STOCK-BASED COMPENSATION - Stock Option Activity (Details) - Employee Stock Option $ / shares in Units, $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($) $ / shares shares
Number of Options
Options outstanding at January 1, 2018 | shares	4,901,748
Options granted | shares	157,133
Options exercised | shares	(566,799)
Options forfeited/cancelled | shares	(27,583)
Options outstanding at June 30, 2018 | shares	4,464,499
Options vested and exercisable at June 30, 2018 | shares	3,530,693
Options expected to vest at June 30, 2018 | shares	879,470
Weighted Average Exercise Price
Options outstanding at January 1, 2018 | $ / shares	$ 40.91
Options granted | $ / shares	112.91
Options exercised | $ / shares	39.34
Options forfeited/cancelled | $ / shares	63.63
Options outstanding at June 30, 2018 | $ / shares	43.50
Options vested and exercisable at June 30, 2018 | $ / shares	35.46
Options expected to vest at June 30, 2018 | $ / shares	$ 73.49
Aggregate Intrinsic Value
Options outstanding at June 30, 2018 | $	$ 360,830
Options vested and exercisable at June 30, 2018 | $	313,776
Options expected to vest at June 30, 2018 | $	$ 44,709
Weighted Average Remaining Contractual Term (in years)
Options outstanding at June 30, 2018	5 years 10 months 6 days
Options vested and exercisable at June 30, 2018	5 years 4 months 2 days
Options expected to vest at June 30, 2018	7 years 8 months 26 days