Stock-Based Compensation

9 Months Ended

Sep. 30, 2020

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,
	2020		2019		2020		2019
Research and development	$	11,236	$	10,185	$	32,653	$	30,214
Selling, general and administrative		6,655		4,608		19,798		18,651
Total stock-based compensation expense	$	17,891	$	14,793	$	52,451	$	48,865

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,
	2020			2019			2020			2019
Stock Options
Expected term (in years)		5.6			5.3			5.7			5.3
Expected volatility		59.0	%		67.8	%		67.7	%		67.9	%
Risk-free interest rate		0.3	%		1.7	%		0.8	%		2.4	%
Expected dividend yield		—			—			—			—
Weighted average estimated fair value	$	22.48		$	25.17		$	18.11		$	32.44

ESPPs
Expected term (in years)	0.5 - 2.0			0.5 - 2.0			0.5 - 2.0			0.5 - 2.0
Expected volatility	50.4 - 77.1		%	48.1 - 62.1		%	49.5 - 77.1		%	48.1 - 62.1		%
Risk-free interest rate	0.2 - 2.9		%	1.6 - 2.9		%	0.2 - 2.9		%	1.3 - 2.9		%
Expected dividend yield		—			—			—			—
Weighted average estimated fair value	$	17.00		$	19.27		$	17.74		$	19.53