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Share-Based Payments - Additional Information (Details) |  | 12 Months Ended
--- | --- | ---
Apr. 05, 2014 GBP (£) | Dec. 31, 2017 USD ($) agreement shares | Dec. 31, 2016 USD ($) shares | Dec. 31, 2015 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 |  |
Compensation cost |  |  |  | $ (6,500,000)
Total tax credit |  | $ 100,000 | $ 500,000 | $ 1,500,000
Percentages for vesting |  | 100.00% |  |
Contract period (in years) |  | 10 years |  |
Options exercised | shares |  | 0 | 29,222 | 189,215
Intrinsic value of options |  | $ 0 | $ 600,000 | $ 4,300,000
Employee Stock Ownership Plan, shares issued during period (in shares) | shares |  | 46,866 | 21,285 | 59,940
Restricted Share Units |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Compensation cost |  | $ (9,400,000) | $ (9,800,000) | $ (8,700,000)
Total tax credit |  | 2,300,000 | 2,400,000 | 1,800,000
Unrecognized share compensation costs, RSUs |  | $ 11,000,000 |  |
Weighted average period of compensation cost recognition |  | 1 year 7 months 6 days |  |
Compensation cost, non employee directors |  | $ 1,500,000 | 1,400,000 | 1,800,000
Tax charge, non employee directors |  | 0 | 0 | 0
Performance Shares |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Compensation cost |  | (400,000) | (2,200,000) |
Unrecognized share compensation costs, RSUs |  | $ 10,000,000 |  |
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 |  | $ (700,000) | (700,000) | (4,300,000)
Unrecognized share compensation costs, RSUs |  | $ 5,700,000 |  |
Weighted average period of compensation cost recognition |  | 1 year 9 months 18 days |  |
Fair value adjustment on compensation cost |  | $ (1,700,000) | (2,900,000) | (3,200,000)
Tax credit recognized by the company |  | $ 200,000 | (700,000) | (1,200,000)
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 |  | $ (400,000) | (400,000) | (300,000)
Total tax credit |  | $ 100,000 | 100,000 |
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 | 0
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 |  | $ 200,000 |  |
Weighted average period of compensation cost recognition |  | 1 month 6 days |  |

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- Definition Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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