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Marketable Securities (Narrative) (Details) - USD ($)
1 Months Ended 12 Months Ended
Jan. 23, 2020
Mar. 12, 2019
Mar. 01, 2019
Jan. 23, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Marketable Securities [Line Items]              
(Decrease) increase in fair value of marketable securities         $ (5,533,000) $ (26,453,000) $ 0
Lexington Realty Trust              
Marketable Securities [Line Items]              
Shares sold (shares)     18,468,969        
Proceeds from sale of equity method investments     $ 167,698,000        
(Decrease) increase in fair value of marketable securities         16,068,000 (26,596,000) 0
PREIT              
Marketable Securities [Line Items]              
Shares issued upon conversion of operating partnership units (shares)   6,250,000          
(Decrease) increase in fair value of marketable securities         (21,649,000) $ 0 $ 0
Interest and Other Investment Income, Net | PREIT              
Marketable Securities [Line Items]              
(Decrease) increase in fair value of marketable securities         $ (21,649,000)    
Subsequent Event | PREIT              
Marketable Securities [Line Items]              
Shares sold (shares) 6,250,000     6,250,000      
Proceeds from sale of equity method investments $ 28,375,000            
(Decrease) increase in fair value of marketable securities       $ (4,938,000)