Stockholders' Equity - Company's Significant Subsidiaries (Details) $ in Thousands	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Simmons First National Corporation
Class of Stock [Line Items]
Capital	$ 2,603,142	$ 2,356,982
Capital to Risk Weighted Assets	0.167	0.168
Capital Required for Capital Adequacy	$ 1,247,014	$ 1,122,372
Capital Required for Capital Adequacy to Risk Weighted Assets	0.080	0.080
Tier One Risk Based Capital	$ 2,147,158	$ 1,884,562
Tier One Risk Based Capital to Risk Weighted Assets	0.138	0.134
Tier One Risk Based Capital Required for Capital Adequacy	$ 933,547	$ 843,834
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets	0.060	0.060
Common Equity Tier One Capital	$ 2,147,158	$ 1,883,795
Common Equity Tier One Capital Ratio	0.138	0.134
Common Equity Tier One Capital Required for Capital Adequacy	$ 700,160	$ 632,618
Common Equity Tier One Capital Required for Capital Adequacy to Risk Weighted Assets	4.50%	4.50%
Tier One Leverage Capital	$ 2,147,158	$ 1,884,562
Tier One Leverage Capital to Average Assets	0.091	0.091
Tier One Leverage Capital Required for Capital Adequacy	$ 943,806	$ 828,379
Tier One Leverage Capital Required for Capital Adequacy to Average Assets	0.040	0.040
Simmons Bank
Class of Stock [Line Items]
Capital	$ 2,389,704	$ 2,136,253
Capital to Risk Weighted Assets	0.154	0.153
Capital Required for Capital Adequacy	$ 1,241,405	$ 1,116,995
Capital Required for Capital Adequacy to Risk Weighted Assets	0.080	0.080
Capital Required to be Well Capitalized	$ 1,551,756	$ 1,396,244
Capital Required to be Well Capitalized to Risk Weighted Assets	0.100	0.100
Tier One Risk Based Capital	$ 2,317,855	$ 2,046,711
Tier One Risk Based Capital to Risk Weighted Assets	0.150	0.146
Tier One Risk Based Capital Required for Capital Adequacy	$ 927,142	$ 841,114
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	$ 1,236,189	$ 1,121,485
Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets	0.080	0.080
Common Equity Tier One Capital	$ 2,317,855	$ 2,046,711
Common Equity Tier One Capital Ratio	0.150	0.146
Common Equity Tier One Capital Required for Capital Adequacy	$ 695,357	$ 630,836
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	$ 1,004,404	$ 911,207
Common Equity Tier One Capital Required to Be Well Capitalized to Risk Weighted Assets	6.50%	6.50%
Tier One Leverage Capital	$ 2,317,855	$ 2,046,711
Tier One Leverage Capital to Average Assets	0.098	0.099
Tier One Leverage Capital Required for Capital Adequacy	$ 946,063	$ 826,954
Tier One Leverage Capital Required for Capital Adequacy to Average Assets	0.040	0.040
Tier One Leverage Capital Required to be Well Capitalized	$ 1,182,579	$ 1,033,692
Tier One Leverage Capital Required to be Well Capitalized to Average Assets	0.050	0.050