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Stock-Based Compensation - Stock-Based Compensation Expense (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended
	May 31, 2017	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options outstanding (in shares)		7,644,939		6,706,994
Weighted-average exercise price of stock options outstanding (in dollars per share)		$ 20.95		$ 22.33
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense		$ 4,491	$ 3,799
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 discount	10.00%
Purchase price of common stock percent of the fair market value	85.00%
Common stock purchased (in shares)		0
2003 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options outstanding (in shares)		376,079
Weighted-average exercise price of stock options outstanding (in dollars per share)		$ 2.96
Stock Incentive Plan 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, number of shares authorized (in shares)		11,896,613
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%
Options outstanding (in shares)		7,268,860
Weighted-average exercise price of stock options outstanding (in dollars per share)		$ 21.88
Research and Development
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense		$ 2,434	1,814
General and Administrative
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense		$ 2,057	$ 1,985

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