Stock-Based Compensation - Total Stock-based Compensation Recognized (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Stock-based compensation expense:
Stock-based compensation expense	$ 3,425	$ 3,513	$ 6,459	$ 7,526
Research and development
Stock-based compensation expense:
Stock-based compensation expense	1,548	1,663	2,885	3,582
General and administrative
Stock-based compensation expense:
Stock-based compensation expense	$ 1,877	$ 1,850	$ 3,574	$ 3,944

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
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-Section 50
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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