EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Basic earnings per share:
NET INCOME		$ 17,667	$ 16,506	$ 47,248	$ 44,507
Weighted-average common shares		29,365,000	29,759,000	29,558,000	29,646,000
Basic earnings per share		$ 0.60	$ 0.55	$ 1.60	$ 1.50
Diluted earnings per share:
NET INCOME		$ 17,667	$ 16,506	$ 47,248	$ 44,507
Weighted-average common shares		29,365,000	29,759,000	29,558,000	29,646,000
Effect of dilutive awards:
Employee stock plans		633,000	855,000	723,000	826,000
Diluted weighted-average common shares		29,998,000	30,614,000	30,281,000	30,472,000
Diluted earnings per share	[1]	$ 0.59	$ 0.54	$ 1.56	$ 1.46
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.