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Share Plans - Additional Information (Details)
$ / shares in Units, $ in Millions
1 Months Ended 12 Months Ended
May 23, 2017
shares
Dec. 31, 2019
USD ($)
$ / shares
shares
May 31, 2019
shares
Mar. 31, 2019
shares
Apr. 30, 2018
shares
Mar. 31, 2018
shares
Jan. 31, 2018
shares
May 31, 2017
shares
Mar. 31, 2017
shares
Dec. 31, 2019
USD ($)
$ / shares
shares
Dec. 31, 2018
USD ($)
$ / shares
Dec. 31, 2017
USD ($)
$ / shares
Dec. 31, 2012
Dec. 31, 2019
£ / shares
Dec. 31, 2018
£ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                              
Loan outstanding from ESOP | $   $ 0.6               $ 0.6 $ 2.0        
Number of awards exercisable (in shares)   8,903               8,903          
Cash received from options exercised | $                   $ 0.5 0.5 $ 0.9      
Tax benefit realized for the tax deductions from option exercises | $                   $ 0.9 $ 0.2 $ 0.3      
Weighted average fair value of options granted (in USD per share) | $ / shares                   $ 17.65 $ 11.02 $ 9.82      
Total intrinsic value of options exercised | $                   $ 11.2 $ 9.3 $ 1.4      
Total unrecognized compensation cost related to share options | $   $ 2.7               $ 2.7 $ 3.8        
Ordinary shares                              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                              
Common stock, par value (in GBP per share) | £ / shares                           £ 0.50 £ 0.50
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 3 months 18 days 1 year 9 months 18 days        
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 expiration period                   5 years          
Award vesting period                   3 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 expiration period   2 years   2 years   2 years     2 years            
Award vesting period   4 years   4 years   3 years     3 years            
Award granted during period (in shares)   6,000   196,320   432,600     139,800            
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   3,981   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   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) | $ / shares   $ 18.51               $ 18.51 $ 17.63        
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