SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) (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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Selected Quarterly Financial Information [Abstract]
Interest income	$ 128,741	$ 117,648	$ 112,423	$ 104,820	$ 103,475	$ 105,278	$ 104,436	$ 99,096	$ 101,007	$ 97,849	$ 97,321	$ 95,301	$ 463,632	$ 412,284	$ 391,477
Interest expense	11,282	8,570	7,360	5,447	5,211	5,068	4,730	4,242	3,836	4,141	4,173	4,258	32,659	19,250	16,408
Interest-bearing deposits	117,459	109,078	105,063	99,373	98,264	100,210	99,706	94,854	97,171	93,708	93,148	91,043	430,973	393,034	375,069
Provision for loan losses	2,500	2,000	2,000	2,000	2,000	2,000	2,000	2,000	2,030	2,000	2,000	0	8,500	8,000	6,030
Net interest income	114,959	107,078	103,063	97,373	96,264	98,210	97,706	92,854	95,141	91,708	91,148	91,043	422,473	385,034	369,039
Non-interest income	21,018	20,411	21,217	21,362	27,675	18,081	20,396	19,048	19,463	23,512	20,537	19,959	84,008	85,200	76,225
Other operating expenses	95,396	81,632	82,637	81,706	82,501	80,331	79,857	76,281	79,857	79,092	79,887	84,034	341,371	318,970	315,624
Income before provision for income taxes	40,581	45,857	41,643	37,029	41,438	35,960	38,245	35,621	34,747	36,128	31,798	26,968	165,110	151,264	129,640
PROVISION FOR INCOME TAXES	3,053	8,084	9,219	8,239	54,986	10,883	12,791	11,828	11,943	12,277	10,841	9,194	28,595	90,488	44,255
NET INCOME	$ 37,528	$ 37,773	$ 32,424	$ 28,790	$ (13,548)	$ 25,077	$ 25,454	$ 23,793	$ 22,804	$ 23,851	$ 20,957	$ 17,774	$ 136,515	$ 60,776	$ 85,385
Basic earnings (loss) per share (in dollars per share)	$ 1.10	$ 1.17	$ 1.01	$ 0.89	$ (0.41)	$ 0.76	$ 0.77	$ 0.72	$ 0.69	$ 0.70	$ 0.62	$ 0.52	$ 4.16	$ 1.85	$ 2.52
Diluted earnings (loss) per share (in dollars per share)	1.09	1.17	1.00	0.89	(0.41)	0.76	0.77	0.72	0.69	0.70	0.61	0.52	4.15	1.84	2.52
Cumulative dividends declared per common share (in dollars per share)	$ 0.38	$ 0.38	$ 0.85	$ 0.35	$ 0.25	$ 0.25	$ 1.25	$ 0.25	$ 0.23	$ 0.23	$ 0.21	$ 0.21	$ 1.96	$ 2.00	$ 0.88