LOANS AND THE ALLOWANCE FOR LOAN LOSSES - Summary of loans (Details) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2021 USD ($) loan	Dec. 31, 2020 USD ($) loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for investment	$ 1,603,383	$ 1,534,185
Deferred fees and unamortized premiums/(unaccreted discounts), net	(333)	(1,352)
Allowance for loan losses	(12,964)	(12,539)
Loans, net	1,590,086	1,520,294
Simmons Bank
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for investment	117,400	127,200
Cash, Securities and Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for investment	293,837	357,020
Construction and Development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for investment	132,141	131,111
1-4 Family Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for investment	502,439	455,038
Non-Owner Occupied CRE
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for investment	358,369	281,943
Owner Occupied CRE
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for investment	167,638	163,042
Commercial and Industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for investment	148,959	146,031
PPP Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for investment	61,900	142,900
PPP Loans \| Cash, Securities and Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for investment	$ 61,900	$ 142,900
Percent of Loans on Total Loan Segment	21.10%	40.00%
MSLP loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for investment	$ 6,800	$ 6,600
COVID-19 \| MSLP loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net	$ 6,800	$ 6,600
Percent of Loans on Total Loan Segment	4.60%	4.50%
Number of loans \| loan	6	6

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Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

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Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements and loans held for sale.

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