Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2021

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Schedule of Recorded Stock-Based Compensation Expense

Stock-based compensation expense was recorded directly to research and development and selling, general and administrative expense as follows (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2021		2020		2021		2020
Research and development	$	8,220	$	11,236	$	31,123	$	32,653
Selling, general and administrative		7,484		6,655		22,966		19,798
Total stock-based compensation expense	$	15,704	$	17,891	$	54,089	$	52,451

Schedule of Assumptions used to Estimate Fair Value of Stock Options Granted and Purchases under 2014 Employee Share Purchase Plan

The assumptions used to estimate the fair value of stock options granted and purchases under the Company’s 2014 Employee Share Purchase Plan (“ESPP”) using the Black-Scholes option valuation model were as follows:

	Three Months Ended September 30,						Nine Months Ended September 30,
	2021			2020			2021			2020
Stock Options
Expected term (in years)		5.8			5.6			5.7			5.7
Expected volatility		68.1	%		59.0	%		61.8	%		67.7	%
Risk-free interest rate		1.2	%		0.3	%		0.9	%		0.8	%
Expected dividend yield		—			—			—			—
Weighted average estimated fair value	$	11.08		$	22.48		$	21.45		$	18.11

ESPPs
Expected term (in years)	0.5 - 2.0			0.5 - 2.0			0.5 - 2.0			0.5 - 2.0
Expected volatility	50.6 - 104.4		%	50.4 - 77.1		%	47.1 - 104.4		%	49.5 - 77.1		%
Risk-free interest rate	0.0 - 1.6		%	0.2 - 2.9		%	0.0 - 2.2		%	0.2 - 2.9		%
Expected dividend yield		—			—			—			—
Weighted average estimated fair value	$	12.37		$	17.00		$	13.57		$	17.74