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Shareholders' Equity - Additional Information (Details)
 12 Months Ended
Dec. 31, 2018
USD ($)
shares
Dec. 31, 2017
USD ($)
shares
Dec. 31, 2016
shares
Dec. 31, 2018
£ / shares
Dec. 31, 2017
£ / 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 0        
Number of treasury shares held (in shares) 378,201 527,616        
Cost of treasury shares held | $ $ 4,300,000 $ 5,800,000        
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  
ESOP shares held by trustees 1,621,301 104,709        
Common stock, shares, outstanding, actively traded 28,376,729 25,929,312        
Deferred shares            
Class of Stock [Line Items]            
Common stock, par value (in GBP per share) | £ / shares       £ 0.0001 £ 0.0001