SECURITIES - Summary of Amortized Cost and Fair Value of Available-For-Sale Securities and Held-To-Maturity Securities (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale Securities, Amortized Cost	$ 924,257	$ 751,591
Available-for-sale Securities, Gross Unrealized Gains	33,918	14,419
Available-for-sale Securities, Gross Unrealized Losses	717	923
Available-for-sale securities, Fair Value	957,458	765,087
U.S Treasury
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale Securities, Amortized Cost	9,080	4,967
Available-for-sale Securities, Gross Unrealized Gains	370	0
Available-for-sale Securities, Gross Unrealized Losses	0	73
Available-for-sale securities, Fair Value	9,450	4,894
Obligations of U.S. government agencies and sponsored entities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale Securities, Amortized Cost	97,463	76,699
Available-for-sale Securities, Gross Unrealized Gains	3,168	1,475
Available-for-sale Securities, Gross Unrealized Losses	28	224
Available-for-sale securities, Fair Value	100,603	77,950
Tax-exempt and taxable obligations of states and municipal subdivisions
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale Securities, Amortized Cost	403,518	253,527
Available-for-sale Securities, Gross Unrealized Gains	13,642	5,602
Available-for-sale Securities, Gross Unrealized Losses	593	147
Available-for-sale securities, Fair Value	416,567	258,982
Mortgage-backed securities - residential
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale Securities, Amortized Cost	238,920	263,229
Available-for-sale Securities, Gross Unrealized Gains	9,263	4,726
Available-for-sale Securities, Gross Unrealized Losses	16	107
Available-for-sale securities, Fair Value	248,167	267,848
Mortgage-backed securities - commercial
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale Securities, Amortized Cost	147,766	125,292
Available-for-sale Securities, Gross Unrealized Gains	6,563	2,398
Available-for-sale Securities, Gross Unrealized Losses	38	223
Available-for-sale securities, Fair Value	154,291	127,467
Corporate obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale Securities, Amortized Cost	27,510	27,877
Available-for-sale Securities, Gross Unrealized Gains	912	218
Available-for-sale Securities, Gross Unrealized Losses	42	149
Available-for-sale securities, Fair Value	$ 28,380	$ 27,946