Stockholders' Equity - Company's Significant Subsidiaries (Details) $ in Thousands	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
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	$ 2,964,917	$ 2,948,490
Capital to Risk Weighted Assets	0.144	0.142
Capital Required for Capital Adequacy	$ 1,647,176	$ 1,661,121
Capital Required for Capital Adequacy to Risk Weighted Assets	0.080	0.080
Tier One Risk Based Capital	$ 2,493,799	$ 2,466,874
Tier One Risk Based Capital to Risk Weighted Assets	0.121	0.119
Tier One Risk Based Capital Required for Capital Adequacy	$ 1,236,595	$ 1,243,802
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets	0.060	0.060
Common Equity Tier One Capital	$ 2,493,799	$ 2,466,874
Common Equity Tier One Capital Ratio	0.121	0.119
Common Equity Tier One Capital Required for Capital Adequacy	$ 927,446	$ 932,852
Common Equity Tier One Capital Required for Capital Adequacy to Risk Weighted Assets	4.50%	4.50%
Tier One Leverage Capital	$ 2,493,799	$ 2,466,874
Tier One Leverage Capital to Average Assets	0.094	0.093
Tier One Leverage Capital Required for Capital Adequacy	$ 1,061,191	$ 1,061,021
Tier One Leverage Capital Required for Capital Adequacy to Average Assets	0.040	0.040
Simmons Bank
Class of Stock [Line Items]
Capital	$ 2,834,126	$ 2,743,625
Capital to Risk Weighted Assets	0.138	0.133
Capital Required for Capital Adequacy	$ 1,642,972	$ 1,650,301
Capital Required for Capital Adequacy to Risk Weighted Assets	0.080	0.080
Capital Required to be Well Capitalized	$ 2,053,714	$ 2,062,876
Capital Required to be Well Capitalized to Risk Weighted Assets	0.100	0.100
Tier One Risk Based Capital	$ 2,663,153	$ 2,628,002
Tier One Risk Based Capital to Risk Weighted Assets	0.130	0.127
Tier One Risk Based Capital Required for Capital Adequacy	$ 1,229,148	$ 1,241,576
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	$ 1,638,863	$ 1,655,434
Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets	0.080	0.080
Common Equity Tier One Capital	$ 2,663,153	$ 2,628,002
Common Equity Tier One Capital Ratio	0.130	0.127
Common Equity Tier One Capital Required for Capital Adequacy	$ 921,861	$ 931,182
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	$ 1,331,577	$ 1,345,040
Common Equity Tier One Capital Required to Be Well Capitalized to Risk Weighted Assets	6.50%	6.50%
Tier One Leverage Capital	$ 2,663,153	$ 2,628,002
Tier One Leverage Capital to Average Assets	0.100	0.100
Tier One Leverage Capital Required for Capital Adequacy	$ 1,065,261	$ 1,051,201
Tier One Leverage Capital Required for Capital Adequacy to Average Assets	0.040	0.040
Tier One Leverage Capital Required to be Well Capitalized	$ 1,331,577	$ 1,314,001
Tier One Leverage Capital Required to be Well Capitalized to Average Assets	0.050	0.050