Stock-Based Compensation

3 Months Ended

Mar. 31, 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 March 31,
	2021		2020
Research and development	$	12,221	$	10,637
Selling, general and administrative		7,163		6,279
Total stock-based compensation expense	$	19,384	$	16,916

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 March 31,
	2021			2020
Stock Options
Expected term (in years)		5.7			5.7
Expected volatility		58.8	%		68.3	%
Risk-free interest rate		0.7	%		0.9	%
Expected dividend yield		—			—
Weighted average estimated fair value	$	27.46		$	17.24

ESPPs
Expected term (in years)	0.5 - 2.0			0.5 - 2.0
Expected volatility	47.5 - 64.4		%	49.5 - 57.7		%
Risk-free interest rate	0.1 - 2.2		%	1.5 - 2.9		%
Expected dividend yield		—			—
Weighted average estimated fair value	$	16.94		$	18.57