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Marketable Securities - Narrative (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Investments, Debt and Equity Securities [Abstract]        
Unrealized Gains   $ 878    
Marketable securities, realized gain $ 143 2,978 $ 378 $ 4,228
Marketable securities, realized loss $ (22) $ (2,432) $ (350) $ (3,755)