Stockholders' Equity - Company's Significant Subsidiaries (Details) $ in Thousands	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)
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	$ 2,992,132	$ 2,964,917
Capital to Risk Weighted Assets	0.146	0.144
Capital Required for Capital Adequacy	$ 1,639,524	$ 1,647,176
Capital Required for Capital Adequacy to Risk Weighted Assets	0.080	0.080
Tier One Risk Based Capital	$ 2,535,527	$ 2,493,799
Tier One Risk Based Capital to Risk Weighted Assets	0.124	0.121
Tier One Risk Based Capital Required for Capital Adequacy	$ 1,226,868	$ 1,236,595
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets	0.060	0.060
Common Equity Tier One Capital	$ 2,535,527	$ 2,493,799
Common Equity Tier One Capital Ratio	0.124	0.121
Common Equity Tier One Capital Required for Capital Adequacy	$ 920,151	$ 927,446
Common Equity Tier One Capital Required for Capital Adequacy to Risk Weighted Assets	4.50%	4.50%
Tier One Leverage Capital	$ 2,535,527	$ 2,493,799
Tier One Leverage Capital to Average Assets	0.097	0.094
Tier One Leverage Capital Required for Capital Adequacy	$ 1,045,578	$ 1,061,191
Tier One Leverage Capital Required for Capital Adequacy to Average Assets	0.040	0.040
Simmons Bank
Class of Stock [Line Items]
Capital	$ 2,795,430	$ 2,834,126
Capital to Risk Weighted Assets	0.137	0.138
Capital Required for Capital Adequacy	$ 1,632,368	$ 1,642,972
Capital Required for Capital Adequacy to Risk Weighted Assets	0.080	0.080
Capital Required to be Well Capitalized	$ 2,040,460	$ 2,053,714
Capital Required to be Well Capitalized to Risk Weighted Assets	0.100	0.100
Tier One Risk Based Capital	$ 2,573,121	$ 2,663,153
Tier One Risk Based Capital to Risk Weighted Assets	0.126	0.130
Tier One Risk Based Capital Required for Capital Adequacy	$ 1,225,296	$ 1,229,148
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	$ 1,633,728	$ 1,638,863
Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets	0.080	0.080
Common Equity Tier One Capital	$ 2,573,121	$ 2,663,153
Common Equity Tier One Capital Ratio	0.126	0.130
Common Equity Tier One Capital Required for Capital Adequacy	$ 918,972	$ 921,861
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	$ 1,327,404	$ 1,331,577
Common Equity Tier One Capital Required to Be Well Capitalized to Risk Weighted Assets	6.50%	6.50%
Tier One Leverage Capital	$ 2,573,121	$ 2,663,153
Tier One Leverage Capital to Average Assets	0.099	0.100
Tier One Leverage Capital Required for Capital Adequacy	$ 1,039,645	$ 1,065,261
Tier One Leverage Capital Required for Capital Adequacy to Average Assets	0.040	0.040
Tier One Leverage Capital Required to be Well Capitalized	$ 1,299,556	$ 1,331,577
Tier One Leverage Capital Required to be Well Capitalized to Average Assets	0.050	0.050