LOANS AND THE ALLOWANCE FOR LOAN LOSSES - Summary of loans (Details) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2021 USD ($) loan	Dec. 31, 2020 USD ($) loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for investment	$ 1,573,015	$ 1,534,185
Deferred fees and unamortized premiums/(unaccreted discounts), net	(1,955)	(1,352)
Allowance for loan losses	(12,552)	(12,539)
Loans, net	1,558,508	1,520,294
Simmons Bank
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for investment	116,100	127,200
Cash, Securities and Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for investment	290,907	357,020
Construction and Development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for investment	127,141	131,111
1-4 Family Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for investment	496,101	455,038
Non-Owner Occupied CRE
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for investment	324,493	281,943
Owner Occupied CRE
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for investment	178,847	163,042
Commercial and Industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for investment	155,526	146,031
PPP Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for investment	103,100	142,900
PPP Loans | Cash, Securities and Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for investment	$ 103,100	$ 142,900
Percent of Loans on Total Loan Segment	35.50%	40.00%
MSLP loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans held for investment	$ 6,700	$ 6,600
COVID-19 | MSLP loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net	$ 6,700	$ 6,600
Percent of Loans on Total Loan Segment	4.30%	4.50%
Number of loans | loan	6	6