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Share-Based Payments - Additional Information (Details) |  | 12 Months Ended
--- | --- | ---
Apr. 05, 2014 GBP (£) | Dec. 31, 2018 USD ($) agreement shares | Dec. 31, 2017 USD ($) shares | Dec. 31, 2016 USD ($) shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Number of arrangements under options and equity incentive plans | agreement |  | 3 |  |
Vesting period (in years) |  | 3 years |  |
Total tax credit |  | $ 100,000 | $ 100,000 | $ 500,000
Percentages for vesting |  | 100.00% |  |
Contract period (in years) |  | 10 years |  |
Options exercised | shares |  | 0 | 0 | 29,222
Intrinsic value of options |  | $ 0 | $ 0 | $ 600,000
Employee Stock Ownership Plan, shares issued during period (in shares) | shares |  | 38,280 | 46,866 | 21,285
Restricted Share Units |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Compensation cost |  | $ (8,900,000) | $ (9,400,000) | $ (9,800,000)
Total tax credit |  | 2,200,000 | 2,300,000 | 2,400,000
Unrecognized share compensation costs, RSUs |  | $ 7,900,000 |  |
Weighted average period of compensation cost recognition |  | 1 year 7 months 6 days |  |
Compensation cost, non employee directors |  | $ 1,500,000 | 1,500,000 | 1,400,000
Tax charge, non employee directors |  | 0 | 0 | 0
Performance Shares |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Compensation cost |  | (500,000) | (400,000) | 2,200,000
Unrecognized share compensation costs, RSUs |  | $ 6,800,000 |  |
Weighted average period of compensation cost recognition |  | 1 year 8 months 12 days |  |
Percentages for vesting |  | 33.33% |  |
Phantom Shares |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Compensation cost |  | $ 0 | (700,000) | (700,000)
Unrecognized share compensation costs, RSUs |  | $ 5,200,000 |  |
Weighted average period of compensation cost recognition |  | 1 year 9 months 18 days |  |
Fair value adjustment on compensation cost |  | $ (800,000) | (1,700,000) | (2,900,000)
Tax credit recognized by the company |  | $ (200,000) | (200,000) | (700,000)
Maximum |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Vesting period (in years) |  | 3 years |  |
Percentages for vesting |  | 200.00% |  |
2013 Share Incentive Plan |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Number of shares available for grant | shares |  | 2,845,683 |  |
2003 Share Incentive Plan |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Number of shares available for grant | shares |  | 595,683 |  |
International Employee Share Purchase Plan |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Employee savings under ESPP |  | $ 500 |  |
Employee contribution period (in years) |  | 2 years |  |
2008 Share Save Scheme |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Employee savings under ESPP | £ | £ 500 |  |  |
Employee contribution period (in years) |  | 3 years |  |
ESPP |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Compensation cost |  | $ (200,000) | (400,000) | (400,000)
Total tax credit |  | $ 100,000 | 100,000 | 0
Purchase price percentage of fair market value |  | (85.00%) |  |
Director Stock Option Plan |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Compensation cost |  | $ 0 | $ 0 | $ 0
Options granted (in shares) | shares |  | 0 |  | 4,447
Non-Employee Directors |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Percentages for vesting |  | 8.33% |  |
Non-Employee Directors | Restricted Share Units |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Unrecognized share compensation costs, RSUs |  | $ 100,000 |  |
Weighted average period of compensation cost recognition |  | 1 month 6 days |  |

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