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Stock Incentive Plans and Stock Based Compensation - Additional Information (Detail)
6 Months Ended 12 Months Ended
Feb. 01, 2018
Feb. 01, 2017
Nov. 30, 2018
$ / shares
shares
May 31, 2018
$ / shares
shares
Dec. 31, 2018
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2016
USD ($)
$ / shares
Dec. 31, 2018
£ / shares
Dec. 31, 2017
£ / shares
Mar. 05, 2015
£ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Common stock, par value | £ / shares               £ 0.50 £ 0.50 £ 0.50
Weighted average fair value of the stock options granted | $ / shares         $ 7.82 $ 3.09 $ 1.62      
Grant date fair value of options vested         $ 7,700,000 $ 7,100,000 $ 6,500,000      
Proceeds from the exercise of options         26,402,000 638,000 287,000      
Intrinsic value of options exercised         69,700,000 500,000 200,000      
Unrecognized stock-based compensation expense related to unvested stock option         $ 25,700,000          
Unrecognized stock-based compensation expense related to unvested stock option, recognition period         2 years 3 months 18 days          
Compensation expense related to stock option         $ 8,200,000 7,000,000 6,600,000      
Compensation expense in relation to restricted stock units         $ 10,600,000 $ 7,000,000 $ 7,000,000      
Ordinary shares, reserved and available for issuance | shares         11,463,000          
Stock Options                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Employee Stock options, Vesting Period         4 years          
Stock Incentive Plan 2011                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Newly authorized Shares available for award | shares         51,500,000          
Stock Incentive Plan 2011 | Stock Options                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Employee Stock options, Vesting Period 4 years 4 years     4 years          
Expire period         10 years          
2017 Employee Stock Purchase Plan                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Ordinary shares, reserved and available for issuance | shares         3,000,000          
Percentage of fair market value to payroll deductions to eligible employees to purchase ordinary shares         85.00%          
Maximum fair market value of stock which can be purchased by participant in calendar year         $ 25,000          
Issuance of common stock under employee stock purchase plan (in shares) | shares     184,385 127,872            
Sale of stock price per share | $ / shares     $ 2.86 $ 2.81            
Reserved for future issuance | shares         2,687,743          
Shares issued | shares           0        
2017 Employee Stock Purchase Plan | Maximum                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Percentage of payroll deductions to eligible employees to purchase ordinary shares         15.00%