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Stockholders' Equity - Common Stock and Warrants (Details)
 9 Months Ended
Jul. 18, 2017
USD ($)
Jul. 12, 2017
USD ($)
$ / shares
shares
Dec. 21, 2016
USD ($)
$ / shares
Sep. 30, 2017
USD ($)
item
shares
Sep. 30, 2016
USD ($)
Dec. 31, 2016
shares
Convertible Debt            
Authorized number of shares of common stock | shares       250,000,000   250,000,000
Voting right for each share held, number | item       1    
Net proceeds from the issuance of common stock       $ 243,036,000 $ 97,068,000  
Common Stock            
Convertible Debt            
Number of common stock issued from underwriting agreement | shares   21,122,449        
Price per share of common stock issued (in dollars per share) | $ / shares   $ 12.25        
Gross proceeds from issue of common stock before deducting underwriting discounts and commissions   $ 258,800,000        
Net proceeds from the issuance of common stock $ 243,000,000     $ 243,000,000    
2016 Convertible Notes | Convertible Notes            
Convertible Debt            
Aggregate principle amount     $ 250,000,000      
Debt conversion ratio (in shares)     0.1633987      
Increment used for debt conversion     $ 1,000      
Conversion price (in dollars per share) | $ / shares     $ 6.12      
2016 Convertible Notes | Convertible Notes | Common Stock            
Convertible Debt            
Debt conversion ratio (in shares)     0.1633987      
Increment used for debt conversion     $ 1,000      
Conversion price (in dollars per share) | $ / shares     $ 6.12