Investments in Unconsolidated Joint Ventures (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Schedule of Equity Method Investments [Line Items]
Accumulated depreciation	$ (1,182,569,000)		$ (1,051,593,000)		$ (1,067,272,000)		$ (953,573,000)
Operating real estate	5,351,099,000		5,216,006,000
Development in Process	277,411,000		209,187,000
Land held for development	269,059,000		233,055,000
Other liabilities	219,418,000		250,819,000
Company's net investment in unconsolidated joint ventures	208,832,000		179,655,000
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity	(3,200,000)		41,700,000
Operating income	257,423,000		203,047,000		189,942,000
Interest	(151,887,000)		(127,115,000)		(103,363,000)
Depreciation and amortization	(236,075,000)		(205,147,000)		(167,421,000)
Equity in earnings (loss) of unconsolidated joint ventures	10,314,000		6,067,000		(681,000)
Liberty Venture I, LP [Member]
Schedule of Equity Method Investments [Line Items]
Real estate assets	173,346,000		129,354,000
Accumulated depreciation	(32,752,000)		(32,617,000)
Operating real estate	140,594,000		96,737,000
Development in Process	0		2,226,000
Land held for development	14,108,000		2,842,000
Other assets	15,523,000		11,531,000
Total assets	170,225,000		113,336,000
Debt	78,748,000		72,121,000
Other liabilities	5,215,000		4,032,000
Equity	86,262,000		37,183,000
Total liabilities and equity	170,225,000		113,336,000
Company's net investment in unconsolidated joint ventures	20,010,000	[1]	8,551,000	[1]
Total revenue	19,277,000		16,238,000		15,328,000	[2]
Operating expense	6,398,000		5,248,000		5,277,000	[2]
Operating income	12,879,000		10,990,000		10,051,000	[2]
Interest	(5,078,000)		(5,318,000)		(5,402,000)	[2]
Depreciation and amortization	(5,757,000)		(4,414,000)		(3,930,000)	[2]
Other income/(expense)	(17,000)		53,000		40,000	[2]
Gain (Loss) on Sale of Assets and Asset Impairment Charges	0		0		0	[2]
Net income (loss)	2,027,000		1,311,000		759,000
Equity in earnings (loss) of unconsolidated joint ventures	788,000		530,000		306,000
Kings Hill Unit Trust [Member]
Schedule of Equity Method Investments [Line Items]
Real estate assets	173,707,000		182,220,000
Accumulated depreciation	(26,494,000)		(25,006,000)
Operating real estate	147,213,000		157,214,000
Development in Process	0		0
Land held for development	0		0
Other assets	14,928,000		20,173,000
Total assets	162,141,000		177,387,000
Debt	91,356,000		97,091,000
Other liabilities	94,504,000		100,112,000
Equity	(23,719,000)		(19,816,000)
Total liabilities and equity	162,141,000		177,387,000
Company's net investment in unconsolidated joint ventures	11,663,000	[1]	12,772,000	[1]
Total revenue	13,221,000		12,701,000		15,642,000	[2]
Operating expense	5,387,000		4,187,000		4,237,000	[2]
Operating income	7,834,000		8,514,000		11,405,000	[2]
Interest	(6,110,000)		(5,133,000)		(6,520,000)	[2]
Depreciation and amortization	(4,135,000)		(3,829,000)		(3,731,000)	[2]
Other income/(expense)	208,000		71,000		(160,000)	[2]
Gain (Loss) on Sale of Assets and Asset Impairment Charges	0		(5,647,000)		0	[2]
Net income (loss)	(2,203,000)		(6,024,000)		994,000
Equity in earnings (loss) of unconsolidated joint ventures	(288,000)		(908,000)		352,000
Liberty Illinois, LP [Member]
Schedule of Equity Method Investments [Line Items]
Real estate assets	260,988,000		261,375,000
Accumulated depreciation	(48,191,000)		(43,304,000)
Operating real estate	212,797,000		218,071,000
Development in Process	17,973,000		0
Land held for development	28,362,000		42,770,000
Other assets	20,182,000		20,346,000
Total assets	279,314,000		281,187,000
Debt	145,752,000		140,400,000
Other liabilities	13,812,000		7,936,000
Equity	119,750,000		132,851,000
