Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2017

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

Schedule of activity under the company's stock option plans

The stock option and related activity under all of the Company’s stock compensation plans is summarized as follows:

	Outstanding Options				Weighted Average Remaining Contractual Term (years)	Aggregate Intrinsic Value (thousands)
	Number of Shares		Weighted Average Exercise Price
Stock Options
Balances, December 31, 2016	5,270,196		$	9.60
Options granted	1,444,675		18.87
Options exercised	(1,183,353	)	7.63
Options forfeited	(383,243	)	16.42
Options expired	(10,691	)	20.29
Balances, December 31, 2017	5,137,584		$	12.12	6.48	$	111,981
Vested and expected to vest, December 31, 2017	4,991,486		$	11.99	6.43	$	109,438
Exercisable, December 31, 2017	3,363,191		$	9.01	5.31	$	83,688

Schedule of restricted stock unit and related activity

The restricted stock unit and related activity under all of the Company’s stock compensation plans is summarized as follows:

	Outstanding Units
			Weighted-Average Grant Date Fair Value
Restricted Stock Units	Number of Shares
Unvested, December 31, 2016	213,361		$	14.66
Granted	349,896		19.97
Vested	(64,471	)	14.96
Forfeited	(71,735	)	16.13
Unvested, December 31, 2017	427,051		$	18.72

Schedule of allocation of total stock-based compensation expense

The following table reflects stock-based compensation expense recognized for the years ended December 31, 2017, 2016, and 2015 (in thousands):

	Years Ended December 31,
	2017		2016		2015
Research and development	$	3,597	$	2,855	$	3,156
Selling, general and administrative	9,770		7,716		6,800
Total stock-based compensation expense	$	13,367	$	10,571	$	9,956

Schedule of assumptions used to estimate fair value of stock options

The Company estimated the fair value of employee stock options and ESPP shares on the date of grant using the Black-Scholes model with the following weighted-average assumptions:

	Years Ended December 31,
	2017	2016	2015
Stock Options
Expected price volatility	68% - 70%	69% - 71%	68% - 80%
Risk-free interest rate	1.83% - 2.17%	1.23% - 1.81%	1.37% - 1.95%
Expected term (in years)	5.50 - 6.25	5.50 - 6.25	5.50 - 6.25
Dividend yield	—	—	—

	Years Ended December 31,
	2017	2016	2015
Employee Stock Purchase Plan
Expected price volatility	51% - 71%	68% - 73%	56% - 62%
Risk-free interest rate	0.60% - 1.45%	0.49% - 0.60%	0.07% - 0.41%
Expected term (in years)	0.50	0.50	0.50
Dividend yield	—	—	—

The Company estimated the fair value of non-employee stock options using the Black-Scholes model with the following weighted-average assumptions:

	Years Ended December 31,
	2017	2016	2015
Expected price volatility	68% - 80%	71% - 77%	76% - 82%
Risk-free interest rate	1.94% - 2.36%	1.47% - 2.46%	1.84% - 2.26%
Expected term (in years)	6.00 - 9.75	7.00 - 9.75	8.00 - 9.00
Dividend yield	—	—	—