Shareholders' Equity (Tables)

6 Months Ended

Jun. 30, 2017

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Summary of Share-based Compensation Expense

Total share-based compensation expense related to all share-based awards for the three and six months ended June 30, 2017 and 2016 was comprised of the following (in thousands):

	Three Months Ended   June 30,				Six Months Ended June 30,
	2017		2016		2017		2016
Research and development:
Stock options	$	2,563	$	1,215	$	5,016	$	2,476
Restricted stock	—		—		—		30
Warrants	16		4		33		15
Research and development share-based compensation expense	2,579		1,219		5,049		2,521
General and administrative:
Stock options	1,993		1,559		3,948		3,269
Restricted stock	—		510		409		1,129
Warrants	139		53		288		127
General and administrative share-based compensation expense	2,132		2,122		4,645		4,525
Total share-based compensation expense included in expenses	$	4,711	$	3,341	$	9,694	$	7,046

Stock Options, Valuation Assumptions

The Company estimated the fair value of each stock option grant on the grant date using the Black-Scholes model with the following weighted-average assumptions:

	Six Months Ended June 30,
	2017		2016
Expected volatility	137	%	143	%
Expected life	6.23 years		5.27 years
Risk-free interest rate	2.0	%	1.3	%
Dividend yield	—		—

Summary of Stock Option Activity

The Company’s 2013 Equity Plan stock option activity for the six months ended June 30, 2017 was comprised of the following:

	Outstanding Stock Options and 2013 Equity Plan
	Shares Underlying Stock Options		Weighted- Average Exercise Price per Share
Outstanding at December 31, 2016	2,627,462		$	21.07
Granted	1,475,425		$	20.54
Exercised	(130,555	)	$	15.89
Forfeited	(74,217	)	$	21.97
Outstanding at June 30, 2017	3,898,115		$	21.03

Summary of Restricted Stock Activity

The Company’s restricted stock award activity for the six months ended June 30, 2017 was comprised of the following:

	Number of Shares		Weighted- Average Grant Date Fair Market Value
Unvested at December 31, 2016	542,680		$	13.22
Vested	(542,680	)	$	13.22
Unvested at June 30, 2017	—		$	—