SECURITIES - Narrative (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Investment [Line Items]
Debt Securities, Held-to-maturity, Nonaccrual	$ 0								$ 0
Debt Securities, Held-to-maturity, Past Due	0								0
Interest receivable	38,708,000				$ 28,452,000				38,708,000	$ 28,452,000
Transferred Securities, Unrealized Loss, Before Tax	2,900,000				3,700,000				2,900,000	3,700,000
Securities Transferred, Unrealized Loss, Net Of Tax	2,300,000				2,900,000				2,300,000	2,900,000
Fair value of securities transferred from AFS to HTM									0	0
Less than 12 Months, fair value	39,898,000				273,963,000				39,898,000	273,963,000
More than 12 Months, fair value	0				30,697,000				0	30,697,000
Equity balance	875,297,000				804,580,000				875,297,000	804,580,000	$ 731,291,000	$ 754,140,000
Available-for-sale Securities, Gross Realized Gain (Loss) [Abstract]
Net gain (loss) on sale of securities available for sale	(24,000)	$ 78,000	$ 2,662,000	$ 5,541,000	42,000	$ 42,000	$ 416,000	$ 256,000	8,257,000	756,000	(1,839,000)
Gross realized gains on AFS securities									8,400,000	5,700,000	2,000,000.0
Gross realized loss on AFS securities									129,000	4,900,000	3,800,000
Carrying value of investment securities pledged as collateral	1,560,000,000				1,120,000,000				1,560,000,000	1,120,000,000
Equity investments	11,905,000				12,331,000				11,905,000	12,331,000
Debt Securities, Held-to-maturity, Sale or Transfer of Investment [Abstract]
Proceeds from sale of held-to-maturity securities									0	0	0
Debt Securities, Available-for-sale, Allowance for Credit Loss	0								0
Debt Securities, Held-to-maturity, Credit Loss Expense (Reversal)									0
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0								0
Debt Securities, Available-for-sale, Allowance for Credit Loss, Period Increase (Decrease)									0
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net, Available-for-sale Securities										0
Other-than-temporarily impaired equity investments	0								0
FHLB Stock with other-than-temporary impairment	0								0
Retained  Earnings
Investment [Line Items]
Equity balance	111,208,000				80,274,000				111,208,000	80,274,000	64,797,000	32,851,000
Impact of CECL adoption
Investment [Line Items]
Equity balance					(7,830,000)					(7,830,000)	(16,452,000)	0
Impact of CECL adoption | Retained  Earnings
Investment [Line Items]
Equity balance					$ (7,830,000)					$ (7,830,000)	$ (16,452,000)	$ (85,000)
Available-for-sale Securities [Member]
Investment [Line Items]
Interest receivable	22,000,000.0								22,000,000.0
Held-to-maturity Securities [Member]
Investment [Line Items]
Interest receivable	$ 298,000								$ 298,000