Document And Entity Information - shares	9 Months Ended
	Sep. 30, 2017	Oct. 31, 2017
Document Information [Line Items]
Entity Registrant Name	SIMMONS FIRST NATIONAL CORP
Entity Central Index Key	0000090498
Trading Symbol	sfnc
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Well-known Seasoned Issuer	Yes
Entity Common Stock, Shares Outstanding (in shares)		45,970,797
Document Type	10-Q
Document Period End Date	Sep. 30, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	Q3
Amendment Flag	false