Cover - USD ($)	12 Months Ended
	Dec. 31, 2019	Mar. 06, 2020	Jun. 28, 2019
Cover page.
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2019
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 per share
Trading Symbol	FSB
Security Exchange Name	NYSE
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
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 Public Float			$ 431,382,862
Entity Common Stock, Shares Outstanding		14,859,704
Documents Incorporated by Reference	None.
Amendment Flag	false
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Entity Central Index Key	0001407067
Current Fiscal Year End Date	--12-31