Segment Reporting (Summary of Net Operating Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Straight line rental income and expenses, net			$ 12,775	$ 10,892
Income (Loss) from Equity Method Investments	$ 15,123	$ 60,826	21,369	63,147
Interest expense	(42,976)	(51,448)	(92,567)	(103,306)
Depreciation and amortization on continuing operations	(78,334)	(88,500)	(160,237)	(176,798)
Gain on sale of properties	107,410	70,318	130,894	86,171
Asset Impairment Charges	(5,470)	(2,523)	(5,470)	(2,523)
Interest and other income, net	1,375	229	1,713	580
General and administrative expenses	(19,238)	(10,365)	(36,242)	(25,059)
Gain on land sales	17,012	3,889	22,437	4,041
Other operating expenses	(1,555)	(1,987)	(3,112)	(4,203)
Gains (Losses) on Extinguishment of Debt	(82,653)	(139)	(82,653)	(139)
Acquisition-related activity	(1,305)	(747)	(1,333)	(761)
Income from continuing operations before income taxes	55,707	129,115	94,626	136,663
Operating Segments [Member]
Property Level Net Operating Income – Cash Basis	138,113	124,823	271,673	242,578
Property Level Net Income, Cash Basis, Continuing Operations	140,864	141,129	283,136	274,180
Operating Segments [Member] | Industrial [Member]
Property Level Net Operating Income – Cash Basis	99,408	87,897	195,456	172,056
Operating Segments [Member] | Medical Office [Member]
Property Level Net Operating Income – Cash Basis	25,820	22,015	51,051	42,592
Operating Segments [Member] | Office [Member]
Property Level Net Operating Income – Cash Basis	12,885	13,611	25,166	25,525
Operating Segments [Member] | All Other Segments [Member]
Property Level Net Operating Income – Cash Basis	0	1,300	0	2,405
Operating Segments [Member] | Service Operations [Member]
Service Operations Net Income (loss)	2,163	5,655	7,960	14,204
Segment Reconciling Items [Member]
Sold Properties Included in Continuing Operations Net Operating Income	2,751	16,306	11,463	31,602
Straight line rental income and expenses, net	3,956	4,077	11,107	9,778
Revenues Related to Lease Buyouts	94	1,525	958	4,220
Amortization of lease incentives and market rents	(490)	(1,389)	(2,203)	(3,600)
Intercompany rents and other adjusting items	(412)	(1,355)	(872)	(2,497)
Corporate, Non-Segment [Member]
Income (Loss) from Equity Method Investments	15,123	60,826	21,369	63,147
Interest expense	(42,976)	(51,448)	(92,567)	(103,306)
Depreciation and amortization on continuing operations	(78,334)	(88,500)	(160,237)	(176,798)
Gain on sale of properties	107,410	70,318	130,894	86,171
Asset Impairment Charges	(5,470)	(2,523)	(5,470)	(2,523)
Interest and other income, net	1,375	229	1,713	580
General and administrative expenses	(19,238)	(10,365)	(36,242)	(25,059)
Gain on land sales	17,012	3,889	22,437	4,041
Other operating expenses	(1,555)	(1,987)	(3,112)	(4,203)
Gains (Losses) on Extinguishment of Debt	(82,653)	(139)	(82,653)	(139)
Acquisition-related activity	(1,305)	(747)	(1,333)	(761)
Other non-segment income	$ 143	$ (80)	$ (259)	$ (772)