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Stock Incentive Plans and Stock Based Compensation - Additional Information (Detail)
12 Months Ended
Feb. 01, 2017
Feb. 01, 2016
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2016
USD ($)
$ / shares
Dec. 31, 2015
USD ($)
$ / shares
Dec. 31, 2017
£ / shares
Dec. 31, 2016
£ / 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     $ 3.09 $ 1.62 $ 2.16      
Grant date fair value of options vested     $ 7,100,000 $ 6,500,000 $ 9,100,000      
Proceeds from the exercise of options     638,000 287,000 31,000      
Intrinsic value of options exercised     500,000 200,000 6,000      
Unrecognized stock-based compensation expense related to unvested stock option     $ 12,700,000          
Unrecognized stock-based compensation expense related to unvested stock option, recognition period     2 years 3 months 18 days          
Compensation expense related to stock option     $ 7,000,000 6,600,000 7,900,000      
Compensation expense in relation to restricted stock units     $ 7,000,000 $ 7,000,000 $ 6,000,000      
Ordinary shares, reserved and available for issuance | shares     16,994,000          
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]                
Compensation expense related to stock option     $ 0          
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%          
Share-based compensation, number of shares purchased | shares     0          
Maximum fair market value of stock which can be purchased by participant in calendar year     $ 25,000          
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%