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Share Plans - Additional Information (Details)
$ / shares in Units, $ in Millions
1 Months Ended 12 Months Ended
Sep. 30, 2020
shares
Jun. 30, 2020
shares
May 31, 2020
shares
Mar. 31, 2020
shares
Dec. 31, 2019
£ / shares
shares
May 31, 2019
shares
Mar. 31, 2019
shares
Apr. 30, 2018
shares
Mar. 31, 2018
shares
Jan. 31, 2018
shares
Dec. 31, 2020
USD ($)
$ / shares
Dec. 31, 2019
USD ($)
$ / shares
Dec. 31, 2018
USD ($)
$ / shares
Dec. 31, 2020
£ / shares
Dec. 31, 2020
USD ($)
$ / shares
Dec. 31, 2019
USD ($)
$ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                
Loan outstanding from ESOP | $                             $ 0.6 $ 0.6
Tax benefit realized for the tax deductions from option exercises | $                     $ 0.6 $ 0.9 $ 0.2      
Weighted average fair value of options granted (in USD per share) | $ / shares                     $ 9.41 $ 17.65 $ 11.02      
Total intrinsic value of options exercised | $                     $ 3.0 $ 11.2 $ 9.3      
Total unrecognized compensation cost related to share options | $                             $ 2.2 $ 2.7
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 3 months 18 days        
Long-Term Umbrella Incentive Plan (LTIP)                                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                
Award expiration period                     6 years          
Award vesting period                     4 years          
Long-Term Umbrella Incentive Plan (LTIP) | Tranche One                                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                
Award vesting percentage                     25.00%          
Long-Term Umbrella Incentive Plan (LTIP) | Tranche Two                                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                
Award vesting percentage                     25.00%          
Long-Term Umbrella Incentive Plan (LTIP) | Tranche Three                                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                
Award vesting percentage                     25.00%          
Long-Term Umbrella Incentive Plan (LTIP) | Tranche Four                                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                
Award vesting percentage                     25.00%          
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   2 years   2 years              
Award vesting period 4 years   4 years 4 years 4 years   4 years   3 years              
Award granted during period (in shares) 3,892     132,900 6,000   196,320   432,600              
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)   27,280 2,000     3,981   11,936                
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   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                             $ 16.42 $ 18.51
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 (in shares)                   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 (in shares)                   5,000            
Number of vesting shares when mid-point target achieved (in shares)                   10,000            
Number of vesting shares when top target achieved (in shares)                   15,000