EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, $ 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
Basic earnings per share:
Net income		$ 22,736	[1]	$ 17,667	$ 15,381	$ 14,200	$ (16,898)	$ 16,506	$ 13,902	$ 14,099	$ 69,984	$ 27,609	$ 48,898
Weighted-average common shares		29,164,000		29,365,000	29,505,000	29,799,000	29,759,000	29,759,000	29,720,000	29,456,000	29,456,000	29,674,000	28,925,000
Basic earnings per share		$ 0.78		$ 0.60	$ 0.52	$ 0.48	$ (0.57)	$ 0.55	$ 0.47	$ 0.48	$ 2.38	$ 0.93	$ 1.69
Diluted earnings per share:
Net income		$ 22,736	[1]	$ 17,667	$ 15,381	$ 14,200	$ (16,898)	$ 16,506	$ 13,902	$ 14,099	$ 69,984	$ 27,609	$ 48,898
Weighted-average common shares		29,164,000		29,365,000	29,505,000	29,799,000	29,759,000	29,759,000	29,720,000	29,456,000	29,456,000	29,674,000	28,925,000
Effect of dilutive securities:
Employee stock plans	[2]										691,000	871,000	694,000
Diluted weighted average common shares	[2]	29,651,000		29,998,000	30,183,000	30,552,000	29,759,000	30,614,000	30,454,000	30,248,000	30,147,000	30,545,000	29,619,000
Diluted earnings per share	[2]	$ 0.77		$ 0.59	$ 0.51	$ 0.46	$ (0.57)	$ 0.54	$ 0.46	$ 0.47	$ 2.32	$ 0.90	$ 1.65
Antidilutive shares attributable to stock-based awards outstanding excluded from computation of diluted earnings per share											0	0	0

[1] In December 2017 the U.S. government enacted comprehensive tax legislation commonly referred to as the Tax Cuts and Jobs Act (Refer to Note 11, Provision (Benefit) for Income Taxes, in the Notes to Consolidated Financial Statements).

[2] 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 included in the 2018, 2017 and 2016 calculations those shares that were contingently issuable upon the satisfaction of the performance conditions as of the end of the respective periods.