LOANS AND THE ALLOWANCE FOR CREDIT LOSSES - Narrative (Details)	3 Months Ended			6 Months Ended
	Jun. 30, 2024 USD ($) loan property	Mar. 31, 2024 loan	Jun. 30, 2023 USD ($) loan	Jun. 30, 2024 USD ($) loan	Jun. 30, 2023 USD ($)	Dec. 31, 2023 USD ($)
Financing Receivable, Allowance for Credit Loss [Line Items]
Total	$ 2,456,063,000			$ 2,456,063,000		$ 2,530,915,000
Loans accounted for under the fair value option	10,190,000			10,190,000		13,726,000
Total, net	2,428,744,000			2,428,744,000		2,506,984,000
Interest income on nonaccrual loans	0		$ 200,000	0	$ 200,000
Accrued interest receivable	10,700,000			10,700,000		10,800,000
Other real estate owned, gross	$ 0			0		0
Protective advances			$ 300,000		$ 500,000
Number of non-accrual loans paid | loan			2
Non-accrual loans paid off			$ 1,800,000
Number of foreclosed properties | property	2
Release of provision on pooled loans				1,900,000
Increase in provision for individually analyzed loans				$ (5,300,000)
Number of loans modified | loan	0			1
Consumer and Other
Financing Receivable, Allowance for Credit Loss [Line Items]
Total	$ 25,614,000			$ 25,614,000		40,754,000
Loans accounted for under the fair value option	10,500,000			10,500,000		14,100,000
PPP Loans | Cash, Securities, and Other
Financing Receivable, Allowance for Credit Loss [Line Items]
Total	$ 3,100,000			$ 3,100,000		$ 4,200,000
Percent of loans on total loan segment	2.30%			2.30%		3.00%
Cash, Securities, and Other
Financing Receivable, Allowance for Credit Loss [Line Items]
Total	$ 133,357,000			$ 133,357,000		$ 139,947,000
MSLP loan
Financing Receivable, Allowance for Credit Loss [Line Items]
Percent of loans on total loan segment						1.50%
Number of loans | loan		3		3
Total, net						$ 5,100,000
MSLP loan | COVID-19
Financing Receivable, Allowance for Credit Loss [Line Items]
Percent of loans on total loan segment	1.80%			1.80%
Total, net	$ 5,100,000			$ 5,100,000		3,800,000
Commercial and Industrial
Financing Receivable, Allowance for Credit Loss [Line Items]
Total	278,106,000			278,106,000		337,180,000
Simmons Bank
Financing Receivable, Allowance for Credit Loss [Line Items]
Total	$ 178,400,000			$ 178,400,000		$ 208,200,000