Share-based Compensation - HCML Share Option Exercises (Details) - HCML Share Option Scheme £ in Thousands, $ in Thousands	6 Months Ended
Jun. 30, 2018 GBP (£)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 GBP (£)	Jun. 30, 2017 USD ($)
Share option values
Cash received from share options exercised \| £	£ 720		£ 174
Total intrinsic value of share options exercised \| $		$ 4,817		$ 1,049

Amount of cash inflow from exercise of stock options granted under share-based compensation arrangement.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3255-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=SL79508275-113901

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration