Stockholders' Equity - Company's Significant Subsidiaries (Details) $ in Thousands	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Class of Stock [Line Items]
Capital Required to be Well Capitalized to Risk Weighted Assets	0.10
Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets	0.08
Tier One Leverage Capital Required to be Well Capitalized to Average Assets	0.05
Simmons First National Corporation
Class of Stock [Line Items]
Capital	$ 2,948,490	$ 2,603,142
Capital to Risk Weighted Assets	0.142	0.167
Capital Required for Capital Adequacy	$ 1,661,121	$ 1,247,014
Capital Required for Capital Adequacy to Risk Weighted Assets	0.080	0.080
Tier One Risk Based Capital	$ 2,466,874	$ 2,147,158
Tier One Risk Based Capital to Risk Weighted Assets	0.119	0.138
Tier One Risk Based Capital Required for Capital Adequacy	$ 1,243,802	$ 933,547
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets	0.060	0.060
Common Equity Tier One Capital	$ 2,466,874	$ 2,147,158
Common Equity Tier One Capital Ratio	0.119	0.138
Common Equity Tier One Capital Required for Capital Adequacy	$ 932,852	$ 700,160
Common Equity Tier One Capital Required for Capital Adequacy to Risk Weighted Assets	4.50%	4.50%
Tier One Leverage Capital	$ 2,466,874	$ 2,147,158
Tier One Leverage Capital to Average Assets	0.093	0.091
Tier One Leverage Capital Required for Capital Adequacy	$ 1,061,021	$ 943,806
Tier One Leverage Capital Required for Capital Adequacy to Average Assets	0.040	0.040
Simmons Bank
Class of Stock [Line Items]
Capital	$ 2,743,625	$ 2,389,704
Capital to Risk Weighted Assets	0.133	0.154
Capital Required for Capital Adequacy	$ 1,650,301	$ 1,241,405
Capital Required for Capital Adequacy to Risk Weighted Assets	0.080	0.080
Capital Required to be Well Capitalized	$ 2,062,876	$ 1,551,756
Capital Required to be Well Capitalized to Risk Weighted Assets	0.100	0.100
Tier One Risk Based Capital	$ 2,628,002	$ 2,317,855
Tier One Risk Based Capital to Risk Weighted Assets	0.127	0.150
Tier One Risk Based Capital Required for Capital Adequacy	$ 1,241,576	$ 927,142
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets	0.060	0.060
Tier One Risk Based Capital Required to be Well Capitalized	$ 1,655,434	$ 1,236,189
Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets	0.080	0.080
Common Equity Tier One Capital	$ 2,628,002	$ 2,317,855
Common Equity Tier One Capital Ratio	0.127	0.150
Common Equity Tier One Capital Required for Capital Adequacy	$ 931,182	$ 695,357
Common Equity Tier One Capital Required for Capital Adequacy to Risk Weighted Assets	4.50%	4.50%
Common Equity Tier One Capital Required to be Well-Capitalized	$ 1,345,040	$ 1,004,404
Common Equity Tier One Capital Required to Be Well Capitalized to Risk Weighted Assets	6.50%	6.50%
Tier One Leverage Capital	$ 2,628,002	$ 2,317,855
Tier One Leverage Capital to Average Assets	0.100	0.098
Tier One Leverage Capital Required for Capital Adequacy	$ 1,051,201	$ 946,063
Tier One Leverage Capital Required for Capital Adequacy to Average Assets	0.040	0.040
Tier One Leverage Capital Required to be Well Capitalized	$ 1,314,001	$ 1,182,579
Tier One Leverage Capital Required to be Well Capitalized to Average Assets	0.050	0.050