Stock-Based Compensation

6 Months Ended

Jun. 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 June 30,				Six Months Ended June 30,
	2021		2020		2021		2020
Research and development	$	10,681	$	10,780	$	22,903	$	21,417
Selling, general and administrative		8,320		6,864		15,482		13,143
Total stock-based compensation expense	$	19,001	$	17,644	$	38,385	$	34,560

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 June 30,						Six Months Ended June 30,
	2021			2020			2021			2020
Stock Options
Expected term (in years)		5.6			5.5			5.7			5.7
Expected volatility		63.4	%		69.2	%		59.4	%		68.4	%
Risk-free interest rate		0.9	%		0.4	%		0.7	%		0.8	%
Expected dividend yield		—			—			—			—
Weighted average estimated fair value	$	11.91		$	21.72		$	25.38		$	17.75

ESPPs
Expected term (in years)	0.5 - 2.0			0.5 - 2.0			0.5 - 2.0			0.5 - 2.0
Expected volatility	47.5 - 104.2		%	49.5 - 77.1		%	47.5 - 104.2		%	49.5 - 77.1		%
Risk-free interest rate	0.0 - 2.2		%	0.2 - 2.9		%	0.0 - 2.2		%	0.2 - 2.9		%
Expected dividend yield		—			—			—			—
Weighted average estimated fair value	$	13.60		$	17.82		$	14.69		$	18.11