Investments in Unconsolidated Joint Ventures (Details) (USD $)	3 Months Ended											12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011		Sep. 30, 2011	Jun. 30, 2011		Mar. 31, 2011	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010	Dec. 31, 2009
Schedule of Equity Method Investments [Line Items]
Real estate assets	$ 5,760,860,000					$ 5,253,064,000						$ 5,760,860,000		$ 5,253,064,000		$ 4,879,278,000	$ 4,810,239,000
Accumulated depreciation	(1,170,030,000)					(1,047,336,000)						(1,170,030,000)		(1,047,336,000)		(923,790,000)	(822,648,000)
Operating real estate	4,083,904,000					3,897,505,000						4,083,904,000		3,897,505,000
Land held for development	258,324,000					219,375,000						258,324,000		219,375,000
Other liabilities	185,956,000					194,995,000						185,956,000		194,995,000
Company's net investment in unconsolidated joint ventures	169,021,000					174,687,000						169,021,000		174,687,000
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity	28,500,000					42,700,000						28,500,000		42,700,000
Total revenue	176,227,000		171,727,000	168,567,000	169,031,000	168,704,000		165,192,000	163,948,000	[1]	165,397,000	685,552,000		663,241,000		652,009,000
Operating income												241,705,000		244,889,000		248,610,000
Interest												(119,630,000)		(120,718,000)		(132,951,000)
Depreciation and amortization												(167,421,000)		(171,714,000)		(174,013,000)
Impairment of Real Estate												(2,300,000)		(7,900,000)		(579,000)
Gain (Loss) on Sale of Properties												15,506,000		65,607,000		11,095,000
Equity in (loss) earnings of unconsolidated joint ventures												(681,000)		3,496,000		2,296,000
Liberty Venture I, LP [Member]
Schedule of Equity Method Investments [Line Items]
Real estate assets	129,296,000					126,996,000						129,296,000		126,996,000
Accumulated depreciation	(28,849,000)					(25,466,000)						(28,849,000)		(25,466,000)
Operating real estate	100,447,000					101,530,000						100,447,000		101,530,000
Land held for development	2,760,000					2,760,000						2,760,000		2,760,000
Other assets	9,845,000					10,386,000						9,845,000		10,386,000
Total assets	113,052,000					114,676,000						113,052,000		114,676,000
Debt	73,426,000					74,651,000						73,426,000		74,651,000
Other liabilities	3,754,000					3,291,000						3,754,000		3,291,000
Equity	35,872,000					36,734,000						35,872,000		36,734,000
Total liabilities and equity	113,052,000					114,676,000						113,052,000		114,676,000
Company's net investment in unconsolidated joint ventures	8,205,000	[2]				8,428,000	[2]					8,205,000	[2]	8,428,000	[2]
Total revenue												15,328,000		17,008,000		17,089,000
Operating expense												5,277,000		5,912,000		7,352,000
Operating income												10,051,000		11,096,000		9,737,000
Interest												(5,402,000)		(5,472,000)		(5,879,000)
Depreciation and amortization												(3,930,000)		(4,088,000)		(5,099,000)
Other income/(expense)												40,000		985,000		11,000
Impairment of Real Estate												0
Gain (Loss) on Sale of Properties														1,515,000
Net income (loss)												759,000		4,036,000		(1,230,000)
Equity in (loss) earnings of unconsolidated joint ventures												306,000		1,212,000		(107,000)
Kings Hill Unit Trust [Member]
Schedule of Equity Method Investments [Line Items]
Real estate assets	193,628,000					183,097,000						193,628,000		183,097,000
Accumulated depreciation	(22,666,000)					(18,510,000)						(22,666,000)		(18,510,000)
Operating real estate	170,962,000					164,587,000						170,962,000		164,587,000
Land held for development	0					0						0		0
Other assets	13,736,000					11,528,000						13,736,000		11,528,000
Total assets	184,698,000					176,115,000						184,698,000		176,115,000
Debt	117,308,000					112,835,000						117,308,000		112,835,000
Other liabilities	77,832,000					71,539,000						77,832,000		71,539,000
Equity	(10,442,000)					(8,259,000)						(10,442,000)		(8,259,000)
Total liabilities and equity	184,698,000					176,115,000						184,698,000		176,115,000
Company's net investment in unconsolidated joint ventures	10,341,000	[2]				9,634,000	[2]					10,341,000	[2]	9,634,000	[2]
Total revenue												15,642,000		16,389,000		15,980,000
Operating expense												4,237,000		3,372,000		2,471,000
Operating income												11,405,000		13,017,000		13,509,000
Interest												(6,520,000)		(5,979,000)		(3,078,000)
Depreciation and amortization												(3,731,000)		(4,219,000)		(4,197,000)
Other income/(expense)												(160,000)		(511,000)		(564,000)
Impairment of Real Estate												0
Gain (Loss) on Sale of Properties														0
Net income (loss)												994,000		2,308,000		5,670,000
Equity in (loss) earnings of unconsolidated joint ventures												352,000		637,000		1,317,000
Liberty Illinois, LP [Member]
Schedule of Equity Method Investments [Line Items]
