Parent Company only Condensed Financial Information - Condensed Statements of Income and Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Income Statements, Captions [Line Items]
Interest expense	$ 15,072	$ 18,269	$ 20,088	$ 20,103	$ 19,125	$ 17,155	$ 15,231	$ 12,931	$ 73,532	$ 64,442	$ 35,407
Income tax benefit	(2,970)	2,117	706	334	122	1,068	2,263	2,459	187	5,912	18,531
Net income	$ (4,584)	$ 11,324	$ 5,181	$ 2,901	$ 3,751	$ 10,549	$ 10,169	$ 10,052	14,822	34,521	28,099
Comprehensive income									31,982	25,966	29,997
Franklin Financial Network, Inc.
Condensed Income Statements, Captions [Line Items]
Dividends from subsidiaries									5,500	5,925	4,000
Other income									249	203	171
Interest expense									4,328	4,328	4,321
Other expense									7,614	5,163	2,890
Loss before income tax and undistributed subsidiaries income									(6,193)	(3,363)	(3,040)
Income tax benefit									(2,855)	(2,229)	(2,671)
Equity in undistributed subsidiaries income									18,160	35,655	28,468
Net income									14,822	34,521	28,099
Comprehensive income									$ 31,982	$ 25,966	$ 29,997