statements. The Company expects to elect the package of practical expedients, the land easement practical expedient and the practical expedient to not separate lease and non-lease components. The Company expects to adopt the new lease standard on January 1, 2019. In August 2016, the FASB issued ASU 2016-15, Classification of Certain Cash Receipts and Cash Payments (“ASU 2016-15”). ASU 2016-15 is designed to clarify how entities should classify cash receipts and cash payments in the statement of cash flows. ASU 2016-15 became effective for the Company beginning January 1, 2018. The standard requires retrospective application. The adoption of the ASU 2016-15 did not have a material impact on the Company's consolidated financial statements. In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows (Topic 230) - Restricted Cash (“ASU 2016-18”) , which requires that restricted cash and cash equivalents be included with cash and cash equivalents when reconciling beginning-of-period and end-of-period total amounts shown on the consolidated statements of cash flows. The new standard became effective for public entities for fiscal years beginning after December 15, 2017 and for interim periods therein. The Company adopted ASU 2016-18 as of December 31, 2017. The impact of the implementation of ASU 2016-18 was as follows (in thousands): Nine months ended September 30, 2017 Net cash provided by operating activities (prior to adoption of ASU 2016-18) $ 261,587 Impact of including restricted cash with cash and cash equivalents 3,330 Net cash provided by operating activities (after adoption of ASU 2016-18) $ 264,917 In February 2017, the FASB issued ASU 2017-05, Clarifying the Scope of Asset Derecognition Guidance and Accounting for Partial Sales of Nonfinancial Assets (“ASU 2017-05”). ASU 2017-05 is designed to provide guidance on how to recognize gain and losses on sales, including partial sale, of nonfinancial assets to noncustomers. The Company adopted ASU 2017-05 effective January 1, 2018 on a modified retrospective method and the adoption did not have an effect on the Company’s consolidated financial statements. In May 2017, the FASB issued ASU 2017-09, Scope of Modification Accounting (“ASU 2017-09”). ASU 2017-09 clarifies when changes to the terms or conditions of a share-based payment award must be accounted for as modifications. ASU 2017-09 became effective for the Company beginning January 1, 2018. The new guidance will be applied prospectively to awards modified on or after the adoption date. The adoption of the ASU 2017-09 did not have a material impact on the Company's consolidated financial statements. In August 2017, the FASB issued ASU 2017-12, Derivatives and Hedging: Targeted Improvements to Accounting for Hedging Activities (“ASU 2017-12”). ASU 2017-12 is designed to better align a company’s financial reporting for hedging activities with the economic objectives of those activities. ASU 2017-12 is effective for the Company beginning January 1, 2019. Early adoption is permitted using a modified retrospective transition method. This adoption method will require the Company to recognize the cumulative effect of initially applying ASU 2017-12 as an adjustment to accumulated other comprehensive income with a corresponding adjustment to the opening balance of retained earnings as of the beginning of the fiscal year that an entity adopts the update. The Company is evaluating the impact ASU 2017-12 will have on the Company's financial position and results of operations. In August 2018, the FASB issued ASU 2018-15, Intangibles - Goodwill and Other - Internal-Use Software: Customer's Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That is a Service Contract ("ASU 2018-15"). ASU 2018-15 amends the definition of a hosting arrangement and requires a customer in a hosting arrangement that is a service contract to capitalize certain implementation costs as if the arrangement was an internal-use software project. ASU 2018-15 is effective for the Company beginning January 1, 2020. Early adoption is permitted. The Company is evaluating the impact ASU 2018-15 will have on the Company's financial position and results of operations.
Systems Implementation Expense [Policy Text Block]	The Company is incurring costs associated with its efforts to implement new financial and operating systems in certain cloud computing arrangements. The Company evaluated the arrangements in accordance with ASU 2015-05, “ Customer’s Accounting for Fees Paid in a Cloud Computing Arrangement” and concluded that such arrangements are service contracts under the standard. Accordingly, the Company expenses substantially all costs as incurred. Certain costs that relate to the software service agreements received over time are set up as prepaid and expensed over the applicable service period. These costs include license costs incurred during the implementation period as well as consulting, personnel and other direct costs related to the implementation.
Revenue Recognition, Policy [Policy Text Block]	In May 2014, the Financial Accounting Standards Board (“FASB”) issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASU 2014-09”), which supersedes nearly all existing revenue recognition guidance (except revenue in the scope of other accounting standards, including standards related to leasing). Subsequently, the FASB issued the following standards related to ASU 2014-09: ASU 2016-08, Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations; ASU 2016-10, Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing ; ASU 2016-12, Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients ; ASU 2017-05 , Gains and losses from the derecognition of nonfinancial assets (Topic 610-20) , and ASU 2017-13, Revenue from Contracts with Customers (Topic 606): Amendments to SEC Paragraphs Pursuant to the Staff Announcement at the July 20, 2017 EITF Meeting and Rescission of Prior SEC Staff Announcements and Observer Comments (collectively, the “New Revenue Standards”). The New Revenue Standards provide a unified model to determine how revenue is recognized. In accordance with the New Revenue Standards, the Company performs the following steps: (i) identify the contract with the customer, (ii) identify the performance obligations within the contract, (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations and (v) recognize revenue when (or as) a performance obligation is satisfied. The Company evaluated each of its revenue streams: lease agreement revenue, development service fee revenue, deferred land sale revenue and gain or loss on sale of nonfinancial assets and adopted the New Revenue Standards effective January 1, 2018 using the modified retrospective approach. The Company concluded that there are no revenue streams from its lease agreements that are covered by the New Revenue Standards with the possible exception of non-lease components as further discussed below. The New Revenue Standards did not have an impact on the amount and timing of recognizing the Company's development service fee income. The Company recognizes development service fee income on a variable basis as a percentage of costs incurred on third party development contracts. Property development services, which are a single performance obligation, continue to be satisfied and recognized over time. The Company measures its progress toward completing the performance obligation under each arrangement. The measurement of the transfer of value to the customer for these services utilizes the input method (actual costs incurred against anticipated project costs) since this method best depicts the actual transfer of value promised to the customer. Estimated expected losses on such contracts are accrued in the period in which they are determinable. The total amount of consideration to be received from these projects is assessed on a quarterly basis. Based on existing contracts, completion is anticipated by mid-2019. The Company recognizes revenues from improving land sites and selling the underlying land on behalf of its development partner to home builders in the United Kingdom. These agreements contain a pre-emption clause and a seller's call option. The Company recognizes revenue as the pre-emption period or seller's call option lapses utilizing the output method. There was $884,000 and $1.5 million in revenue recognized for such contracts during the three and nine months ended September 30, 2018, respectively. The New Revenue Standards did not have an impact on the gain or loss on sale of nonfinancial assets. The New Revenue Standards require the Company to derecognize nonfinancial assets once it transfers control of a distinct nonfinancial asset or distinct in-substance nonfinancial asset. Additionally, when the Company transfers its controlling interest in a nonfinancial asset, but retains a noncontrolling ownership interest, the Company is required to measure any noncontrolling interest it receives or retains at fair value. The guidance requires companies to recognize a full gain or loss on the transaction. See Notes 5 and 7 for further discussion of sales of nonfinancial assets during the three and nine months ended September 30, 2018. Estimated gross revenue related to the remaining performance obligations under existing contracts (before allocation of related costs and expenses) as of September 30, 2018 that will be recognized as revenue in future periods was approximately $54.2 million , which is expected to be recognized in 2018 through 2020 . The Company adopted the practical expedient to assess the recognition of revenue for open contracts during the transition period. There was no adjustment to the opening balance of retained earnings recorded at January 1, 2018.

Segment Information (Policies)

Segment Information (Policies)	9 Months Ended
	Sep. 30, 2018
Segment Reporting [Abstract]
Segment Reporting, Policy [Policy Text Block]	The Company evaluates the performance of its reportable segments based on segment net operating income (“SNOI”). SNOI is defined as net operating income (rental revenue and operating expense reimbursements less rental property and real estate tax expenses) less amortization of lease transaction costs and other operating expenses which relate directly to the management and operation of the assets within each reportable segment. The Company's accounting policies for the segments are the same as those used in the Company's consolidated financial statements.

Impairment and Disposal of Lo_2

Impairment and Disposal of Long-Lived Assets Impairment and Disposal of Long-Lived Assets (Policies)	9 Months Ended
	Sep. 30, 2018
Discontinued Operations and Disposal Groups [Abstract]
Discontinued Operations, Policy [Policy Text Block]	The Company determined that the strategic shift would have a major effect on its operations and financial results. As such, properties sold or those that meet the criteria to be classified as held for sale within the new corporate strategy were classified within discontinued operations. Consistent with the held for sale criteria these properties are expected to be sold within one year . As the result of the classification within discontinued operations, the in-service assets and liabilities of this portfolio are required to be presented as held for sale for all prior periods presented in our Consolidated Balance Sheets. Operating results pertaining to these properties were reclassified to discontinued operations for all prior periods presented in our Consolidated Statements of Comprehensive Income.

