Equity Incentive Plans (Tables)

6 Months Ended

Jun. 30, 2020

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Activity Under Stock Option Plans

Activity under the Company’s stock option plans during the six months ended June 30, 2020 was as follows:

				Weighted-		Weighted-		Aggregate
	Options			Average		Average		Intrinsic
	Outstanding			Exercise		Remaining		Value
	(000’s)			Price		Term		(000’s)
Balance at December 31, 2019		3,127		$	11.52
Granted		1,019		$	21.58
Exercised		(84	)	$	1.52
Forfeited and expired		(73	)	$	20.81
Balance at June 30, 2020		3,989		$	14.13		8.4	$	24,208
Exercisable at June 30, 2020		1,628		$	9.55		7.7	$	15,549

Weighted Average Assumptions Used to Estimate Fair Value of Stock Options Granted to Employees

The Company utilizes the Black-Scholes option pricing model to value awards under its equity plans. The following table summarizes the weighted average assumptions used to estimate the fair value of stock options granted under the Company’s stock option plans and the shares purchasable under the ESPP during the periods presented:

Stock Option Plans	2020	2019
Expected option term	6.0 years	5.9 years
Expected volatility	77%	78%
Risk free interest rate	1.1%	2.4%
Expected dividend yield	—%	—%

ESPP	2020	2019
Expected option term	2.0 years	1.3 years
Expected volatility	82%	62%
Risk free interest rate	0.0%	2.3%
Expected dividend yield	—%	—%

Summary of Stock-based Compensation Expense for the Equity Awards Issued to Employees and Non-Employees

Stock-based compensation expense for the equity awards issued by the Company to employees and non-employees for the periods presented below was as follows (in thousands):

	Three months ended June 30,				Six months ended June 30,
	2020		2019		2020		2019
Included in research and development	$	1,227	$	839	$	2,313	$	1,378
Included in general and administrative		1,301		742		2,362		1,231
Total stock-based compensation expense	$	2,528	$	1,581	$	4,675	$	2,609