EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Basic earnings per share:
NET INCOME		$ 14,200	$ 14,099
Weighted-average common shares		29,799,000	29,456,000
Basic earnings per share		$ 0.48	$ 0.48
Diluted earnings per share:
NET INCOME		$ 14,200	$ 14,099
Weighted-average common shares		29,799,000	29,456,000
Effect of dilutive awards:
Employee stock plans		753,000	792,000
Diluted weighted-average common shares		30,552,000	30,248,000
Diluted earnings per share	[1]	$ 0.46	$ 0.47
Stock awards excluded in the computation of diluted earnings per share		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.