Disclosure of Fair Value of F_2

Disclosure of Fair Value of Financial Instruments Fair Value of Financial Instruments (Policies)	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Fair Value of Financial Instruments, Policy [Policy Text Block]	ASC 820, Fair Value Measurements and Disclosures, provides guidance on the fair value measurement of a financial asset or liability. Inputs used to develop fair value are classified in one of three categories: Level 1 inputs (quoted prices (unadjusted) in active markets for identical assets or liabilities), Level 2 inputs (inputs other than quoted prices that are observable for the asset or liability, either directly or indirectly) and Level 3 inputs (unobservable inputs for the asset or liability). The following disclosure of estimated fair value was determined by management using available market information and appropriate valuation methodologies. However, considerable judgment is necessary to interpret market data and develop estimated fair value. Accordingly, the following estimates are not necessarily indicative of the amounts the Company could have realized on disposition of the financial instruments at September 30, 2018 and December 31, 2017 . The use of different market assumptions and/or estimation methodologies may have a material effect on the estimated fair value amounts. The carrying value of cash and cash equivalents, restricted cash, accounts receivable, accounts payable, accrued interest, dividend and distributions payable and other liabilities are reasonable estimates of fair value because of the short-term nature of these instruments. The carrying value of the outstanding amounts under the Company's credit facilities is a reasonable estimate of fair value because interest rates float at a rate based on LIBOR. The Company determines the fair value of its interest rate swaps by using the standard methodology of netting discounted future fixed cash payments with the discounted expected variable cash receipts. These variable cash receipts of interest rate swaps are based on expectations of future LIBOR interest rates (forward curves) estimated by observing market LIBOR interest rate curves. This is a Level 2 fair value calculation. Also, credit valuation adjustments are factored into the fair value calculations to account for potential nonperformance risk. These credit valuation adjustments were concluded to be not significant inputs for the fair value calculations for the periods presented. See Note 13 - Derivative Instruments. The Company used a discounted cash flow model to determine the estimated fair value of its debt as of September 30, 2018 and December 31, 2017 . This is a Level 3 fair value calculation. The inputs used in preparing the discounted cash flow model include actual maturity dates and scheduled cash flows as well as estimates for market value discount rates. The Company updates the discounted cash flow model on a quarterly basis to reflect any changes in the Company's debt holdings and changes to discount rate assumptions.

Organization and Basis of Pre_3

Organization and Basis of Presentation Basis of Presentation (Tables)	9 Months Ended
	Sep. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of Adjustments to Previously Reported Financial Statements	The impact of the implementation of ASU 2016-18 was as follows (in thousands): Nine months ended September 30, 2017 Net cash provided by operating activities (prior to adoption of ASU 2016-18) $ 261,587 Impact of including restricted cash with cash and cash equivalents 3,330 Net cash provided by operating activities (after adoption of ASU 2016-18) $ 264,917

Income per Common Share of th_2

Income per Common Share of the Trust (Tables)	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Schedule of Earnings Per Share, Basic and Diluted [Table Text Block]	The following table sets forth the computation of basic and diluted income per common share of the Trust (in thousands except per share amounts): For the Three Months Ended For the Three Months Ended September 30, 2018 September 30, 2017 Income (Numerator) Weighted Average Shares (Denominator) Per Share Income (Numerator) Weighted Average Shares (Denominator) Per Share Income from continuing operations net of noncontrolling interest - basic $ 48,595 147,324 $ 0.33 $ 56,487 146,811 $ 0.39 Dilutive shares for long-term compensation plans — 947 — 785 Income from continuing operations net of noncontrolling interest - diluted $ 48,595 148,271 $ 0.33 $ 56,487 147,596 $ 0.38 Discontinued operations net of noncontrolling interest - basic $ 101,548 147,324 $ 0.69 $ 3,052 146,811 $ 0.02 Dilutive shares for long-term compensation plans — 947 — 785 Discontinued operations net of noncontrolling interest - diluted $ 101,548 148,271 $ 0.68 $ 3,052 147,596 $ 0.02 Net income available to common shareholders - basic $ 150,143 147,324 $ 1.02 $ 59,539 146,811 $ 0.41 Dilutive shares for long-term compensation plans — 947 — 785 Net income available to common shareholders - diluted $ 150,143 148,271 $ 1.01 $ 59,539 147,596 $ 0.40 For the Nine Months Ended For the Nine Months Ended September 30, 2018 September 30, 2017 Income Weighted Per Share Income Weighted Per Share Income from continuing operations net of noncontrolling interest - basic $ 104,885 147,241 $ 0.71 $ 136,460 146,678 $ 0.93 Dilutive shares for long-term compensation plans 919 752 Income from continuing operations net of noncontrolling interest - diluted $ 104,885 148,160 $ 0.71 $ 136,460 147,430 $ 0.92 Discontinued operations net of noncontrolling interest - basic $ 205,053 147,241 $ 1.39 $ 17,522 146,678 $ 0.12 Dilutive shares for long-term compensation plans 919 752 Discontinued operations net of noncontrolling interest - diluted $ 205,053 148,160 $ 1.38 $ 17,522 147,430 $ 0.12 Net income available to common shareholders - basic $ 309,938 147,241 $ 2.10 $ 153,982 146,678 $ 1.05 Dilutive shares for long-term compensation plans 919 752 Net income available to common shareholders - diluted $ 309,938 148,160 $ 2.09 $ 153,982 147,430 $ 1.04

Income per Common Unit of the_2

Income per Common Unit of the Operating Partnership (Tables)	9 Months Ended
	Sep. 30, 2018
Earnings Per Unit, Basic and Diluted [Line Items]
Schedule of Earnings Per Share, Basic and Diluted [Table Text Block]	The following table sets forth the computation of basic and diluted income per common share of the Trust (in thousands except per share amounts): For the Three Months Ended For the Three Months Ended September 30, 2018 September 30, 2017 Income (Numerator) Weighted Average Shares (Denominator) Per Share Income (Numerator) Weighted Average Shares (Denominator) Per Share Income from continuing operations net of noncontrolling interest - basic $ 48,595 147,324 $ 0.33 $ 56,487 146,811 $ 0.39 Dilutive shares for long-term compensation plans — 947 — 785 Income from continuing operations net of noncontrolling interest - diluted $ 48,595 148,271 $ 0.33 $ 56,487 147,596 $ 0.38 Discontinued operations net of noncontrolling interest - basic $ 101,548 147,324 $ 0.69 $ 3,052 146,811 $ 0.02 Dilutive shares for long-term compensation plans — 947 — 785 Discontinued operations net of noncontrolling interest - diluted $ 101,548 148,271 $ 0.68 $ 3,052 147,596 $ 0.02 Net income available to common shareholders - basic $ 150,143 147,324 $ 1.02 $ 59,539 146,811 $ 0.41 Dilutive shares for long-term compensation plans — 947 — 785 Net income available to common shareholders - diluted $ 150,143 148,271 $ 1.01 $ 59,539 147,596 $ 0.40 For the Nine Months Ended For the Nine Months Ended September 30, 2018 September 30, 2017 Income Weighted Per Share Income Weighted Per Share Income from continuing operations net of noncontrolling interest - basic $ 104,885 147,241 $ 0.71 $ 136,460 146,678 $ 0.93 Dilutive shares for long-term compensation plans 919 752 Income from continuing operations net of noncontrolling interest - diluted $ 104,885 148,160 $ 0.71 $ 136,460 147,430 $ 0.92 Discontinued operations net of noncontrolling interest - basic $ 205,053 147,241 $ 1.39 $ 17,522 146,678 $ 0.12 Dilutive shares for long-term compensation plans 919 752 Discontinued operations net of noncontrolling interest - diluted $ 205,053 148,160 $ 1.38 $ 17,522 147,430 $ 0.12 Net income available to common shareholders - basic $ 309,938 147,241 $ 2.10 $ 153,982 146,678 $ 1.05 Dilutive shares for long-term compensation plans 919 752 Net income available to common shareholders - diluted $ 309,938 148,160 $ 2.09 $ 153,982 147,430 $ 1.04
Liberty Property Limited Partnership [Member]
Earnings Per Unit, Basic and Diluted [Line Items]
Schedule of Earnings Per Share, Basic and Diluted [Table Text Block]	The following table sets forth the computation of basic and diluted income per common unit of the Operating Partnership (in thousands, except per unit amounts): For the Three Months Ended For the Three Months Ended September 30, 2018 September 30, 2017 Income (Numerator) Weighted Average Units (Denominator) Per Unit Income (Numerator) Weighted Average Units (Denominator) Per Unit Income from continuing operations - net of noncontrolling interest - consolidated joint ventures $ 49,868 $ 57,959 Less: Preferred unit distributions (118 ) (118 ) Income from continuing operations available to common unitholders - basic $ 49,750 150,844 $ 0.33 $ 57,841 150,339 $ 0.39 Dilutive units for long-term compensation plans — 947 — 785 Income from continuing operations available to common unitholders - diluted $ 49,750 151,791 $ 0.33 $ 57,841 151,124 $ 0.38 Income from discontinued operations - basic $ 103,971 150,844 $ 0.69 $ 3,125 150,339 $ 0.02 Dilutive units for long-term compensation plans — 947 — 785 Income from discontinued operations - diluted $ 103,971 151,791 $ 0.68 $ 3,125 151,124 $ 0.02 Income available to common unitholders - basic $ 153,721 150,844 $ 1.02 $ 60,966 150,339 $ 0.41 Dilutive units for long-term compensation plans — 947 — 785 Income available to common unitholders - diluted $ 153,721 151,791 $ 1.01 $ 60,966 151,124 $ 0.40 For the Nine Months Ended For the Nine Months Ended September 30, 2018 September 30, 2017 Income (Numerator) Weighted Average Units (Denominator) Per Unit Income (Numerator) Weighted Average Units (Denominator) Per Unit Net income - net of noncontrolling interest - consolidated joint ventures $ 107,731 $ 140,084 Less: Preferred unit distributions (354 ) (354 ) Income from continuing operations available to common unitholders - basic 107,377 150,761 $ 0.71 139,730 150,206 $ 0.93 Dilutive units for long-term compensation plans — 919 — 752 Income from continuing operations available to common unitholders - diluted $ 107,377 151,680 $ 0.71 $ 139,730 150,958 $ 0.92 Income from discontinued operations - basic $ 209,945 150,761 $ 1.39 $ 17,942 150,206 $ 0.12 Dilutive units for long-term compensation plans — 919 — 752 Income from discontinued operations - diluted $ 209,945 151,680 $ 1.38 $ 17,942 150,958 $ 0.12 Net income available to common unitholders - basic $ 317,322 150,761 $ 2.10 $ 157,672 150,206 $ 1.05 Dilutive units for long-term compensation plans — 919 — 752 Net income available to common unitholders - diluted $ 317,322 151,680 $ 2.09 $ 157,672 150,958 $ 1.04

Accumulated Other Comprehensi_2

Accumulated Other Comprehensive Income (Tables)	9 Months Ended
	Sep. 30, 2018
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block]	The following table sets forth the components of Accumulated Other Comprehensive Loss (in thousands): As of and for the nine months ended September 30, 2018 2017 Foreign Currency Translation: Beginning balance $ (38,701 ) $ (56,767 ) Translation adjustment (9,221 ) 16,314 Ending balance (47,922 ) (40,453 ) Derivative Instruments: Beginning balance 150 (455 ) Unrealized gain 620 (25 ) Reclassification adjustment (1) (127 ) 391 Ending balance 643 (89 ) Total accumulated other comprehensive loss (47,279 ) (40,542 ) Less: portion included in noncontrolling interest – operating partnership 957 801 Total accumulated other comprehensive loss included in shareholders' equity/owners' equity $ (46,322 ) $ (39,741 ) (1) Amounts reclassified out of Accumulated Other Comprehensive Loss/General & Limited Partner's Equity into contractual interest expense.

