STOCK-BASED COMPENSATION (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Stock-based compensation
Stock-based compensation expense	$ 12,500	$ 6,100
Valuation assumptions
Risk-free interest rates (as a percent)	2.79%
Expected volatility (as a percent)	61.80%
Additional disclosures
Market price (in dollars per share)	$ 40.33
Total intrinsic value of options exercised	$ 19,100	200
Weighted Average Grant Date Fair Value
Unrecognized compensation cost	$ 137,600
Weighted average period for recognition of unrecognized compensation cost	3 years 2 months 12 days
Share based compensation expense	$ 12,500	$ 6,100
Stock Options
Valuation assumptions
Risk-free interest rates (as a percent)		2.13%
Expected term	6 years 5 months 9 days	6 years 7 months 2 days
Expected volatility (as a percent)		62.90%
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)	$ 26.84	$ 13.20
Shares
Outstanding at the beginning of the period (in shares)	3,360,461
Granted (in shares)	343,566
Exercised (in shares)	(420,129)
Outstanding at the end of the period (in shares)	3,283,898		3,360,461
Exercisable at the end of the period (in shares)	1,757,384
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)	3.31
Outstanding at the end of the period (in dollars per share)	16.39		$ 11.89
Exercisable at the end of the period (in dollars per share)	$ 10.80
Weighted Average Remaining Contractual Term
Outstanding at the end of the period	7 years		6 years 4 months 24 days
Exercisable at the end of the period	5 years 4 months 24 days
Aggregate Intrinsic Value
Outstanding at the end of the period	$ 80,098
Exercisable at the end of the period	$ 51,901
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)	$ 45.16
Restricted Shares
Outstanding at the beginning of the period (in shares)	6,148,778
Granted (in shares)	1,079,223
Released (in shares)	(880,022)
Forfeited (in shares)	(33,307)
Outstanding at the end of the period (in shares)	6,314,672		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)	45.16
Released (in dollars per share)	17.67
Forfeited (in dollars per share)	26.37
Outstanding at the end of the period (in dollars per share)	$ 20.36		$ 15.76
ASU 2016-09
Stock-based compensation
Cumulative-effect adjustment		$ 400