Stock-Based Compensation

3 Months Ended

Mar. 31, 2020

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

6.	Stock-Based Compensation

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

	Three Months Ended March 31,
	2020		2019
Research and development	$	10,637	$	9,578
Selling, general and administrative		6,279		6,852
Total stock-based compensation expense	$	16,916	$	16,430

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,
	2020			2019
Stock Options
Expected term (in years)		5.7			5.3
Expected volatility		68.3	%		67.9	%
Risk-free interest rate		0.9	%		2.6	%
Expected dividend yield		—			—
Weighted average estimated fair value	$	17.24		$	34.14

ESPPs
Expected term (in years)	0.5 - 2.0			0.5 - 2.0
Expected volatility	49.5 - 57.7		%	48.1 - 62.1		%
Risk-free interest rate	1.5 - 2.9		%	1.3 - 2.9		%
Expected dividend yield		—			—
Weighted average estimated fair value	$	18.57		$	20.57