Quarterly report pursuant to Section 13 or 15(d)

Redeemable Convertible Preferred Stock - Narrative (Details)

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Redeemable Convertible Preferred Stock - Narrative (Details) - USD ($)
4 Months Ended 6 Months Ended 12 Months Ended
Sep. 22, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Jun. 09, 2017
Temporary Equity [Line Items]          
Payments of issuance costs     $ (12,625,000)    
Series A convertible preferred stock          
Temporary Equity [Line Items]          
Agreement to sell shares (in shares)     480,000    
Share price (in dollars per share)     $ 1,000    
Agreement to sell shares     $ 480,000,000.0    
Payments of issuance costs $ (29,300,000) $ (29,318,000)   $ (29,318,000)  
Proceeds from issuance of redeemable convertible preferred stock $ 450,700,000        
Series A convertible preferred stock | Initial closing          
Temporary Equity [Line Items]          
Agreement to sell shares (in shares)         172,500
Agreement to sell shares         $ 172,500,000
Series A convertible preferred stock | Second closing          
Temporary Equity [Line Items]          
Agreement to sell shares (in shares) 307,500        
Agreement to sell shares $ 307,500,000