EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
EARNINGS PER SHARE [Abstract]
Weighted-average anti-dilutive stock options outstanding (in shares)	300	300	200
Basic earnings per share [Abstract]
Net income	$ 103,812	$ 91,230	$ 91,353
Income attributable to unvested stock-based compensation awards	(550)	(453)	(456)
Income available to common shareholders	$ 103,262	$ 90,777	$ 90,897
Weighted-average common shares outstanding - basic (in shares)	44,091	40,996	40,548
Basic earnings per share (in dollars per share)	$ 2.34	$ 2.21	$ 2.24
Diluted earnings per share [Abstract]
Net income	$ 103,812	$ 91,230	$ 91,353
Income attributable to unvested stock-based compensation awards	(550)	(453)	(456)
Income available to common shareholders	$ 103,262	$ 90,777	$ 90,897
Weighted-average common shares outstanding (in shares)	44,091	40,996	40,548
Assumed exercise of stock options (in shares)	394	405	481
Weighted-average common shares outstanding - diluted (in shares)	44,485	41,401	41,029
Diluted earnings per share (in dollars per share)	$ 2.32	$ 2.19	$ 2.22
Cash dividends declared per share (in dollars per share)	$ 1.26	$ 1.22	$ 1.16
Stock Repurchase Program [Abstract]
Number of common shares authorized to be repurchased (in shares)	2,200	2,200
Number of common shares repurchased (in shares)	0	300	100