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Share-Based Payments - Narrative (Details) | 9 Months Ended |  |
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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% |  |  |

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