EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
EARNINGS PER SHARE [Abstract]
Weighted-average anti-dilutive stock options outstanding (in shares)	500,000	400,000
Basic earnings per share [Abstract]
Net income	$ 41,946	$ 40,106
Income attributable to unvested stock-based compensation awards	(109)	(141)
Income available to common shareholders	$ 41,837	$ 39,965
Weighted-average common shares outstanding - basic (in shares)	51,520,000	50,934,000
Basic earnings per share (in dollars per share)	$ 0.81	$ 0.78
Diluted earnings per share [Abstract]
Net income	$ 41,946	$ 40,106
Income attributable to unvested stock-based compensation awards	(109)	(141)
Income available to common shareholders	$ 41,837	$ 39,965
Weighted-average common shares outstanding - basic (in shares)	51,520,000	50,934,000
Assumed exercise of stock options (in shares)	541,000	563,000
Weighted-average common shares outstanding - diluted (in shares)	52,061,000	51,497,000
Diluted earnings per share (in dollars per share)	$ 0.80	$ 0.78
Stock Repurchase Program [Abstract]
Number of common shares authorized to be repurchased (in shares)			2,500,000	2,500,000
Number of common shares repurchased (in shares)	0	0