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Share Plans - Additional Information (Details) - USD ($)
$ / shares in Units, $ in Millions
1 Months Ended 12 Months Ended
May 23, 2017
May 24, 2016
Apr. 30, 2018
Mar. 31, 2018
Jan. 31, 2018
May 31, 2017
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Loan outstanding from ESOP                 $ 2.0 $ 2.6    
Number of awards exercisable (in shares)                 320,882      
Weighted- average remaining contractual life (years)                 1 year 11 months 2 years 5 months    
Cash received from options exercised                 $ 0.5 $ 0.9 $ 0.1  
Tax benefit realized for the tax deductions from option exercises                 $ 0.2 $ 0.3 $ 0.3  
Weighted average fair value of options granted (in USD per share)                 $ 11.02 $ 9.82 $ 9.39  
Total intrinsic value of options exercised                 $ 9.3 $ 1.4 $ 1.4  
Total unrecognized compensation cost related to share options                 $ 3.8 $ 3.4    
Options                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Total unrecognized compensation cost related to share options, period for recognition                 1 year 9 months      
Market Value Options                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Number of awards exercisable (in shares)                 126,000      
Market Value Options | Tranche One                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Award vestiing percentage                       40.00%
Market Value Options | Tranche Two                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Award vestiing percentage                       20.00%
Market Value Options | Tranche Three                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Award vestiing percentage                       20.00%
Market Value Options | Tranche Four                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Award vestiing percentage                       20.00%
Executive Share Option Plan                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Award expiration period                 10 years      
Long-Term Umbrella Incentive Plan (LTIP)                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Award vesting period                 3 years      
Award expiration period                 5 years      
Long-Term Umbrella Incentive Plan (LTIP) | Tranche One                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Award vestiing percentage                 33.33%      
Long-Term Umbrella Incentive Plan (LTIP) | Tranche Two                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Award vestiing percentage                 33.33%      
Long-Term Umbrella Incentive Plan (LTIP) | Tranche Three                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Award vestiing percentage                 33.33%      
Long-Term Umbrella Incentive Plan (LTIP) | Restricted Stock Units and Options                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Award vesting period       3 years     3 years 3 years        
Award expiration period       2 years     2 years 2 years        
Award granted during period (in shares)       432,600     139,800 95,140        
Non-Executive Directors Equity Incentive Plan (Director EIP)                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Award vesting period                 1 year      
Non-Executive Directors Equity Incentive Plan (Director EIP) | Restricted Stock Units and Options                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Award granted during period (in shares) 21,814 12,520 11,936                  
Award fully vested and settled period after grant date             1 year          
Non-Executive Directors Equity Incentive Plan (Director EIP) | Restricted Stock Units and Options | Tranche One                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Award vesting period     3 years                  
Award granted during period (in shares)     2,000                  
Non-Executive Directors Equity Incentive Plan (Director EIP) | Restricted Stock Units and Options | Tranche Two                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Award granted during period (in shares)     9,936                  
Award fully vested and settled period after grant date     1 year                  
Employee Benefit Trust | Ordinary shares                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Market value of shares held by ESOP (in USD per share)                 $ 17.63 $ 15.80    
Chief Financial Officer                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Number of shares to be acquired within twelve months of appointment         21,000              
Chief Financial Officer | Restricted Stock Units (RSUs)                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Award vesting period         3 years              
Chief Financial Officer | Performance Shares                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Equity instruments other than options, grants in period (over) (in shares)         30,000              
Number of vesting shares when lower target achieved         5,000              
Number of vesting shares when mid-point target achieved         10,000              
Number of vesting shares when top target achieved         15,000              
CEO                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Number of shares to be acquired within twelve months of appointment           22,500            
CEO | Restricted Stock Units (RSUs)                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Award vesting period           4 years            
Equity instruments other than options, grants in period (over) (in shares)           60,000            
CEO | Performance Shares                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Number of vesting shares when lower target achieved           30,000            
Number of vesting shares when mid-point target achieved           40,000            
Number of vesting shares when top target achieved           50,000            
Within Twelve Months of Appointment | CEO | Restricted Stock Units (RSUs)                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Award vesting period           3 years            
Equity instruments other than options, grants in period (over) (in shares)           45,000