EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
EARNINGS PER SHARE [Abstract]
Weighted-average anti-dilutive stock options outstanding (in shares)	200	300	300
Basic earnings per share [Abstract]
Net income	$ 150,717	$ 103,812	$ 91,230
Income attributable to unvested stock-based compensation awards	(597)	(550)	(453)
Income available to common shareholders	$ 150,120	$ 103,262	$ 90,777
Weighted-average common shares outstanding - basic (in shares)	48,843	44,091	40,996
Basic earnings per share (in dollars per share)	$ 3.07	$ 2.34	$ 2.21
Diluted earnings per share [Abstract]
Net income	$ 150,717	$ 103,812	$ 91,230
Income attributable to unvested stock-based compensation awards	(597)	(550)	(453)
Income available to common shareholders	$ 150,120	$ 103,262	$ 90,777
Weighted-average common shares outstanding (in shares)	48,843	44,091	40,996
Assumed exercise of stock options (in shares)	627	394	405
Weighted-average common shares outstanding - diluted (in shares)	49,470	44,485	41,401
Diluted earnings per share (in dollars per share)	$ 3.03	$ 2.32	$ 2.19
Cash dividends declared per share (in dollars per share)	$ 1.32	$ 1.26	$ 1.22
Stock Repurchase Program [Abstract]
Number of common shares authorized to be repurchased (in shares)	2,500	2,200
Number of common shares repurchased (in shares)	0	0	300