STOCK-BASED COMPENSATION (Tables)

12 Months Ended

Dec. 31, 2016

Summary of Stock Option Activity

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

	For the Year Ended December 31,
	2016					2015					2014
				Weighted					Weighted					Weighted
				Average					Average					Average
				Exercise					Exercise					Exercise
	Shares			Price		Shares			Price		Shares			Price
Grants outstanding at beginning of the period		6,515,976		$	23.91		5,216,203		$	24.45		4,190,367		$	23.46
Granted		1,285,051			14.89		2,830,078			20.28		1,694,560			25.67
Exercised		(1,056,821	)		18.31		(816,696	)		12.26		(86,007	)		11.40
Cancelled		(1,307,255	)		24.61		(713,609	)		26.78		(582,717	)		22.79
Grants outstanding at end of the period		5,436,951		$	22.70		6,515,976		$	23.91		5,216,203		$	24.45
Grants exercisable at end of the period		2,942,624		$	24.42		2,617,167		$	23.85		2,019,514		$	18.69
Grants vested and expected to vest at end of the period		5,116,409		$	22.83		5,908,213		$	23.94		4,462,100		$	23.27

The weighted-average grant date fair value per share of stock options granted during the years ended December 31, 2016, 2015 and 2014 was $12.46, $14.98 and $18.59, respectively. 

			Weighted
	Aggregate		Average
	Intrinsic		Remaining
	Value		Contractual
	(in thousands)		Life (Years)
Options outstanding at December 31, 2016	$	38,660	7.41
Options exercisable at December 31, 2016	$	17,483		6.45
Options vested and expected to vest at    December 31, 2016	$	35,955	7.33

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,
	2016		2015		2014
	(in thousands)
Aggregate grant date fair value of stock options vested	$	30,651	$	27,858	$	17,672
Aggregate intrinsic value of stock options exercised	$	30,610	$	18,138	$	1,497

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,
	2016						2015					2014
					Weighted					Weighted				Weighted
					Average					Average				Average
					Exercise					Exercise				Exercise
	Shares				Price		Shares			Price		Shares		Price
Grants outstanding at beginning of the period		161,320			$	21.50		6,000		$	34.92		6,000	$	34.92
Granted		117,553		(1)		41.22		181,783			20.80		—		—
Vested		(59,703	)			13.62		(24,463	)		20.21		—		—
Forfeited		(66,000	)	(2)		33.51		(2,000	)		13.90		—		—
Grants outstanding at end of the period		153,170			$	34.53		161,320		$	21.50		6,000	$	34.92

(1) In September 2016, the Company granted executives RSAs with certain sales targets. If the sales targets are achieved within the required time frame, the number of RSAs may be increased from 71,925 to 89,906 shares. If and when deemed probable that such performance milestones may be achieved within the required time frame, the Company may recognize up to $4.4 million of stock-based compensation related to these grants.

(2) In September 2015, the Company granted executives 65,000 RSAs with performance conditions. As a result of the performance conditions not being met, these grants were cancelled during 2016.

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,
	2016					2015				2014
				Weighted				Weighted				Weighted
				Average				Average				Average
				Exercise				Exercise				Exercise
	Shares			Price		Shares		Price		Shares		Price
Grants outstanding at beginning of the period		170,000		$	18.18		170,000	$	18.18		170,000	$	18.18
Exercised		(67,812	)		10.08		—		—		—		—
Forfeited		(2,188	)		10.08		—		—		—		—
Grants outstanding at end of the period		100,000			23.85		170,000		18.18		170,000		18.18
Grants exercisable at end of the period		100,000		$	23.85		141,249	$	17.59		92,916	$	17.80
Grants vested and expected to vest at end of    the period		100,000		$	23.85		167,813	$	18.29		170,000	$	18.18
				Weighted
	Aggregate			Average
	Intrinsic			Remaining
	Value			Contractual
	(in thousands)			Life (Years)
SARs outstanding, exercisable and vested at December 31, 2016	$	358,000		5.85

Summary of Employee Stock Purchase Plan Activity and Expense

The following table summarizes the Company’s ESPP activity and expense for each of the periods indicated:

	For the Year Ended December 31,
	2016		2015
Number of shares purchased		137,113		75,539
Proceeds received (in millions)	$	1.6	$	0.9

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,
	2016		2015		2014
	(in thousands)
Research and development	$	9,499	$	10,403	$	8,269
Selling, general and administrative		20,463		21,714		12,076
Total stock-based compensation	$	29,962	$	32,117	$	20,345

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,
	2016		2015		2014
	(in thousands)
Stock options	$	26,320	$	29,014	$	18,388
Employee stock purchase plan		2,380		2,165		1,165
Restricted stock awards		872		446		204
Other		390		492		588
Total stock-based compensation	$	29,962	$	32,117	$	20,345

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,
	2016	2015	2014
Risk-free interest rate (1)	1.1 – 1.8%	1.1 – 1.7%	1.4 – 1.7 %
Expected dividend yield (2)	—	—	—
Expected terms (3)	4.2 – 4.8  years	4.7 – 5.0  years	4.7 – 4.9  years
Expected volatility (4)	78.2 – 137.1 %	94.3 – 111.1 %	93.0 – 103.0 %

(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 terms 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.