Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2020

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 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

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 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