STOCK COMPENSATION (Tables)

12 Months Ended

Dec. 31, 2013

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Stock Option Activity

The Company’s stock option activity consisted of the following as of the dates indicated:

	For the Year Ended December 31,
	2013					2012					2011
	Shares			Weighted Average Exercise Price		Shares			Weighted Average Exercise Price		Shares			Weighted Average Exercise Price
Options outstanding at beginning of year		2,522,522		$	11.76		2,417,659		$	11.18		1,415,009		$	12.84
Granted		2,283,719			34.18		1,269,470			12.92		1,595,375			9.18
Exercised		(241,056	)		11.31		(371,353	)		10.18		(25,291	)		6.54
Canceled or expired		(374,818	)		16.83		(793,254	)		12.59		(567,434	)		9.90
Options outstanding at end of year		4,190,367		$	23.46		2,522,522		$	11.76		2,417,659		$	11.18
Exercisable at year end		1,051,329		$	11.91		615,394		$	12.71		742,211		$	15.60
Vested at December 31, 2013 and expected to vest		3,467,069		$	21.50

	Aggregate Intrinsic Value		Weighted Average Remaining Contractual Life (Years)
Options outstanding at end of year	$	18,544,313		8.58
Exercisable at end of year		9,856,124		7.04
Vested at December 31, 2013 and expected to vest		18,069,138		8.41

Assumptions for Measuring Fair Values of Stock Options Granted

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:

	Year Ended December 31,
	2013		2012		2011
Risk-free interest rate		0.7% - 1.7%		0.6% - 1.1%		0.9% - 2.4%
Expected dividend yield		0%		0%		0%
Expected lives		4.8 - 5.0 years		4.8 - 5.3 years		5.2 - 8.9 years
Expected volatility		80.0% - 90.7%		79.7% - 108.6%		78.2% - 81.6%

Summary of Restricted Stock Activity

The following table sets forth restricted stock activity for the years shown:

	For the Year Ended December 31,
	2013					2012					2011
	Shares			Weighted Average Grant Date Fair Value		Shares			Weighted Average Grant Date Fair Value		Shares			Weighted Average Grant Date Fair Value
Restricted Stock Awards outstanding at beginning of year		4,998		$	10.08		5,000		$	8.46		3,333		$	7.80
Granted		6,000			34.92		4,998			10.08		5,000			8.46
Vested		(4,998	)		10.08		(5,000	)		8.46		(3,333	)		7.80
Forfeited or canceled		—			—		—			—		—			—
Restricted Stock Awards outstanding at end of year		6,000		$	34.92		4,998		$	10.08		5,000		$	8.46

Restricted Stock Unit Activity

The following table sets forth restricted stock unit activity for the period shown:

	For the Year Ended December 31,
	2013					2012
	Shares			Weighted Average      Grant Date Fair Value		Shares			Weighted Average      Grant Date Fair Value
Restricted Stock Units outstanding at beginning of year		38,260		$	6.32		—		$	—
Granted		—			—		39,877			6.28
Vested		(31,379	)		6.52		—			—
Forfeited or canceled		(374	)		5.40		(1,617	)		5.40
Restricted Stock Units outstanding at end of year		6,507		$	5.40		38,260		$	6.32

Stock Appreciation Rights Activity

The following table sets forth stock appreciation rights activity for the period shown:

	For the Year Ended December 31,
	2013				2012
	Shares		Weighted Average Exercise Price		Shares		Weighted Average Exercise Price
Stock Appreciation Rights outstanding at beginning of year		170,000	$	18.18		—	$	—
Granted		—		—		170,000		18.18
Stock Appreciation Rights outstanding at end of year		170,000	$	18.18		170,000	$	18.18
Exercisable at end of year end		50,416	$	17.48
Vested at December 31, 2013 and expected to vest		170,000	$	18.18

	Aggregate Intrinsic Value		Weighted Average Remaining Contractual Life (Years)
Stock Appreciation Rights outstanding at end of year	$	720,300		8.76
Exercisable at end of year		240,097		8.75
Vested at December 31, 2013 and expected to vest		720,300		8.76

Summary of Stock Based Compensation Expense Recognized in Statement of Operations

A summary of the stock-based compensation expense recognized in the statement of operations and comprehensive loss is as follows:

	Year Ended December 31,
	2013		2012		2011
	(in thousands)
Research and development	$	3,888	$	1,173	$	1,279
General and administrative		7,239		1,905		1,850
Total	$	11,127	$	3,078	$	3,129