Investments in Unconsolidated Joint Ventures (Details) (USD $)	3 Months Ended										12 Months Ended
	Dec. 31, 2011		Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010		Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009	Dec. 31, 2008
Schedule of Equity Method Investments [Line Items]
Real estate assets	$ 4,973,962,000					$ 4,704,162,000					$ 4,973,962,000		$ 4,704,162,000
Accumulated depreciation	(1,062,230,000)					(938,044,000)					(1,062,230,000)		(938,044,000)		(836,146,000)	(733,688,000)
Operating real estate	3,911,732,000					3,766,118,000					3,911,732,000		3,766,118,000
Land held for development	219,375,000					203,811,000					219,375,000		203,811,000
Other liabilities	194,995,000					156,803,000					194,995,000		156,803,000
Company's net investment in unconsolidated joint ventures	174,687,000					171,916,000					174,687,000		171,916,000
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity	42,700,000										42,700,000
Total revenue	169,801,000		166,247,000	165,027,000	166,519,000	164,530,000		166,316,000	161,789,000	164,366,000	667,594,000		657,001,000		644,696,000
Operating income											245,991,000		249,893,000		246,805,000
Interest											(122,018,000)		(134,384,000)		(133,997,000)
Depreciation and amortization											(171,714,000)		(174,013,000)		(172,575,000)
Gain (loss) on sale											5,025,000		4,616,000		1,687,000
Impairment charges - investment in unconsolidated joint ventures and other											0		(378,000)		(82,552,000)
Equity in earnings of unconsolidated joint ventures											3,496,000		2,296,000		2,161,000
Liberty Venture I, LP [Member]
Schedule of Equity Method Investments [Line Items]
Real estate assets	126,996,000					133,443,000					126,996,000		133,443,000
Accumulated depreciation	(25,466,000)					(23,764,000)					(25,466,000)		(23,764,000)
Operating real estate	101,530,000					109,679,000					101,530,000		109,679,000
Land held for development	2,760,000					2,741,000					2,760,000		2,741,000
Other assets	10,386,000					11,475,000					10,386,000		11,475,000
Total assets	114,676,000					123,895,000					114,676,000		123,895,000
Debt	74,651,000					75,801,000					74,651,000		75,801,000
Other liabilities	3,291,000					2,686,000					3,291,000		2,686,000
Equity	36,734,000					45,408,000					36,734,000		45,408,000
Total liabilities and equity	114,676,000					123,895,000					114,676,000		123,895,000
Company's net investment in unconsolidated joint ventures	8,428,000	[1]				10,600,000	[1]				8,428,000	[1]	10,600,000	[1]
Total revenue											17,008,000		17,089,000		18,407,000
Operating expense											5,912,000		7,352,000		6,559,000
Operating income											11,096,000		9,737,000		11,848,000
Interest											(5,472,000)		(5,879,000)		(4,992,000)
Depreciation and amortization											(4,088,000)		(5,099,000)		(4,255,000)
Other income/(expense)											985,000		11,000		131,000
Gain (loss) on sale											1,515,000
Impairment charges - investment in unconsolidated joint ventures and other															0
Net income (loss)											4,036,000		(1,230,000)		2,732,000
Equity in earnings of unconsolidated joint ventures											1,212,000		(107,000)		905,000
Kings Hill Unit Trust [Member]
Schedule of Equity Method Investments [Line Items]
Real estate assets	183,097,000					183,763,000					183,097,000		183,763,000
Accumulated depreciation	(18,510,000)					(15,861,000)					(18,510,000)		(15,861,000)
Operating real estate	164,587,000					167,902,000					164,587,000		167,902,000
Land held for development	0					0					0		0
Other assets	11,528,000					6,480,000					11,528,000		6,480,000
Total assets	176,115,000					174,382,000					176,115,000		174,382,000
Debt	112,835,000					140,859,000					112,835,000		140,859,000
Other liabilities	71,539,000					41,058,000					71,539,000		41,058,000
Equity	(8,259,000)					(7,535,000)					(8,259,000)		(7,535,000)
Total liabilities and equity	176,115,000					174,382,000					176,115,000		174,382,000
Company's net investment in unconsolidated joint ventures	9,634,000	[1]				3,286,000	[1]				9,634,000	[1]	3,286,000	[1]
Total revenue											16,389,000		15,980,000		16,197,000
Operating expense											3,372,000		2,471,000		2,649,000
Operating income											13,017,000		13,509,000		13,548,000
Interest											(5,979,000)		(3,078,000)		(7,674,000)
Depreciation and amortization											(4,219,000)		(4,197,000)		(4,559,000)
Other income/(expense)											(511,000)		(564,000)		(602,000)
Gain (loss) on sale											0
Impairment charges - investment in unconsolidated joint ventures and other															0
Net income (loss)											2,308,000		5,670,000		713,000
Equity in earnings of unconsolidated joint ventures											637,000		1,317,000		334,000
Liberty Illinois, LP [Member]
Schedule of Equity Method Investments [Line Items]
Real estate assets	256,441,000					252,140,000					256,441,000		252,140,000
