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, 2018	Mar. 31, 2017	Dec. 31, 2017
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)	34.90%	36.60%
Risk-free interest rate (as a percent)	2.80%	2.10%
Expected term	6 years	6 years 1 month 6 days
Stock Options, Number of Shares
Balance at the beginning of the period (in shares)	17,819
Granted (in shares)	2,615
Exercised (in shares)	(415)
Cancelled or forfeited (in shares)	(192)
Balance at the end of the period (in shares)	19,827		17,819
Vested and expected to vest in the future at the end of the period (in shares)	18,689
Exercisable at the end of the period (in shares)	10,834
Stock options, Weighted-Average Exercise Price Per Share
Balance at the beginning of the period (in dollars per share)	$ 29.62
Granted (in dollars per share)	58.76
Exercised (in dollars per share)	15.68
Cancelled or forfeited (in dollars per share)	43.02
Balance at the end of the period (in dollars per share)	33.62		$ 29.62
Vested and expected to vest in the future at the end of the period (in dollars per share)	32.62
Exercisable at the end of the period (in dollars per share)	$ 21.99
Weighted-Average Remaining Contractual Term
Balance at the beginning of the period	6 years 4 months 24 days		6 years 1 month 6 days
Balance at the end of the period	6 years 4 months 24 days		6 years 1 month 6 days
Vested and expected to vest in the future at the end of the period	6 years 3 months 18 days
Exercisable at the end of the period	4 years 8 months 12 days
Aggregate Intrinsic Value
Balance at the beginning of the period	$ 600,032
Balance at the end of the period	472,160		$ 600,032
Vested and expected to vest in the future at the end of the period	463,173
Exercisable at the end of the period	$ 381,567