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Description of Business (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 15, 2018
Feb. 15, 2018
Mar. 31, 2018
Dec. 31, 2017
Corporate Information, Status of Operations and Management Plans        
Net proceeds from the issuance of common stock after deducting underwriting discounts, commissions and offering expenses     $ 294,584  
Accumulated deficit     $ (1,113,143) $ (1,063,610)
Common Stock        
Corporate Information, Status of Operations and Management Plans        
Number of common stock shares issued in underwriting agreement 885,000 19,354,839 20,239,839  
Net proceeds from the issuance of common stock after deducting underwriting discounts, commissions and offering expenses $ 294,600   $ 294,600