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Equity Incentive Schemes and Stock Compensation Charges - Outstanding and Exercisable Share Options (Details)	12 Months Ended
	Dec. 31, 2018 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 125.74
Number of Shares (in shares) | shares	920,746
Weighted Average Remaining Contractual Life	5 years 3 days
Weighted Average Exercise Price (usd per share)	$ 74.32
Number of Shares (in shares) | shares	397,923
Weighted Average Exercise Price (usd per share)	$ 56.10
Minimum Range Exercise Price (usd per share)	20.28
Range 1
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 20.28
Number of Shares (in shares) | shares	18,190
Weighted Average Remaining Contractual Life	1 month 27 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	18,190
Weighted Average Exercise Price (usd per share)
Range 2
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 22.30
Number of Shares (in shares) | shares	52,811
Weighted Average Remaining Contractual Life	1 year 3 months 26 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	52,811
Weighted Average Exercise Price (usd per share)
Range 3
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 23.66
Number of Shares (in shares) | shares	1,161
Weighted Average Remaining Contractual Life	1 year 6 months 25 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	1,161
Weighted Average Exercise Price (usd per share)
Range 4
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 26.71
Number of Shares (in shares) | shares	450
Weighted Average Remaining Contractual Life	1 year 8 months 9 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	450
Weighted Average Exercise Price (usd per share)
Range 5
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 32.37
Number of Shares (in shares) | shares	20,848
Weighted Average Remaining Contractual Life	2 years 3 months 29 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	20,848
Weighted Average Exercise Price (usd per share)
Range 6
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 36.22
Number of Shares (in shares) | shares	2,141
Weighted Average Remaining Contractual Life	2 years 5 months 16 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	2,141
Weighted Average Exercise Price (usd per share)
Range 7
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 37.90
Number of Shares (in shares) | shares	920
Weighted Average Remaining Contractual Life	2 years 11 months 4 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	920
Weighted Average Exercise Price (usd per share)
Range 8
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 40.83
Number of Shares (in shares) | shares	40,502
Weighted Average Remaining Contractual Life	3 years 4 months 20 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	25,360
Weighted Average Exercise Price (usd per share)
Range 9
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 47.03
Number of Shares (in shares) | shares	22,685
Weighted Average Remaining Contractual Life	3 years 2 months 1 day
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	14,020
Weighted Average Exercise Price (usd per share)
Range 10
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 48.67
Number of Shares (in shares) | shares	50,896
Weighted Average Remaining Contractual Life	3 years 2 months 16 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	34,906
Weighted Average Exercise Price (usd per share)
Range 11
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 51.35
Number of Shares (in shares) | shares	2,030
Weighted Average Remaining Contractual Life	3 years 7 months 6 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	1,224
Weighted Average Exercise Price (usd per share)
Range 12
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 65.60
Number of Shares (in shares) | shares	66,752
Weighted Average Remaining Contractual Life	5 years 4 months 17 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	24,543
Weighted Average Exercise Price (usd per share)
Range 13
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 66.47
Number of Shares (in shares) | shares	4,267
Weighted Average Remaining Contractual Life	4 years 4 months 20 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	1,572
Weighted Average Exercise Price (usd per share)
Range 14
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 66.97
Number of Shares (in shares) | shares	1,249
Weighted Average Remaining Contractual Life	4 years 5 months 12 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	0
Weighted Average Exercise Price (usd per share)
Range 15
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 68.39
Number of Shares (in shares) | shares	135,400
Weighted Average Remaining Contractual Life	4 years 2 months 5 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	79,069
Weighted Average Exercise Price (usd per share)
Range 16
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 71.95
Number of Shares (in shares) | shares	127,905
Weighted Average Remaining Contractual Life	5 years 2 months 1 day
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	74,473
Weighted Average Exercise Price (usd per share)
Range 17
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 83.47
Number of Shares (in shares) | shares	127,136
Weighted Average Remaining Contractual Life	6 years 2 months 1 day
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	22,461
Weighted Average Exercise Price (usd per share)
Range 18
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 90.03
Number of Shares (in shares) | shares	77,846
Weighted Average Remaining Contractual Life	6 years 4 months 17 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	23,774
Weighted Average Exercise Price (usd per share)
Range 19
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 115.11
Number of Shares (in shares) | shares	107,794
Weighted Average Remaining Contractual Life	7 years 2 months 1 day
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	0
Weighted Average Exercise Price (usd per share)
Range 20
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 125.74
Number of Shares (in shares) | shares	59,763
Weighted Average Remaining Contractual Life	7 years 4 months 17 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	0
Weighted Average Exercise Price (usd per share)

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
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