UNAUDITED QUARTERLY FINANCIAL INFORMATION (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
UNAUDITED QUARTERLY FINANCIAL INFORMATION [Abstract]
Net interest income	$ 38,732		$ 38,458		$ 38,053		$ 37,487		$ 37,898		$ 35,347		$ 35,610		$ 34,631		$ 152,730	$ 143,486	$ 128,564
Provision (credit) for loan losses	(1,000)		23		1,047		450		529		1,168		442		(21)		520	2,118	(1,330)
Net interest income after provision for loan losses	39,732		38,435		37,006		37,037		37,369		34,179		35,168		34,652		152,210	141,368	129,894
Non-interest income	13,706		4,283		1,747		1,778		1,817		2,071		2,305		69,741		21,514	75,934	8,616
Non-interest expense	22,573		22,175		19,469		20,769		29,638		18,232		18,092		17,869		84,986	83,831	62,493
Income before income taxes	30,865		20,543		19,284		18,046		9,548		18,018		19,381		86,524		88,738	133,471	76,017
Income tax expense	15,442		7,230		7,295		6,889		8,816		7,481		8,173		36,487		36,856	60,957	31,245
Net income	$ 15,423		$ 13,313		$ 11,989		$ 11,157		$ 732		$ 10,537		$ 11,208		$ 50,037		$ 51,882	$ 72,514	$ 44,772
EPS [Abstract]
Basic (in dollars per share)	$ 0.41	[1]	$ 0.36	[1]	$ 0.32	[1]	$ 0.30	[1]	$ 0.02	[1]	$ 0.29	[1]	$ 0.30	[1]	$ 1.37	[1]	$ 1.38	$ 1.97	$ 1.24
Diluted (in dollars per share)	$ 0.41	[1]	$ 0.35	[1]	$ 0.32	[1]	$ 0.30	[1]	$ 0.02	[1]	$ 0.29	[1]	$ 0.30	[1]	$ 1.36	[1]	$ 1.38	$ 1.97	$ 1.23

[1] The quarterly EPS amounts, when added, may not coincide with the full fiscal year EPS reported on the Consolidated Statements of Operations due to differences in the computed weighted average shares outstanding as well as rounding differences.