Cover - shares	3 Months Ended
	Mar. 31, 2020	Apr. 29, 2020
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2020
Document Transition Report	false
Entity File Number	001-36895
Entity Registrant Name	FRANKLIN FINANCIAL NETWORK, INC.
Entity Incorporation, State or Country Code	TN
Entity Tax Identification Number	20-8839445
Entity Address, Address Line One	722 Columbia Avenue
Entity Address, City or Town	Franklin
Entity Address, State or Province	TN
Entity Address, Postal Zip Code	37064
City Area Code	615
Local Phone Number	236-2265
Title of 12(b) Security	Common Stock, no par value
Trading Symbol	FSB
Security Exchange Name	NYSE
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		14,919,433
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
Entity Central Index Key	0001407067
Current Fiscal Year End Date	--12-31