Stock-Based Compensation

6 Months Ended

Jun. 30, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

5.	Stock-Based Compensation

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

	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
Research and development	$	7,894	$	5,621	$	14,264	$	10,521
General and administrative		5,337		3,950		9,845		7,459
Total stock-based compensation expense	$	13,231	$	9,571	$	24,109	$	17,980

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,
	2018			2017			2018			2017
Stock Options
Expected term (in years)		5.4			5.4			5.4			5.7
Expected volatility		67.5	%		71.0	%		67.8	%		71.6	%
Risk-free interest rate		2.8	%		1.9	%		2.7	%		2.3	%
Expected dividend yield		—			—			—			—
Weighted average estimated fair value	$	31.35		$	17.06		$	32.12		$	16.26

ESPPs
Expected term (in years)	0.5 - 2.0			0.5 - 2.0			0.5 - 2.0			0.5 - 2.0
Expected volatility	47.3 - 75.3		%	52.8 - 77.2		%	47.3 - 75.3		%	52.8 - 77.2		%
Risk-free interest rate	0.8 - 2.6		%	0.5 - 1.3		%	0.8 - 2.6		%	0.5 - 1.3		%
Expected dividend yield		—			—			—			—
Weighted average estimated fair value	$	15.25		$	9.10		$	13.67		$	8.98