Accumulated depreciation	(30,633,000)					(24,436,000)					(30,633,000)		(24,436,000)
Operating real estate	225,808,000					227,704,000					225,808,000		227,704,000
Land held for development	42,670,000					42,698,000					42,670,000		42,698,000
Other assets	12,667,000					13,514,000					12,667,000		13,514,000
Total assets	281,145,000					283,916,000					281,145,000		283,916,000
Debt	140,400,000					140,400,000					140,400,000		140,400,000
Other liabilities	6,683,000					5,897,000					6,683,000		5,897,000
Equity	134,062,000					137,619,000					134,062,000		137,619,000
Total liabilities and equity	281,145,000					283,916,000					281,145,000		283,916,000
Company's net investment in unconsolidated joint ventures	21,348,000	[1]				21,959,000	[1]				21,348,000	[1]	21,959,000	[1]
Total revenue											20,245,000		20,160,000		22,191,000
Operating expense											8,055,000		7,782,000		8,350,000
Operating income											12,190,000		12,378,000		13,841,000
Interest											(8,348,000)		(7,966,000)		(7,469,000)
Depreciation and amortization											(7,342,000)		(7,051,000)		(7,117,000)
Other income/(expense)											(56,000)		(48,000)		(50,000)
Gain (loss) on sale											0
Impairment charges - investment in unconsolidated joint ventures and other															0
Net income (loss)											(3,556,000)		(2,687,000)		(795,000)
Equity in earnings of unconsolidated joint ventures											(394,000)		(174,000)		181,000
Blythe Valley JV Sarl [Member]
Schedule of Equity Method Investments [Line Items]
Real estate assets	192,847,000					202,544,000					192,847,000		202,544,000
Accumulated depreciation	(18,781,000)					(14,887,000)					(18,781,000)		(14,887,000)
Operating real estate	174,066,000					187,657,000					174,066,000		187,657,000
Land held for development	36,868,000					36,897,000					36,868,000		36,897,000
Other assets	10,640,000					12,221,000					10,640,000		12,221,000
Total assets	221,574,000					236,775,000					221,574,000		236,775,000
Debt	184,436,000					192,132,000					184,436,000		192,132,000
Other liabilities	73,737,000					67,060,000					73,737,000		67,060,000
Equity	(36,599,000)					(22,417,000)					(36,599,000)		(22,417,000)
Total liabilities and equity	221,574,000					236,775,000					221,574,000		236,775,000
Company's net investment in unconsolidated joint ventures	3,663,000	[1]				2,298,000	[1]				3,663,000	[1]	2,298,000	[1]
Total revenue											13,950,000		13,270,000		13,325,000
Operating expense											3,942,000		3,401,000		6,120,000
Operating income											10,008,000		9,869,000		7,205,000
Interest											(14,991,000)		(14,783,000)		(13,084,000)
Depreciation and amortization											(4,951,000)		(5,315,000)		(5,576,000)
Other income/(expense)											(191,000)		(289,000)		(175,000)
Gain (loss) on sale											(1,605,000)
Impairment charges - investment in unconsolidated joint ventures and other															0
Net income (loss)											(11,730,000)		(10,518,000)		(11,630,000)
Equity in earnings of unconsolidated joint ventures											(1,898,000)		(1,980,000)		(1,318,000)
Liberty Washington, LP [Member]
Schedule of Equity Method Investments [Line Items]
Real estate assets	917,879,000					909,485,000					917,879,000		909,485,000
Accumulated depreciation	(93,569,000)					(71,513,000)					(93,569,000)		(71,513,000)
Operating real estate	824,310,000					837,972,000					824,310,000		837,972,000
Land held for development	2,000,000					2,000,000					2,000,000		2,000,000
Other assets	58,125,000					55,138,000					58,125,000		55,138,000
Total assets	884,435,000					895,110,000					884,435,000		895,110,000
Debt	349,234,000					347,990,000					349,234,000		347,990,000
Other liabilities	23,995,000					34,161,000					23,995,000		34,161,000
Equity	511,206,000					512,959,000					511,206,000		512,959,000
Total liabilities and equity	884,435,000					895,110,000					884,435,000		895,110,000
Company's net investment in unconsolidated joint ventures	74,893,000	[1]				74,345,000	[1]				74,893,000	[1]	74,345,000	[1]
Total revenue											76,811,000		72,824,000		74,850,000
Operating expense											27,074,000		25,614,000		26,159,000
Operating income											49,737,000		47,210,000		48,691,000
Interest											(22,998,000)		(20,486,000)		(17,086,000)
Depreciation and amortization											(28,618,000)		(29,132,000)		(30,727,000)
Other income/(expense)											125,000		165,000		280,000
Gain (loss) on sale											0
Impairment charges - investment in unconsolidated joint ventures and other															(39,380,000)
Net income (loss)											(1,754,000)		(2,243,000)		(38,222,000)
Equity in earnings of unconsolidated joint ventures											1,889,000		1,624,000		1,649,000
Liberty/Commerz 1701 JFK Boulevard, LP [Member]
Schedule of Equity Method Investments [Line Items]
Real estate assets	493,737,000					493,196,000					493,737,000		493,196,000
Accumulated depreciation	(55,588,000)					(41,733,000)					(55,588,000)		(41,733,000)