Real Estate (Tables)

Real Estate (Tables)	9 Months Ended
	Sep. 30, 2018
Property, Plant and Equipment [Abstract]
Acquisitions and Dispositions of Real Estate [Table Text Block]	Information on the Operating Properties and land parcels the Company acquired during the three and nine months ended September 30, 2018 is as follows: Three Months Ended September 30, 2018 Nine Months Ended September 30, 2018 Number of Buildings Acres of Developable Land Leaseable Square Feet Purchase Price (in thousands) Number of Buildings Acres of Developable Land Leaseable Square Feet Purchase Price (in thousands) Florida — — — $ — — 28.4 — $ 14,728 Lehigh/Central PA — 16.4 — 3,000 — 170.2 — 34,043 United Kingdom — 1.6 — 709 — 8.7 — 4,949 Other: Dallas 1 — 900,043 52,491 1 — 900,043 52,491 New Jersey 2 7.5 412,553 61,375 3 7.5 584,569 84,875 Southern California — 12.9 — 33,125 3 36.0 1,002,471 231,061 3 38.4 1,312,596 $ 150,700 7 250.8 2,487,083 $ 422,147 Information on the Operating Properties and land parcels the Company sold during the three and nine months ended September 30, 2018 is as follows: Three Months Ended September 30, 2018 Nine Months Ended September 30, 2018 Number of Buildings Acres of Developable Land Leaseable Square Feet Gross Proceeds (in thousands) Number of Buildings Acres of Developable Land Leaseable Square Feet Gross Proceeds (in thousands) Carolinas/Richmond — — — $ — 1 1.5 80,000 $ 7,094 Chicago/Minneapolis — — — — — 8.3 — 2,714 Lehigh/Central PA — 65.8 — 9,700 — 65.8 — 9,700 Philadelphia — — — — 1 3.1 207,779 130,850 Southeastern PA — — — — 23 — 1,420,515 183,808 Other Arizona 5 — 805,746 $ 255,000 5 — 805,746 $ 255,000 5 65.8 805,746 $ 264,700 30 78.7 2,514,040 $ 589,166

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2018
Segment Reporting [Abstract]
Schedule of Segment Reporting Information, by Segment, Revenue and Net Operating Income [Table Text Block]	The operating information by reportable segment is as follows (in thousands): Three Months Ended Nine Months Ended September 30, 2018 September 30, 2017 September 30, 2018 September 30, 2017 Operating revenue Carolinas/Richmond $ 19,417 $ 18,910 $ 58,723 $ 55,192 Chicago/Minneapolis 17,274 16,147 50,414 47,789 Florida 15,847 15,072 47,117 43,802 Houston 16,561 15,167 48,514 44,720 Lehigh/Central PA 39,188 40,698 115,799 122,048 Philadelphia 10,598 11,137 33,659 33,501 Southeastern PA 6,697 15,422 23,573 45,165 United Kingdom 3,523 3,773 11,579 10,204 Other 43,118 33,288 124,419 95,826 Segment-level operating revenue 172,223 169,614 513,797 498,247 Reconciliation to total operating revenues Development service fee income 12,956 24,176 59,132 53,920 Discontinued operations (14,013 ) (22,066 ) (45,511 ) (64,184 ) Other 18 (137 ) (372 ) (18 ) Total operating revenue $ 171,184 $ 171,587 $ 527,046 $ 487,965 SNOI Carolinas/Richmond $ 14,254 $ 13,621 $ 42,572 $ 39,698 Chicago/Minneapolis 10,292 9,554 30,417 28,913 Florida 11,148 10,176 32,734 29,686 Houston 9,514 8,701 28,225 23,263 Lehigh/Central PA 29,182 29,995 85,297 88,656 Philadelphia 8,678 8,743 27,398 25,987 Southeastern PA 5,316 9,246 16,721 25,866 United Kingdom 1,873 1,781 5,879 5,288 Other 30,000 21,901 85,082 64,089 SNOI 120,257 113,718 354,325 331,446 Reconciliation to income from continuing operations Interest expense (1) (24,670 ) (23,060 ) (71,331 ) (67,345 ) Development service fee income 12,956 24,176 59,132 53,920 Development service fee expense (12,924 ) (23,665 ) (120,799 ) (52,497 ) Depreciation/amortization expense (1) (2) (32,243 ) (33,521 ) (97,582 ) (99,829 ) Impairment charges - real estate assets (1) — (9,650 ) (26,000 ) (9,650 ) Gain on property dispositions 2,002 23,840 54,705 30,542 Equity in earnings of unconsolidated joint ventures 6,766 4,305 20,958 14,026 General and administrative expense (1) (2) (9,615 ) (7,962 ) (30,954 ) (31,073 ) Expensed pursuit costs (1,292 ) (4,772 ) (1,675 ) (4,957 ) Systems implementation expense (1,177 ) (142 ) (3,352 ) (157 ) Discontinued operations excluding (loss) gain on property dispositions (9,093 ) (3,125 ) (25,256 ) (17,942 ) Income taxes (2) (70 ) (64 ) (89 ) (240 ) Other (797 ) (2,044 ) (3,341 ) (5,965 ) Income from continuing operations $ 50,100 $ 58,034 $ 108,741 $ 140,279 (1) Includes activity in discontinued operations. (2) Excludes costs that are included in determining SNOI.
Assets by Segment and Reconciliation to Consolidated [Table Text Block]	The Company's total assets by reportable segment as of September 30, 2018 and December 31, 2017 is as follows (in thousands): September 30, 2018 December 31, 2017 Carolinas/Richmond $ 524,967 $ 543,922 Chicago/Minnesota 600,838 615,186 Florida 546,526 533,861 Houston 537,500 498,584 Lehigh/Central PA 1,220,160 1,210,746 Philadelphia 640,790 665,843 Southeastern PA 145,778 241,128 United Kingdom 400,321 251,824 Other 2,063,248 1,807,653 Segment-level total assets 6,680,128 6,368,747 Corporate Other 138,947 71,010 Total assets $ 6,819,075 $ 6,439,757

Impairment and Disposal of Lo_3

Impairment and Disposal of Long-Lived Assets Impairment and Disposal of Long-Lived Assets (Tables)	9 Months Ended
	Sep. 30, 2018
Discontinued Operations and Disposal Groups [Abstract]
Disposal Groups, Including Discontinued Operations [Table Text Block]	The following table illustrates the number of sold or held for sale properties included in, or excluded from, discontinued operations: Held for Sale as of September 30, 2018 Sold during the nine months ended September 30, 2018 Sold during the year ended December 31, 2017 Total Properties included in discontinued operations 13 28 2 43 Properties included in continuing operations 1 2 8 11 Properties sold or classified as held for sale 14 30 10 54 The following table illustrates aggregate balance sheet information for all held for sale properties (in thousands): September 30, 2018 December 31, 2017 Included in Continuing Operations Included in Discontinued Operations Total Included in Continuing Operations Included in Discontinued Operations Total Land and land improvements $ 1,301 $ 111,517 $ 112,818 $ 3,476 $ 152,708 $ 156,184 Buildings and improvements 5,638 222,983 228,621 80,738 471,628 552,366 Development in progress — — — — 45,035 45,035 Land held for development 10,227 — 10,227 863 — 863 Accumulated depreciation (1,546 ) (67,821 ) (69,367 ) (11,785 ) (172,604 ) (184,389 ) Deferred financing and leasing costs, net 58 4,433 4,491 2,210 14,914 17,124 Other assets 165 7,236 7,401 5,137 18,291 23,428 Total assets held for sale $ 15,843 $ 278,348 $ 294,191 $ 80,639 $ 529,972 $ 610,611 Total liabilities held for sale $ 191 $ 3,795 $ 3,986 $ 1,153 $ 13,129 $ 14,282 A summary of the results of operations for the properties classified as discontinued operations is as follows (in thousands): Three Months Ended Nine Months Ended September 30, 2018 September 30, 2017 September 30, 2018 September 30, 2017 Revenues $ 14,013 $ 22,066 $ 45,511 $ 64,184 Operating expenses (3,239 ) (6,834 ) (11,099 ) (21,783 ) Depreciation and amortization (295 ) (4,828 ) (4,444 ) (13,907 ) Impairment expense — (5,714 ) — (5,714 ) Interest and other income 10 47 37 98 Interest expense (1,396 ) (1,612 ) (4,749 ) (4,936 ) Income from discontinued operations before gain on property dispositions 9,093 3,125 25,256 17,942 Gain on property dispositions 94,878 — 184,689 — Income from discontinued operations 103,971 3,125 209,945 17,942 Noncontrolling interest - operating partnership (2,423 ) (73 ) (4,892 ) (420 ) Income from discontinued operations available to common shareholders $ 101,548 $ 3,052 $ 205,053 $ 17,522

