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Significant Accounting Policies - Additional Information (Details)
12 Months Ended
Oct. 04, 2017
USD ($)
Dec. 31, 2017
USD ($)
Segment
$ / shares
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Significant Accounting Policies [Line Items]          
Number of operating segments | Segment   1      
Tax benefits recognized for stock-based compensation         $ 0
Realized gains (losses) recognized from investment     $ 0 $ 0  
Interest or penalties accrued for uncertain tax positions     0    
Warrants exercise price (in dollars per share) | $ / shares   $ 0.05      
Research and development $ 24,802,000 $ 72,078,000 52,394,000 44,451,000  
Income tax effect   (1,758,000) $ 193,000 $ 139,000  
Long-term Receivable | Accounting Standards Update 2016-16, Income Tax Effect Deferred From Past Intercompany Transactions          
Significant Accounting Policies [Line Items]          
Income tax effect   $ 2,100,000