EARNINGS PER SHARE (Details) (USD $) In Thousands, except Share data, unless otherwise specified	1 Months Ended	12 Months Ended
	Jan. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
EARNINGS PER SHARE [Abstract]
Weighted-average anti-dilutive stock options outstanding (in shares)		200,000	300,000	500,000
Basic earnings per share [Abstract]
Net income		$ 91,353	$ 78,829	$ 77,068
Income attributable to unvested stock-based compensation awards		(456)	(433)	(499)
Income available to common shareholders		90,897	78,396	76,569
Weighted-average common shares outstanding - basic (in shares)		40,548,000	40,000,000	39,192,000
Basic earnings per share (in dollars per share)		$ 2.24	$ 1.96	$ 1.95
Diluted earnings per share [Abstract]
Net income		91,353	78,829	77,068
Income attributable to unvested stock-based compensation awards		(456)	(433)	(499)
Income available to common shareholders		90,897	78,396	76,569
Weighted-average common shares outstanding (in shares)		40,548,000	40,000,000	39,192,000
Assumed exercise of stock options (in shares)		481,000	504,000	479,000
Weighted-average common shares outstanding - diluted (in shares)		41,029,000	40,504,000	39,671,000
Diluted earnings per share (in dollars per share)		$ 2.22	$ 1.94	$ 1.93
Cash dividends declared per share (in dollars per share)		$ 1.16	$ 1.10	$ 1.06
Common stock issuance	57,500	9,989	15,239	64,145
Common stock issuance (in shares)	2,130,000
Net proceeds from offerings	$ 54,900			$ 54,917
Stock Repurchase Program [Abstract]
Number of common shares authorized to be repurchased (in shares)		2,000,000	2,000,000	1,500,000
Number of common shares repurchased		123,000