Fair Value of Financial Instruments - Schedule of Changes in Fair Value Associated with Commercial Loans Held for Sale (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Change in fair value:
Fair Value Recurring Basis Unobservable Input Reconciliation Asset Gain Loss Statement Of Income Extensible List Not Disclosed Flag	Changes in valuation included in other noninterest income	Changes in valuation included in other noninterest income	Changes in valuation included in other noninterest income	Changes in valuation included in other noninterest income
Franklin Financial Network, Inc. \| Commercial and industrial \| Aggregate Unpaid Principal Balance
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Carrying value at beginning of period	$ 12,232	$ 47,462	$ 34,357	$ 86,762
Change in fair value:
Pay-downs and pay-offs		(3,706)	(22,125)	(43,006)
Write-offs to discount		(8,729)		(8,729)
Changes in valuation included in other noninterest income	0	0	0	0
Carrying value at end of period	12,232	35,027	12,232	35,027
Franklin Financial Network, Inc. \| Commercial and industrial \| Fair Value Discount
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Carrying value at beginning of period	(2,965)	(9,647)	(3,867)	(7,463)
Change in fair value:
Pay-downs and pay-offs		0	0	0
Write-offs to discount		8,729		8,729
Changes in valuation included in other noninterest income	(7)	(387)	895	(2,571)
Carrying value at end of period	(2,972)	(1,305)	(2,972)	(1,305)
Franklin Financial Network, Inc. \| Commercial and industrial \| Fair Value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Carrying value at beginning of period	9,267	37,815	30,490	79,299
Change in fair value:
Pay-downs and pay-offs		(3,706)	(22,125)	(43,006)
Write-offs to discount		0		0
Changes in valuation included in other noninterest income	(7)	(387)	895	(2,571)
Carrying value at end of period	$ 9,260	$ 33,722	$ 9,260	$ 33,722

Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Write-offs To Discount

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- Definition

Fair Value Recurring Basis Unobservable Input Reconciliation Asset Gain Loss Statement Of Income Extensible List Not Disclosed Flag

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No definition available.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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- Definition

Amount of gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-Publisher FASB
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- Definition

Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
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-Subparagraph (c)(2)
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Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)
-SubTopic 10
-Topic 820
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482106/820-10-50-2

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