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
Basic earnings per share:
NET INCOME		$ 15,381	$ 13,902	$ 29,581	$ 28,001
Weighted-average common shares		29,505,000	29,720,000	29,651,000	29,589,000
Basic earnings per share		$ 0.52	$ 0.47	$ 1.00	$ 0.95
Diluted earnings per share:
NET INCOME		$ 15,381	$ 13,902	$ 29,581	$ 28,001
Weighted-average common shares		29,505,000	29,720,000	29,651,000	29,589,000
Effect of dilutive awards:
Employee stock plans		678,000	734,000	736,000	781,000
Diluted weighted-average common shares		30,183,000	30,454,000	30,387,000	30,370,000
Diluted earnings per share	[1]	$ 0.51	$ 0.46	$ 0.97	$ 0.92
Stock awards excluded in the computation of diluted earnings per share		0	0	0	0

[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 the shares underlying the outstanding performance-based awards in the 2018 and 2017 calculations as the shares were not contingently issuable as of the end of the reporting periods.