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Share-Based Compensation - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Apr. 24, 2018
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Mar. 20, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Share-based compensation expense   $ 4.8 $ 4.3 $ 19.5 $ 12.8  
Performance Shares [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Share-based compensation expense       4.8    
Share-based compensation expense related to unvested performance-based stock options not yet recognized   0.0   0.0    
Term Based Stock Options [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Share-based compensation expense related to unvested performance-based stock options not yet recognized   $ 49.7   $ 49.7    
Share-based compensation expense related to unvested performance-based stock options expected to be recognized over a weighted average period       1 year 10 months 24 days    
Restricted Stock Units (RSUs) [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Share-based compensation expense related to unvested performance-based stock options expected to be recognized over a weighted average period       2 years 9 months 18 days    
Stock options, granted       159,323    
Stock options, vested       230,990    
Shares issued upon vesting of RSUs       105,990    
Remaining vested and releasable   125,000   125,000    
Unvested restricted stock units outstanding   534,323   534,323    
Share-based compensation expense related to unvested performance-based stock options not yet recognized   $ 10.6   $ 10.6    
Restricted Stock Units (RSUs) [Member] | IPO [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Stock options, vested       29,800    
Restricted Stock Units (RSUs) [Member] | Minimum [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Restricted stock vesting period       3 years    
Restricted Stock Units (RSUs) [Member] | Maximum [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Restricted stock vesting period       4 years    
Hcm Stock Incentive Plan 2007 [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Number of shares authorized   10,540,540   10,540,540    
Common shares outstanding   5,000   5,000    
Hcm Stock Incentive Plan 2013 [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Number of shares authorized   12,500,000   12,500,000   15,000,000
Common shares outstanding   10,766,257   10,766,257    
Share based compensation arrangement by share based payment award exercise period after employment termination       90 days    
Stock option awards, contractual term       10 years    
Hcm Stock Incentive Plan 2013 [Member] | Minimum [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Restricted stock vesting period       4 years    
Hcm Stock Incentive Plan 2013 [Member] | Maximum [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Restricted stock vesting period       5 years    
Hcm Stock Incentive Plan 2013 [Member] | Stock Appreciation Rights (SARs) [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Number of shares authorized   600,000   600,000    
Common shares outstanding       0    
Share-based compensation expense       $ 1.5    
Hcm Equity Incentive Plan 2018 [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Number of shares authorized 13,500,000          
Common shares outstanding   5,125,110   5,125,110    
Restricted stock vesting period 4 years          
Share based compensation arrangement by share based payment award exercise period after employment termination 90 days          
Stock option awards, contractual term 10 years          
Shares available for future grants of equity awards   8,374,890   8,374,890