Parent Company only Condensed Financial Information - Condensed Statements of Cash Flows (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
Cash flows from operating activities
Net income	$ (4,584)	$ 11,324	$ 5,181	$ 2,901	$ 3,751	$ 10,549	$ 10,169	$ 10,052	$ 14,822	$ 34,521	$ 28,099
Adjustments:
Amortization of debt issuance costs									179	178	178
Stock-based compensation									5,917	6,569	2,802
Net cash from operating activities									23,811	50,307	55,638
Cash flows from investing activities
Net cash acquired from acquisition (See Note 2)									0	24,660	0
Net cash from investing activities									366,753	(219,532)	(759,628)
Cash flows from financing activities
Proceeds from exercise of common stock warrants									0	0	150
Proceeds from exercise of common stock options									5,307	3,047	1,615
Proceeds from issuance of common stock, net of offering costs									0	(242)	0
Divestment of common stock issued to 401(k) plan									0	(308)	(256)
Purchase of Treasury Stock									717	0	0
Noncontrolling interest distributions									16	16	16
Net cash from financing activities									(435,785)	197,894	864,606
Net change in cash and cash equivalents									(45,221)	28,669	160,616
Cash and cash equivalents at beginning of period				280,212				251,543	280,212	251,543	90,927
Cash and cash equivalents at end of period	234,991				280,212				234,991	280,212	251,543
Franklin Financial Network, Inc.
Cash flows from operating activities
Net income									14,822	34,521	28,099
Adjustments:
Equity in undistributed subsidiaries income									(18,160)	(35,655)	(28,468)
Amortization of debt issuance costs									179	178	178
Stock-based compensation									851	1,008	219
Change in other assets									(208)	(1,367)	728
Change in other liabilities									839	(1,908)	686
Net cash from operating activities									(1,677)	(3,223)	1,442
Cash flows from investing activities
Investments in subsidiaries									0	(26,512)	(1,359)
Net cash acquired from acquisition (See Note 2)									0
Net cash from investing activities									0	(1,852)	(1,359)
Cash flows from financing activities
Proceeds from exercise of common stock warrants									0	0	150
Proceeds from exercise of common stock options									5,307	3,047	1,615
Proceeds from issuance of common stock, net of offering costs									0	(242)	0
Divestment of common stock issued to 401(k) plan									0	(308)	(256)
Redemption of Series A preferred stock									0	0	0
Purchase of Treasury Stock									717	0	0
Dividends paid on common stock									(2,636)	0	0
Noncontrolling interest distributions									16	16	0
Net cash from financing activities									1,938	2,481	1,509
Net change in cash and cash equivalents									261	(2,594)	1,592
Cash and cash equivalents at beginning of period				$ 3,364				$ 5,958	3,364	5,958	4,366
Cash and cash equivalents at end of period	$ 3,625				$ 3,364				3,625	3,364	5,958
Non-cash supplemental information:
Transfers from subsidiary stock based compensation expense to parent company only additional paid-in capital									$ 5,066	$ 5,561	$ 2,583