LOANS AND THE ALLOWANCE FOR CREDIT LOSSES - Narrative (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2023 USD ($) loan	Dec. 31, 2022 USD ($) loan
Financing Receivable, Allowance for Credit Loss [Line Items]
Total	$ 2,530,459	$ 2,469,413
Loans accounted for under the fair value option	15,464	23,321
Total, net	2,507,284	2,452,230
Protective advances	500
Interest income on nonaccrual loans	200
Accrued interest receivable	10,800	9,800
Release of provision on pooled loans	1,500
Allowance increase on analyzed loans	$ 4,000
COVID-19
Financing Receivable, Allowance for Credit Loss [Line Items]
Delinquent over period (in days)	30 days
Payments period (in years)	2 years
Number of loans modified | loan	43
Outstanding balance of modified loans	$ 72,900
Number of loans modified | loan	14
Loan deferral amount	$ 3,000
Consumer and Other
Financing Receivable, Allowance for Credit Loss [Line Items]
Total	38,928	49,391
Loans accounted for under the fair value option	16,100	23,400
PPP Loans | Cash, Securities, and Other
Financing Receivable, Allowance for Credit Loss [Line Items]
Total	$ 4,800	$ 6,900
Percent of loans on total loan segment	3.20%	4.20%
Cash, Securities, and Other
Financing Receivable, Allowance for Credit Loss [Line Items]
Total	$ 148,618	$ 165,559
MSLP loan | COVID-19
Financing Receivable, Allowance for Credit Loss [Line Items]
Percent of loans on total loan segment	1.50%	1.60%
Number of loans | loan	4	5
Total, net	$ 5,100	$ 5,900
Commercial and Industrial
Financing Receivable, Allowance for Credit Loss [Line Items]
Total	$ 349,981	$ 361,791
Number of loans, 90 days delinquent and accruing interest | loan	0	1
Allowance increase on analyzed loans	$ 4,000
Simmons Bank
Financing Receivable, Allowance for Credit Loss [Line Items]
Total	$ 212,200	$ 230,400