EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator:
Net Income	$ 28,740	$ 20,570	$ 66,090	$ 39,507
Denominator:
Weighted average basic shares (in shares)	31,567,000	31,443,000	31,497,000	31,425,000
Dilutive potential common shares (in shares)	697,000	869,000	729,000	899,000
Weighted average fully diluted shares (in shares)	32,264,000	32,312,000	32,226,000	32,324,000
Earnings per share:
Basic (in dollars per share)	$ 0.91	$ 0.65	$ 2.10	$ 1.26
Diluted (in dollars per share)	$ 0.89	$ 0.64	$ 2.05	$ 1.22
Outstanding common stock equivalents having no dilutive effect (in shares)	11,000	0	23,000	10,000