EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net income	$ 38,935	$ 20,569	$ 105,025	$ 60,076
Denominator:
Weighted average basic shares (in shares)	31,800	31,420	31,599	31,424
Dilutive potential common shares (in shares)	548	848	663	856
Weighted average fully diluted shares (in shares)	32,348	32,268	32,262	32,280
Earnings per share:
Basic (in dollars per share)	$ 1.22	$ 0.65	$ 3.32	$ 1.91
Diluted (in dollars per share)	$ 1.20	$ 0.64	$ 3.26	$ 1.86
Outstanding common stock equivalents having no dilutive effect (in shares)	11	0	16	5