Segment Reporting (Summary of Net Operating Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Straight line rental income and expenses, net	$ 2,928	$ 2,928
Income from Equity Method Investments	4,749	21,860
Interest expense	(30,505)	(37,730)
Depreciation and amortization on continuing operations	(81,557)	(77,798)
Gain on sale of properties	37,046	15,577
Impairment Charges	(859)	(6,405)
Interest and other income, net	533	2,523
General and administrative expenses	(19,232)	(18,098)
Gain on land sales	1,505	130
Other operating expenses	(738)	(1,237)
Gain on Extinguishment of Debt	25	0
Acquisition-related activity	0	(3)
Income from continuing operations before income taxes	72,913	43,948
Operating Segments [Member]
Property Level Net Operating Income – Cash Basis	145,615	128,100
Property Level Net Income, Cash Basis, Continuing Operations	148,404	140,838
Operating Segments [Member] | Industrial [Member]
Property Level Net Operating Income – Cash Basis	115,572	100,983
Operating Segments [Member] | Medical Office [Member]
Property Level Net Operating Income – Cash Basis	28,851	25,519
Operating Segments [Member] | All Other Segments [Member]
Property Level Net Operating Income – Cash Basis	1,192	1,598
Operating Segments [Member] | Service Operations [Member]
Service Operations Net Income (loss)	1,775	2,231
Segment Reconciling Items [Member]
Sold Properties Included in Continuing Operations Net Operating Income	2,789	12,738
Straight line rental income and expenses, net	2,928	2,923
Revenues Related to Lease Buyouts	9,785	165
Amortization of lease incentives and market rents	(543)	(633)
Intercompany rents and other adjusting items	180	7
Corporate, Non-Segment [Member]
Income from Equity Method Investments	4,749	21,860
Interest expense	(30,505)	(37,730)
Depreciation and amortization on continuing operations	(81,557)	(77,798)
Gain on sale of properties	37,046	15,577
Impairment Charges	(859)	(6,405)
Interest and other income, net	533	2,523
General and administrative expenses	(19,232)	(18,098)
Gain on land sales	1,505	130
Other operating expenses	(738)	(1,237)
Gain on Extinguishment of Debt	25	0
Acquisition-related activity	0	(3)
Other non-segment income	$ (583)	$ (402)