Stockholders' Equity - Company's Significant Subsidiaries (Details) $ in Thousands	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Simmons First National Corporation
Capital	$ 2,356,982	$ 2,272,858
Capital to Risk Weighted Assets	0.168	0.137
Capital Required for Capital Adequacy	$ 1,122,372	$ 1,327,216
Capital Required for Capital Adequacy to Risk Weighted Assets	0.080	0.080
Tier One Risk Based Capital	$ 1,884,562	$ 1,807,954
Tier One Risk Based Capital to Risk Weighted Assets	0.134	0.109
Tier One Risk Based Capital Required for Capital Adequacy	$ 843,834	$ 995,204
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets	0.060	0.060
Common Equity Tier One Capital	$ 1,883,795	$ 1,807,187
Common Equity Tier One Capital Ratio	0.134	0.109
Common Equity Tier One Capital Required for Capital Adequacy	$ 632,618	$ 746,086
Common Equity Tier One Capital Required for Capital Adequacy to Risk Weighted Assets	4.50%	4.50%
Tier One Leverage Capital	$ 1,884,562	$ 1,807,954
Tier One Leverage Capital to Average Assets	0.091	0.096
Tier One Leverage Capital Required for Capital Adequacy	$ 828,379	$ 753,314
Tier One Leverage Capital Required for Capital Adequacy to Average Assets	0.040	0.040
Simmons Bank
Capital	$ 2,136,253	$ 1,852,880
Capital to Risk Weighted Assets	0.153	0.129
Capital Required for Capital Adequacy	$ 1,116,995	$ 1,149,073
Capital Required for Capital Adequacy to Risk Weighted Assets	0.080	0.080
Capital Required to be Well Capitalized	$ 1,396,244	$ 1,436,341
Capital Required to be Well Capitalized to Risk Weighted Assets	0.100	0.100
Tier One Risk Based Capital	$ 2,046,711	$ 1,777,602
Tier One Risk Based Capital to Risk Weighted Assets	0.146	0.123
Tier One Risk Based Capital Required for Capital Adequacy	$ 841,114	$ 867,123
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	$ 1,121,485	$ 1,156,164
Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets	0.080	0.080
Common Equity Tier One Capital	$ 2,046,711	$ 1,777,602
Common Equity Tier One Capital Ratio	0.146	0.123
Common Equity Tier One Capital Required for Capital Adequacy	$ 630,836	$ 650,342
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	$ 911,207	$ 939,383
Common Equity Tier One Capital Required to Be Well Capitalized to Risk Weighted Assets	6.50%	6.50%
Tier One Leverage Capital	$ 2,046,711	$ 1,777,602
Tier One Leverage Capital to Average Assets	0.099	0.107
Tier One Leverage Capital Required for Capital Adequacy	$ 826,954	$ 664,524
Tier One Leverage Capital Required for Capital Adequacy to Average Assets	0.040	0.040
Tier One Leverage Capital Required to be Well Capitalized	$ 1,033,692	$ 830,655
Tier One Leverage Capital Required to be Well Capitalized to Average Assets	0.050	0.050
Landmark Bank (1)
Capital		$ 291,378
Capital to Risk Weighted Assets		0.139
Capital Required for Capital Adequacy		$ 167,700
Capital Required for Capital Adequacy to Risk Weighted Assets		0.080
Capital Required to be Well Capitalized		$ 209,624
Capital Required to be Well Capitalized to Risk Weighted Assets		0.100
Tier One Risk Based Capital		$ 290,016
Tier One Risk Based Capital to Risk Weighted Assets		0.138
Tier One Risk Based Capital Required for Capital Adequacy		$ 126,094
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets		0.060
Tier One Risk Based Capital Required to be Well Capitalized		$ 168,125
Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets		0.080
Common Equity Tier One Capital		$ 270,016
Common Equity Tier One Capital Ratio		0.129
Common Equity Tier One Capital Required for Capital Adequacy		$ 94,192
Common Equity Tier One Capital Required for Capital Adequacy to Risk Weighted Assets		4.50%
Common Equity Tier One Capital Required to be Well-Capitalized		$ 136,055
Common Equity Tier One Capital Required to Be Well Capitalized to Risk Weighted Assets		6.50%
Tier One Leverage Capital		$ 290,016
Tier One Leverage Capital to Average Assets		0.088
Tier One Leverage Capital Required for Capital Adequacy		$ 131,825
Tier One Leverage Capital Required for Capital Adequacy to Average Assets		0.040
Tier One Leverage Capital Required to be Well Capitalized		$ 164,782
Tier One Leverage Capital Required to be Well Capitalized to Average Assets		0.050