Right-Of-Use Lease Assets and Lease Liabilities - Lease Expense and Supplemental Information (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Leases [Abstract]
Right-of-use lease assets	$ 32,528	$ 40,675
Lease liabilities	$ 32,804	$ 40,854
Weighted average remaining lease term	8 years 7 months 6 days	8 years 4 months 13 days
Weighted average discount rate	3.26%	3.27%
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	sfnc:PropertyPlantandEquipmentandRightofUseAssetafterAccumulatedDepreciationandAmortization	sfnc:PropertyPlantandEquipmentandRightofUseAssetafterAccumulatedDepreciationandAmortization
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:AccountsPayableAndAccruedLiabilitiesCurrentAndNoncurrent	us-gaap:AccountsPayableAndAccruedLiabilitiesCurrentAndNoncurrent