STOCK-BASED COMPENSATION (Tables)

12 Months Ended

Dec. 31, 2014

The following tables summarize the Company’s stock option activity for each of the periods indicated:


	For the Year Ended December 31,
	2014					2013					2012
	Shares			Weighted Average Exercise Price		Shares			Weighted Average Exercise Price		Shares			Weighted Average Exercise Price
Grants outstanding at beginning of the period		4,190,367		$	23.46		2,522,522		$	11.76		2,417,659		$	11.18
Granted		1,694,560			25.67		2,283,719			34.18		1,269,470			12.92
Exercised		(86,007	)		11.40		(241,056	)		11.31		(371,353	)		10.18
Canceled or expired		(582,717	)		22.79		(371,818	)		16.83		(793,254	)		12.59

Grants outstanding at end of the period		5,216,203		$	24.45		4,190,367		$	23.46		2,522,522		$	11.76

Grants exercisable at end of the period		2,019,514		$	18.69		1,051,329		$	11.91		615,394		$	12.71
Grants vested and expected to vest at end of the period		4,462,100		$	23.27		3,467,069		$	21.50		2,343,086		$	11.73

The weighted-average fair value per share of stock options granted during the years ended December 31, 2014, 2013 and 2012 was $18.59, $22.86 and $9.54, respectively.


	Aggregate Intrinsic Value (in thousands)		Weighted Average Remaining Contractual Life (Years)
Options outstanding at December 31, 2014	$	7,794		8.04
Options exercisable at December 31, 2014	$	5,973		7.04
Options vested and expected to vest at December 31, 2014	$	7,671		7.94

Summary of Company's Stock Options Vested and Exercised

The following table summarizes the Company’s stock options vested and exercised for each of the periods indicated:


	For the Year Ended December 31,
	2014		2013		2012
	(in thousands)
Aggregate grant date fair value of stock options vested	$	17,672	$	4,872	$	3,749
Aggregate intrinsic value of stock options exercised	$	1,497	$	5,444	$	4,964

Summary of Restricted Stock Award Activity

The following table summarizes the Company’s RSA activity for each of the periods indicated:


	For the Year Ended December 31,
	2014				2013					2012
	Shares		Weighted Average Grant Date Fair Value		Shares			Weighted Average Grant Date Fair Value		Shares			Weighted Average Grant Date Fair Value
Grants outstanding at beginning of the period		6,000	$	34.92		4,998		$	10.08		5,000		$	8.46
Granted		—		—		6,000			34.92		4,998			10.08
Vested		—		—		(4,998	)		10.08		(5,000	)		8.46
Canceled or expired		—		—		—			—		—			—

Grants outstanding at end of the period		6,000	$	34.92		6,000		$	34.92		4,998		$	10.08

Summary of Restricted Stock Unit Activity

The following table summarizes the Company’s RSU activity for each of the periods indicated:


	For the Year Ended December 31,
	2014					2013					2012
	Shares			Weighted Average Grant Date Fair Value		Shares			Weighted Average Grant Date Fair Value		Shares			Weighted Average Grant Date Fair Value
Grants outstanding at beginning of the period		6,507		$	5.40		38,260		$	6.32		—		$	—
Granted		—			—		—			—		39,877			6.28
Vested		(6,507	)		5.40		(31,379	)		6.52		—			—
Canceled or expired		—			—		(374	)		5.40		(1,617	)		5.40

Grants outstanding at end of the period		—		$	—		6,507		$	5.40		38,260		$	6.32

Summary of Stock Appreciation Rights Activity

The following table summarizes the Company’s SAR activity for each of the periods indicated:


	For the Year Ended December 31,
	2014				2013				2012
	Shares		Weighted Average Exercise Price		Shares		Weighted Average Exercise Price		Shares		Weighted Average Exercise Price
Grants outstanding at beginning of the period		170,000	$	18.18		170,000	$	18.18		—	$	—
Granted		—		—		—		—		170,000		18.18

Grants outstanding at end of the period		170,000	$	18.18		170,000	$	18.18		170,000	$	18.18

Grants exercisable at end of the period		92,916	$	17.80		50,416	$	17.48		—	$	—
Grants vested and expected to vest at end of the period		170,000	$	18.18		170,000	$	18.18		170,000	$	18.18


	Aggregate Intrinsic Value (in thousands)		Weighted Average Remaining Contractual Life (Years)
SARs outstanding at December 31, 2014	$	307		7.76
SARs exercisable at December 31, 2014	$	179		7.76
SARs vested and expected to vest at December 31, 2014	$	307		7.76

Summary of Stock-Based Compensation Expense by Function Included within Consolidated Statement of Operations and Comprehensive Loss

The following table summarizes stock-based compensation expense by function included within the consolidated statements of operations and comprehensive loss:


	For the Year Ended December 31,
	2014		2013		2012
	(in thousands)
Research and development	$	8,269	$	3,888	$	1,173
General and administrative		12,076		7,239		1,905

Total	$	20,345	$	11,127	$	3,078

Summary of Stock-Based Compensation Expense by Grant Type Included within Consolidated Statement of Operations and Comprehensive Loss

The following table summarizes stock-based compensation expense by grant type included within the consolidated statements of operations and comprehensive loss:


	For the Year Ended December 31,
	2014		2013		2012
	(in thousands)
Stock options	$	18,388	$	9,632	$	2,867
Restricted stock awards		204		149		38
Restricted stock units		1		269		63
Stock appreciation rights		587		593		110
Employee stock purchase plan		1,165		484		—

Total	$	20,345	$	11,127	$	3,078

Stock Options [Member]

Assumptions for Measuring Fair Values of Stocks

The fair values of stock options granted during the periods presented were measured on the date of grant using the Black-Scholes-Merton option-pricing model, with the following assumptions:

For the Year Ended December 31,

2014

2013

2012

Risk-free interest rate (1)

1.4 – 1.7

0.7 – 1.7

0.6 – 1.1

Expected dividend yield (2)

—

Expected lives (3)

4.7 – 4.9 years

4.8 – 5.0 years

4.8 – 5.3 years

Expected volatility (4)

93.0 – 103.0

80.0 – 90.7

79.7 – 108.6

(1)	The risk-free interest rate is estimated using an average of Treasury bill interest rates over a historical period commensurate with the expected term of the option that correlates to the prevailing interest rates at the time of grant.

(2)	The expected dividend yield is zero as the Company has not paid any dividends to date and does not expect to pay dividends in the future.

(3)	The expected lives are estimated using historical exercise behavior.

(4)	The expected volatility is estimated using a blend of calculated volatility of the Company’s common stock over a historical period and implied volatility in exchange-traded options of the Company’s common stock.