Equity Incentive Schemes and Stock Compensation Charges - Summary of RSU and PSU Activity (Details) - $ / shares	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Performance Share Unit (PSUs)
Outstanding Number of Shares
Outstanding at beginning of period (in shares)	511,026
Granted (in shares)	71,906
Shares vested (in shares)	(215,826)
Forfeited (in shares)	(116,053)
Outstanding at ending of period (in shares)	251,053	511,026
Weighted Average Fair Value
Outstanding at beginning of period (usd per share)	$ 72.07
Granted (usd per share)	116.02
Shares vested (usd per share)	68.28
Forfeited (usd per share)	70.89
Outstanding at end of period (usd per share)	$ 85.95	$ 72.07
Weighted Average Remaining Contractual Life
Outstanding at end of period (usd per share)	11 months 15 days	11 months 5 days
Restricted Stock Units (RSUs)
Outstanding Number of Shares
Outstanding at beginning of period (in shares)	715,970
Granted (in shares)	160,113
Shares vested (in shares)	(276,495)
Forfeited (in shares)	(64,911)
Outstanding at ending of period (in shares)	534,677	715,970
Weighted Average Fair Value
Outstanding at beginning of period (usd per share)	$ 72.65
Granted (usd per share)	123.42
Shares vested (usd per share)	67.99
Forfeited (usd per share)	78.92
Outstanding at end of period (usd per share)	$ 89.50	$ 72.65
Weighted Average Remaining Contractual Life
Outstanding at end of period (usd per share)	1 year 2 months 19 days	1 year 3 months 11 days

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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

+ 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 (c)(1)(iv)(3)
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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

+ 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 (c)(2)(iii)(3)
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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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The weighted average fair value at grant date for nonvested equity-based awards issued 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).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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No definition available.

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Weighted average remaining contractual term for equity-based awards excluding options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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-Publisher FASB
-Name Accounting Standards Codification
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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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-Publisher FASB
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