STOCK-BASED COMPENSATION - Fair Value Assumptions (Details) - Stock options - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Weighted-average assumptions used to estimate the fair value of options granted
Dividend yield (as a percent)	0.00%	0.00%
Expected volatility (as a percent)	30.20%	34.90%
Risk-free interest rate (as a percent)	2.40%	2.80%
Expected term	6 years	6 years
Stock options, Number of Shares
Balance at the beginning of the period (in shares)	18,890
Granted (in shares)	1,570
Exercised (in shares)	(3,627)
Cancelled or forfeited (in shares)	(158)
Balance at the end of the period (in shares)	16,675		18,890
Vested and expected to vest in the future at the end of the period (in shares)	15,564
Exercisable at the end of the period (in shares)	8,680
Stock options, Weighted-Average Exercise Price Per Share
Balance at the beginning of the period (in dollars per share)	$ 34.61
Granted (in dollars per share)	59.52
Exercised (in dollars per share)	9.69
Cancelled or forfeited (in dollars per share)	50.86
Balance at the end of the period (in dollars per share)	42.23		$ 34.61
Vested and expected to vest in the future at the end of the period (in dollars per share)	41.40
Exercisable at the end of the period (in dollars per share)	$ 33.49
Weighted-Average Remaining Contractual Term (in years)
Balance at the beginning of the period	6 years 10 months 24 days		5 years 9 months 18 days
Balance at the end of the period	6 years 10 months 24 days		5 years 9 months 18 days
Vested and expected to vest in the future at the end of the period	6 years 9 months 18 days
Exercisable at the end of the period	5 years 7 months 6 days
Aggregate Intrinsic Value
Balance at the beginning of the period	$ 303,627
Balance at the end of the period	224,846		$ 303,627
Vested and expected to vest in the future at the end of the period	221,040
Exercisable at the end of the period	$ 184,608