Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Schedule of Allocated Stock-Based Compensation Expense

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

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,
	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