Stock-Based Compensation

6 Months Ended

Jun. 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 June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Research and development	$	10,450	$	7,894	$	20,028	$	14,264
Selling, general and administrative		7,192		5,337		14,044		9,845
Total stock-based compensation expense	$	17,642	$	13,231	$	34,072	$	24,109

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,
	2019			2018			2019			2018
Stock Options
Expected term (in years)		5.3			5.4			5.3			5.4
Expected volatility		68.2	%		67.5	%		67.9	%		67.8	%
Risk-free interest rate		2.0	%		2.8	%		2.5	%		2.7	%
Expected dividend yield		—			—			—			—
Weighted average estimated fair value	$	24.40		$	31.35		$	33.20		$	32.12

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 - 75.3		%	48.1 - 62.1		%	47.3 - 75.3		%
Risk-free interest rate	1.3 - 2.9		%	0.8 - 2.6		%	1.3 - 2.9		%	0.8 - 2.6		%
Expected dividend yield		—			—			—			—
Weighted average estimated fair value	$	19.07		$	15.25		$	19.65		$	13.67