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v3.20.2

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% |  |  |

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