Stock Based Compensation (Tables)

3 Months Ended

Mar. 31, 2015

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Option Activities

The following table summarizes option activity under our 2013 Equity Incentive Plan and related information during the three months ended March 31, 2015:

	Shares
	Subject to			Weighted-
	Outstanding			Average Exercise
	Options			Price Per Share
Balance at December 31, 2014		4,249,168		$	14.77
Options granted		1,016,766			32.36
Options exercised		(379,146	)		10.14
Options canceled		(98,919	)		24.37
Balance at March 31, 2015		4,787,869		$	18.67

Fair Values of Employee Stock Options

The estimated grant date fair values of the employee stock options were calculated using the Black Scholes valuation model, based on the following assumptions:

	Three Month Ended March 31,
	2015		2014
Risk-free interest rate		1.54%		1.88%
Expected life	6.0 years		6.0 years
Expected volatility		66%		79%
Dividend yield	–		–

Classification of Stock-Based Compensation Expense

The table below sets forth the functional classification of stock-based compensation expense, net of estimated forfeitures, for the periods presented (in thousands):  

	Three Months Ended March 31,
	2015		2014
Research and development	$	2,209	$	1,066
Selling, general and administrative		2,963		1,236
Total stock-based compensation	$	5,172	$	2,302

Assumptions used to Determine Weighted-Average Fair Value of Performance Stock Units

	Three Month Ended March 31, 2015
	PSUs
Number of PSUs granted		140,000
Grant price at date of grant	$	29.72
Risk-free interest rate		1.10%
Volatility		63%
Dividend yield	–
Weighted- average fair value per share of PSUs granted ($50 Vesting Hurdle)	$	23.10
Weighted- average fair value per share of PSUs granted ($60 Vesting Hurdle)	$	20.87