Stock-Based Compensation (Tables)

9 Months Ended

Sep. 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 September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018
Research and development	$	10,185	$	8,465	$	30,214	$	22,729
Selling, general and administrative		4,608		5,858		18,651		15,703
Total stock-based compensation expense	$	14,793	$	14,323	$	48,865	$	38,432

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 September 30,						Nine Months Ended September 30,
	2019			2018			2019			2018
Stock Options
Expected term (in years)		5.3			5.3			5.3			5.4
Expected volatility		67.8	%		68.1	%		67.9	%		67.8	%
Risk-free interest rate		1.7	%		2.9	%		2.4	%		2.7	%
Expected dividend yield		—			—			—			—
Weighted average estimated fair value	$	25.17		$	35.99		$	32.44		$	32.37

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 - 72.8		%	48.1 - 62.1		%	47.3 - 75.3		%
Risk-free interest rate	1.6 - 2.9		%	1.0 - 2.6		%	1.3 - 2.9		%	0.8 - 2.6		%
Expected dividend yield		—			—			—			—
Weighted average estimated fair value	$	19.27		$	20.10		$	19.53		$	15.27