Segment Reporting (Summary of Net Operating Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Straight line rental income and expenses, net			$ 5,768	$ 12,775
Income from Equity Method Investments	$ 3,534	$ 15,123	25,394	21,369
Gain on dissolution of unconsolidated company	30,697	0	30,697	0
Promote income	24,087	0	24,087	0
Interest expense	(37,184)	(42,976)	(74,914)	(92,567)
Depreciation and amortization on continuing operations	(80,161)	(78,334)	(157,959)	(160,237)
Gain on sale of properties	39,314	107,410	54,891	130,894
Impairment Charges	(5,651)	(5,470)	(12,056)	(5,470)
Interest and other income, net	567	1,375	3,090	1,713
General and administrative expenses	(11,584)	(19,238)	(29,682)	(36,242)
Gain on land sales	707	17,012	837	22,437
Other operating expenses	(836)	(1,555)	(2,072)	(3,112)
Gains (Losses) on Extinguishment of Debt	(2,430)	(82,653)	(2,430)	(82,653)
Acquisition-related activity	(72)	(1,305)	(75)	(1,333)
Income from continuing operations before income taxes	109,647	55,707	153,595	94,626
Operating Segments [Member]
Property Level Net Operating Income – Cash Basis	141,823	132,319	280,288	260,401
Property Level Net Income, Cash Basis, Continuing Operations	142,452	140,864	283,282	283,136
Operating Segments [Member] | Industrial [Member]
Property Level Net Operating Income – Cash Basis	104,741	96,629	206,798	191,588
Operating Segments [Member] | Medical Office [Member]
Property Level Net Operating Income – Cash Basis	28,179	25,820	55,421	51,051
Operating Segments [Member] | All Other Segments [Member]
Property Level Net Operating Income – Cash Basis	8,903	9,870	18,069	17,762
Operating Segments [Member] | Service Operations [Member]
Service Operations Net Income (loss)	3,816	2,163	6,047	7,960
Segment Reconciling Items [Member]
Sold Properties Included in Continuing Operations Net Operating Income	629	8,545	2,994	22,735
Straight line rental income and expenses, net	2,893	3,956	5,824	11,107
Revenues Related to Lease Buyouts	69	94	234	958
Amortization of lease incentives and market rents	(425)	(490)	(1,058)	(2,203)
Intercompany rents and other adjusting items	(226)	(412)	(219)	(872)
Corporate, Non-Segment [Member]
Income from Equity Method Investments	3,534	15,123	25,394	21,369
Gain on dissolution of unconsolidated company	30,697	0	30,697	0
Promote income	24,087	0	24,087	0
Interest expense	(37,184)	(42,976)	(74,914)	(92,567)
Depreciation and amortization on continuing operations	(80,161)	(78,334)	(157,959)	(160,237)
Gain on sale of properties	39,314	107,410	54,891	130,894
Impairment Charges	(5,651)	(5,470)	(12,056)	(5,470)
Interest and other income, net	567	1,375	3,090	1,713
General and administrative expenses	(11,584)	(19,238)	(29,682)	(36,242)
Gain on land sales	707	17,012	837	22,437
Other operating expenses	(836)	(1,555)	(2,072)	(3,112)
Gains (Losses) on Extinguishment of Debt	(2,430)	(82,653)	(2,430)	(82,653)
Acquisition-related activity	(72)	(1,305)	(75)	(1,333)
Other non-segment income	$ 80	$ 143	$ (323)	$ (259)