Stock-Based Compensation

9 Months Ended

Sep. 30, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

6.	Stock-Based Compensation

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

	Three Months Ended September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018
Research and development	$	10,185	$	8,465	$	30,214	$	22,729
Selling, general and administrative		4,608		5,858		18,651		15,703
Total stock-based compensation expense	$	14,793	$	14,323	$	48,865	$	38,432

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,
	2019			2018			2019			2018
Stock Options
Expected term (in years)		5.3			5.3			5.3			5.4
Expected volatility		67.8	%		68.1	%		67.9	%		67.8	%
Risk-free interest rate		1.7	%		2.9	%		2.4	%		2.7	%
Expected dividend yield		—			—			—			—
Weighted average estimated fair value	$	25.17		$	35.99		$	32.44		$	32.37

ESPPs
Expected term (in years)	0.5 - 2.0			0.5 - 2.0			0.5 - 2.0			0.5 - 2.0
Expected volatility	48.1 - 62.1		%	47.3 - 72.8		%	48.1 - 62.1		%	47.3 - 75.3		%
Risk-free interest rate	1.6 - 2.9		%	1.0 - 2.6		%	1.3 - 2.9		%	0.8 - 2.6		%
Expected dividend yield		—			—			—			—
Weighted average estimated fair value	$	19.27		$	20.10		$	19.53		$	15.27