STOCK-BASED COMPENSATION (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
	Apr. 25, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Mar. 31, 2017
Stock-based compensation
Stock-based compensation expense		$ 16,500	$ 10,800	$ 44,600	$ 23,000
Non-cash stock-based compensation expense		$ 1,400		$ 2,500
Additional disclosures
Market price (in dollars per share)		$ 78.92		$ 78.92
Total intrinsic value of options exercised				$ 40,100	$ 11,200
Weighted Average Grant Date Fair Value
Unrecognized compensation cost		$ 127,800		$ 127,800
Weighted average period for recognition of unrecognized compensation cost				2 years 10 months 24 days
Employee And Non Employees Stock Option
Stock-based compensation
Accelerated vesting, shares	69,950
Valuation assumptions
Risk-free interest rates, minimum (as a percent)				2.73%	2.06%
Risk-free interest rates, maximum (as a percent)				2.79%	2.13%
Risk-free interest rates (as a percent)		2.06%
Expected term		6 years 6 months 22 days
Expected volatility (as a percent)		62.50%
Expected volatility, minimum (as a percent)				61.82%	62.50%
Expected volatility, maximum (as a percent)				66.17%	62.90%
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)		$ 27.03		$ 24.55	$ 25.18
Shares
Outstanding at the beginning of the period (in shares)				3,360,461
Granted (in shares)				343,566
Exercised (in shares)				(848,061)
Outstanding at the end of the period (in shares)		2,855,966		2,855,966		3,360,461
Exercisable at the end of the period (in shares)		1,329,452		1,329,452
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)				$ 11.89
Granted (in dollars per share)				44.37
Exercised (in dollars per share)				7.52
Outstanding at the end of the period (in dollars per share)		$ 17.10		17.10		$ 11.89
Exercisable at the end of the period (in dollars per share)		$ 10.52		$ 10.52
Weighted Average Remaining Contractual Term
Outstanding at the end of the period				6 years 8 months 12 days		6 years 4 months 24 days
Exercisable at the end of the period				4 years 9 months 18 days
Aggregate Intrinsic Value
Outstanding at the end of the period		$ 176,558		$ 176,558
Exercisable at the end of the period		$ 90,931		$ 90,931
Employee And Non Employees Stock Option | Minimum
Valuation assumptions
Expected term				5 years 5 months 12 days	6 years 6 months 22 days
Employee And Non Employees Stock Option | Maximum
Valuation assumptions
Expected term				6 years 5 months 5 days	6 years 7 months 2 days
Employee Stock
Valuation assumptions
Risk-free interest rates, minimum (as a percent)				2.05%	0.98%
Risk-free interest rates, maximum (as a percent)				2.50%	1.28%
Expected volatility, minimum (as a percent)				51.75%	66.40%
Expected volatility, maximum (as a percent)				65.39%	85.50%
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)				$ 18.68	$ 13.05
Employee Stock | Minimum
Valuation assumptions
Expected term				6 months	6 months
Employee Stock | Maximum
Valuation assumptions
Expected term				2 years	2 years
Restricted Shares and RSUs
Stock-based compensation
Accelerated vesting, shares	54,350
Valuation assumptions
Weighted average fair value per share of stock purchase rights granted during the period (in dollars per share)				$ 48.55
Restricted Shares
Outstanding at the beginning of the period (in shares)				6,148,778
Granted (in shares)				1,504,700
Released (in shares)				(1,209,608)
Forfeited (in shares)				(164,161)
Outstanding at the end of the period (in shares)		6,279,709		6,279,709		6,148,778
Weighted Average Grant Date Fair Value
Outstanding at the beginning of the period (in dollars per share)				$ 15.76
Granted (in dollars per share)				48.55
Released (in dollars per share)				20.84
Forfeited (in dollars per share)				32.78
Outstanding at the end of the period (in dollars per share)		$ 22.14		$ 22.14		$ 15.76
ASU 2016-09
Stock-based compensation
Cumulative-effect adjustment - ASU 2016-09 adoption							$ 400