Stock-Based Awards (Tables)

6 Months Ended

Jun. 30, 2020

Share-based Payment Arrangement [Abstract]

Summary of Stock Option Activity

The following table summarizes option award activity under the Company's 2017 Equity Incentive Plan and the 2015 Stock Incentive Plan: 

	Number of Options	Weighted- Average Exercise Price
Balance at December 31, 2019	11,640,734	$	12.96
Granted	3,244,298	24.78
Exercised	(461,968)	7.67
Forfeited	(232,093)	19.08
Balance at June 30, 2020	14,190,971	$	15.74
Vested and expected to vest at June 30, 2020	12,446,239	$	17.85
Exercisable at June 30, 2020	5,318,846	$	13.52

Summary of Assumptions Used for Estimating the Fair Value of Stock Granted

The estimated fair value of stock options granted to employees were calculated using the Black-Scholes option-pricing model using the following assumptions:

	Six Months Ended June 30,
	2020	2019
Expected term (in years)	5.50 - 6.08	5.50 - 6.08
Volatility	65.2% - 67.1%	66.9% - 77.8%
Risk-free interest rate	0.4% - 1.7%	2.0% - 2.6%
Dividend yield	—	—

Restricted Stock Activity

Aggregated information regarding restricted stock for the six months ended June 30, 2020 is summarized below:

	Share Awards & Units	Weighted-Average Fair Value at Date of Grant per Share
Unvested at December 31, 2019	882,636	$	18.67
Granted	957,249	24.62
Vested and released	(118,421)	19.13
Forfeited	(48,412)	21.52
Unvested at June 30, 2020	1,673,052	$	21.98
Expected to vest at June 30, 2020	1,673,052	$	21.98

Summary of Stock-Based Compensation Expense

The Company’s results of operations include expenses relating to stock-based compensation as follows (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2020		2019		2020		2019
Research and development	$	7,146	$	5,295	$	13,200	$	9,277
General and administrative	5,338		8,419		10,069		11,311
Total	$	12,484	$	13,714	$	23,269	$	20,588