EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Basic earnings per share:
Net income		$ 12,701	$ 11,513	$ 8,590	$ 6,343	$ 14,354	$ 16,111	$ 16,716	$ 12,363	$ 39,147	$ 59,544	$ 57,266
Weighted-average common shares		28,483,000	28,855,000	29,368,000	29,309,000	29,350,000	30,013,000	30,310,000	30,239,000	29,001,000	29,976,000	29,421,000
Basic earnings per share		$ 0.45	$ 0.40	$ 0.29	$ 0.22	$ 0.49	$ 0.54	$ 0.55	$ 0.41	$ 1.35	$ 1.99	$ 1.95
Diluted earnings per share:
Net income		$ 12,701	$ 11,513	$ 8,590	$ 6,343	$ 14,354	$ 16,111	$ 16,716	$ 12,363	$ 39,147	$ 59,544	$ 57,266
Weighted-average common shares		28,483,000	28,855,000	29,368,000	29,309,000	29,350,000	30,013,000	30,310,000	30,239,000	29,001,000	29,976,000	29,421,000
Effect of dilutive securities:
Employee stock plans	[1]									695,000	853,000	999,000
Diluted weighted average common shares	[1]	29,126,000	29,298,000	30,034,000	30,058,000	30,051,000	30,757,000	31,110,000	31,167,000	29,696,000	30,829,000	30,420,000
Diluted earnings per share	[1]	$ 0.44	$ 0.39	$ 0.29	$ 0.21	$ 0.48	$ 0.52	$ 0.54	$ 0.40	$ 1.32	$ 1.93	$ 1.88
Antidilutive shares attributable to stock-based awards outstanding excluded from computation of diluted earnings per share										8,000	37,000	122,000

[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 2015 and 2013 calculations those shares that were contingently issuable upon the satisfaction of the performance conditions as of the end of the respective periods.