Noncontrolling Interest - Ope_2

Noncontrolling Interest - Operating Partnership/Limited Partners' Equity - Preferred Units (Tables)	9 Months Ended
	Sep. 30, 2018
Temporary Equity [Line Items]
Schedule of Temporary Equity [Table Text Block]	As of September 30, 2018 , the Company had outstanding the following cumulative preferred units of the Operating Partnership: ISSUE AMOUNT UNITS LIQUIDATION PREFERENCE DIVIDEND RATE (in 000’s) Series I-2 $ 7,537 301 $25 6.25 %

Disclosure of Fair Value of F_3

Disclosure of Fair Value of Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2018
Debt Instrument, Fair Value Disclosure [Abstract]
Fair Value, by Balance Sheet Grouping [Table Text Block]	The following summarizes the fair value of the Company's mortgage loans and unsecured notes as of December 31, 2017 and September 30, 2018 (in thousands): Mortgage Loans Unsecured Notes Carrying Value Fair Value Carrying Value Fair Value As of December 31, 2017 $ 267,093 $ 267,298 $ 2,283,513 $ 2,359,998 As of September 30, 2018 $ 233,682 $ 230,987 $ 2,285,565 $ 2,272,500

Derivative Instruments (Tables)

Derivative Instruments (Tables)	9 Months Ended
	Sep. 30, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Instruments, Gain (Loss) [Table Text Block]	The following table presents the location in the financial statements of the gains or losses recognized related to the Company’s cash flow hedges for the three and nine months ended September 30, 2018 and 2017 (in thousands): Three Months Ended Nine Months Ended September 30, 2018 September 30, 2017 September 30, 2018 September 30, 2017 Amount of gain (loss) related to the effective portion recognized in other comprehensive (loss) income $ 69 $ 19 $ 619 $ (7 ) Amount of loss related to the effective portion subsequently reclassified to interest expense $ (66 ) $ (75 ) $ (127 ) $ (391 ) Amount of (loss) gain related to the ineffective portion recognized in interest expense $ — $ (8 ) $ (27 ) $ 5

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Future Minimum Rental Payments	Future minimum rental payments under the terms of all non-cancelable operating ground leases under which the Company is the lessee, as of September 30, 2018 , were as follows (in thousands): Year Amount 2018 (remaining) $ 170 2019 934 2020 934 2021 934 2022 934 2023 and thereafter 32,388 Total $ 36,294

Supplemental Disclosure to St_2

Supplemental Disclosure to Statements of Cash Flows (Tables)	9 Months Ended
	Sep. 30, 2018
Supplemental Cash Flow Information [Abstract]
Supplemental Disclosure to Statements of Cash Flows	The following is a reconciliation of the Company's cash and cash equivalents and restricted cash at the beginning and end of the nine months ended September 30, 2018 and 2017 (amounts in thousands): 2018 2017 Cash and cash equivalents at beginning of period $ 11,882 $ 43,642 Restricted cash at beginning of period 13,803 12,383 Cash and cash equivalents and restricted cash at beginning of period $ 25,685 $ 56,025 Cash and cash equivalents at end of period $ 17,770 $ 47,666 Restricted cash at end of period 13,371 16,824 Cash and cash equivalents and restricted cash at end of period $ 31,141 $ 64,490 The following are supplemental disclosures to the consolidated statements of cash flows for the nine months ended September 30, 2018 and 2017 (amounts in thousands): 2018 2017 Write-off of fully depreciated/amortized property and deferred costs - properties included in continuing operations $ 18,507 $ 17,672 Write-off of fully depreciated/amortized property and deferred costs - properties included in discontinued operations $ 479 $ 8,937 Write-off of depreciated property and deferred costs due to sale/demolition - properties included in continuing operations $ 20,273 $ 13,127 Write-off of depreciated property and deferred costs due to sale - properties included in discontinued operations $ 99,072 $ — Changes in accrued development capital expenditures - properties included in continuing operations $ 9,636 $ 17,744 Changes in accrued development capital expenditures - properties included in discontinued operations $ (1,903 ) $ 511 Unrealized gain on cash flow hedge $ 493 $ 366 Capitalized equity-based compensation $ 727 $ 877 Redemption of noncontrolling interests - common units $ — $ 27

Organization and Basis of Pre_4

Organization and Basis of Presentation (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Variable Interest Entity [Line Items]
Ownership interest in operating partnership - Limited Partnership	97.70%
Liberty Comcast 1701 JFK Boulevard, LP [Member]
Variable Interest Entity [Line Items]
Ownership percentage of equity method investment	20.00%
Liberty Comcast 1701 JFK Boulevard, LP [Member] | Variable Interest Entity, Not Primary Beneficiary [Member] | Other Investments [Member]
Variable Interest Entity [Line Items]
Equity investment in joint venture	$ 75.9	$ 17.3
Maximum exposure to loss	$ 75.9	$ 17.3
Liberty Comcast 1701 JFK Boulevard, LP [Member] | Philadelphia [Member]
Variable Interest Entity [Line Items]
Ownership percentage of equity method investment	20.00%

Organization and Basis of Pre_5

Organization and Basis of Presentation Development Fee Contracts (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Loss Contingency Accrual	$ 67,300,000		$ 67,300,000
Profit (loss) on development fee contracts	$ 32,000	$ 511,000	$ 61,700,000	$ 1,400,000

Organization and Basis of Pre_6

Organization and Basis of Presentation Systems Implementation Expense (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Systems implementation expense	$ 1,177,000	$ 142,000	$ 3,352,000	$ 157,000

Organization and Basis of Pre_7

Organization and Basis of Presentation Expensed Pursuit Costs (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Expensed pursuit costs	$ 1,292	$ 4,772	$ 1,675	$ 4,957

Organization and Basis of Pre_8

Organization and Basis of Presentation Prior period adjustments (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Prior Period Adjustments [Line Items]
Net cash provided by operating activities	$ 296,445	$ 264,917
Accounting Standards Update 2016-18 [Member]
Prior Period Adjustments [Line Items]
Net cash provided by operating activities		264,917
Previously Reported [Member] | Accounting Standards Update 2016-18 [Member]
Prior Period Adjustments [Line Items]
Net cash provided by operating activities		261,587
Restatement Adjustment [Member] | Accounting Standards Update 2016-18 [Member]
Prior Period Adjustments [Line Items]
Net cash provided by operating activities		$ 3,330

Organization and Basis of Pre_9

Organization and Basis of Presentation Revenue Recognition (Details) - Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01 $ in Millions	Sep. 30, 2018USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation	$ 54.2
Remaining performance obligation through years	2,020

Organization and Basis of Pr_10

Organization and Basis of Presentation Revenue Recognition - Land Sales (Details) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018
Land sold to homebuilders [Member]
Disaggregation of Revenue [Line Items]
Revenue from land sold to homebuilders	$ 884,000	$ 1,500,000

Income per Common Share of th_3

Income per Common Share of the Trust (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income (Numerator) [Abstract]
Income from continuing operations net of noncontrolling interest	$ 48,595	$ 56,487	$ 104,885	$ 136,460
Income from discontinued operations net of noncontrolling interest	101,548	3,052	205,053	17,522
Net income available to common shareholders	$ 150,143	$ 59,539	$ 309,938	$ 153,982
Weighted Average Shares (Denominator) [Abstract]
Weighted Average Number of Shares or Units Outstanding, Basic	147,324	146,811	147,241	146,678
Dilutive shares for long-term compensation plans	947	785	919	752
Weighted Average Number of Shares or Units Outstanding, Diluted	148,271	147,596	148,160	147,430
Common shares [Member]
Earnings Per Share, Basic [Abstract]
Income from continuing operations	$ 0.33	$ 0.39	$ 0.71	$ 0.93
Income from discontinued operations	0.69	0.02	1.39	0.12
Income per common share or unit - basic	1.02	0.41	2.10	1.05
Earnings Per Share, Diluted [Abstract]
Income from continuing operations	0.33	0.38	0.71	0.92
Income from discontinued operations	0.68	0.02	1.38	0.12
Income per common share or unit - diluted	$ 1.01	$ 0.40	$ 2.09	$ 1.04

Income per Common Share of th_4

Income per Common Share of the Trust Income per Common Share of the Trust - Anti-dilutive Options (Details) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Stock Option [Member]
Antidilutive Options Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Options Excluded from Computation of Earnings Per Share	0	0	0	0

Income per Common Share of th_5

Income per Common Share of the Trust - Options Exercised (Details) - shares	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Stock Option [Member] | Common shares [Member]
Stock Option Exercises [Line Items]
Amount of Options Exercises in Period	5,000	106,000	193,000

Income per Common Share of th_6

Income per Common Share of the Trust Share Repurchase (Details) - 2017 Share Repurchase Program [Member] - Common shares [Member] $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018USD ($)shares	Sep. 30, 2018USD ($)shares
Equity, Class of Treasury Stock [Line Items]
Stock Repurchase Program, Authorized Amount | $	$ 250	$ 250
Stock Repurchased During Period, Shares | shares	0	0

