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Impairment and Disposal of Long-Lived Assets Impairment Charges - Real Estate Assets (Details) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Impaired Long-Lived Assets Held and Used [Line Items]        
Valuation adjustment $ 0   $ 0  
Impairment charges - real estate assets $ 0 $ 3,936,000 26,000,000 $ 3,936,000
Land Held for Development [Member] | Philadelphia [Member]        
Impaired Long-Lived Assets Held and Used [Line Items]        
Impairment charges - real estate assets     $ 26,000,000  
Land, Buildings and Improvements [Member] | Houston [Member]        
Impaired Long-Lived Assets Held and Used [Line Items]        
Impairment charges - real estate assets   $ 9,700,000   $ 9,700,000