Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2019

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 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

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,
	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