Share-Based Payments - Narrative (Details)	3 Months Ended
Mar. 31, 2018 USD ($) Agreement shares	Mar. 31, 2018 GBP (£) Agreement shares	Mar. 31, 2017 USD ($) 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	206,073
Compensation costs	$ 2,000,000		$ 2,700,000
Tax benefit (expense) from compensation expense	$ 300,000		$ 500,000
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	174,284
Compensation costs	$ 1,100,000		$ 2,200,000
Tax benefit (expense) from compensation expense	$ (200,000)		$ (400,000)
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	221,943	221,943	174,711
Compensation costs	$ 2,100,000		$ 2,500,000
Tax benefit (expense) from compensation expense	200,000		600,000
Tax charge, non-employee directors	$ 0		$ 0
2003 Share Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available to grant under incentive plan (in shares) \| shares	595,683	595,683
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	4,797	4,797	2,310
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	$ 200,000		$ 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	24,456
Compensation costs	$ 300,000		$ 400,000

Employee service share based compensation tax expenses recognized for stock options.

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Employee share purchase plan maximum saving by employee.

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Share based compensation arrangement by share based payment award number of arrangements.

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Share Based Compensation Arrangement By Share Based Payment Award Purchase Price Percentage Of Fair Market Value

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- Definition

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Represents the compensation cost capitalized during the period arising from equity-based compensation arrangements (for example, shares of stock, units, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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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.

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Percentage of vesting of share-based compensation awards.

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