Total liabilities and equity	279,314,000		281,187,000
Company's net investment in unconsolidated joint ventures	18,692,000	[1]	21,688,000	[1]
Total revenue	25,181,000		24,455,000		22,156,000	[2]
Operating expense	8,965,000		8,353,000		8,093,000	[2]
Operating income	16,216,000		16,102,000		14,063,000	[2]
Interest	(7,932,000)		(8,348,000)		(8,348,000)	[2]
Depreciation and amortization	(7,380,000)		(7,382,000)		(7,395,000)	[2]
Other income/(expense)	39,000		38,000		31,000	[2]
Gain (Loss) on Sale of Assets and Asset Impairment Charges	187,000		0		0	[2]
Net income (loss)	1,130,000		410,000		(1,649,000)
Equity in earnings (loss) of unconsolidated joint ventures	824,000		618,000		106,000
Blythe Valley JV Sarl [Member]
Schedule of Equity Method Investments [Line Items]
Total revenue					14,278,000	[2]
Operating expense					3,921,000	[2]
Operating income					10,357,000	[2]
Interest					(12,130,000)	[2]
Depreciation and amortization					(4,148,000)	[2]
Other income/(expense)					211,000	[2]
Gain (Loss) on Sale of Assets and Asset Impairment Charges					(77,026,000)	[2]
Net income (loss)					(82,736,000)
Equity in earnings (loss) of unconsolidated joint ventures					(5,610,000)
Liberty Washington, LP [Member]
Schedule of Equity Method Investments [Line Items]
Real estate assets	603,413,000		887,452,000
Accumulated depreciation	(35,384,000)		(131,064,000)
Operating real estate	568,029,000		756,388,000
Development in Process	0		0
Land held for development	2,000,000		2,000,000
Other assets	50,657,000		58,701,000
Total assets	620,686,000		817,089,000
Debt	318,367,000		319,856,000
Other liabilities	12,022,000		15,012,000
Equity	290,297,000		482,221,000
Total liabilities and equity	620,686,000		817,089,000
Company's net investment in unconsolidated joint ventures	69,230,000	[1]	72,981,000	[1]
Total revenue	72,824,000		75,821,000		81,128,000	[2]
Operating expense	28,349,000		27,549,000		27,901,000	[2]
Operating income	44,475,000		48,272,000		53,227,000	[2]
Interest	(18,612,000)		(18,946,000)		(21,260,000)	[2]
Depreciation and amortization	(27,112,000)		(28,392,000)		(28,749,000)	[2]
Other income/(expense)	375,000		122,000		149,000	[2]
Gain (Loss) on Sale of Assets and Asset Impairment Charges	(172,691,000)		(8,731,000)		0	[2]
Net income (loss)	(173,565,000)		(7,675,000)		3,367,000
Equity in earnings (loss) of unconsolidated joint ventures	2,251,000		3,748,000		3,243,000
Liberty Commerz 1701 JFK Boulevard, LP [Member]
Schedule of Equity Method Investments [Line Items]
Real estate assets	494,678,000		494,150,000
Accumulated depreciation	(97,189,000)		(83,461,000)
Operating real estate	397,489,000		410,689,000
Development in Process	0		0
Land held for development	0		0
Other assets	52,393,000		50,680,000
Total assets	449,882,000		461,369,000
Debt	317,934,000		321,401,000
Other liabilities	9,657,000		9,544,000
Equity	122,291,000		130,424,000
Total liabilities and equity	449,882,000		461,369,000
Company's net investment in unconsolidated joint ventures	23,153,000	[1]	24,740,000	[1]
Total revenue	63,580,000		62,411,000		62,484,000	[2]
Operating expense	23,557,000		23,074,000		22,935,000	[2]
Operating income	40,023,000		39,337,000		39,549,000	[2]
Interest	(20,179,000)		(20,391,000)		(20,501,000)	[2]
Depreciation and amortization	(14,591,000)		(14,734,000)		(15,411,000)	[2]
Other income/(expense)	(278,000)		(233,000)		(80,000)	[2]
Gain (Loss) on Sale of Assets and Asset Impairment Charges	0		0		0	[2]
Net income (loss)	4,975,000		3,979,000		3,557,000
Equity in earnings (loss) of unconsolidated joint ventures	1,633,000		1,406,000		1,304,000
Liberty Property 18th & Arch [Member] [Member]
Schedule of Equity Method Investments [Line Items]
Real estate assets	0	[3]
Accumulated depreciation	0	[3]
