Stock Based Awards (Tables)

12 Months Ended

Dec. 31, 2014

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, 2013		168,571			462,471		$	1.31
Additional shares authorized		803,590			—			—
Options granted		(974,459	)		974,459			2.16
Options exercised		—			(21,444	)		1.27
Options cancelled		11,831			(11,831	)		1.30
Balances, December 31, 2013		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		8.8	$	36,006
Vested and expected to vest as of December 31, 2014		—			2,651,182		$	10.27		8.8	$	35,141
Exercisable as of December 31, 2014		—			648,058		$	3.04		7.7	$	12,858

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, 2014:

					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		769,755		7.5		471,420		7.5
2.53		570,927		9.0		146,484		9.0
3.34 - 18.66		576,987		9.2		1,087		9.2
18.73 - 31.96		795,697		9.6		29,067		9.6
34.00		10,000		9.5		—		9.5
		2,723,366				648,058

Summary of Fair Value of Employee Stock Options

The fair value of employee stock options was estimated using the following weighted average assumptions:

	Year Ended December 31,
	2014		2013		2012
Expected volatility		84.9%		89.1%		91.3%
Risk-free interest rate		1.9%		1.7%		1.1%
Dividend yield		0.0%		0.0%		0.0%
Expected life (in years)		6.1		6.1		6.1

Schedule of Estimated Stock-Based Compensation Expense

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

							Cumulative
							Period From
							December 10,
							2008 (Date of
							Inception) to
	Year Ended December 31,						December 31,
	2014		2013		2012		2014
Operating Expenses
Research and development	$	1,230	$	124	$	91	$	1,599
General and administrative		3,411		91		50		3,590
Total	$	4,641	$	215	$	141	$	5,189

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

We 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,
	2014		2013
Expected volatility		54.9%	-
Risk-free interest rate		0.06%	-
Dividend yield		0.00%	-
Expected life (in years)		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, 2013		—	$	—
Granted		185,514		26.77
Vested		—		—
Forfeited/canceled		—		—
Unvested at December 31, 2014		185,514	$	26.77