STOCK-BASED COMPENSATION (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Stock-based compensation
Stock-based compensation expense	$ 10,800	$ 6,200
Valuation assumptions
Risk-free interest rates (as a percent)	2.06%
Expected volatility (as a percent)	62.50%
Dividend yield (as a percent)			0.00%
Weighted average fair value per share of options granted during the period (in dollars per share)			$ 25.18
Additional disclosures
Market price (in dollars per share)	$ 47.12		$ 47.12
Total intrinsic value of options exercised			$ 11,200	$ 5,600
Weighted Average Grant Date Fair Value
Unrecognized compensation cost	$ 100,800		$ 100,800
Weighted average period for recognition of unrecognized compensation cost			3 years
Share based compensation expense	$ 10,800	$ 6,200
Stock Options
Valuation assumptions
Risk-free interest rates, minimum (as a percent)				1.48%
Risk-free interest rates, maximum (as a percent)				1.69%
Expected term	6 years 6 months 22 days
Expected volatility, minimum (as a percent)				58.90%
Expected volatility, maximum (as a percent)				59.40%
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)	$ 27.03			$ 3.17
Shares
Outstanding at the beginning of the period (in shares)			3,505,481
Granted (in shares)			942,997
Exercised (in shares)			(382,967)
Forfeited (in shares)			(23,879)
Outstanding at the end of the period (in shares)	4,041,632		4,041,632		3,505,481
Exercisable at the end of the period (in shares)	2,281,645		2,281,645
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)			$ 7.00
Granted (in dollars per share)			21.68
Exercised (in dollars per share)			8.75
Forfeited (in dollars per share)			14.19
Outstanding at the end of the period (in dollars per share)	$ 10.22		10.22		$ 7.00
Exercisable at the end of the period (in dollars per share)	$ 5.81		$ 5.81
Weighted Average Remaining Contractual Term
Outstanding at the end of the period			5 years 10 months 24 days		5 years 6 months
Exercisable at the end of the period			3 years 7 months 6 days
Aggregate Intrinsic Value
Outstanding at the end of the period	$ 149,127		$ 149,127
Exercisable at the end of the period	$ 94,250		$ 94,250
Stock Options | Minimum
Valuation assumptions
Expected term			6 years 6 months 22 days	6 years 3 months
Stock Options | Maximum
Valuation assumptions
Expected term			6 years 7 months 2 days	6 years 8 months 27 days
Market Measure-Based Shares
Valuation assumptions
Risk-free interest rates, minimum (as a percent)				0.76%
Risk-free interest rates, maximum (as a percent)			0.91%
Risk-free interest rates (as a percent)		0.76%
Expected term		2 years 5 months 5 days
Expected volatility (as a percent)		79.60%
Expected volatility, minimum (as a percent)				68.30%
Expected volatility, maximum (as a percent)			79.60%
Dividend yield (as a percent)		0.00%		0.00%
Weighted average fair value per share of stock purchase rights granted during the period (in dollars per share)		$ 13.42		$ 3.77
Weighted Average Grant Date Fair Value
Granted (in dollars per share)		$ 13.42		$ 3.77
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)			0.98%	0.41%
Risk-free interest rates, maximum (as a percent)			1.28%	0.80%
Expected volatility, minimum (as a percent)			66.40%	70.10%
Expected volatility, maximum (as a percent)			85.50%	92.70%
Dividend yield (as a percent)			0.00%	0.00%
Weighted average fair value per share of stock purchase rights granted during the period (in dollars per share)			$ 13.05	$ 3.08
Weighted Average Grant Date Fair Value
Granted (in dollars per share)			$ 13.05	$ 3.08
Employee Stock | Minimum
Valuation assumptions
Expected term			6 months	6 months
Employee Stock | Maximum
Valuation assumptions
Expected term			2 years	2 years
Restricted Stock Awards And Restricted Stock Units RSU
Valuation assumptions
Weighted average fair value per share of stock purchase rights granted during the period (in dollars per share)			$ 31.81
Restricted Shares
Outstanding at the beginning of the period (in shares)			5,601,316
Granted (in shares)			1,946,736
Released (in shares)			(1,075,538)
Forfeited (in shares)			(308,471)
Outstanding at the end of the period (in shares)	6,164,043		6,164,043		5,601,316
Weighted Average Grant Date Fair Value
Outstanding at the beginning of the period (in dollars per share)			$ 9.19
Granted (in dollars per share)			31.81
Released (in dollars per share)			14.14
Forfeited (in dollars per share)			18.60
Outstanding at the end of the period (in dollars per share)	$ 15.16		$ 15.16		$ 9.19