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Significant Accounting Policies - Investment in Marketable Securities (Details) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2017
Jun. 30, 2018
Dec. 31, 2017
Investment in Marketable Securities        
Marketable securities     $ 28,570,000 $ 28,579,000
Proceeds from sale of marketable securities $ 54,400,000      
Marketable Securities, Realized Gain (Loss)   $ 3,300,000    
Fair Value        
Investment in Marketable Securities        
Marketable securities     28,570,000 28,579,000
Equity marketable securities        
Investment in Marketable Securities        
Cost basis     0 0
Commercial mortgage-backed securities        
Investment in Marketable Securities        
Marketable securities     28,570,000 28,579,000
Cost basis     $ 27,500,000 $ 27,500,000