Share Based Compensation - Classification of expense (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Share-based compensation
Share-based compensation expense	$ 15,080	$ 5,701
Expenses expected to be recognized over a weighted average remaining period		3 years 1 month 6 days
Research and development expenses
Share-based compensation
Share-based compensation expense	5,746	$ 2,285
General and administrative expenses
Share-based compensation
Share-based compensation expense	9,334	$ 3,416
Stock options
Share-based compensation
Total unrecognized compensation cost	$ 44,185
Expenses expected to be recognized over a weighted average remaining period	3 years 1 month 6 days
Time-based RSUs
Share-based compensation
Share-based compensation expense	$ 65
Total unrecognized compensation cost	$ 1,185
Expenses expected to be recognized over a weighted average remaining period	1 year
First RSU Award
Share-based compensation
Share-based compensation expense	$ 0
Total unrecognized compensation cost	$ 2,994

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5047-113901

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.F)
-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

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Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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-Topic 718
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-Section 50
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-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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