Income per Common Unit of the_3

Income per Common Unit of the Operating Partnership - Diluted (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income (Numerator) [Abstract]
Income from continuing operations net of noncontrolling interest - consolidated joint ventures	$ 48,595	$ 56,487	$ 104,885	$ 136,460
Income from discontinued operations	103,971	3,125	209,945	17,942
Net income available to common shareholders or unitholders	$ 150,143	$ 59,539	$ 309,938	$ 153,982
Weighted Average Units (Denominator) [Abstract]
Weighted Average Number of Shares or Units Outstanding, Basic	147,324	146,811	147,241	146,678
Dilutive units for long-term compensation plans	947	785	919	752
Weighted Average Number of Shares or Units Outstanding, Diluted	148,271	147,596	148,160	147,430
Liberty Property Limited Partnership [Member]
Income (Numerator) [Abstract]
Income from continuing operations net of noncontrolling interest - consolidated joint ventures	$ 49,868	$ 57,959	$ 107,731	$ 140,084
Less: Preferred unit distributions	(118)	(118)	(354)	(354)
Income from continuing operations available to common unitholders	49,750	57,841	107,377	139,730
Income from discontinued operations	103,971	3,125	209,945	17,942
Net income available to common shareholders or unitholders	$ 153,721	$ 60,966	$ 317,322	$ 157,672
Weighted Average Units (Denominator) [Abstract]
Weighted Average Number of Shares or Units Outstanding, Basic	150,844	150,339	150,761	150,206
Dilutive units for long-term compensation plans	947	785	919	752
Weighted Average Number of Shares or Units Outstanding, Diluted	151,791	151,124	151,680	150,958
Liberty Property Limited Partnership [Member] | Common Units [Member]
Earnings Per Unit, Basic [Abstract]
Income from continuing operations	$ 0.33	$ 0.39	$ 0.71	$ 0.93
Income from discontinued operations	0.69	0.02	1.39	0.12
Income per common share or unit - basic	1.02	0.41	2.10	1.05
Earnings Per Unit, Diluted [Abstract]
Income from continuing operations	0.33	0.38	0.71	0.92
Income from discontinued operations	0.68	0.02	1.38	0.12
Income per common share or unit - diluted	$ 1.01	$ 0.40	$ 2.09	$ 1.04

Income per Common Unit of the_4

Income per Common Unit of the Operating Partnership - Anti-dilutive Options (Details) - Stock Option [Member] - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Antidilutive Options Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Options Excluded from Computation of Earnings Per Share	0	0	0	0
Liberty Property Limited Partnership [Member]
Antidilutive Options Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Options Excluded from Computation of Earnings Per Share	0	0	0	0

Income per Common Unit of the_5

Income per Common Unit of the Operations Partnership - Options Exercised (Details) - shares	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Liberty Property Limited Partnership [Member] | Stock Option [Member] | Common Units [Member]
Stock Option Exercises [Line Items]
Amount of Options Exercises in Period	5,000	106,000	193,000

Income per Common Unit of the_6

Income per Common Unit of the Operating Partnership Unit Repurchase (Details) - Liberty Property Limited Partnership [Member] - 2017 Share Repurchase Program [Member] - Common Units [Member] $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018USD ($)shares	Sep. 30, 2018USD ($)shares
Equity, Class of Treasury Stock [Line Items]
Stock Repurchase Program, Authorized Amount | $	$ 250	$ 250
Stock Repurchased During Period, Shares | shares	0	0

Accumulated Other Comprehensi_3

Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance		$ 3,148,366
Ending Balance		3,289,584
Total accumulated other comprehensive loss		3,148,366		$ 3,289,584	$ 3,148,366
Total accumulated other comprehensive loss included in shareholders' equity/owners' equity				3,225,100	3,087,358
Accumulated Foreign Currency Adjustment Including Portion Attributable to Noncontrolling Interest [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance		(38,701)	$ (56,767)
Unrealized income (loss)		(9,221)	16,314
Ending Balance		(47,922)	(40,453)
Total accumulated other comprehensive loss		(38,701)	(56,767)	(47,922)	(38,701)	$ (40,453)
Accumulated Net Gain (Loss) from Cash Flow Hedges Including Portion Attributable to Noncontrolling Interest [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance		150	(455)
Unrealized income (loss)		620	(25)
Reclassification adjustment	[1]	(127)	391
Ending Balance		643	(89)
Total accumulated other comprehensive loss		150	(455)	643	150	(89)
AOCI Including Portion Attributable to Noncontrolling Interest [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Ending Balance		(47,279)	(40,542)
Total accumulated other comprehensive loss		(47,279)	(40,542)	(47,279)		(40,542)
AOCI Attributable to Noncontrolling Interest [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Ending Balance		957	801
Total accumulated other comprehensive loss		957	$ 801	957		801
AOCI Attributable to Parent [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance		(37,797)
Ending Balance		(46,322)
Total accumulated other comprehensive loss		$ (37,797)		(46,322)	$ (37,797)
Total accumulated other comprehensive loss included in shareholders' equity/owners' equity				$ (46,322)		$ (39,741)
[1]	Amounts reclassified out of Accumulated Other Comprehensive Loss/General & Limited Partner's Equity into contractual interest expense.

