STOCK-BASED COMPENSATION - Fair Value and Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended				78 Months Ended
	Sep. 30, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2016	Aug. 02, 2016	Jul. 24, 2015	Dec. 16, 2014	Apr. 02, 2014	Dec. 31, 2009
Stock-based compensation
Forfeiture rate (as a percent)		3.48%	4.99%	4.99%
Additional disclosures
Options outstanding that had exercise prices that were lower than the market price of common stock (in shares)		3,505,481
Options exercisable that had exercise prices that were lower than the market price of common stock (in shares)		2,265,691
Market price (in dollars per share)		$ 13.36				$ 13.36	$ 15.50	$ 25.50	$ 25.75	$ 12.75
Total intrinsic value of options exercised		$ 30,500	$ 3,600	$ 29,200
Number of shares of common stock covered by warrants issued in connection with a consulting agreement											75,000
Number of shares that vested during the period	75,000
Expense related to stock warrants	$ 1,300
Weighted Average Grant Date Fair Value
Unrecognized compensation cost		$ 41,500				$ 41,500
Weighted average period for recognition of unrecognized compensation cost		2 years 4 months 24 days
Proceeds from stock option exercises		$ 3,400	$ 1,200	$ 2,600
Stock Options
Valuation assumptions
Risk-free interest rates, minimum (as a percent)		1.48%	1.50%	1.96%
Risk-free interest rates, maximum (as a percent)		1.69%	1.92%	2.01%
Expected term			6 years	6 years 3 months
Expected volatility, minimum (as a percent)		58.90%	67.10%	77.60%
Expected volatility, maximum (as a percent)		59.40%	73.20%	80.80%
Dividend yield (as a percent)		0.00%	0.00%	0.00%
Weighted average fair value per share of options granted during the period (in dollars per share)		$ 3.17	$ 15.81	$ 10.05
Stock Options | Minimum
Valuation assumptions
Expected term		6 years 3 months	6 years 3 months
Stock Options | Maximum
Valuation assumptions
Expected term		6 years 8 months 27 days	6 years 7 months 6 days
Market Measure-Based Shares
Valuation assumptions
Risk-free interest rates, minimum (as a percent)		0.76%
Risk-free interest rates (as a percent)			1.12%
Risk-free interest rates, maximum (as a percent)		0.91%
Expected term			3 years 1 month 28 days
Expected volatility, minimum (as a percent)		68.30%
Expected volatility (as a percent)			64.30%
Expected volatility, maximum (as a percent)		79.60%
Dividend yield (as a percent)		0.00%	0.00%
Weighted average fair value per share of options granted during the period (in dollars per share)		$ 3.77	$ 5.91
Market Measure-Based Shares | Minimum
Valuation assumptions
Expected term		2 years 5 months 5 days
Market Measure-Based Shares | Maximum
Valuation assumptions
Expected term		2 years 10 months 2 days
Employee Stock
Valuation assumptions
Risk-free interest rates, minimum (as a percent)		0.41%	0.25%	0.10%
Risk-free interest rates, maximum (as a percent)		0.83%	0.75%	0.50%
Expected volatility, minimum (as a percent)		70.10%	51.20%	42.50%
Expected volatility, maximum (as a percent)		92.70%	110.00%	62.70%
Dividend yield (as a percent)		0.00%	0.00%	0.00%
Weighted average fair value per share of options granted during the period (in dollars per share)		$ 3.30	$ 4.67	$ 6.30
Shares
Outstanding at the beginning of the period (in shares)		4,936,594	4,934,317	6,062,587
Granted (in shares)		883,889	340,978	266,477
Exercised (in shares)		(2,255,959)	(281,315)	(1,378,372)
Forfeited (in shares)		(59,043)	(57,386)	(16,375)
Outstanding at the end of the period (in shares)		3,505,481	4,936,594	4,934,317	6,062,587	3,505,481
Exercisable at the end of the period (in shares)		2,265,691				2,265,691
Vested and expected to vest at the end of the period (in shares)		3,429,333				3,429,333
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)		$ 4.80	$ 3.63	$ 2.78
Granted (in dollars per share)		5.48	23.51	14.28
Exercised (in dollars per share)		1.52	4.44	1.91
Forfeited (in dollars per share)		9.75	16.99	6.37
Outstanding at the end of the period (in dollars per share)		7.00	$ 4.80	$ 3.63	$ 2.78	$ 7.00
Exercisable at the end of the period (in dollars per share)		5.42				5.42
Vested and expected to vest at the end of the period (in dollars per share)		$ 3.97				$ 3.97
Weighted Average Remaining Contractual Term
Outstanding at the end of the period		5 years 6 months	4 years 6 months	5 years 2 months 12 days	6 years 7 months 6 days
Exercisable at the end of the period		3 years 9 months 18 days
Vested and expected to vest at the end of the period		5 years 6 months
Aggregate Intrinsic Value
Outstanding at the end of the period		$ 25,700				$ 25,700
Exercisable at the end of the period		18,943				18,943
Vested and expected to vest at the end of the period		$ 25,201				$ 25,201
Employee Stock | Minimum
Valuation assumptions
Expected term		6 months	6 months	6 months
Employee Stock | Maximum
Valuation assumptions
Expected term		2 years	2 years	2 years
Restricted Stock Awards And Restricted Stock Units RSU
Restricted Shares
Outstanding at the beginning of the period (in shares)		3,444,694	1,541,114	1,150,694
Granted (in shares)		3,960,583	2,895,818	926,171
Released (in shares)		(796,168)	(578,033)	(491,370)
Forfeited (in shares)		(1,007,793)	(414,205)	(44,381)
Outstanding at the end of the period (in shares)		5,601,316	3,444,694	1,541,114	1,150,694	5,601,316
Weighted Average Grant Date Fair Value
Outstanding at the beginning of the period (in dollars per share)		$ 14.19	$ 13.86	$ 11.23
Granted (in dollars per share)		6.90	15.23	15.61
Released (in dollars per share)		16.95	13.77	11.17
Forfeited (in dollars per share)		9.57	20.84	12.44
Outstanding at the end of the period (in dollars per share)		$ 9.19	$ 14.19	$ 13.86	$ 11.23	$ 9.19
2010 Employee Stock Purchase Plan
Weighted Average Grant Date Fair Value
Stock issued under the Company's stock purchase plan (in shares)		356,823	176,785	88,166		793,431
Proceeds from stock issued under the Company's stock purchase plan		$ 2,100	$ 1,700	$ 800