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) - Series A convertible preferred stock - USD ($)
3 Months Ended 4 Months Ended 12 Months Ended
Mar. 31, 2018
Sep. 22, 2017
Dec. 31, 2017
Jun. 30, 2017
Jun. 09, 2017
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 Stock Issuance Costs $ 29,318,000 $ 29,300,000 $ 29,318,000    
Proceeds from issuance of redeemable convertible preferred stock   $ 450,700,000      
Initial closing          
Temporary Equity [Line Items]          
Agreement to sell shares (in shares)         172,500
Agreement to sell shares         $ 172,500,000
Second closing          
Temporary Equity [Line Items]          
Agreement to sell shares (in shares)   307,500      
Agreement to sell shares   $ 307,500,000