Segment Reporting (Summary of Net Operating Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Straight line rental income and expenses, net			$ 18,498	$ 16,419
Income (Loss) from Equity Method Investments	$ (5,088)	$ 19,178	16,281	82,325
Interest expense	(41,615)	(47,421)	(134,576)	(145,628)
Depreciation and amortization on continuing operations	(79,898)	(85,772)	(240,135)	(262,570)
Gain on sale of properties	71,259	47,143	202,153	133,617
Asset Impairment Charges	(2,426)	(6,368)	(7,896)	(8,891)
Interest and other income, net	1,343	356	3,056	936
General and administrative expenses	(11,340)	(10,573)	(47,582)	(35,632)
Gain on land sales	1,659	3,167	24,096	7,208
Other operating expenses	(1,467)	(1,829)	(4,579)	(6,032)
Gains (Losses) on Extinguishment of Debt	64	0	(82,589)	(139)
Acquisition-related activity	(5,660)	(110)	(6,993)	(871)
Income from continuing operations before income taxes	73,835	67,719	168,067	209,786
Operating Segments [Member]
Property Level Net Operating Income – Cash Basis	136,854	126,768	405,229	365,809
Property Level Net Income, Cash Basis, Continuing Operations	139,565	139,930	422,700	413,079
Operating Segments [Member] | Industrial [Member]
Property Level Net Operating Income – Cash Basis	99,236	88,654	291,987	258,863
Operating Segments [Member] | Medical Office [Member]
Property Level Net Operating Income – Cash Basis	25,827	23,456	76,878	65,820
Operating Segments [Member] | Office [Member]
Property Level Net Operating Income – Cash Basis	11,791	13,688	36,364	37,851
Operating Segments [Member] | All Other Segments [Member]
Property Level Net Operating Income – Cash Basis	0	970	0	3,275
Operating Segments [Member] | Service Operations [Member]
Service Operations Net Income (loss)	3,905	7,211	11,865	21,415
Segment Reconciling Items [Member]
Sold Properties Included in Continuing Operations Net Operating Income	2,711	13,162	17,471	47,270
Straight line rental income and expenses, net	5,723	5,466	16,830	16,183
Revenues Related to Lease Buyouts	408	145	1,366	4,365
Amortization of lease incentives and market rents	(357)	(919)	(2,559)	(4,820)
Intercompany rents and other adjusting items	(434)	(1,616)	(1,306)	(3,718)
Corporate, Non-Segment [Member]
Income (Loss) from Equity Method Investments	(5,088)	19,178	16,281	82,325
Interest expense	(41,615)	(47,421)	(134,576)	(145,628)
Depreciation and amortization on continuing operations	(79,898)	(85,772)	(240,135)	(262,570)
Gain on sale of properties	71,259	47,143	202,153	133,617
Asset Impairment Charges	(2,426)	(6,368)	(7,896)	(8,891)
Interest and other income, net	1,343	356	3,056	936
General and administrative expenses	(11,340)	(10,573)	(47,582)	(35,632)
Gain on land sales	1,659	3,167	24,096	7,208
Other operating expenses	(1,467)	(1,829)	(4,579)	(6,032)
Gains (Losses) on Extinguishment of Debt	64	0	(82,589)	(139)
Acquisition-related activity	(5,660)	(110)	(6,993)	(871)
Other non-segment income	$ (1,806)	$ (269)	$ (2,065)	$ (1,041)