SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	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, 2019	Dec. 31, 2018	Dec. 31, 2017
Selected Quarterly Financial Information [Abstract]
Interest income	$ 133,409	$ 131,436	$ 130,838	$ 129,996	$ 128,741	$ 117,648	$ 112,423	$ 104,820	$ 103,475	$ 105,278	$ 104,436	$ 99,096	$ 525,679	$ 463,632	$ 412,284
Interest expense	13,918	14,815	14,143	13,892	11,282	8,570	7,360	5,447	5,211	5,068	4,730	4,242	56,768	32,659	19,250
Interest-bearing deposits	119,491	116,621	116,695	116,104	117,459	109,078	105,063	99,373	98,264	100,210	99,706	94,854	468,911	430,973	393,034
Provision for loan losses	4,000	2,000	2,000	2,000	2,500	2,000	2,000	2,000	2,000	2,000	2,000	2,000	10,000	8,500	8,000
Net interest income	115,491	114,621	114,695	114,104	114,959	107,078	103,063	97,373	96,264	98,210	97,706	92,854	458,911	422,473	385,034
Non-interest income	20,282	20,866	22,676	18,125	21,018	20,411	21,217	21,362	27,675	18,081	20,396	19,048	81,949	84,008	85,200
Other operating expenses	93,690	87,308	86,716	90,014	95,396	81,632	82,637	81,706	82,501	80,331	79,857	76,281	357,728	341,371	318,970
Income before provision for income taxes	42,083	48,179	50,655	42,215	40,581	45,857	41,643	37,029	41,438	35,960	38,245	35,621	183,132	165,110	151,264
PROVISION FOR INCOME TAXES	8,428	8,602	10,955	8,869	3,053	8,084	9,219	8,239	54,986	10,883	12,791	11,828	36,854	28,595	90,488
NET INCOME	$ 33,655	$ 39,577	$ 39,700	$ 33,346	$ 37,528	$ 37,773	$ 32,424	$ 28,790	$ (13,548)	$ 25,077	$ 25,454	$ 23,793	$ 146,278	$ 136,515	$ 60,776
Basic earnings (loss) per share (in dollars per share)	$ 0.96	$ 1.15	$ 1.14	$ 0.95	$ 1.10	$ 1.17	$ 1.01	$ 0.89	$ (0.41)	$ 0.76	$ 0.77	$ 0.72	$ 4.20	$ 4.16	$ 1.85
Diluted earnings (loss) per share (in dollars per share)	0.95	1.15	1.14	0.95	1.09	1.17	1.00	0.89	(0.41)	0.76	0.77	0.72	4.18	4.15	1.84
Cumulative dividends declared per common share (in dollars per share)	$ 1.41	$ 0.41	$ 0.41	$ 0.41	$ 0.38	$ 0.38	$ 0.85	$ 0.35	$ 0.25	$ 0.25	$ 1.25	$ 0.25	$ 2.64	$ 1.96	$ 2.00