UNAUDITED QUARTERLY FINANCIAL INFORMATION (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
UNAUDITED QUARTERLY FINANCIAL INFORMATION [Abstract]
Net interest income	$ 37,150		$ 35,028		$ 36,134		$ 38,015		$ 38,732		$ 38,458		$ 38,053		$ 37,487		$ 146,326	$ 152,730	$ 143,486
Provision (credit) for loan losses	603		335		1,113		193		(1,000)		23		1,047		450		2,244	520	2,118
Net interest income after provision for loan losses	36,547		34,693		35,021		37,822		39,732		38,435		37,006		37,037		144,082	152,210	141,368
Non-interest income	1,821		2,221		2,237		3,244		13,706		4,283		1,747		1,778		9,523	21,514	75,934
Non-interest expense	22,745		21,585		20,827		21,734		22,573		22,175		19,469		20,769		86,890	84,986	83,831
Income before income taxes	15,623		15,329		16,431		19,332		30,865		20,543		19,284		18,046		66,715	88,738	133,471
Income tax expense	3,183		3,547		4,110		4,587		15,442		7,230		7,295		6,889		15,427	36,856	60,957
Net income	$ 12,440		$ 11,782		$ 12,321		$ 14,745		$ 15,423		$ 13,313		$ 11,989		$ 11,157		$ 51,288	$ 51,882	$ 72,514
EPS [Abstract]
Basic (in dollars per share)	$ 0.34	[1]	$ 0.32	[1]	$ 0.33	[1]	$ 0.39	[1]	$ 0.41	[1]	$ 0.36	[1]	$ 0.32	[1]	$ 0.30	[1]	$ 1.38	$ 1.38	$ 1.97
Diluted (in dollars per share)	$ 0.34	[1]	$ 0.32	[1]	$ 0.33	[1]	$ 0.39	[1]	$ 0.41	[1]	$ 0.35	[1]	$ 0.32	[1]	$ 0.30	[1]	$ 1.38	$ 1.38	$ 1.97

[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.