Segment Reporting (Summary on Revenues and Funds from Operations) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Rental and related revenue	$ 203,432	$ 190,986
General contractor and service fee revenue (''Service Operations'')	68,968	146,547
Total Segment Revenues	271,271	335,550
Consolidated Revenue from continuing operations	272,400	337,533
Discontinued Operations	1,234	52,170
Consolidated Revenue	273,634	389,703
Net earnings excluding depreciation and Non-Segment Items	142,114	132,718
Interest expense	(61,086)	(52,124)
Interest and other income	146	87
Other operating expenses	(265)	(85)
General and administrative expenses	(11,839)	(11,197)
Undeveloped land carrying costs	(2,298)	(2,309)
Acquisition-related activity	(580)	(589)
Other non-segment income	352	981
Net (income) loss attributable to noncontrolling interests	643	(1,083)
Noncontrolling interest share of FFO adjustments	(2,060)	(569)
Joint venture items	10,095	8,610
Dividends on preferred shares	(13,193)	(15,974)
Adjustments for redemption/repurchase of preferred shares	(5,730)	(163)
Discontinued operations	(106)	11,756
FFO attributable to common shareholders	56,193	70,059
Depreciation and amortization on continuing operations	(91,613)	(77,822)
Depreciation and amortization on discontinued operations	(643)	(17,159)
Company's share of joint venture adjustments	(8,586)	(7,628)
Earnings from depreciated property sales on continuing operations	(277)	67,856
Earnings from depreciated property sales on discontinued operations	6,476	11,603
Earnings from depreciated property sales - share of joint venture	0	91
Noncontrolling Interest Share of FFO Adjustments, Reconciliation Add-Back	2,060	569
Net income (loss) attributable to common shareholders	(36,390)	47,569
Industrial [Member]
Rental and related revenue	108,487	94,262
Net earnings excluding depreciation and Non-Segment Items	79,902	66,726
Office [Member]
Rental and related revenue	67,417	75,602
Net earnings excluding depreciation and Non-Segment Items	39,490	43,100
Medical Office [Member]
Rental and related revenue	20,741	13,484
Net earnings excluding depreciation and Non-Segment Items	13,667	7,802
Non-reportable Rental Operations segments [Member]
Rental and related revenue	5,658	5,655
Net earnings excluding depreciation and Non-Segment Items	4,008	4,207
Other Revenue [Member]
Rental and related revenue	1,129	1,983
Service Operations [Member]
Net earnings excluding depreciation and Non-Segment Items	$ 5,047	$ 10,883