Annual report pursuant to Section 13 and 15(d)

Properties, net (Details)

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Properties, net (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Properties        
Gross $ 3,811,950 [1],[2] $ 3,859,960 $ 4,038,932 $ 3,948,487
Operating properties, net 2,702,693 2,597,666 2,597,666  
Operating properties, net
        
Properties        
Less: accumulated depreciation (597,649) (555,975)    
Operating properties, net | Land
        
Properties        
Gross 430,472 427,766    
Operating properties, net | Buildings and improvements
        
Properties        
Gross $ 2,869,870 $ 2,725,875    
[1] The aggregate cost of these assets for Federal income tax purposes was approximately $3.3 billion at December 31, 2013.
[2] As discussed in Note 3 to our Consolidated Financial Statements, we recognized impairment losses of $5.9 million in connection with two properties in the Greater Baltimore region that we determined no longer met our strategic investment criteria.