Equity (Tables)

6 Months Ended

Jun. 30, 2018

Equity [Abstract]

Summary of Stock Option Activity

A summary of the stock option activity under the Company’s plans for the six months ended June 30, 2018 is as follows:

	Number of shares	Weighted average price
Options outstanding at December 31, 2017	6,342,526	$	6.51
Options granted	2,168,602	$	8.34
Less:
Options exercised	1,030,576	$	5.53
Options canceled or expired	25,498	$	8.72
Options outstanding at June 30, 2018	7,455,054	$	7.18
Exercisable at June 30, 2018	2,958,222	$	6.14

Summary of Fair Value of Each Stock Option Award Granted

The fair value of each stock option award granted during the six months ended June 30, 2018 was estimated as of the grant date using a trinomial lattice model with the following weighted average assumptions:

	Six Months Ended June 30, 2018
Expected term (in years)	2.0 - 4.0
Risk-free interest rate (%)	2.4%
Expected volatility (%)	35.6% - 45.5%
Dividend yield (%)	—
Weighted average fair value/share at grant date	$2.61

Summary of Restricted Stock Activity

A summary of the restricted stock activity under the Company’s plans for the six months ended June 30, 2018 is as follows:

	Number of shares		Weighted average price
Nonvested at December 31, 2017	327,211		$	7.27
Granted	41,382		$	11.60
Vested	(119,180	)	7.27
Nonvested at June 30, 2018	249,413		$	7.99

Summary of Stock-Based Compensation Expense Recognized for Stock Options and Restricted Stock Included in Consolidated Statements of Operations

Stock based compensation expense recognized for stock options and restricted stock and included in the consolidated statements of operations was allocated as follows (in thousands): 

	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
Research and development expense	$	251	$	285	$	269	$	327
General and administrative expense	2,082		1,637		3,688		2,725
Total stock based compensation expense	$	2,333	$	1,922	$	3,957	$	3,052