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Stockholders' Equity - Common Stock (Details) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Sep. 22, 2016
Nov. 25, 2015
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Class of Stock [Line Items]          
Common stock, shares authorized (in shares)     500,000,000 500,000,000  
Issuance of common stock, net of offering costs       $ 49,044 $ 76,056
Proceeds from sale of common stock, net of offering costs     $ 0 49,044 76,056
Payments of stock issuance costs       $ 1,000 $ 1,000
Private Placement          
Class of Stock [Line Items]          
Common stock, shares authorized (in shares)   3,400,000      
Proceeds from sale of common stock, net of offering costs   $ 74,800      
Proceeds from issuance of common stock, net   $ 73,800      
Bridge Loan | Rights Offering          
Class of Stock [Line Items]          
Shares subscribed but unissued 4,000,000        
Issuance of common stock, net of offering costs $ 49,900        
Proceeds from issuance of common stock, net $ 49,000        
Common Stock | Private Placement          
Class of Stock [Line Items]          
Price per share (in dollars per share)   $ 22.00      
Payments of stock issuance costs   $ 1,000      
Common Stock | Bridge Loan | Rights Offering          
Class of Stock [Line Items]          
Price per share (in dollars per share) $ 12.25        
Payments of stock issuance costs $ 900