Stock Based Awards (Tables)

12 Months Ended

Dec. 31, 2017

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Activity under the Company’s stock option plans is set forth below:

									Weighted
									Average
							Weighted		Remaining		Aggregate
	Shares						Average		Contractual		Intrinsic
	Available			Number of			Exercise		Life		Value
	for Grant			Shares			Price		(in years)		(in thousands)
Balance at January 1, 2015		991,401			2,723,367		$	10.33
Additional shares authorized		1,091,045			—			—
Options granted		(664,100	)		664,100			15.34
Restricted stock units granted		(123,450	)		—			—
Options exercised		—			(90,851	)		1.34
Options cancelled		55,647			(55,647	)		15.68
Balances, December 31, 2015		1,350,543			3,240,969		$	11.51
Additional shares authorized		1,323,911			—			—
Options granted		(1,399,522	)		1,399,522			10.04
Restricted stock units granted		(332,964	)		—			—
Options exercised		—			(85,646	)		1.64
Options cancelled		102,145			(102,145	)		14.90
Balances, December 31, 2016		1,044,113			4,452,700		$	11.16
Additional shares authorized		1,567,975			—			—
Options granted		(1,364,700	)		1,364,700			15.92
Restricted stock units granted		(2,568,333	)		—			—
Options exercised		—			(850,995	)		2.94
Options cancelled		1,320,949			(1,320,949	)		14.65
Balances, December 31, 2017		4			3,645,456		$	13.60		6.3	$	38
Vested and expected to vest as of December 31, 2017		—			3,569,972		$	13.56		6.2	$	38
Exercisable as of December 31, 2017		—			2,345,851		$	12.85		5.0	$	31

Stock Options Outstanding and Exercisable under Stock Option Plans

The following table summarizes information with respect to stock options outstanding and currently exercisable and vested as of December 31, 2017:

					Options Exercisable
	Options Outstanding				and Vested
			Weighted Average				Weighted Average
			Remaining				Remaining
Range of	Number		Contractual		Number		Contractual
Exercise Prices	Outstanding		Life (in Years)		Outstanding		Life (in Years)
$1.27-$8.17		819,299		4.6		765,419		4.6
8.17-10.87		757,509		6.1		458,172		6.1
10.88-15.19		1,003,148		7.6		522,356		7.6
15.22-21.30		778,791		7.7		335,805		7.7
22.24-34.00		286,709		3.1		264,099		3.1
		3,645,456				2,345,851

Summary of Fair Value of Employee Stock Options

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

	Year Ended December 31,
	2017		2016		2015
Expected volatility	78.0%		77.9%		79.7%
Risk-free interest rate	2.1%		1.5%		1.7%
Dividend yield	0.0%		0.0%		0.0%
Expected life (in years)		6.2		6.0		6.0

Schedule of Estimated Stock-Based Compensation Expense

Stock-based compensation expense, net of estimate forfeitures, is reflected in the statements of operations and comprehensive loss as follows (in thousands):

	Year Ended December 31,
	2017		2016		2015
Operating Expenses
Research and development	$	4,549	$	3,837	$	3,032
General and administrative		8,800		7,086		7,667
Total	$	13,349	$	10,923	$	10,699

Summary of Fair Value of Employees Stock Purchase Rights Under ESPP Using Black-Scholes Model with Following Weighted-Average Assumptions

The Company estimated the fair value of our employees’ stock purchase rights under the ESPP using the Black-Scholes model with the following weighted-average assumptions:

	Year Ended December 31,
	2017		2016		2015
Expected volatility	162.3%		73.6%		72.1%
Risk-free interest rate	1.21%		0.51%		0.26%
Dividend yield	0.0%		0.0%		0.0%
Expected life (in years)		0.5		0.5		0.5

Summary of Restricted Stock Activity

A summary of the Company’s restricted stock activity is presented in the following tables:

				Weighted
				Average
	Number of			Grant Date
	Shares			Fair Value
Restricted Stock Units
Unvested at December 31, 2016		502,027		$	14.43
Granted		2,568,333			5.84
Vested		(169,938	)		13.24
Forfeited/canceled		(381,682	)		16.32
Unvested at December 31, 2017		2,518,740		$	5.43