Segment Reporting (Summary of Net Operating Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Straight line rental income and expenses, net			$ 16,419	$ 12,421
Income (Loss) from Equity Method Investments	19,178	(27)	82,325	50,442
Interest expense	(53,343)	(56,618)	(163,479)	(171,365)
Depreciation and amortization on continuing operations	(95,000)	(101,191)	(290,700)	(289,508)
Gains on depreciated property sales on continuing operations	47,143	0	133,617	1,108
Asset Impairment Charges	(6,368)	0	(8,891)	(3,777)
Interest and other income, net	356	145	936	1,219
Other Operating Income (Expense), Net	(56)	(47)	(277)	(150)
General and administrative expenses	(10,573)	(10,373)	(35,632)	(33,225)
Earnings from sales of land	3,167	3,365	7,208	3,365
Undeveloped land carrying costs	(1,773)	(2,108)	(5,755)	(6,837)
Gains (Losses) on Extinguishment of Debt	0	0	(139)	0
Acquisition-related activity	(110)	(726)	(871)	(2,506)
Income (Loss) from Continuing Operations before Income Taxes, Extraordinary Items, Noncontrolling Interest	69,976	(13,102)	216,133	8,195
Operating Segments [Member]
Property Level Net Operating Income – Cash Basis	156,375	139,276	453,254	404,710
Operating Segments [Member] | Industrial [Member]
Property Level Net Operating Income – Cash Basis	97,324	88,968	284,191	254,849
Operating Segments [Member] | Office [Member]
Property Level Net Operating Income – Cash Basis	34,894	30,982	100,237	94,959
Operating Segments [Member] | Medical Office [Member]
Property Level Net Operating Income – Cash Basis	23,228	18,239	65,592	51,764
Operating Segments [Member] | All Other Segments [Member]
Property Level Net Operating Income – Cash Basis	929	1,087	3,234	3,138
Operating Segments [Member] | Service Operations [Member]
Service Operations Net Income (loss)	7,211	3,415	21,415	18,079
Segment Reconciling Items [Member]
Straight line rental income and expenses, net	5,527	2,942	16,929	11,040
Revenues Related to Lease Buyouts	145	2,936	4,365	9,615
Amortization of lease incentives and market rents	(1,220)	(2,267)	(5,376)	(6,485)
Intercompany rents and other adjusting items	(1,221)	(1,125)	(3,323)	(3,470)
Sold Properties Included in Continuing Operations Net Operating Income	807	9,332	11,568	25,449
Corporate, Non-Segment [Member]
Income (Loss) from Equity Method Investments	19,178	(27)	82,325	50,442
Interest expense	(53,343)	(56,618)	(163,479)	(171,365)
Depreciation and amortization on continuing operations	(95,000)	(101,191)	(290,700)	(289,508)
Gains on depreciated property sales on continuing operations	47,143	0	133,617	1,108
Asset Impairment Charges	(6,368)	0	(8,891)	(3,777)
Interest and other income, net	356	145	936	1,219
Other Operating Income (Expense), Net	(56)	(47)	(277)	(150)
General and administrative expenses	(10,573)	(10,373)	(35,632)	(33,225)
Earnings from sales of land	3,167	3,365	7,208	3,365
Undeveloped land carrying costs	(1,773)	(2,108)	(5,755)	(6,837)
Gains (Losses) on Extinguishment of Debt	0	0	(139)	0
Acquisition-related activity	(110)	(726)	(871)	(2,506)
Other non-segment income	$ (269)	$ (31)	$ (1,041)	$ 491