Segment Reporting (Summary on Revenues and Funds from Operations) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Rental and related revenue	$ 752,478,000	$ 678,795,000	$ 634,455,000
General contractor and service fee revenue (''Service Operations'')	521,796,000	515,361,000	449,509,000
Total Segment Revenues	1,262,730,000	1,183,062,000	1,071,587,000
Consolidated Revenue from continuing operations	1,274,274,000	1,194,156,000	1,083,964,000
Discontinued Operations	184,571,000	238,772,000	264,240,000
Consolidated Revenue	1,458,845,000	1,432,928,000	1,348,204,000
Net earnings excluding depreciation and Non-Segment Items	536,766,000	472,093,000	436,111,000
Interest expense	(223,053,000)	(189,094,000)	(151,605,000)
Impairment charges on non-depreciable properties	(12,931,000)	(9,834,000)	(168,511,000)
Interest and other income	658,000	534,000	1,229,000
Other operating expenses	(1,237,000)	(1,231,000)	(1,017,000)
General and administrative expenses	(43,107,000)	(41,329,000)	(47,937,000)
Earnings from sales of land	0	0	357,000
Undeveloped land carrying costs	(8,934,000)	(9,203,000)	(10,403,000)
Gain (loss) on debt transactions	0	(16,349,000)	20,700,000
Acquisition-related activity	(1,188,000)	55,820,000	(1,062,000)
Income tax benefit	194,000	1,126,000	6,070,000
Other non-segment income	6,131,000	8,132,000	5,905,000
Net (income) loss attributable to noncontrolling interests	(744,000)	536,000	11,340,000
Noncontrolling interest share of FFO adjustments	(6,644,000)	(7,771,000)	(15,826,000)
Joint venture items	38,161,000	40,346,000	46,862,000
Dividends on preferred shares	(60,353,000)	(69,468,000)	(73,451,000)
Adjustments for redemption/repurchase of preferred shares	(3,796,000)	(10,438,000)	0
Discontinued operations	54,693,000	74,085,000	83,835,000
FFO attributable to common shareholders	274,616,000	297,955,000	142,597,000
Depreciation and amortization on continuing operations	(330,450,000)	(279,606,000)	(245,456,000)
Depreciation and amortization on discontinued operations	(55,229,000)	(80,578,000)	(94,670,000)
Company's share of joint venture adjustments	(33,687,000)	(34,674,000)	(36,966,000)
Impairment charges on depreciable properties	0	0	(134,055,000)
Earnings from depreciated property sales on continuing operations	68,549,000	39,662,000	12,337,000
Earnings from depreciated property sales on discontinued operations	100,882,000	33,054,000	6,786,000
Earnings from depreciated property sales - share of joint venture	91,000	2,308,000	0
Noncontrolling interest share of FFO adjustments	6,644,000	7,771,000	15,826,000
Net income (loss) attributable to common shareholders	31,416,000	(14,108,000)	(333,601,000)
Office [Member]
Rental and related revenue	271,137,000	312,036,000	321,506,000
Net earnings excluding depreciation and Non-Segment Items	159,529,000	184,915,000	192,561,000
Industrial [Member]
Rental and related revenue	388,828,000	289,946,000	249,555,000
Net earnings excluding depreciation and Non-Segment Items	283,850,000	215,751,000	188,277,000
Non-reportable Rental Operations segments [Member]
Rental and related revenue	80,969,000	65,719,000	51,017,000
Net earnings excluding depreciation and Non-Segment Items	52,071,000	42,931,000	33,430,000
Other Revenue [Member]
Rental and related revenue	11,544,000	11,094,000	12,377,000
Service Operations [Member]
Net earnings excluding depreciation and Non-Segment Items	$ 41,316,000	$ 28,496,000	$ 21,843,000