Segment Reporting (Summary of Net Operating Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Straight line rental income and expenses, net			$ 10,892	$ 8,789
Income (Loss) from Equity Method Investments	60,826	1,091	63,147	50,469
Interest expense	(54,872)	(57,019)	(110,129)	(114,343)
Depreciation and amortization on continuing operations	(97,641)	(95,322)	(195,700)	(188,316)
Gains on depreciated property sales on continuing operations	70,318	940	86,171	1,108
Asset Impairment Charges	(2,523)	(3,777)	(2,523)	(3,777)
Interest and other income, net	229	921	580	1,074
Other Operating Income (Expense), Net	(129)	(35)	(221)	(103)
General and administrative expenses	(10,365)	(9,707)	(25,059)	(22,852)
Earnings from sales of land	3,889	0	4,041	0
Undeveloped land carrying costs	(1,858)	(2,531)	(3,982)	(4,729)
Gains (Losses) on Extinguishment of Debt	(139)	0	(139)	0
Acquisition-related activity	(747)	(2,423)	(761)	(1,780)
Income (Loss) from Continuing Operations before Income Taxes, Extraordinary Items, Noncontrolling Interest	134,259	(13,953)	145,861	21,702
Operating Segments [Member]
Property Level Net Operating Income – Cash Basis	153,518	136,194	299,413	267,513
Operating Segments [Member] | Industrial [Member]
Property Level Net Operating Income – Cash Basis	95,564	85,257	187,308	166,401
Operating Segments [Member] | Office [Member]
Property Level Net Operating Income – Cash Basis	34,967	32,851	67,436	65,549
Operating Segments [Member] | Medical Office [Member]
Property Level Net Operating Income – Cash Basis	21,787	17,022	42,364	33,525
Operating Segments [Member] | All Other Segments [Member]
Property Level Net Operating Income – Cash Basis	1,200	1,064	2,305	2,038
Operating Segments [Member] | Service Operations [Member]
Service Operations Net Income (loss)	5,655	5,601	14,204	14,664
Segment Reconciling Items [Member]
Straight line rental income and expenses, net	4,940	3,827	11,402	8,097
Revenues Related to Lease Buyouts	1,525	4,686	4,220	6,679
Amortization of lease incentives and market rents	(1,689)	(2,165)	(4,156)	(4,062)
Intercompany rents and other adjusting items	(960)	(1,311)	(2,102)	(2,345)
Sold Properties Included in Continuing Operations Net Operating Income	4,362	6,866	8,227	13,883
Corporate, Non-Segment [Member]
Income (Loss) from Equity Method Investments	60,826	1,091	63,147	50,469
Interest expense	(54,872)	(57,019)	(110,129)	(114,343)
Depreciation and amortization on continuing operations	(97,641)	(95,322)	(195,700)	(188,316)
Gains on depreciated property sales on continuing operations	70,318	940	86,171	1,108
Asset Impairment Charges	(2,523)	(3,777)	(2,523)	(3,777)
Interest and other income, net	229	921	580	1,074
Other Operating Income (Expense), Net	(129)	(35)	(221)	(103)
General and administrative expenses	(10,365)	(9,707)	(25,059)	(22,852)
Earnings from sales of land	3,889	0	4,041	0
Undeveloped land carrying costs	(1,858)	(2,531)	(3,982)	(4,729)
Gains (Losses) on Extinguishment of Debt	(139)	0	(139)	0
Acquisition-related activity	(747)	(2,423)	(761)	(1,780)
Other non-segment income	$ (80)	$ 211	$ (772)	$ 522