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Employee Benefits and Incentive Plans - Qualified Retirement Plan and Non-Qualified Retirement Plans - Additional Information (Detail)
$ in Millions
 6 Months Ended 12 Months Ended 30 Months Ended
Jun. 30, 2014
Dec. 31, 2016
USD ($)
Dec. 31, 2016
GBP (£)
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Dec. 31, 2016
USD ($)
Retirement Plan [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Employees age limit   18 years 18 years      
Employees minimum working hours   1000 hours 1000 hours      
Employees Contribution Percentage   50.00% 50.00%      
Percentage of employer matching contribution   14.00% 14.00%      
Percentage of amounts in notional equity shares, vesting in each year   25.00% 25.00%      
Attainment of vested employees salary contribution   100.00% 100.00%      
Vested maturity period   5 years 5 years      
Total employer contribution under the plan   $ 9.8   $ 8.5 $ 7.4  
Pension Plans, Defined Benefit [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Employees age limit   22 years 22 years      
Total employer contribution under the plan   $ 1.3   1.2 1.3  
Minimum employee earnings | £     £ 10,000      
Employees successful probationary completion period   3 months 3 months      
Pension Plans, Defined Benefit [Member] | Minimum [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Percentage of employer matching contribution   2.00% 2.00%      
Pension Plans, Defined Benefit [Member] | Maximum [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Percentage of employer matching contribution   3.00% 3.00%      
Deferred Profit Sharing Plan [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Employees Contribution Percentage   50.00% 50.00%      
Percentage of employer matching contribution   3.00% 3.00%      
Attainment of vested employees salary contribution   100.00% 100.00%      
Total employer contribution under the plan   $ 0.3   0.3 0.3  
Personal Superannuation Account [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Employees Contribution Percentage 9.50%         10.50%
Total employer contribution under the plan   $ 0.6   0.7 0.8  
Non Qualified Retirement Savings Plan [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Employees Contribution Percentage   50.00% 50.00%      
Attainment of vested employees salary contribution   100.00% 100.00%      
Vested maturity period   3 years 3 years      
Total employer contribution under the plan   $ 2.1   2.6 1.9  
Assets Under Trust   $ 51.6   46.9   $ 51.6
Non Qualified Deferred Compensation Plan [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Employees Contribution Percentage   50.00% 50.00%      
Percentage of amounts in notional equity shares, vesting in each year   20.00% 20.00%      
Attainment of vested employees salary contribution   100.00% 100.00%      
Vested maturity period   5 years 5 years      
Total employer contribution under the plan   $ 0.8   1.7 $ 1.1  
Assets Under Trust   9.8   9.2   $ 9.8
Non Qualified Deferred Compensation Plan [Member] | Notional [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Notional equity shares in the deferred Compensation Plan   $ 34.9   $ 28.4