Borrowings - Subordinated Debt (Details)	9 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($) issue	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2003 USD ($) trust
Debt Instrument [Line Items]
Issuance of subordinated debt	$ 100,000,000	$ 0
Net proceeds from sale of common stock under employee stock purchase program	$ 978,000	804,000
Period for loss of tier 2 capital treatment	5 years
Subordinated debt
Debt Instrument [Line Items]
Number of separate trusts | trust				2
Subordinated debt | Franklin Financial Network, Inc.
Debt Instrument [Line Items]
Debt instrument, face amount	$ 60,000,000
Preferred securities issued to form the trust		60,000,000
Number of issues | issue	2
Subordinated debt | Minimum | Franklin Financial Network, Inc.
Debt Instrument [Line Items]
Debt instrument, interest rate, phase two period	6 years
Subordinated debt | Maximum | Franklin Financial Network, Inc.
Debt Instrument [Line Items]
Debt instrument, interest rate, phase two period	10 years
Subordinated debt | Floating Rate Trust Preferred Securities Trust
Debt Instrument [Line Items]
Preferred securities issued to form the trust		30,000,000	$ 30,000,000
Distribution payment period of occurrence of a special event	120 days
Subordinated debt | Floating Rate Trust Preferred Securities Trust I
Debt Instrument [Line Items]
Preferred securities issued to form the trust				$ 9,000,000
Issuance of subordinated debt				9,280,000
Net proceeds from sale of common stock under employee stock purchase program				280,000
Debt instrument, interest rate	3.56%		5.19%
Subordinated debt | Floating Rate Trust Preferred Securities Trust I | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	3.25%
Subordinated debt | Floating Rate Trust Preferred Securities Trust II
Debt Instrument [Line Items]
Preferred securities issued to form the trust				21,000,000
Issuance of subordinated debt				21,650,000
Net proceeds from sale of common stock under employee stock purchase program				$ 650,000
Debt instrument, interest rate	3.43%		5.10%
Subordinated debt | Floating Rate Trust Preferred Securities Trust II | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	3.15%
Subordinated debt | Fixed To Floating Rate Note
Debt Instrument [Line Items]
Debt instrument, face amount	$ 100,000,000
Preferred securities issued to form the trust		$ 100,000,000
Debt instrument, basis spread on variable rate	4.39%
Debt instrument, interest rate	4.50%
Long-term debt, term	10 years
Debt instrument, interest rate, phase one period	5 years
Percentage of tier 2 capital treatment lost	20.00%
Subordinated debt | Fixed To Floating Rate Note | Minimum
Debt Instrument [Line Items]
Debt instrument, interest rate, phase two period	6 years
Subordinated debt | Fixed To Floating Rate Note | Maximum
Debt Instrument [Line Items]
Debt instrument, interest rate, phase two period	10 years
Subordinated debt | March 2026 Subordinated Notes | Franklin Financial Network, Inc.
Debt Instrument [Line Items]
Preferred securities issued to form the trust	$ 40,000,000
Debt instrument, interest rate	6.875%
Subordinated debt | March 2026 Subordinated Notes | London Interbank Offered Rate (LIBOR) | Franklin Financial Network, Inc.
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	5.636%
Subordinated debt | July 2026 Subordinated Notes
Debt Instrument [Line Items]
Preferred securities issued to form the trust	$ 20,000
Debt instrument, interest rate	7.00%
Subordinated debt | July 2026 Subordinated Notes | London Interbank Offered Rate (LIBOR) | Franklin Financial Network, Inc.
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	6.04%