Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Schedule of weighted-average assumptions used to estimate grant date fair value of stock options

The weighted-average assumptions used to estimate grant date fair value of stock options using the Black-Scholes option pricing model were as follows for the years ended December 31, 2018, 2017 and 2016:

	Years ended December 31,
	2018	2017	2016
Expected common stock price volatility	85%	81%	71%
Risk-free interest rate	2.39% - 2.95%	1.82% - 2.38%	1.14% - 2.37%
Expected term of options (years)	6.0	6.1	6.1
Expected dividend yield	—	—	—

Schedule of share-based compensation expense

Share-based compensation expense includes expenses related to stock options and RSUs granted to employees, non-employee directors and consultants, as well as the option granted to employees to purchase shares under the ESPP, all of which have been reported in the Company’s Statements of Operations as follows:

	Years ended December 31,
	2018		2017		2016
Research and development	$	4,967	$	11,114	$	21,380
General and administrative	6,105		7,057		10,280
Total	$	11,072	$	18,171	$	31,660