Accounting for the Impairment or Disposal of Long-Lived Assets - Results of Operations for the Properties Held for Sale and Disposed (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012 sqft properties	Sep. 30, 2011	Sep. 30, 2012 sqft properties	Sep. 30, 2011
Long Lived Assets Held-for-sale [Line Items]
Number Properties Held For Sale	2		2
Property Square Feet Held For Sale	257,000		257,000
Revenues	$ 635,000	$ 11,915,000	$ 12,156,000	$ 51,978,000
Operating expenses	(260,000)	(5,446,000)	(5,373,000)	(22,738,000)
Interest and other income	4,000	43,000	35,000	196,000
Interest expense	(601,000)	(2,353,000)	(3,228,000)	(9,700,000)
Depreciation and Amortization, Discontinued Operations	125,000	2,780,000	608,000	11,743,000
Income before property dispositions	(347,000)	1,379,000	2,982,000	7,993,000
Gain on property dispositions	(1,540,000)	4,095,000	2,505,000	54,722,000
Net income	(1,887,000)	5,474,000	5,487,000	62,715,000
Proceeds from Sale of Property, Plant, and Equipment	0	70,600,000	210,800,000	340,200,000
Asset held for sale, Sold subsequent to reporting period [Member]
Long Lived Assets Held-for-sale [Line Items]
Number Properties Held For Sale	1		1
Long Lived Assets Held-for-sale, Proceeds from Sale	$ 14,500,000		$ 14,500,000