Stock-Based Compensation

9 Months Ended

Sep. 30, 2021

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

7.	Stock-Based Compensation

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

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