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Share-Based Payments - Narrative (Details) | 6 Months Ended
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Jun. 30, 2018 USD ($) Agreement shares | Jun. 30, 2018 GBP (£) Agreement shares | Jun. 30, 2017 USD ($) shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Number of arrangements under options and other equity incentives | Agreement | 3 | 3 |
Vesting period | 3 years | 3 years |
Award vesting rights, percentage | 100.00% | 100.00% |
Maximum |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Award vesting rights, percentage | 200.00% | 200.00% |
Performance Shares |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Granted, number of shares (in shares) | shares | 215,273 | 215,273 | 206,076
Compensation costs | $ 2,700,000 |  | $ 1,200,000
Tax benefit (expense) from compensation expense | $ 600,000 |  | $ 300,000
Award vesting rights, percentage | 33.33% | 33.33% |
Phantom Shares |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Granted, number of shares (in shares) | shares | 150,185 | 150,185 | 173,619
Compensation costs | $ 1,600,000 |  | $ 2,100,000
Tax benefit (expense) from compensation expense | $ 600,000 |  | $ 100,000
Restricted Share Units |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Vesting period | 3 years | 3 years |
Granted, number of shares (in shares) | shares | 230,766 | 230,766 | 174,488.707095856
Compensation costs | $ 4,600,000 |  | $ 5,000,000
Tax benefit (expense) from compensation expense | 1,100,000 |  | 1,200,000
Tax charge, non-employee directors | $ 0 |  | $ 0
2013 Share Incentive Plan |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Shares available to grant under incentive plan (in shares) | shares | 2,845,683 | 2,845,683 |
2016 Share Incentive Plan |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Shares available to grant under incentive plan (in shares) | shares | 263,695 | 263,695 |
Sharesave Scheme |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Employee contribution period | 3 years | 3 years |
Purchase price of fair market value | 85.00% | 85.00% |
Employee savings under ESPP | £ |  | £ 500 |
International Employee Share Purchase Plan |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Employee contribution period | 2 years | 2 years |
Employee savings under ESPP | $ 500 |  |
Shares issued (in shares) | shares | 31,684 | 31,684 | 9,979
Employee Share Purchase Plans |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Tax benefit (expense) from compensation expense | $ 0 |  | $ 0
Compensation costs charged in ESPP | $ 100,000 |  | $ 200,000
Non-Employee Directors |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Award vesting rights, percentage | 8.33% | 8.33% |
Non-Employee Directors | Restricted Share Units |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Granted, number of shares (in shares) | shares | 29,025 | 29,025 | 22,230
Compensation costs | $ 800,000 |  | $ 700,000

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