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Share-Based Payments - Narrative (Details)
 9 Months Ended
Sep. 30, 2017
USD ($)
Agreement
shares
Sep. 30, 2017
GBP (£)
Agreement
shares
Sep. 30, 2016
USD ($)
shares
Feb. 08, 2016
shares
Feb. 10, 2015
shares
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]          
Vesting period 3 years 3 years      
Options grants in period (in shares) | shares 0 0 0    
Options exercised (in shares) | shares 0 0 29,222    
Compensation costs $ 0   $ 0    
Award vesting rights, percentage 8.33% 8.33%      
Percentage of restricted share units vested and issued 100.00% 100.00%      
Achievement One          
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]          
Achieved and required performance growth, percentage 5.00% 5.00%      
Percentage of initial shares granted 33.33% 33.33%      
Achievement Two | Minimum          
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]          
Achieved and required performance growth, percentage 5.00% 5.00%      
Eligible for vesting, percent 10.00% 10.00%      
Achievement Two | Maximum          
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]          
Achieved and required performance growth, percentage 10.00% 10.00%      
Eligible for vesting, percent 100.00% 100.00%      
Achievement Three | Minimum          
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]          
Achieved and required performance growth, percentage 10.00% 10.00%      
Eligible for vesting, percent 100.00% 100.00%      
Achievement Three | Maximum          
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]          
Achieved and required performance growth, percentage 20.00% 20.00%      
Eligible for vesting, percent 200.00% 200.00%      
2003 Share Incentive Plan          
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]          
Shares available to grant under incentive plan (in shares) | shares 595,683 595,683      
2013 Share Incentive Plan          
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]          
Shares available to grant under incentive plan (in shares) | shares 2,845,683 2,845,683      
2016 Share Incentive Plan          
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]          
Shares available to grant under incentive plan (in shares) | shares 263,695 263,695      
Sharesave Scheme          
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]          
Employee contribution period 3 years 3 years      
Employee savings under ESPP | £   £ 500      
Purchase price of fair market value 85.00% 85.00%      
International Employee Share Purchase Plan          
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]          
Employee contribution period 2 years 2 years      
Employee savings under ESPP $ 500        
Shares issued (in shares) | shares 10,545 10,545 13,342    
Employee Share Purchase Plans          
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]          
Compensation costs charged in ESPP $ 300,000   $ 300,000    
Investor Options          
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]          
Number of arrangements under options and other equity incentives | Agreement 3 3      
Stock Options          
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]          
Vesting period 3 years 3 years      
Exercise period 10 years 10 years      
Performance Shares          
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]          
Vesting period 3 years 3 years      
Compensation costs $ (400,000)   $ (2,500,000)    
Granted, number of shares (in shares) | shares 216,878 216,878 278,477    
Tax benefit from compensation expense $ (100,000)   $ (500,000)    
Phantom Shares          
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]          
Compensation costs $ (700,000)   $ (800,000)    
Granted, number of shares (in shares) | shares 173,619 173,619 147,513    
Tax benefit from compensation expense $ (200,000)   $ (600,000)    
Fair value adjustment capitalized amount $ 1,800,000   3,800,000    
Restricted Share Units          
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]          
Vesting period 3 years 3 years      
Compensation costs $ (7,200,000)   $ (7,200,000)    
Granted, number of shares (in shares) | shares 200,020.707095856 200,020.707095856 328,551    
Tax benefit from compensation expense $ (1,700,000)   $ (1,700,000)    
Fair value adjustment capitalized amount 0   600,000    
Restricted Share Units | Director          
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]          
Compensation costs $ (1,100,000)   $ (1,100,000)    
RSUs for non-employee directors (in shares) | shares       22,230 24,456
Restricted Share Units | Board of Directors Chairman          
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]          
RSUs for non-employee directors (in shares) | shares       8,892 10,952
Grant Year 2017 and 2018 | Performance Shares          
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]          
Vesting rule, threshold percent of eligible shares 100.00% 100.00%      
Vesting rule, maximum vesting percent of eligible shares 100.00% 100.00%