SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES , NET INCOME PER SHARE (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Numerator:
Net income	$ 85,169	$ 52,231
Denominator:
Denominator for basic net income per share - weighted-average outstanding shares (in shares)	36,237	35,893
Effect of dilutive securities:
Stock options and restricted stock awards	330	457
Denominator for diluted net income per share — weighted average outstanding shares (in shares)	36,567	36,350
Net income per share - basic (in dollars per share)	$ 2.35	$ 1.46
Net income per share - diluted (in dollars per share)	$ 2.33	$ 1.44
Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	89	84
Employee Stock Option [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	52	83