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
 1 Months Ended
Apr. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Total | Recurring        
Assets and Liabilities Measured at Fair Value        
Cash equivalents   $ 65,521 $ 76,304  
Quoted prices in active markets for identical assets (Level 1) | Recurring        
Assets and Liabilities Measured at Fair Value        
Cash equivalents   $ 65,521 $ 76,304  
Series C common stock | Zero-strike call option | Call Option        
Assets and Liabilities Measured at Fair Value        
Derivative underlying share amount 610,325     704,908
Derivative premium paid $ 47,800