Investments in Unconsolidated Joint Ventures (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2013
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Real estate assets		$ 1,694,165		$ 1,775,173
Accumulated depreciation		(285,535)		(249,661)
Real estate assets, net		1,408,630		1,525,512
Development in Process		398,892		133,583
Land held for development		77,556		83,908
Other assets		209,271		193,204
Total assets		2,094,349		1,936,207
Equity Method Investment, Summarized Financial Information, Liabilities and Equity [Abstract]
Debt		1,061,218		995,271
Other liabilities		136,847		162,914
Equity		896,284		778,022
Total liabilities and equity		2,094,349		1,936,207
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Equity in earnings of unconsolidated joint ventures		3,149		10,314		$ 6,067
Company's net investment in unconsolidated joint ventures		218,454		208,832	[1]
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity		4,500		(3,200)
Cambridge Medipark Ltd [Member] | Leaseholds and Leasehold Improvements [Member]
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Gain (loss) on sale/impairment		9,900		9,100
Liberty Venture I, LP [Member]
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Real estate assets		168,690		173,346
Accumulated depreciation		(37,473)		(32,752)
Real estate assets, net		131,217		140,594
Development in Process		34,878		0
Land held for development		4,471		14,108
Other assets		14,860		15,523
Total assets		185,426		170,225
Equity Method Investment, Summarized Financial Information, Liabilities and Equity [Abstract]
Debt		121,793		78,748
Other liabilities		10,571		5,215
Equity		53,062		86,262
Total liabilities and equity		185,426		170,225
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Total revenue		23,708		19,277		16,238
Operating expense		7,977		6,398		5,248
Operating income (loss)		15,731		12,879		10,990
Interest		(5,415)		(5,078)		(5,318)
Depreciation and amortization		(6,399)		(5,757)		(4,414)
Other income/(expense)		68		(17)		53
Gain (loss) on sale/impairment		760		0		0
Net income (loss)		4,745		2,027		1,311
Equity in earnings of unconsolidated joint ventures		1,540		788		530
Company's net investment in unconsolidated joint ventures		11,563		20,010	[1]
Kings Hill Unit Trust [Member]
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Real estate assets		164,603		173,707
Accumulated depreciation		(28,513)		(26,494)
Real estate assets, net		136,090		147,213
Development in Process		0		0
Land held for development		0		0
Other assets		8,967		14,928
Total assets		145,057		162,141
Equity Method Investment, Summarized Financial Information, Liabilities and Equity [Abstract]
Debt		99,603		91,356
Other liabilities		27,408		94,504
Equity		18,046		(23,719)
Total liabilities and equity		145,057		162,141
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Total revenue		12,602		13,221		12,701
Operating expense		5,261		5,387		4,187
Operating income (loss)		7,341		7,834		8,514
Interest		(6,116)		(6,110)		(5,133)
Depreciation and amortization		(3,954)		(4,135)		(3,829)
Other income/(expense)		45,604		208		71
Gain (loss) on sale/impairment		0		0		(5,647)
Net income (loss)		42,875		(2,203)		(6,024)
Equity in earnings of unconsolidated joint ventures		(425)		(288)		(908)
Company's net investment in unconsolidated joint ventures		6,789		11,663	[1]
Liberty Illinois, LP [Member]
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Real estate assets		263,552		260,988
Accumulated depreciation		(54,308)		(48,191)
Real estate assets, net		209,244		212,797
Development in Process		22,998		17,973
Land held for development		28,590		28,362
Other assets		20,668		20,182
Total assets		281,500		279,314
Equity Method Investment, Summarized Financial Information, Liabilities and Equity [Abstract]
Debt		155,601		145,752
Other liabilities		8,594		13,812
Equity		117,305		119,750
Total liabilities and equity		281,500		279,314
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Total revenue		26,085		25,181		24,455
Operating expense		9,303		8,965		8,353
Operating income (loss)		16,782		16,216		16,102
Interest		(8,441)		(7,932)		(8,348)
Depreciation and amortization		(7,406)		(7,380)		(7,382)
Other income/(expense)		29		39		38
Gain (loss) on sale/impairment		0		187		0
Net income (loss)		964		1,130		410
Equity in earnings of unconsolidated joint ventures		807		824		618
Company's net investment in unconsolidated joint ventures		18,221		18,692	[1]
Liberty Washington, LP [Member]
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Real estate assets		534,132		603,413
Accumulated depreciation		(43,224)		(35,384)
Real estate assets, net		490,908		568,029
Development in Process		0		0
Land held for development		2,000		2,000
Other assets		52,429		50,657
Total assets		545,337		620,686
Equity Method Investment, Summarized Financial Information, Liabilities and Equity [Abstract]
Debt		314,080		318,367
Other liabilities		10,634		12,022
Equity		220,623		290,297
Total liabilities and equity		545,337		620,686
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Total revenue		71,101		72,824		75,821
Operating expense		27,384		28,349		27,549
Operating income (loss)		43,717		44,475		48,272
Interest		(17,353)		(18,612)		(18,946)
Depreciation and amortization		(20,725)		(27,112)		(28,392)
Other income/(expense)		531		375		122
Gain (loss) on sale/impairment		(56,792)		(172,691)		(8,731)
Net income (loss)		(50,622)		(173,565)		(7,675)
Equity in earnings of unconsolidated joint ventures		(7,314)		2,251		3,748
