Quarterly report pursuant to Section 13 or 15(d)

Assets and Liabilities Measured at Fair Value (Details)

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Assets and Liabilities Measured at Fair Value (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Realized and Unrealized Gains (Losses) on Financial Instruments        
Realized and unrealized gains (losses) $ (2,019) $ 1,370 $ (2,019) $ 2,351
Zero-strike call option        
Realized and Unrealized Gains (Losses) on Financial Instruments        
Realized and unrealized gains (losses) $ (2,019) $ 1,370 $ (2,019) $ 2,351