LOANS AND THE ALLOWANCE FOR CREDIT LOSSES - Narrative (Details)	3 Months Ended	6 Months Ended	12 Months Ended
	Mar. 31, 2024 USD ($) loan	Jun. 30, 2023 loan	Dec. 31, 2023 USD ($) loan
Financing Receivable, Allowance for Credit Loss [Line Items]
Total	$ 2,475,524,000		$ 2,530,915,000
Loans accounted for under the fair value option	11,922,000		13,726,000
Total, net	2,450,894,000		2,506,984,000
Interest income on nonaccrual loans	0
Accrued interest receivable	11,100,000		10,800,000
Release of provision on pooled loans	1,600,000		500,000
Other real estate owned, gross	0		0
Consumer and Other
Financing Receivable, Allowance for Credit Loss [Line Items]
Total	30,372,000		40,754,000
Loans accounted for under the fair value option	12,300,000		14,100,000
PPP Loans | Cash, Securities, and Other
Financing Receivable, Allowance for Credit Loss [Line Items]
Total	$ 3,700,000		$ 4,200,000
Percent of loans on total loan segment	2.50%		3.00%
Cash, Securities, and Other
Financing Receivable, Allowance for Credit Loss [Line Items]
Total	$ 150,969,000		$ 139,947,000
MSLP loan
Financing Receivable, Allowance for Credit Loss [Line Items]
Percent of loans on total loan segment	1.60%		1.50%
Number of loans | loan	3		3
Total, net	$ 4,900,000		$ 5,100,000
MSLP loan | COVID-19
Financing Receivable, Allowance for Credit Loss [Line Items]
Total, net			3,800,000
Commercial and Industrial
Financing Receivable, Allowance for Credit Loss [Line Items]
Total	$ 297,842,000		337,180,000
Number of loans, 90 days delinquent and accruing interest | loan		1
Secured by Cash and Securities
Financing Receivable, Allowance for Credit Loss [Line Items]
Number of loans, 90 days delinquent and accruing interest | loan	1
Simmons Bank
Financing Receivable, Allowance for Credit Loss [Line Items]
Total	$ 204,300,000		$ 208,200,000