Stock-Based Compensation

3 Months Ended

Mar. 31, 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 March 31,
	2018		2017
Research and development	$	6,370	$	4,901
General and administrative		4,508		3,508
Total stock-based compensation expense	$	10,878	$	8,409

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 March 31,
	2018			2017
Stock Options
Expected term (in years)		5.4			5.8
Expected volatility		67.9	%		71.7	%
Risk-free interest rate		2.7	%		2.3	%
Expected dividend yield		—			—
Weighted average estimated fair value	$	32.18		$	16.16

ESPPs
Expected term (in years)	0.5 - 2.0			0.5 - 2.0
Expected volatility	54.9 - 75.3		%	63.7 - 77.2		%
Risk-free interest rate	0.8 - 1.6		%	0.5 - 1.0		%
Expected dividend yield		—			—
Weighted average estimated fair value	$	10.93		$	8.86