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Share-Based Compensation - Classification of Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based compensation
Share-based compensation expense	$ 20,875	$ 15,080	$ 5,701
Expenses expected to be recognized over a weighted average remaining period	2 years 9 months 14 days
Research and development expenses
Share-based compensation
Share-based compensation expense	$ 8,866	5,746	2,285
General and administrative expenses
Share-based compensation
Share-based compensation expense	12,009	$ 9,334	$ 3,416
Stock options
Share-based compensation
Total unrecognized compensation cost	64,715
First RSU Award
Share-based compensation
Total unrecognized compensation cost	$ 2,754