Acquisitions and Dispositions (

Acquisitions and Dispositions (Details) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2018USD ($)ft²abldg	Sep. 30, 2018USD ($)ft²abldg
Disposal Group, Not Discontinued Operations [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	5	5
Leaseable square feet | ft²	805,746	805,746
Disposal Group, Not Discontinued Operations [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Gross proceeds | $	$ 264,700
Disposal Group, Not Discontinued Operations [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	65.8	65.8
Disposal Group, Not Discontinued Operations [Member] | Operating Segments [Member] | 2018 sales [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	30	30
Leaseable square feet | ft²	2,514,040	2,514,040
Disposal Group, Not Discontinued Operations [Member] | Operating Segments [Member] | 2018 sales [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Gross proceeds | $		$ 589,166
Disposal Group, Not Discontinued Operations [Member] | Operating Segments [Member] | 2018 sales [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	78.7	78.7
Disposal Group, Not Discontinued Operations [Member] | Carolinas Richmond [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	0	0
Leaseable square feet | ft²	0	0
Disposal Group, Not Discontinued Operations [Member] | Carolinas Richmond [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Gross proceeds | $	$ 0
Disposal Group, Not Discontinued Operations [Member] | Carolinas Richmond [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	0	0
Disposal Group, Not Discontinued Operations [Member] | Carolinas Richmond [Member] | Operating Segments [Member] | 2018 sales [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	1	1
Leaseable square feet | ft²	80,000	80,000
Disposal Group, Not Discontinued Operations [Member] | Carolinas Richmond [Member] | Operating Segments [Member] | 2018 sales [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Gross proceeds | $		$ 7,094
Disposal Group, Not Discontinued Operations [Member] | Carolinas Richmond [Member] | Operating Segments [Member] | 2018 sales [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	1.5	1.5
Disposal Group, Not Discontinued Operations [Member] | Chicago Minneapolis [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	0	0
Leaseable square feet | ft²	0	0
Disposal Group, Not Discontinued Operations [Member] | Chicago Minneapolis [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Gross proceeds | $	$ 0
Disposal Group, Not Discontinued Operations [Member] | Chicago Minneapolis [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	0	0
Disposal Group, Not Discontinued Operations [Member] | Chicago Minneapolis [Member] | Operating Segments [Member] | 2018 sales [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	0	0
Leaseable square feet | ft²	0	0
Disposal Group, Not Discontinued Operations [Member] | Chicago Minneapolis [Member] | Operating Segments [Member] | 2018 sales [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Gross proceeds | $		$ 2,714
Disposal Group, Not Discontinued Operations [Member] | Chicago Minneapolis [Member] | Operating Segments [Member] | 2018 sales [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	8.3	8.3
Disposal Group, Not Discontinued Operations [Member] | Southeastern PA [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	0	0
Disposal Group, Not Discontinued Operations [Member] | Southeastern PA [Member] | Operating Segments [Member] | 2018 sales [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	0	0
Disposal Group, Not Discontinued Operations [Member] | Lehigh Central PA [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	0	0
Leaseable square feet | ft²	0	0
Disposal Group, Not Discontinued Operations [Member] | Lehigh Central PA [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Gross proceeds | $	$ 9,700
Disposal Group, Not Discontinued Operations [Member] | Lehigh Central PA [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	65.8	65.8
Disposal Group, Not Discontinued Operations [Member] | Lehigh Central PA [Member] | Operating Segments [Member] | 2018 sales [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	0	0
Leaseable square feet | ft²	0	0
Disposal Group, Not Discontinued Operations [Member] | Lehigh Central PA [Member] | Operating Segments [Member] | 2018 sales [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Gross proceeds | $		$ 9,700
Disposal Group, Not Discontinued Operations [Member] | Lehigh Central PA [Member] | Operating Segments [Member] | 2018 sales [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	65.8	65.8
Disposal Group, Not Discontinued Operations [Member] | Philadelphia [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	0	0
Leaseable square feet | ft²	0	0
Disposal Group, Not Discontinued Operations [Member] | Philadelphia [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Gross proceeds | $	$ 0
Disposal Group, Not Discontinued Operations [Member] | Philadelphia [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	0	0
Disposal Group, Not Discontinued Operations [Member] | Philadelphia [Member] | Operating Segments [Member] | 2018 sales [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	1	1
Leaseable square feet | ft²	207,779	207,779
Disposal Group, Not Discontinued Operations [Member] | Philadelphia [Member] | Operating Segments [Member] | 2018 sales [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Gross proceeds | $		$ 130,850
Disposal Group, Not Discontinued Operations [Member] | Philadelphia [Member] | Operating Segments [Member] | 2018 sales [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	3.1	3.1
Disposal Group, Not Discontinued Operations [Member] | Arizona [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	0	0
Disposal Group, Not Discontinued Operations [Member] | Arizona [Member] | Operating Segments [Member] | 2018 sales [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	0	0
Discontinued Operations [Member] | Southeastern PA [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	0	0
Leaseable square feet | ft²	0	0
Discontinued Operations [Member] | Southeastern PA [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Gross proceeds | $	$ 0
Discontinued Operations [Member] | Southeastern PA [Member] | Operating Segments [Member] | 2018 sales [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	23	23
Leaseable square feet | ft²	1,420,515	1,420,515
Discontinued Operations [Member] | Southeastern PA [Member] | Operating Segments [Member] | 2018 sales [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Gross proceeds | $		$ 183,808
Discontinued Operations [Member] | Arizona [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	5	5
Leaseable square feet | ft²	805,746	805,746
Discontinued Operations [Member] | Arizona [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Gross proceeds | $	$ 255,000
Discontinued Operations [Member] | Arizona [Member] | Operating Segments [Member] | 2018 sales [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	5	5
Leaseable square feet | ft²	805,746	805,746
Discontinued Operations [Member] | Arizona [Member] | Operating Segments [Member] | 2018 sales [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Gross proceeds | $		$ 255,000
2018 acquisitions current quarter [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	3	3
Leaseable square feet | ft²	1,312,596	1,312,596
2018 acquisitions current quarter [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $	$ 150,700
2018 acquisitions current quarter [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	38.4	38.4
2018 acquisitions current quarter [Member] | Dallas [Member] | Operating Segments [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	1	1
Leaseable square feet | ft²	900,043	900,043
2018 acquisitions current quarter [Member] | Dallas [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $	$ 52,491
2018 acquisitions current quarter [Member] | Dallas [Member] | Operating Segments [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	0	0
2018 acquisitions current quarter [Member] | United Kingdom [Member] | Operating Segments [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	0	0
Leaseable square feet | ft²	0	0
2018 acquisitions current quarter [Member] | United Kingdom [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $	$ 709
2018 acquisitions current quarter [Member] | United Kingdom [Member] | Operating Segments [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	1.6	1.6
2018 acquisitions current quarter [Member] | Lehigh Central PA [Member] | Operating Segments [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	0	0
Leaseable square feet | ft²	0	0
2018 acquisitions current quarter [Member] | Lehigh Central PA [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $	$ 3,000
2018 acquisitions current quarter [Member] | Lehigh Central PA [Member] | Operating Segments [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	16.4	16.4
2018 acquisitions current quarter [Member] | Florida [Member] | Operating Segments [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	0	0
Leaseable square feet | ft²	0	0
2018 acquisitions current quarter [Member] | Florida [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $	$ 0
2018 acquisitions current quarter [Member] | Florida [Member] | Operating Segments [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	0	0
2018 acquisitions current quarter [Member] | New Jersey [Member] | Operating Segments [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	2	2
Leaseable square feet | ft²	412,553	412,553
2018 acquisitions current quarter [Member] | New Jersey [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $	$ 61,375
2018 acquisitions current quarter [Member] | New Jersey [Member] | Operating Segments [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	7.5	7.5
2018 acquisitions current quarter [Member] | Southern California [Member] | Operating Segments [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	0	0
Leaseable square feet | ft²	0	0
2018 acquisitions current quarter [Member] | Southern California [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $	$ 33,125
2018 acquisitions current quarter [Member] | Southern California [Member] | Operating Segments [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	12.9	12.9
2018 acquisitions [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	7	7
Leaseable square feet | ft²	2,487,083	2,487,083
2018 acquisitions [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	250.8	250.8
2018 acquisitions [Member] | Dallas [Member] | Operating Segments [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	1	1
Leaseable square feet | ft²	900,043	900,043
2018 acquisitions [Member] | Dallas [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $		$ 52,491
2018 acquisitions [Member] | Dallas [Member] | Operating Segments [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	0	0
2018 acquisitions [Member] | United Kingdom [Member] | Operating Segments [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	0	0
Leaseable square feet | ft²	0	0
2018 acquisitions [Member] | United Kingdom [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $		$ 4,949
2018 acquisitions [Member] | United Kingdom [Member] | Operating Segments [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	8.7	8.7
2018 acquisitions [Member] | Lehigh Central PA [Member] | Operating Segments [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	0	0
Leaseable square feet | ft²	0	0
2018 acquisitions [Member] | Lehigh Central PA [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $		$ 34,043
2018 acquisitions [Member] | Lehigh Central PA [Member] | Operating Segments [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	170.2	170.2
2018 acquisitions [Member] | Florida [Member] | Operating Segments [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	0	0
Leaseable square feet | ft²	0	0
2018 acquisitions [Member] | Florida [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $		$ 14,728
2018 acquisitions [Member] | Florida [Member] | Operating Segments [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	28.4	28.4
2018 acquisitions [Member] | New Jersey [Member] | Operating Segments [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	3	3
Leaseable square feet | ft²	584,569	584,569
2018 acquisitions [Member] | New Jersey [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $		$ 84,875
2018 acquisitions [Member] | New Jersey [Member] | Operating Segments [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	7.5	7.5
2018 acquisitions [Member] | Southern California [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $		$ 422,147
2018 acquisitions [Member] | Southern California [Member] | Operating Segments [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	3	3
Leaseable square feet | ft²	1,002,471	1,002,471
2018 acquisitions [Member] | Southern California [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $		$ 231,061
2018 acquisitions [Member] | Southern California [Member] | Operating Segments [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	36	36

Segment Information - Income St

Segment Information - Income Statement (Details) - USD ($)		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Revenues		$ 171,184,000	$ 171,587,000	$ 527,046,000	$ 487,965,000
Net operating income		61,645,000	50,185,000	93,059,000	154,161,000
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense		(23,274,000)	(21,448,000)	(66,582,000)	(62,409,000)
Development service fee income		12,956,000	24,176,000	59,132,000	53,920,000
Development service fee expense		(12,924,000)	(23,665,000)	(120,799,000)	(52,497,000)
Depreciation and amortization expense		(43,375,000)	(41,754,000)	(127,280,000)	(123,924,000)
Impairment charges - real estate assets		0	(3,936,000)	(26,000,000)	(3,936,000)
Gain on property dispositions		2,002,000	23,840,000	54,705,000	30,542,000
Equity in earnings of unconsolidated joint ventures		6,766,000	4,305,000	20,958,000	14,026,000
General and administrative expense		(13,121,000)	(11,873,000)	(43,406,000)	(44,069,000)
Expensed pursuit costs		(1,292,000)	(4,772,000)	(1,675,000)	(4,957,000)
Systems implementation expense		(1,177,000)	(142,000)	(3,352,000)	(157,000)
Income taxes		(472,000)	(582,000)	(1,967,000)	(1,528,000)
Income from continuing operations		50,100,000	58,034,000	108,741,000	140,279,000
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues		0	0	0	0
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues		172,223,000	169,614,000	513,797,000	498,247,000
Net operating income		120,257,000	113,718,000	354,325,000	331,446,000
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Revenues		18,000	(137,000)	(372,000)	(18,000)
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense	[1]	(24,670,000)	(23,060,000)	(71,331,000)	(67,345,000)
Development service fee income		12,956,000	24,176,000	59,132,000	53,920,000
Development service fee expense		(12,924,000)	(23,665,000)	(120,799,000)	(52,497,000)
Depreciation and amortization expense	[1],[2]	(32,243,000)	(33,521,000)	(97,582,000)	(99,829,000)
Impairment charges - real estate assets	[1]	0	(9,650,000)	(26,000,000)	(9,650,000)
Gain on property dispositions		2,002,000	23,840,000	54,705,000	30,542,000
Equity in earnings of unconsolidated joint ventures		6,766,000	4,305,000	20,958,000	14,026,000
General and administrative expense	[1],[2]	(9,615,000)	(7,962,000)	(30,954,000)	(31,073,000)
Expensed pursuit costs		(1,292,000)	(4,772,000)	(1,675,000)	(4,957,000)
Systems implementation expense		(1,177,000)	(142,000)	(3,352,000)	(157,000)
Discontinued operations excluding (loss) gain on property dispositions		(9,093,000)	(3,125,000)	(25,256,000)	(17,942,000)
Income taxes	[2]	(70,000)	(64,000)	(89,000)	(240,000)
Other		(797,000)	(2,044,000)	(3,341,000)	(5,965,000)
Discontinued operations | Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Revenues		(14,013,000)	(22,066,000)	(45,511,000)	(64,184,000)
Development Service Fee Income [Member] | Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Revenues		12,956,000	24,176,000	59,132,000	53,920,000
Carolinas Richmond [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Revenues		19,417,000	18,910,000	58,723,000	55,192,000
Net operating income		14,254,000	13,621,000	42,572,000	39,698,000
Chicago Minneapolis [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Revenues		17,274,000	16,147,000	50,414,000	47,789,000
Net operating income		10,292,000	9,554,000	30,417,000	28,913,000
Florida [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Revenues		15,847,000	15,072,000	47,117,000	43,802,000
Net operating income		11,148,000	10,176,000	32,734,000	29,686,000
Houston [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Revenues		16,561,000	15,167,000	48,514,000	44,720,000
Net operating income		9,514,000	8,701,000	28,225,000	23,263,000
Lehigh Central PA [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Revenues		39,188,000	40,698,000	115,799,000	122,048,000
Net operating income		29,182,000	29,995,000	85,297,000	88,656,000
Philadelphia [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Revenues		10,598,000	11,137,000	33,659,000	33,501,000
Net operating income		8,678,000	8,743,000	27,398,000	25,987,000
Southeastern PA [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Revenues		6,697,000	15,422,000	23,573,000	45,165,000
Net operating income		5,316,000	9,246,000	16,721,000	25,866,000
United Kingdom [Member] | Operating Segments [Member] | UNITED KINGDOM
Segment Reporting Information [Line Items]
Revenues		3,523,000	3,773,000	11,579,000	10,204,000
Net operating income		1,873,000	1,781,000	5,879,000	5,288,000
Other Segments [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Revenues		43,118,000	33,288,000	124,419,000	95,826,000
Net operating income		$ 30,000,000	$ 21,901,000	$ 85,082,000	$ 64,089,000
[1]	Includes activity in discontinued operations.
[2]	Excludes costs that are included in determining SNOI.

