Real Estate (Details 2) (USD $)	3 Months Ended		9 Months Ended			3 Months Ended	9 Months Ended			9 Months Ended	0 Months Ended	3 Months Ended		9 Months Ended
	Sep. 30, 2013 sqft	Sep. 30, 2012	Sep. 30, 2013 sqft	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013 Great Bend, KS	Sep. 30, 2013 Great Bend, KS	Sep. 30, 2012 Great Bend, KS	Nov. 30, 2012 Great Bend, KS	Sep. 30, 2012 In-place lease intangibles Great Bend, KS	Jun. 12, 2013 Pittsburgh, PA sqft	Sep. 30, 2013 Pittsburgh, PA	Sep. 30, 2012 Pittsburgh, PA	Sep. 30, 2013 Pittsburgh, PA	Sep. 30, 2012 Pittsburgh, PA
Disposal of real estate properties
Net rentable square feet	190,000		190,000								53,183
Carrying value of property sold	$ 942,376,000		$ 942,376,000		$ 770,052,000						$ 4,400,000
Sales price											5,100,000
Net proceeds from sales of discontinued operations			4,843,000	3,216,000							4,800,000
Gain on sales of real estate			464,000	219,000							500,000
Amount contributed to total revenue by the property sold	34,883,000	21,102,000	97,163,000	57,846,000								0	100,000	200,000	400,000
Loss on impairment						0	0	3,900,000		700,000
Purchase price under purchase and sale agreement to sell the building									$ 4,000,000