Segment Reporting (Summary of Net Operating Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Straight line rental income and expenses, net	$ (5,974)	$ (4,839)
Income (Loss) from Equity Method Investments	2,321	49,378
Interest expense	(55,257)	(57,181)
Depreciation and amortization on continuing operations	(98,059)	(92,993)
Gains on depreciated property sales on continuing operations	15,853	168
Interest and other income, net	351	153
Other Operating Income (Expense), Net	(92)	(68)
General and administrative expenses	(14,694)	(13,145)
Earnings from sales of land	152	0
Undeveloped land carrying costs	(2,124)	(2,198)
Acquisition-related activity	(14)	643
Income (Loss) from Continuing Operations before Income Taxes, Extraordinary Items, Noncontrolling Interest	11,602	35,798
Operating Segments [Member]
Property Level Net Operating Income – Cash Basis	149,855	135,568
Operating Segments [Member] | Industrial [Member]
Property Level Net Operating Income – Cash Basis	91,909	81,279
Operating Segments [Member] | Office [Member]
Property Level Net Operating Income – Cash Basis	36,264	36,816
Operating Segments [Member] | Medical Office [Member]
Property Level Net Operating Income – Cash Basis	20,577	16,502
Operating Segments [Member] | All Other Segments [Member]
Property Level Net Operating Income – Cash Basis	1,105	971
Operating Segments [Member] | Service Operations [Member]
Service Operations Net Income (loss)	8,549	9,063
Segment Reconciling Items [Member]
Straight line rental income and expenses, net	6,463	4,270
Revenues Related to Lease Buyouts	2,695	1,993
Amortization of lease incentives and market rents	(2,467)	(1,897)
Intercompany rents and other adjusting items	(1,142)	(1,034)
Sold Properties Included in Continuing Operations Net Operating Income	(96)	2,767
Corporate, Non-Segment [Member]
Income (Loss) from Equity Method Investments	2,321	49,378
Interest expense	(55,257)	(57,181)
Depreciation and amortization on continuing operations	(98,059)	(92,993)
Gains on depreciated property sales on continuing operations	15,853	168
Interest and other income, net	351	153
Other Operating Income (Expense), Net	(92)	(68)
General and administrative expenses	(14,694)	(13,145)
Earnings from sales of land	152	0
Undeveloped land carrying costs	(2,124)	(2,198)
Acquisition-related activity	(14)	643
Other non-segment income	$ (692)	$ 311