LOANS AND THE ALLOWANCE FOR CREDIT LOSSES - Narrative (Details)	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2024 USD ($) loan	Sep. 30, 2023 USD ($)	Jun. 30, 2024 USD ($) loan	Sep. 30, 2024 USD ($) loan	Sep. 30, 2023 USD ($)	Dec. 31, 2023 USD ($)
Financing Receivable, Allowance for Credit Loss [Line Items]
Total	$ 2,383,199,000			$ 2,383,199,000		$ 2,530,915,000
Loans accounted for under the fair value option	8,646,000			8,646,000		13,726,000
Total, net	2,364,403,000			2,364,403,000		2,506,984,000
Interest income on nonaccrual loans	0	$ 200,000		0	$ 200,000
Accrued interest receivable	$ 9,700,000			9,700,000		10,800,000
Release of provision on pooled loans				(1,600,000)
Increase in provision for individually analyzed loans				3,500,000
Number of loans modified | loan	0
Other real estate owned, net	$ 37,036,000		$ 11,400,000	37,036,000		0
Additional OREO amount	25,600,000
Protective advances		$ 0			$ 500,000
Fair Value Carrying Amount	200,000			200,000		200,000
Significant Unobservable Inputs (Level 3)
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss on non-performing loans	36,000			36,000		3,800,000
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	300,000			300,000		3,800,000
Consumer and Other
Financing Receivable, Allowance for Credit Loss [Line Items]
Total	20,839,000			20,839,000		40,754,000
Loans accounted for under the fair value option	8,900,000			8,900,000		14,100,000
PPP Loans | Cash, Securities, and Other
Financing Receivable, Allowance for Credit Loss [Line Items]
Total	$ 2,600,000			$ 2,600,000		$ 4,200,000
Percent of loans on total loan segment	2.10%			2.10%		3.00%
Cash, Securities, and Other
Financing Receivable, Allowance for Credit Loss [Line Items]
Total	$ 119,284,000			$ 119,284,000		$ 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	1
Total, net						$ 5,100,000
MSLP loan | COVID-19
Financing Receivable, Allowance for Credit Loss [Line Items]
Percent of loans on total loan segment	0.80%			0.80%
Total, net	$ 2,000,000			$ 2,000,000
Commercial and Industrial
Financing Receivable, Allowance for Credit Loss [Line Items]
Total	239,830,000			239,830,000		337,180,000
Simmons Bank
Financing Receivable, Allowance for Credit Loss [Line Items]
Total	$ 172,600,000			$ 172,600,000		$ 208,200,000