Company's net investment in unconsolidated joint ventures		55,552		69,230	[1]
Liberty Comcast 1701 JFK Boulevard, LP [Member]
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Real estate assets		495,248		494,678
Accumulated depreciation		(111,067)		(97,189)
Real estate assets, net		384,181		397,489
Development in Process		0		0
Land held for development		0		0
Other assets		47,441		52,393
Total assets		431,622		449,882
Equity Method Investment, Summarized Financial Information, Liabilities and Equity [Abstract]
Debt		313,402		317,934
Other liabilities		9,618		9,657
Equity		108,602		122,291
Total liabilities and equity		431,622		449,882
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Total revenue		68,444		63,580		62,411
Operating expense		29,692		23,557		23,074
Operating income (loss)		38,752		40,023		39,337
Interest		(19,936)		(20,179)		(20,391)
Depreciation and amortization		(14,442)		(14,591)		(14,734)
Other income/(expense)		(227)		(278)		(233)
Gain (loss) on sale/impairment		0		0		0
Net income (loss)		4,147		4,975		3,979
Equity in earnings of unconsolidated joint ventures		2,053		1,633		1,406
Company's net investment in unconsolidated joint ventures		20,465		23,153	[1],[2]
Liberty Property 18th & Arch [Member]
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Real estate assets		0	[3]	0	[4]
Accumulated depreciation		0	[3]	0	[4]
Real estate assets, net		0	[3]	0	[4]
Development in Process		323,615	[3]	111,244	[4]
Land held for development		0	[3]	0	[4]
Other assets		15,594	[3]	1,306	[4]
Total assets		339,209	[3]	112,550	[4]
Equity Method Investment, Summarized Financial Information, Liabilities and Equity [Abstract]
Debt		0		0	[4]
Other liabilities		40,958		13,398	[4]
Equity		298,251	[3]	99,152	[4]
Total liabilities and equity		339,209	[3]	112,550	[4]
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Total revenue	[4]	0		0
Operating expense	[4]	221		199
Operating income (loss)	[4]	(221)		(199)
Interest	[4]	0		0
Depreciation and amortization	[4]	0		0
Other income/(expense)	[4]	30		(13)
Gain (loss) on sale/impairment	[4]	0		0
Net income (loss)	[4]	(191)		(212)
Equity in earnings of unconsolidated joint ventures	[4]	(29)		(33)
Company's net investment in unconsolidated joint ventures		62,677		21,292	[1],[4]
Other Joint Ventures [Member]
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Real estate assets		67,940		69,041
Accumulated depreciation		(10,950)		(9,651)
Real estate assets, net		56,990		59,390
Development in Process		17,401		4,366
Land held for development		42,495		39,438
Other assets		49,312		38,215
Total assets		166,198		141,409
Equity Method Investment, Summarized Financial Information, Liabilities and Equity [Abstract]
Debt		56,739		43,114
Other liabilities		29,064		14,306
Equity		80,395		83,989
Total liabilities and equity		166,198		141,409
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Total revenue		9,387	[5]	8,923	[5]	8,415
Operating expense		2,727	[5]	2,897	[5]	2,636
Operating income (loss)		6,660	[5]	6,026	[5]	5,779
Interest		(2,248)	[5]	(3,415)	[5]	(2,933)
Depreciation and amortization		(1,840)	[5]	(1,891)	[5]	(1,870)
Other income/(expense)		9,925	[5]	9,090	[5]	(37)
Gain (loss) on sale/impairment		0	[5]	0	[5]	0
Net income (loss)		12,497	[5]	9,810	[5]	939
Equity in earnings of unconsolidated joint ventures		6,517	[5]	5,139	[5]	673
Company's net investment in unconsolidated joint ventures		43,187		44,792	[1]
Equity Method Investee [Member]
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Total revenue		211,327		203,006		200,041
Operating expense		82,565		75,752		71,047
Operating income (loss)		128,762		127,254		128,994
Interest		(59,509)		(61,326)		(61,069)
Depreciation and amortization		(54,766)		(60,866)		(60,621)
Other income/(expense)		55,960		9,404		14
Gain (loss) on sale/impairment		(56,032)		(172,504)		(14,378)
Net income (loss)		14,415		(158,038)		(7,060)
Equity in earnings of unconsolidated joint ventures		$ 3,149		$ 10,314		$ 6,067

[1] Differences between the Company's net investment in unconsolidated joint ventures and its underlying equity in the net assets of the venture are primarily a result of impairments related to the Company's investment in unconsolidated joint ventures, the deferral of gains associated with the sales of properties to joint ventures in which the Company retains an ownership interest and loans made to the joint ventures by the Company. These adjustments have resulted in an aggregate difference reducing the Company's investments in unconsolidated joint ventures by $4.5 million as of December 31, 2015 and increasing the Company's investment in unconsolidated joint ventures by $3.2 million as of December 31, 2014. Differences between historical cost basis and the basis reflected at the joint venture level (other than loans) are typically depreciated over the life of the related asset.

[2] The Company's maximum exposure to loss is equal to the Company's net investment in unconsolidated joint ventures as of December 31, 2015 and 2014, respectively.

[3] Other income/(expense) for this joint venture reflects forgiveness of related party debt between the joint venture and its investment partners consistent with each partner's equity ownership.

[4] Represents the combined results of two joint ventures related to the property at 18th and Arch Streets, Philadelphia.

[5] Other income/(expense) for this group of joint ventures reflects gains related to the sales of land leasehold interests totaling $9.9 million and $9.1 million for the years ended December 31, 2015 and 2014, respectively.