Segment Information - Total Ass

Segment Information - Total Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Assets	$ 6,819,075	$ 6,439,757
Operating Segments [Member]
Segment Reporting Information [Line Items]
Assets	6,680,128	6,368,747
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Assets	138,947	71,010
North America [Member] | Operating Segments [Member] | Carolinas Richmond [Member]
Segment Reporting Information [Line Items]
Assets	524,967	543,922
North America [Member] | Operating Segments [Member] | Chicago Minneapolis [Member]
Segment Reporting Information [Line Items]
Assets	600,838	615,186
North America [Member] | Operating Segments [Member] | Florida [Member]
Segment Reporting Information [Line Items]
Assets	546,526	533,861
North America [Member] | Operating Segments [Member] | Houston [Member]
Segment Reporting Information [Line Items]
Assets	537,500	498,584
North America [Member] | Operating Segments [Member] | Lehigh Central PA [Member]
Segment Reporting Information [Line Items]
Assets	1,220,160	1,210,746
North America [Member] | Operating Segments [Member] | Philadelphia [Member]
Segment Reporting Information [Line Items]
Assets	640,790	665,843
North America [Member] | Operating Segments [Member] | Southeastern PA [Member]
Segment Reporting Information [Line Items]
Assets	145,778	241,128
North America [Member] | Operating Segments [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Assets	2,063,248	1,807,653
UNITED KINGDOM | Operating Segments [Member] | United Kingdom [Member]
Segment Reporting Information [Line Items]
Assets	$ 400,321	$ 251,824

Impairment and Disposal of Lo_4

Impairment and Disposal of Long-Lived Assets Discontinued Operations (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2018USD ($)bldg	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)bldg	Sep. 30, 2017USD ($)	Dec. 31, 2017bldg
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Gain on property dispositions	$ 94,878,000	$ 0	$ 184,689,000	$ 0
Net income	103,971,000	3,125,000	209,945,000	17,942,000
Income from discontinued operations available to common shareholders	$ 101,548,000	3,052,000	$ 205,053,000	17,522,000
Discontinued Operations, Held-for-sale [Member]
Discontined Operations Income Statement [Line Items]
Number of Real Estate Properties | bldg	13		13
Discontinued Operations, Disposed of by Sale [Member]
Discontined Operations Income Statement [Line Items]
Number of Real Estate Properties | bldg	28		28		2
Discontinued Operations [Member]
Discontined Operations Income Statement [Line Items]
Expected timing of disposal			one year
Capital expenditures	$ 759,000	14,000,000	$ 5,400,000	46,400,000
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Revenues	14,013,000	22,066,000	45,511,000	64,184,000
Operating expenses	(3,239,000)	(6,834,000)	(11,099,000)	(21,783,000)
Depreciation and amortization	(295,000)	(4,828,000)	(4,444,000)	(13,907,000)
Disposal Group, Including Discontinued Operation, Impairment Expense	0	(5,714,000)	0	(5,714,000)
Interest and other income	10,000	47,000	37,000	98,000
Interest expense	(1,396,000)	(1,612,000)	(4,749,000)	(4,936,000)
Net income before (loss) gain on property dispositions	9,093,000	3,125,000	25,256,000	17,942,000
Gain on property dispositions	94,878,000	0	184,689,000	0
Net income	103,971,000	3,125,000	209,945,000	17,942,000
Noncontrolling interest - operating partnership	(2,423,000)	(73,000)	(4,892,000)	(420,000)
Income from discontinued operations available to common shareholders	$ 101,548,000	$ 3,052,000	$ 205,053,000	$ 17,522,000
Florida [Member] | Discontinued Operations, Held-for-sale [Member]
Discontined Operations Income Statement [Line Items]
Number of Real Estate Properties | bldg	3		3
Southeastern PA [Member] | Discontinued Operations, Held-for-sale [Member]
Discontined Operations Income Statement [Line Items]
Number of Real Estate Properties | bldg	8		8
DC Metro [Member] | Discontinued Operations, Held-for-sale [Member]
Discontined Operations Income Statement [Line Items]
Number of Real Estate Properties | bldg	2		2

Impairment and Disposal of Lo_5

Impairment and Disposal of Long-Lived Assets Assets Held for Sale (Details) $ in Thousands	Sep. 30, 2018USD ($)bldg	Dec. 31, 2017USD ($)bldg
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Assets held for sale	$ 294,191	$ 610,611
Liabilities held for sale	$ 3,986	14,282
Properties sold or classified as held for sale in the prior year and current year to date [Member]
Balance Sheet Disclosures by Disposal Groups [Line Items]
Number of Real Estate Properties | bldg	54
Disposal Group, Held-for-sale, Not Discontinued Operations [Member]
Balance Sheet Disclosures by Disposal Groups [Line Items]
Number of Real Estate Properties | bldg	1
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Land and land improvements	$ 1,301	3,476
Buildings and improvements	5,638	80,738
Development in progress	0	0
Land held for development	10,227	863
Accumulated depreciation	(1,546)	(11,785)
Deferred financing and leasing costs, net	58	2,210
Other assets	165	5,137
Assets held for sale	15,843	80,639
Liabilities held for sale	$ 191	1,153
Discontinued Operations, Held-for-sale [Member]
Balance Sheet Disclosures by Disposal Groups [Line Items]
Number of Real Estate Properties | bldg	13
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Land and land improvements	$ 111,517	152,708
Buildings and improvements	222,983	471,628
Development in progress	0	45,035
Land held for development	0	0
Accumulated depreciation	(67,821)	(172,604)
Deferred financing and leasing costs, net	4,433	14,914
Other assets	7,236	18,291
Assets held for sale	278,348	529,972
Liabilities held for sale	$ 3,795	13,129
Held for Sale, Includes Disposal Groups and Discontinued Operations [Member]
Balance Sheet Disclosures by Disposal Groups [Line Items]
Number of Real Estate Properties | bldg	14
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Land and land improvements	$ 112,818	156,184
Buildings and improvements	228,621	552,366
Development in progress	0	45,035
Land held for development	10,227	863
Accumulated depreciation	(69,367)	(184,389)
Deferred financing and leasing costs, net	4,491	17,124
Other assets	7,401	23,428
Assets held for sale	294,191	610,611
Liabilities held for sale	$ 3,986	$ 14,282
Discontinued Operations, Disposed of by Sale [Member]
Balance Sheet Disclosures by Disposal Groups [Line Items]
Number of Real Estate Properties | bldg	28	2
Discontinued Operations, Held-for-sale or Disposed of by Sale [Member]
Balance Sheet Disclosures by Disposal Groups [Line Items]
Number of Real Estate Properties | bldg	43
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member]
Balance Sheet Disclosures by Disposal Groups [Line Items]
Number of Real Estate Properties | bldg	2	8
Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations [Member]
Balance Sheet Disclosures by Disposal Groups [Line Items]
Number of Real Estate Properties | bldg	11
Disposed of by Sale, Discontinued Operations and Not Discontinued Operations [Member]
Balance Sheet Disclosures by Disposal Groups [Line Items]
Number of Real Estate Properties | bldg	30	10

Impairment and Disposal of Lo_6

Impairment and Disposal of Long-Lived Assets Impairment Charges - Real Estate Assets (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Impaired Long-Lived Assets Held and Used [Line Items]
Valuation adjustment	$ 0		$ 0
Impairment charges - real estate assets	$ 0	$ 3,936,000	26,000,000	$ 3,936,000
Land Held for Development [Member] | Philadelphia [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment charges - real estate assets			$ 26,000,000
Land, Buildings and Improvements [Member] | Houston [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment charges - real estate assets		$ 9,700,000		$ 9,700,000

Noncontrolling Interests of t_2

Noncontrolling Interests of the Trust- Narrative (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Redeemable Noncontrolling Interest [Line Items]
Limited Partners' common units outstanding	3,520,205	3,520,205
Liberty Property Limited Partnership [Member]
Redeemable Noncontrolling Interest [Line Items]
Limited Partners' common units outstanding	3,520,205	3,520,205
Market value of common units based on closing price	$ 148.7

Limited Partners' Equity of t_2

Limited Partners' Equity of the Operating Partnership - Narrative (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Limited Partners' Capital Account [Line Items]
Limited Partners' common units outstanding	3,520,205	3,520,205
Liberty Property Limited Partnership [Member]
Limited Partners' Capital Account [Line Items]
Limited Partners' common units outstanding	3,520,205	3,520,205
Market value of common units based on closing price	$ 148.7

Noncontrolling Interest - Ope_3

Noncontrolling Interest - Operating Partnership/Limited Partners' Equity - Preferred Units (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Noncontrolling interest - operating partnership - 301,483 preferred units outstanding as of September 30, 2018 and December 31, 2017	$ 7,537	$ 7,537
Series I 2 [Member]
Temporary Equity [Line Items]
Noncontrolling interest - operating partnership - 301,483 preferred units outstanding as of September 30, 2018 and December 31, 2017	$ 7,537
Units	301,483	301,483
Liquidation Preference	$ 25
Dividend Rate	6.25%

Disclosure of Fair Value of F_4

Disclosure of Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage loans, net	$ 233,682	$ 267,093
Unsecured notes, net	2,285,565	2,283,513
Unsecured Debt [Member] | Fair Value, Inputs, Level 3 [Member] | Long-term Debt [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of debt	2,272,500	2,359,998
Mortgage Loans | Fair Value, Inputs, Level 3 [Member] | Long-term Debt [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of debt	$ 230,987	$ 267,298

