Leases - Narrative (Details) ft² in Thousands, $ in Thousands	9 Months Ended
	Sep. 30, 2020 USD ($) operatingLease branch lease_renew_option	Dec. 31, 2022 USD ($) ft²	Aug. 15, 2020 USD ($)	Dec. 31, 2019 USD ($)
Lessee, Lease, Description [Line Items]
Lessee, number of operating leases | operatingLease	60
Lessee, number of finance leases | branch	1
Lessee, number of total operating leases, noncurrent | operatingLease	54
Lessee, number of total finance leases, noncurrent | branch	1
Lessee, operating and finance lease, number of options to renew | lease_renew_option	1
Operating lease right-of-use assets	$ 52,410			$ 32,539
Operating lease liabilities	56,705			35,525
Right-of-use assets - finance	1,616			$ 0
Franklin Financial Network, Inc.
Lessee, Lease, Description [Line Items]
Operating lease right-of-use assets			$ 24,330
Operating lease liabilities			24,330
Right-of-use assets - finance			$ 1,630
Operating lease, impairment loss	$ 1,038
Forecast | Corporate headquarters
Lessee, Lease, Description [Line Items]
Operating lease right-of-use assets		$ 29,000
Operating lease liabilities		$ 30,000
Net rentable area (square feet) | ft²		52
Minimum
Lessee, Lease, Description [Line Items]
Lessee, operating and finance lease, term of contract	1 year
Lessee, operating and finance lease, renewal term	20 years
Maximum
Lessee, Lease, Description [Line Items]
Lessee, operating and finance lease, term of contract	36 years