EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Basic earnings per share:
Net income		$ 13,617	$ 14,165	$ 11,265	$ 8,843	$ 12,701	$ 11,513	$ 8,590	$ 6,343	$ 47,890	$ 39,147	$ 59,544
Weighted-average common shares		29,196,000	28,972,000	28,850,000	28,679,000	28,483,000	28,855,000	29,368,000	29,309,000	28,925,000	29,001,000	29,976,000
Basic earnings per share		$ 0.47	$ 0.49	$ 0.39	$ 0.31	$ 0.45	$ 0.40	$ 0.29	$ 0.22	$ 1.66	$ 1.35	$ 1.99
Diluted earnings per share:
Net income		$ 13,617	$ 14,165	$ 11,265	$ 8,843	$ 12,701	$ 11,513	$ 8,590	$ 6,343	$ 47,890	$ 39,147	$ 59,544
Weighted-average common shares		29,196,000	28,972,000	28,850,000	28,679,000	28,483,000	28,855,000	29,368,000	29,309,000	28,925,000	29,001,000	29,976,000
Effect of dilutive securities:
Employee stock plans	[1]									694,000	695,000	853,000
Diluted weighted average common shares	[1]	29,914,000	29,625,000	29,422,000	29,244,000	29,126,000	29,298,000	30,034,000	30,058,000	29,619,000	29,696,000	30,829,000
Diluted earnings per share	[1]	$ 0.46	$ 0.48	$ 0.38	$ 0.30	$ 0.44	$ 0.39	$ 0.29	$ 0.21	$ 1.62	$ 1.32	$ 1.93
Antidilutive shares attributable to stock-based awards outstanding excluded from computation of diluted earnings per share										0	8,000	37,000

[1] The Company includes the shares underlying performance-based awards in the calculation of diluted earnings per share if the performance conditions have been satisfied as of the end of the reporting period and excludes such shares when the necessary conditions have not been met. The Company has excluded all performance-based awards in the 2014 calculation as the performance conditions for those awards were not met as of the end of the period. The Company has included in the 2016 and 2015 calculations those shares that were contingently issuable upon the satisfaction of the performance conditions as of the end of the respective periods.