Stock-Based Compensation (Tables)

6 Months Ended

Jun. 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 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

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 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