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Impairment and Disposal of Long-Lived Assets - Impairment Charges - Real Estate Assets (Details) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Impaired Long-Lived Assets Held and Used [Line Items]        
Impairment charges - real estate assets $ 0 $ 26,000,000 $ 99,000 $ 26,000,000
Valuation adjustment 0   0  
DC Metro        
Impaired Long-Lived Assets Held and Used [Line Items]        
Impairment charges - real estate assets $ 4,000,000.0   $ 14,400,000  
Philadelphia        
Impaired Long-Lived Assets Held and Used [Line Items]        
Impairment charges - real estate assets   $ 26,000,000.0   $ 26,000,000