Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2020

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Schedule of Recorded Stock-Based Compensation Expense

Stock-based compensation expense was recorded to research and development and selling, general and administrative expense as follows (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2020		2019		2020		2019
Research and development	$	10,780	$	10,450	$	21,417	$	20,028
Selling, general and administrative		6,864		7,192		13,143		14,044
Total stock-based compensation expense	$	17,644	$	17,642	$	34,560	$	34,072

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,
	2020			2019			2020			2019
Stock Options
Expected term (in years)		5.5			5.3			5.7			5.3
Expected volatility		69.2	%		68.2	%		68.4	%		67.9	%
Risk-free interest rate		0.4	%		2.0	%		0.8	%		2.5	%
Expected dividend yield		—			—			—			—
Weighted average estimated fair value	$	21.72		$	24.40		$	17.75		$	33.20

ESPPs
Expected term (in years)	0.5 – 2.0			0.5 – 2.0			0.5 – 2.0			0.5 – 2.0
Expected volatility	49.5 – 77.1		%	48.1 – 62.1		%	49.5 – 77.1		%	48.1 – 62.1		%
Risk-free interest rate	0.2 – 2.9		%	1.3 – 2.9		%	0.2 – 2.9		%	1.3 – 2.9		%
Expected dividend yield		—			—			—			—
Weighted average estimated fair value	$	17.82		$	19.07		$	18.11		$	19.65