Real estate assets	259,786,000					256,441,000						259,786,000		256,441,000
Accumulated depreciation	(36,978,000)					(30,633,000)						(36,978,000)		(30,633,000)
Operating real estate	222,808,000					225,808,000						222,808,000		225,808,000
Land held for development	42,734,000					42,670,000						42,734,000		42,670,000
Other assets	14,974,000					12,667,000						14,974,000		12,667,000
Total assets	280,516,000					281,145,000						280,516,000		281,145,000
Debt	140,400,000					140,400,000						140,400,000		140,400,000
Other liabilities	7,675,000					6,683,000						7,675,000		6,683,000
Equity	132,441,000					134,062,000						132,441,000		134,062,000
Total liabilities and equity	280,516,000					281,145,000						280,516,000		281,145,000
Company's net investment in unconsolidated joint ventures	21,331,000	[2]				21,348,000	[2]					21,331,000	[2]	21,348,000	[2]
Total revenue												22,156,000		20,245,000		20,160,000
Operating expense												8,093,000		8,055,000		7,782,000
Operating income												14,063,000		12,190,000		12,378,000
Interest												(8,348,000)		(8,348,000)		(7,966,000)
Depreciation and amortization												(7,395,000)		(7,342,000)		(7,051,000)
Other income/(expense)												31,000		(56,000)		(48,000)
Impairment of Real Estate												0
Gain (Loss) on Sale of Properties														0
Net income (loss)												(1,649,000)		(3,556,000)		(2,687,000)
Equity in (loss) earnings of unconsolidated joint ventures												106,000		(394,000)		(174,000)
Blythe Valley JV Sarl [Member]
Schedule of Equity Method Investments [Line Items]
Real estate assets	124,204,000					192,847,000						124,204,000		192,847,000
Accumulated depreciation	(23,764,000)					(18,781,000)						(23,764,000)		(18,781,000)
Operating real estate	100,440,000					174,066,000						100,440,000		174,066,000
Land held for development	38,683,000					36,868,000						38,683,000		36,868,000
Other assets	10,934,000					10,640,000						10,934,000		10,640,000
Total assets	150,057,000					221,574,000						150,057,000		221,574,000
Debt	192,803,000					184,436,000						192,803,000		184,436,000
Other liabilities	80,326,000					73,737,000						80,326,000		73,737,000
Equity	(123,072,000)					(36,599,000)						(123,072,000)		(36,599,000)
Total liabilities and equity	150,057,000					221,574,000						150,057,000		221,574,000
Company's net investment in unconsolidated joint ventures	0	[2]				3,663,000	[2]					0	[2]	3,663,000	[2]
Total revenue												14,278,000		13,950,000		13,270,000
Operating expense												3,921,000		3,942,000		3,401,000
Operating income												10,357,000		10,008,000		9,869,000
Interest												(12,130,000)		(14,991,000)		(14,783,000)
Depreciation and amortization												(4,148,000)		(4,951,000)		(5,315,000)
Other income/(expense)												211,000		(191,000)		(289,000)
Impairment of Real Estate												77,026,000
Gain (Loss) on Sale of Properties														(1,605,000)
Impairment												(4,600,000)
Net income (loss)												(82,736,000)		(11,730,000)		(10,518,000)
Equity in (loss) earnings of unconsolidated joint ventures												(5,610,000)		(1,898,000)		(1,980,000)
Liberty Washington, LP [Member]
Schedule of Equity Method Investments [Line Items]
Real estate assets	929,790,000					917,879,000						929,790,000		917,879,000
Accumulated depreciation	(116,072,000)					(93,569,000)						(116,072,000)		(93,569,000)
Operating real estate	813,718,000					824,310,000						813,718,000		824,310,000
Land held for development	2,000,000					2,000,000						2,000,000		2,000,000
Other assets	62,647,000					58,125,000						62,647,000		58,125,000
Total assets	878,365,000					884,435,000						878,365,000		884,435,000
Debt	341,804,000					349,234,000						341,804,000		349,234,000
Other liabilities	21,989,000					23,995,000						21,989,000		23,995,000
Equity	514,572,000					511,206,000						514,572,000		511,206,000
Total liabilities and equity	878,365,000					884,435,000						878,365,000		884,435,000
Company's net investment in unconsolidated joint ventures	76,965,000	[2]				74,893,000	[2]					76,965,000	[2]	74,893,000	[2]
Total revenue												81,128,000		76,811,000		72,824,000
Operating expense												27,901,000		27,074,000		25,614,000
Operating income												53,227,000		49,737,000		47,210,000
Interest												(21,260,000)		(22,998,000)		(20,486,000)
Depreciation and amortization												(28,749,000)		(28,618,000)		(29,132,000)
Other income/(expense)												149,000		125,000		165,000
Impairment of Real Estate												0
Gain (Loss) on Sale of Properties														0
Net income (loss)												3,367,000		(1,754,000)		(2,243,000)
Equity in (loss) earnings of unconsolidated joint ventures												3,243,000		1,889,000		1,624,000
Liberty Commerz 1701 JFK Boulevard, LP [Member]
Schedule of Equity Method Investments [Line Items]
Real estate assets	494,045,000					493,737,000						494,045,000		493,737,000
