EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
EARNINGS PER SHARE [Abstract]
Weighted-average anti-dilutive stock options outstanding (in shares)	400	200	300
Basic earnings per share [Abstract]
Net income	$ 168,641	$ 150,717	$ 103,812
Income attributable to unvested stock-based compensation awards	(744)	(597)	(550)
Income available to common shareholders	$ 167,897	$ 150,120	$ 103,262
Weighted-average common shares outstanding - basic (in shares)	51,165	48,843	44,091
Basic earnings per share (in dollars per share)	$ 3.28	$ 3.07	$ 2.34
Diluted earnings per share [Abstract]
Net income	$ 168,641	$ 150,717	$ 103,812
Income attributable to unvested stock-based compensation awards	(744)	(597)	(550)
Income available to common shareholders	$ 167,897	$ 150,120	$ 103,262
Weighted-average common shares outstanding (in shares)	51,165	48,843	44,091
Assumed exercise of stock options (in shares)	583	627	394
Weighted-average common shares outstanding - diluted (in shares)	51,748	49,470	44,485
Diluted earnings per share (in dollars per share)	$ 3.24	$ 3.03	$ 2.32
Cash dividends declared per share (in dollars per share)	$ 1.44	$ 1.32	$ 1.26
Stock Repurchase Program [Abstract]
Number of common shares authorized to be repurchased (in shares)	2,500	2,500
Number of common shares repurchased (in shares)	0	0