Stock Based Awards (Tables)

12 Months Ended

Dec. 31, 2015

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, 2014		9,533			1,403,655		$	1.90
Additional shares authorized		2,531,915			—			—
Options granted		(1,471,142	)		1,471,142			18.68
Restricted stock units granted		(185,514	)
Options exercised		—			(44,822	)		1.32
Options cancelled		106,609			(106,609	)		18.43
Balances, December 31, 2014		991,401			2,723,366		$	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		8.1	$	15,306
Vested and expected to vest as of December 31, 2015		—			3,131,822		$	11.39		8.0	$	15,056
Exercisable as of December 31, 2015		—			1,360,952		$	8.02		7.3	$	9,754

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, 2015:

					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-$1.61		678,905		6.6		533,010		6.6
2.53-8.17		1,074,463		7.8		535,847		7.8
10.59-18.67		656,850		9.4		10,969		9.4
18.72-31.96		820,751		8.5		277,376		8.5
34.00		10,000		8.5		3,750		9.5
		3,240,969				1,360,952

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,
	2015		2014		2013
Expected volatility		79.7%		84.9%		89.1%
Risk-free interest rate		1.7%		1.9%		1.7%
Dividend yield		0.0%		0.0%		0.0%
Expected life (in years)		6.0		6.1		6.1

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,
	2015		2014		2013
Operating Expenses
Research and development	$	3,032	$	1,230	$	124
General and administrative		7,667		3,411		91
Total	$	10,699	$	4,641	$	215

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,
	2015		2014
Expected volatility		72.1%		54.9%
Risk-free interest rate		0.26%		0.06%
Dividend yield		0.0%		0.0%
Expected life (in years)		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, 2014		185,514		$	26.77
Granted		123,450			15.09
Vested		(48,890	)		26.73
Forfeited/canceled		(6,007	)		27.10
Unvested at December 31, 2015		254,067		$	21.11