Accumulated depreciation	(69,572,000)					(55,588,000)						(69,572,000)		(55,588,000)
Operating real estate	424,473,000					438,149,000						424,473,000		438,149,000
Land held for development	0					0						0		0
Other assets	50,638,000					51,043,000						50,638,000		51,043,000
Total assets	475,111,000					489,192,000						475,111,000		489,192,000
Debt	324,000,000					324,000,000						324,000,000		324,000,000
Other liabilities	9,257,000					11,069,000						9,257,000		11,069,000
Equity	141,854,000					154,123,000						141,854,000		154,123,000
Total liabilities and equity	475,111,000					489,192,000						475,111,000		489,192,000
Company's net investment in unconsolidated joint ventures	27,305,000	[2]				31,615,000	[2]					27,305,000	[2]	31,615,000	[2]
Total revenue												62,484,000		62,225,000		61,444,000
Operating expense												22,935,000		20,575,000		21,417,000
Operating income												39,549,000		41,650,000		40,027,000
Interest												(20,501,000)		(20,445,000)		(20,445,000)
Depreciation and amortization												(15,411,000)		(15,494,000)		(15,479,000)
Other income/(expense)												(80,000)		(2,046,000)		(781,000)
Impairment of Real Estate												0
Gain (Loss) on Sale of Properties														0
Net income (loss)												3,557,000		3,665,000		3,322,000
Equity in (loss) earnings of unconsolidated joint ventures												1,304,000		1,314,000		1,157,000
Other Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Real estate assets	69,680,000					68,363,000						69,680,000		68,363,000
Accumulated depreciation	(6,853,000)					(5,242,000)						(6,853,000)		(5,242,000)
Operating real estate	62,827,000					63,121,000						62,827,000		63,121,000
Land held for development	23,193,000					14,929,000						23,193,000		14,929,000
Other assets	14,294,000					24,852,000						14,294,000		24,852,000
Total assets	100,314,000					102,902,000						100,314,000		102,902,000
Debt	43,946,000					44,691,000						43,946,000		44,691,000
Other liabilities	6,173,000					8,204,000						6,173,000		8,204,000
Equity	50,195,000					50,007,000						50,195,000		50,007,000
Total liabilities and equity	100,314,000					102,902,000						100,314,000		102,902,000
Company's net investment in unconsolidated joint ventures	24,874,000	[2]				25,106,000	[2]					24,874,000	[2]	25,106,000	[2]
Total revenue												7,481,000		7,212,000		7,276,000
Operating expense												2,392,000		1,869,000		1,271,000
Operating income												5,089,000		5,343,000		6,005,000
Interest												(2,989,000)		(3,169,000)		(3,485,000)
Depreciation and amortization												(1,885,000)		(1,793,000)		(1,979,000)
Other income/(expense)												21,000		(509,000)		20,000
Impairment of Real Estate												0
Gain (Loss) on Sale of Properties														1,253,000
Net income (loss)												236,000		1,125,000		561,000
Equity in (loss) earnings of unconsolidated joint ventures												(382,000)		736,000		459,000
Equity Method Investee [Member]
Schedule of Equity Method Investments [Line Items]
Real estate assets	2,200,429,000					2,239,360,000						2,200,429,000		2,239,360,000
Accumulated depreciation	(304,754,000)					(247,789,000)						(304,754,000)		(247,789,000)
Operating real estate	1,895,675,000					1,991,571,000						1,895,675,000		1,991,571,000
Land held for development	109,370,000					99,227,000						109,370,000		99,227,000
Other assets	177,068,000					179,241,000						177,068,000		179,241,000
Total assets	2,182,113,000					2,270,039,000						2,182,113,000		2,270,039,000
Debt	1,233,687,000					1,230,247,000						1,233,687,000		1,230,247,000
Other liabilities	207,006,000					198,518,000						207,006,000		198,518,000
Equity	741,420,000					841,274,000						741,420,000		841,274,000
Total liabilities and equity	2,182,113,000					2,270,039,000						2,182,113,000		2,270,039,000
Company's net investment in unconsolidated joint ventures	169,021,000	[2]				174,687,000	[2]					169,021,000	[2]	174,687,000	[2]
Total revenue												218,497,000		213,840,000		208,043,000
Operating expense												74,756,000		70,799,000		69,308,000
Operating income												143,741,000		143,041,000		138,735,000
Interest												(77,150,000)		(81,402,000)		(76,122,000)
Depreciation and amortization												(65,249,000)		(66,505,000)		(68,252,000)
Other income/(expense)												212,000		(2,203,000)		(1,486,000)
Impairment of Real Estate												77,026,000
Gain (Loss) on Sale of Properties														1,163,000
Net income (loss)												(75,472,000)		(5,906,000)		(7,125,000)
Equity in (loss) earnings of unconsolidated joint ventures												$ (681,000)		$ 3,496,000		$ 2,296,000

[1] Includes gain on sale included in discontinued operations of $50.2 million.

[2] 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 $28.5 million and $42.7 million as of December 31, 2012 and 2011, respectively. 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.