Segment Reporting (Summary on Revenues and Funds from Operations) (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Rental and related revenue	$ 227,708,000	$ 205,588,000	$ 467,744,000	$ 417,084,000
General contractor and service fee revenue ("Service Operations")	135,362,000	168,398,000	281,909,000	282,039,000
Total Segment Revenues	359,805,000	370,412,000	744,405,000	693,283,000
Consolidated Revenue from continuing operations	363,070,000	373,986,000	749,653,000	699,123,000
Discontinued Operations	365,000	11,234,000	3,485,000	23,997,000
Consolidated Revenue	363,435,000	385,220,000	753,138,000	723,120,000
Net earnings excluding depreciation and Non-Segment Items	157,760,000	138,461,000	315,968,000	275,132,000
Interest expense	(66,846,000)	(58,044,000)	(132,950,000)	(114,300,000)
Impairment charges		(7,974,000)		(7,974,000)
Interest and other income	284,000	204,000	371,000	355,000
Other operating expenses	(26,000)	(145,000)	(111,000)	(422,000)
General and administrative expenses	(8,541,000)	(9,151,000)	(19,738,000)	(22,695,000)
Undeveloped land carrying costs	(2,453,000)	(2,542,000)	(4,762,000)	(4,793,000)
Loss on debt transactions		(15,773,000)		(16,127,000)
Acquisition costs	(594,000)		(1,183,000)
Other non-segment income	1,541,000	1,973,000	2,522,000	3,722,000
Net (income) loss attributable to noncontrolling interests	790,000	1,104,000	(293,000)	1,555,000
Noncontrolling interest share of FFO adjustments	(2,802,000)	(2,315,000)	(3,371,000)	(4,593,000)
Joint venture items	10,352,000	12,384,000	18,962,000	24,572,000
Dividends on preferred shares	(15,974,000)	(18,363,000)	(31,948,000)	(36,726,000)
Adjustments for repurchase of preferred shares		(4,492,000)	(163,000)	(4,492,000)
Discontinued operations	212,000	4,289,000	458,000	8,440,000
FFO attributable to common shareholders	73,703,000	39,616,000	143,762,000	101,654,000
Depreciation and amortization on continuing operation	(100,058,000)	(78,956,000)	(194,743,000)	(159,481,000)
Depreciation and amortization on discontinued operations	(55,000)	(3,049,000)	(351,000)	(6,692,000)
Earnings from depreciated property sales on continuing operations	493,000	4,973,000	68,348,000	7,042,000
Company's share of joint venture adjustments	(8,639,000)	(10,372,000)	(16,267,000)	(19,935,000)
Earnings from depreciated property sales - share of joint venture		4,000	91,000	2,308,000
Earnings from depreciated property sales on discontinued operations	2,712,000	3,078,000	14,316,000	12,856,000
Noncontrolling interest share of FFO adjustments	2,802,000	2,315,000	3,371,000	4,593,000
Net income (loss) attributable to common shareholders	(29,042,000)	(42,391,000)	18,527,000	(57,655,000)
Office [Member]
Rental and related revenue	109,671,000	122,790,000	231,787,000	249,703,000
Net earnings excluding depreciation and Non-Segment Items	63,250,000	71,588,000	130,376,000	142,867,000
Industrial [Member]
Rental and related revenue	95,062,000	62,499,000	191,345,000	128,583,000
Net earnings excluding depreciation and Non-Segment Items	69,093,000	47,515,000	136,918,000	95,850,000
Non-reportable Rental Operations segments [Member]
Rental and related revenue	19,710,000	16,725,000	39,364,000	32,958,000
Net earnings excluding depreciation and Non-Segment Items	13,024,000	11,577,000	25,398,000	22,155,000
Other Revenue [Member]
Rental and related revenue	3,265,000	3,574,000	5,248,000	5,840,000
Service Operations [Member]
Net earnings excluding depreciation and Non-Segment Items	$ 12,393,000	$ 7,781,000	$ 23,276,000	$ 14,260,000