Stock-Based Compensation - Compensation expense (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Stock-based compensation expense
Stock-based compensation expense	$ 8,488	$ 9,478
Unrecognized compensation cost	$ 68,400
Unrecognized compensation cost expected to be recognized over a weighted average period	2 years 3 months 18 days
Research and development expenses
Stock-based compensation expense
Stock-based compensation expense	$ 2,111	1,991
General and administrative expenses
Stock-based compensation expense
Stock-based compensation expense	$ 6,377	$ 7,487

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

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