Liberty Property 18th & Arch LP

Liberty Property 18th & Arch LP and Liberty Property 18th & Arch, Hotel, LP (Details) $ in Thousands	Sep. 30, 2018USD ($)ft²	Dec. 31, 2017USD ($)	Jun. 30, 2014ft²roomstoryinvestment
Schedule of Equity Method Investments [Line Items]
Loss Contingency Accrual	$ 67,300
Investments in and advances to unconsolidated joint ventures	357,970	$ 288,456
Liberty Property 18th & Arch [Member]
Schedule of Equity Method Investments [Line Items]
Investments in and advances to unconsolidated joint ventures	190,500
Philadelphia [Member] | Liberty Property 18th & Arch [Member]
Schedule of Equity Method Investments [Line Items]
Number of equity method investments | investment			2
Anticipated Aggregate Investment	960,000
Philadelphia [Member] | Construction in Progress [Member] | Liberty Property 18th & Arch [Member]
Schedule of Equity Method Investments [Line Items]
Number of stories in a high-rise building | story			60
Leaseable square feet | ft²			1,300,000
Number of hotel rooms | room			217
Anticipated Aggregate Investment	$ 223,800
Philadelphia [Member] | Building and Building Improvements [Member] | Liberty Property 18th & Arch [Member]
Schedule of Equity Method Investments [Line Items]
Leaseable square feet | ft²	250,000
Anticipated Aggregate Investment	$ 138,300
Other Liabilities [Member] | Philadelphia [Member] | Liberty Property 18th & Arch [Member] | Guarantee Obligations [Member]
Schedule of Equity Method Investments [Line Items]
Loss Contingency Accrual	$ 67,300	$ 5,600

Unconsolidated Joint Ventures L

Unconsolidated Joint Ventures Liberty/Comcast 1701 JFK Boulevard, LP (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Schedule of Equity Method Investments [Line Items]
Repayments of mortgage loans	$ 31,954	$ 5,974
Liberty Comcast 1701 JFK Boulevard, LP [Member]
Schedule of Equity Method Investments [Line Items]
Ownership percentage of equity method investment	20.00%
Philadelphia [Member] | Liberty Comcast 1701 JFK Boulevard, LP [Member]
Schedule of Equity Method Investments [Line Items]
Ownership percentage of equity method investment	20.00%
Equity Method Investee [Member] | Liberty Comcast 1701 JFK Boulevard, LP [Member] | Philadelphia [Member]
Schedule of Equity Method Investments [Line Items]
Repayments of mortgage loans	$ 305,200

Unconsolidated Joint Ventures C

Unconsolidated Joint Ventures Cambridge Medipark Ltd (Details) - Cambridge Medipark Ltd [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Schedule of Equity Method Investments [Line Items]
Ownership percentage of equity method investment	50.00%		50.00%
Joint venture, 2018 sales current quarter [Member] | Leaseholds and Leasehold Improvements [Member] | Other Income [Member] | Disposal Group, Not Discontinued Operations [Member]
Schedule of Equity Method Investments [Line Items]
Gain on sale of leasehold interests	$ 1.2
Joint venture, 2018 sales [Member] | Leaseholds and Leasehold Improvements [Member] | Other Income [Member] | Disposal Group, Not Discontinued Operations [Member]
Schedule of Equity Method Investments [Line Items]
Gain on sale of leasehold interests			$ 5
Joint venture, 2017 sales current quarter [Member] | Leaseholds and Leasehold Improvements [Member] | Other Income [Member] | Disposal Group, Not Discontinued Operations [Member]
Schedule of Equity Method Investments [Line Items]
Gain on sale of leasehold interests		$ 1.3
Joint venture, 2017 sales [Member] | Leaseholds and Leasehold Improvements [Member] | Other Income [Member] | Disposal Group, Not Discontinued Operations [Member]
Schedule of Equity Method Investments [Line Items]
Gain on sale of leasehold interests				$ 5.8

Derivative Instruments (Details

Derivative Instruments (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Apr. 01, 2018	Dec. 31, 2017USD ($)
Interest Rate Derivatives [Abstract]
Number of interest rate swaps	2		2
Fair value	$ 700		$ 700			$ 2,200
Aggregate notional amount	67,400		67,400			$ 96,200
Reclassification during next 12 months	600		600
Termination value	700		700
Interest Rate Swap [Member] | Other Comprehensive Income (Loss) [Member] | Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) related to the effective portion recognized in other comprehensive (loss) income	69	$ 19	619	$ (7)
Interest Rate Swap [Member] | Interest Expense [Member] | Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of loss related to the effective portion subsequently reclassified to interest expense	(66)	(75)	(127)	(391)
Amount of (loss) gain related to the ineffective portion recognized in interest expense	$ 0	$ (8)	$ (27)	$ 5
Debt Repayment [Member]
Interest Rate Derivatives [Abstract]
Number of interest rate swaps					1

Commitments and Contingencies -

Commitments and Contingencies - Future Minimum Rental Payments (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Commitments and Contingencies Disclosure [Abstract]
2018 (remaining)	$ 170,000		$ 170,000
2,019	934,000		934,000
2,020	934,000		934,000
2,021	934,000		934,000
2,022	934,000		934,000
2023 and thereafter	32,388,000		32,388,000
Total	36,294,000		36,294,000
Other Expenses [Abstract]
Operating Lease, Expense	$ 250,000	$ 313,000	$ 1,200,000	$ 936,000

Commitments and Contingencies_2

Commitments and Contingencies - Loss Contingencies (Details) $ in Millions	Sep. 30, 2018USD ($)
Legal Matters [Abstract]
Loss Contingency, Pending Claims, Number	0
Other [Abstract]
Letter of credit obligations	$ 5.8

Commitments and Contingencies_3

Commitments and Contingencies - Supply Commitments (Details) $ in Thousands, ft² in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018USD ($)ft²bldg	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)ft²bldg	Sep. 30, 2017USD ($)	Dec. 31, 2017USD ($)
Supply Commitment [Line Items]
Development in progress	$ 440,503		$ 440,503		$ 333,437
Costs incurred to date	12,924	$ 23,665	120,799	$ 52,497
Building and Building Improvements [Member] | American Water Works, Inc [Member]
Supply Commitment [Line Items]
Costs incurred to date			151,500
Supply Commitment [Member] | Tenant Improvements [Member]
Supply Commitment [Line Items]
Remaining maximum amount committed for tenant improvements	17,100		17,100
Supply Commitment [Member] | Building and Building Improvements [Member] | American Water Works, Inc [Member]
Supply Commitment [Line Items]
Fee development commitment	$ 172,900		$ 172,900
Supply Commitment [Member] | Construction in Progress [Member]
Supply Commitment [Line Items]
Leaseable square feet | ft²	8.2		8.2
Number of Real Estate Properties | bldg	27		27
Total anticipated development cost	$ 698,800		$ 698,800
Deferred leasing costs in development	13,200		13,200
Supply Commitment [Member] | Land, Buildings and Improvements [Member]
Supply Commitment [Line Items]
Remaining amount committed or obligated to pay	$ 15,300		$ 15,300
Equity Method Investments [Member] | Supply Commitment [Member] | Construction in Progress [Member]
Supply Commitment [Line Items]
Number of Real Estate Properties | bldg	3		3
Remaining amount committed or obligated to pay	$ 255,300		$ 255,300

Commitments and Contingencies_4

Commitments and Contingencies - Purchase Commitments (Details) $ in Millions	Sep. 30, 2018USD ($)
Land and Land Improvements [Member] | Land Purchases [Member]
Long-term Purchase Commitment [Line Items]
Remaining amount committed to future acquisitions	$ 53.4

Supplemental Disclosure to St_3

Supplemental Disclosure to Statements of Cash Flows (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Continuing Operations [Member]
Other Significant Noncash Transactions [Line Items]
Write off of fully depreciated property and deferred costs	$ 18,507	$ 17,672
Write Off Of Depreciated Property And Deferred Costs Due To Sale	20,273	13,127
Change in accrued development capital expenditures	9,636	17,744
Unrealized gain (loss) on cash flow hedge, non cash transaction	493	366
Employee Service Share-based Compensation, Allocation of Recognized Period Costs, Capitalized Amount	727	877
Conversion of Stock, Amount Converted	0	27
Discontinued Operations [Member]
Other Significant Noncash Transactions [Line Items]
Write off of fully depreciated property and deferred costs	479	8,937
Write Off Of Depreciated Property And Deferred Costs Due To Sale	99,072	0
Change in accrued development capital expenditures	$ (1,903)	$ 511

Supplemental Disclosure to St_4

Supplemental Disclosure to Statements of Cash Flows Cash and Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Abstract]
Cash and cash equivalents	$ 17,770	$ 11,882	$ 47,666	$ 43,642
Restricted cash	13,371	13,803	16,824	12,383
Cash and cash equivalents and restricted cash	$ 31,141	$ 25,685	$ 64,490	$ 56,025

Subsequent Events (Details)

Subsequent Events (Details) - 2018 acquisitions [Member] $ in Thousands	Oct. 01, 2018USD ($)ft²	Sep. 30, 2018USD ($)ft²bldg
Building and Building Improvements [Member]
Subsequent Event [Line Items]
Number of Real Estate Properties | bldg		7
Leaseable square feet		2,487,083
United Kingdom [Member] | Building and Building Improvements [Member] | Operating Segments [Member]
Subsequent Event [Line Items]
Number of Real Estate Properties | bldg		0
Leaseable square feet		0
United Kingdom [Member] | Building and Building Improvements [Member] | Operating Segments [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Number of Real Estate Properties	0
Leaseable square feet	1,100,000
United Kingdom [Member] | Land, Buildings and Improvements [Member] | Operating Segments [Member]
Subsequent Event [Line Items]
Purchase price | $		$ 4,949
United Kingdom [Member] | Land, Buildings and Improvements [Member] | Operating Segments [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Purchase price | $	$ 144,700