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Equity-based compensation - Additional Information (Detail) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Liquidation value $ 0  
PICU    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Weighted-average grant-date fare value per unit $ 442.30 $ 0.00
Compensation expense $ 30,961.00 $ 0.00
Unrecognized compensation cost 2,654  
Net proceeds from issue of share 99,960  
Fair value of units vested during the period $ 130,921 $ 0
PICU | Unvested restricted common stock    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Shares issued, unvested 8,359 12,485