NET INCOME (LOSS) PER SHARE (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net income	$ 83,509	$ 58,759	$ 43,835	$ 52,231	$ 44,204	$ 34,177	$ 22,184	$ 22,130	$ 238,334	$ 122,695	$ 85,071
Denominator:
Denominator for basic net income (loss) per share — weighted-average outstanding shares									36,058	33,200	32,167	32,167
Effect of dilutive securities:
Stock options and restricted stock awards (in shares)									390	359	269
Denominator for diluted net income (loss) per share — weighted-average outstanding shares									36,448	33,559	32,436
Net income (loss) per share — basic (in dollars per share)	$ 2.31	$ 1.63	$ 1.22	$ 1.46	$ 1.24	$ 1.05	$ 0.68	$ 0.69	$ 6.61	$ 3.70	$ 2.64
Net income (loss) per share — diluted (in dollars per share)	$ 2.29	$ 1.61	$ 1.20	$ 1.44	$ 1.22	$ 1.04	$ 0.68	$ 0.68	$ 6.54	$ 3.66	$ 2.62
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)									54	59	60
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)									64	87	194
Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)									53	58	59
Restricted Stock Units (RSUs) [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)									1	1	1