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Stock-Based Compensation - Stock-Based Compensation Expense (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	May 31, 2017	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Oct. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options outstanding (in shares)		8,530,589		8,530,589		7,258,353
Weighted-average exercise price of stock options outstanding (in dollars per share)		$ 21.52		$ 21.52		$ 21.48
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense		$ 6,358	$ 5,832	$ 17,714	$ 15,419
2016 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, number of shares authorized (in shares)	800,000
Payroll deduction percentage	10.00%
Purchase price of common stock percent of the fair market value	85.00%
Common stock purchased (in shares)				12,305
2003 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options outstanding (in shares)		188,944		188,944
Weighted-average exercise price of stock options outstanding (in dollars per share)		$ 2.94		$ 2.94
Stock Incentive Plan 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, number of shares authorized (in shares)		13,856,781		13,856,781
Options outstanding (in shares)		8,341,645		8,341,645
Weighted-average exercise price of stock options outstanding (in dollars per share)		$ 21.94		$ 21.94
Potential annual increase in shares reserved for future issuance (in shares)							1,960,168
Potential annual increase in shares reserved for future issuance as percentage of outstanding share				4.00%
Research and Development
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense		$ 3,109	3,059	$ 8,894	8,082
General and Administrative
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense		$ 3,249	$ 2,773	$ 8,820	$ 7,337

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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