Operating real estate	0	[3]
Development in Process	111,244,000	[3]
Land held for development	0	[3]
Other assets	1,306,000	[3]
Total assets	112,550,000	[3]
Debt	0	[3]
Other liabilities	13,398,000	[3]
Equity	99,152,000	[3]
Total liabilities and equity	112,550,000	[3]
Company's net investment in unconsolidated joint ventures	21,292,000	[1],[3]
Total revenue	0	[3]
Operating expense	199,000	[3]
Operating income	(199,000)	[3]
Interest	0	[3]
Depreciation and amortization	0	[3]
Other income/(expense)	(13,000)	[3]
Gain (Loss) on Sale of Assets and Asset Impairment Charges	0	[3]
Net income (loss)	(212,000)	[3]
Equity in earnings (loss) of unconsolidated joint ventures	(33,000)	[3]
Other Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Real estate assets	69,041,000		70,274,000
Accumulated depreciation	(9,651,000)		(8,456,000)
Operating real estate	59,390,000		61,818,000
Development in Process	4,366,000		0
Land held for development	39,438,000		47,549,000
Other assets	38,215,000		15,911,000
Total assets	141,409,000		125,278,000
Debt	43,114,000		43,151,000
Other liabilities	14,306,000		9,846,000
Equity	83,989,000		72,281,000
Total liabilities and equity	141,409,000		125,278,000
Company's net investment in unconsolidated joint ventures	44,792,000	[1]	38,923,000	[1]
Total revenue	8,923,000		8,415,000		7,481,000	[2]
Operating expense	2,897,000		2,636,000		2,392,000	[2]
Operating income	6,026,000		5,779,000		5,089,000	[2]
Interest	(3,415,000)		(2,933,000)		(2,989,000)	[2]
Depreciation and amortization	(1,891,000)		(1,870,000)		(1,885,000)	[2]
Other income/(expense)	9,090,000	[4]	(37,000)		21,000	[2]
Gain (Loss) on Sale of Assets and Asset Impairment Charges	0		0		0	[2]
Net income (loss)	9,810,000		939,000		236,000
Equity in earnings (loss) of unconsolidated joint ventures	5,139,000		673,000		(382,000)
Equity Method Investee [Member]
Schedule of Equity Method Investments [Line Items]
Real estate assets	1,775,173,000		2,024,825,000
Accumulated depreciation	(249,661,000)		(323,908,000)
Operating real estate	1,525,512,000		1,700,917,000
Development in Process	133,583,000		2,226,000
Land held for development	83,908,000		95,161,000
Other assets	193,204,000		177,342,000
Total assets	1,936,207,000		1,975,646,000
Debt	995,271,000		994,020,000
Other liabilities	162,914,000		146,482,000
Equity	778,022,000		835,144,000
Total liabilities and equity	1,936,207,000		1,975,646,000
Company's net investment in unconsolidated joint ventures	208,832,000	[1]	179,655,000	[1]
Total revenue	203,006,000		200,041,000		218,497,000	[2]
Operating expense	75,752,000		71,047,000		74,756,000	[2]
Operating income	127,254,000		128,994,000		143,741,000	[2]
Interest	(61,326,000)		(61,069,000)		(77,150,000)	[2]
Depreciation and amortization	(60,866,000)		(60,621,000)		(65,249,000)	[2]
Other income/(expense)	9,404,000		14,000		212,000	[2]
Gain (Loss) on Sale of Assets and Asset Impairment Charges	(172,504,000)		(14,378,000)		(77,026,000)	[2]
Net income (loss)	(158,038,000)		(7,060,000)		(75,472,000)
Equity in earnings (loss) of unconsolidated joint ventures	$ 10,314,000		$ 6,067,000		$ (681,000)

[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 increasing the Company's investments in unconsolidated joint ventures by $3.2 million as of December 31, 2014 and reducing the Company's investment in unconsolidated joint ventures by $41.7 million as of December 31, 2013. 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 year ended December 31, 2012 has not been restated for discontinued operations.

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

[4] Other income/(expense) for this group of joint ventures reflects gains related to the sales of land leasehold interests totaling $9.1 million.