Operating real estate	438,149,000					451,463,000					438,149,000		451,463,000
Land held for development	0					0					0		0
Other assets	51,043,000					49,457,000					51,043,000		49,457,000
Total assets	489,192,000					500,920,000					489,192,000		500,920,000
Debt	324,000,000					324,000,000					324,000,000		324,000,000
Other liabilities	11,069,000					10,775,000					11,069,000		10,775,000
Equity	154,123,000					166,145,000					154,123,000		166,145,000
Total liabilities and equity	489,192,000					500,920,000					489,192,000		500,920,000
Company's net investment in unconsolidated joint ventures	31,615,000	[1]				34,355,000	[1]				31,615,000	[1]	34,355,000	[1]
Total revenue											62,225,000		61,444,000		61,094,000
Operating expense											20,575,000		21,417,000		21,170,000
Operating income											41,650,000		40,027,000		39,924,000
Interest											(20,445,000)		(20,445,000)		(20,455,000)
Depreciation and amortization											(15,494,000)		(15,479,000)		(15,498,000)
Other income/(expense)											(2,046,000)		(781,000)		(378,000)
Gain (loss) on sale											0
Impairment charges - investment in unconsolidated joint ventures and other															0
Net income (loss)											3,665,000		3,322,000		3,593,000
Equity in earnings of unconsolidated joint ventures											1,314,000		1,157,000		1,241,000
Other Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Real estate assets	68,363,000					68,614,000					68,363,000		68,614,000
Accumulated depreciation	(5,242,000)					(3,786,000)					(5,242,000)		(3,786,000)
Operating real estate	63,121,000					64,828,000					63,121,000		64,828,000
Land held for development	14,929,000					21,848,000					14,929,000		21,848,000
Other assets	24,852,000					28,959,000					24,852,000		28,959,000
Total assets	102,902,000					115,635,000					102,902,000		115,635,000
Debt	44,691,000					53,996,000					44,691,000		53,996,000
Other liabilities	8,204,000					10,927,000					8,204,000		10,927,000
Equity	50,007,000					50,712,000					50,007,000		50,712,000
Total liabilities and equity	102,902,000					115,635,000					102,902,000		115,635,000
Company's net investment in unconsolidated joint ventures	25,106,000	[1]				25,073,000	[1]				25,106,000	[1]	25,073,000	[1]
Total revenue											7,212,000		7,276,000		4,928,000
Operating expense											1,869,000		1,271,000		2,435,000
Operating income											5,343,000		6,005,000		2,493,000
Interest											(3,169,000)		(3,485,000)		(2,915,000)
Depreciation and amortization											(1,793,000)		(1,979,000)		(1,594,000)
Other income/(expense)											(509,000)		20,000		156,000
Gain (loss) on sale											1,253,000
Impairment charges - investment in unconsolidated joint ventures and other															0
Net income (loss)											1,125,000		561,000		(1,860,000)
Equity in earnings of unconsolidated joint ventures											736,000		459,000		(831,000)
Equity Method Investee [Member]
Schedule of Equity Method Investments [Line Items]
Real estate assets	2,239,360,000					2,243,185,000					2,239,360,000		2,243,185,000
Accumulated depreciation	(247,789,000)					(195,980,000)					(247,789,000)		(195,980,000)
Operating real estate	1,991,571,000					2,047,205,000					1,991,571,000		2,047,205,000
Land held for development	99,227,000					106,184,000					99,227,000		106,184,000
Other assets	179,241,000					177,244,000					179,241,000		177,244,000
Total assets	2,270,039,000					2,330,633,000					2,270,039,000		2,330,633,000
Debt	1,230,247,000					1,275,178,000					1,230,247,000		1,275,178,000
Other liabilities	198,518,000					172,564,000					198,518,000		172,564,000
Equity	841,274,000					882,891,000					841,274,000		882,891,000
Total liabilities and equity	2,270,039,000					2,330,633,000					2,270,039,000		2,330,633,000
Company's net investment in unconsolidated joint ventures	174,687,000	[1]				171,916,000	[1]				174,687,000	[1]	171,916,000	[1]
Total revenue											213,840,000		208,043,000		210,992,000
Operating expense											70,799,000		69,308,000		73,442,000
Operating income											143,041,000		138,735,000		137,550,000
Interest											(81,402,000)		(76,122,000)		(73,675,000)
Depreciation and amortization											(66,505,000)		(68,252,000)		(69,326,000)
Other income/(expense)											(2,203,000)		(1,486,000)		(638,000)
Gain (loss) on sale											1,163,000
Impairment charges - investment in unconsolidated joint ventures and other															(39,380,000)
Net income (loss)											(5,906,000)		(7,125,000)		(45,469,000)
Equity in earnings of unconsolidated joint ventures											$ 3,496,000		$ 2,296,000		$ 2,161,000

[1] Differences between the Company's net investment in unconsolidated joint ventures and its underlying equity in the net assets of the venture is 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 $42.7 million as of December 31, 2011. 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.