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Shareholders' Equity - Additional Information (Details)
 12 Months Ended
Dec. 31, 2019
USD ($)
shares
Dec. 31, 2017
shares
Dec. 31, 2019
£ / shares
Dec. 31, 2018
USD ($)
shares
Dec. 31, 2018
£ / shares
Jun. 30, 2015
USD ($)
shares
Class of Stock [Line Items]            
Deferred shares, multiple of amount paid up on ordinary shares 100          
Share buy-back program, authorized amount | $           $ 10,000,000
Share buy-back program, number of shares authorized           2,700,000
Number of ordinary shares repurchased (in shares) 0          
Number of treasury shares held (in shares) 352,499     378,201    
Cost of treasury shares held | $ $ 4,000,000.0     $ 4,300,000    
Own shares held by ESOP            
Class of Stock [Line Items]            
Number of ordinary shares repurchased (in shares)   100,000        
Ordinary shares            
Class of Stock [Line Items]            
Common stock, par value (in GBP per share) | £ / shares     £ 0.50   £ 0.50  
Common stock, shares, outstanding, actively traded 27,431,283     28,376,729    
Employee stock ownership plan, shares held by trustees 1,216,220     1,621,301    
Deferred shares            
Class of Stock [Line Items]            
Common stock, par value (in GBP per share) | £ / shares     £ 0.0001   £ 0.0001