Segment Reporting (Summary of Net Operating Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Straight line rental income and expenses, net			$ 10,832	$ 18,498
Income from Equity Method Investments	$ 12,010	$ (5,088)	37,404	16,281
Gain on dissolution of unconsolidated company	0	0	30,697	0
Promote income	2,212	0	26,299	0
Interest expense	(34,606)	(41,615)	(109,520)	(134,576)
Depreciation and amortization on continuing operations	(80,688)	(79,898)	(238,647)	(240,135)
Gain on sale of properties	82,698	71,259	137,589	202,153
Impairment Charges	(3,042)	(2,426)	(15,098)	(7,896)
Interest and other income, net	507	1,343	3,597	3,056
General and administrative expenses	(12,534)	(11,340)	(42,216)	(47,582)
Gain on land sales	1,601	1,659	2,438	24,096
Other operating expenses	(1,424)	(1,467)	(3,496)	(4,579)
Gains (Losses) on Extinguishment of Debt	(6,243)	64	(8,673)	(82,589)
Acquisition-related activity	(7)	(5,660)	(82)	(6,993)
Income from continuing operations before income taxes	112,104	73,835	265,699	168,067
Operating Segments [Member]
Property Level Net Operating Income – Cash Basis	142,834	127,429	411,444	376,065
Property Level Net Income, Cash Basis, Continuing Operations	144,674	139,565	427,956	422,700
Operating Segments [Member] | Industrial [Member]
Property Level Net Operating Income – Cash Basis	109,350	96,966	314,349	285,087
Operating Segments [Member] | Medical Office [Member]
Property Level Net Operating Income – Cash Basis	29,401	25,827	84,822	76,878
Operating Segments [Member] | All Other Segments [Member]
Property Level Net Operating Income – Cash Basis	4,083	4,636	12,273	14,100
Operating Segments [Member] | Service Operations [Member]
Service Operations Net Income (loss)	2,169	3,905	8,216	11,865
Segment Reconciling Items [Member]
Sold Properties Included in Continuing Operations Net Operating Income	1,840	12,136	16,512	46,635
Straight line rental income and expenses, net	5,008	5,723	10,832	16,830
Revenues Related to Lease Buyouts	1,491	408	1,725	1,366
Amortization of lease incentives and market rents	(303)	(357)	(1,361)	(2,559)
Intercompany rents and other adjusting items	(27)	(434)	(246)	(1,306)
Corporate, Non-Segment [Member]
Income from Equity Method Investments	12,010	(5,088)	37,404	16,281
Gain on dissolution of unconsolidated company	0	0	30,697	0
Promote income	2,212	0	26,299	0
Interest expense	(34,606)	(41,615)	(109,520)	(134,576)
Depreciation and amortization on continuing operations	(80,688)	(79,898)	(238,647)	(240,135)
Gain on sale of properties	82,698	71,259	137,589	202,153
Impairment Charges	(3,042)	(2,426)	(15,098)	(7,896)
Interest and other income, net	507	1,343	3,597	3,056
General and administrative expenses	(12,534)	(11,340)	(42,216)	(47,582)
Gain on land sales	1,601	1,659	2,438	24,096
Other operating expenses	(1,424)	(1,467)	(3,496)	(4,579)
Gains (Losses) on Extinguishment of Debt	(6,243)	64	(8,673)	(82,589)
Acquisition-related activity	(7)	(5,660)	(82)	(6,993)
Other non-segment income	$ (1,392)	$ (1,806)	$ (1,715)	$ (2,065)