Quarterly report pursuant to Section 13 or 15(d)

Properties, Net (Details)

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Properties, Net (Details) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2014
Sep. 30, 2015
Dec. 31, 2014
Investments in real estate      
Operating properties, net   $ 2,932,843 $ 2,751,488
Greater Philadelphia      
Investments in real estate      
Depreciation expense on reclassified assets $ 12,900    
Operating properties, net      
Investments in real estate      
Less: accumulated depreciation   (675,747) (703,083)
Operating properties, net | Land      
Investments in real estate      
Gross   466,701 439,355
Operating properties, net | Buildings and improvements      
Investments in real estate      
Gross   $ 3,141,889 $ 3,015,216