Basis of presentation - Narrative (Details)	12 Months Ended
	Dec. 31, 2021 USD ($) branch method	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of full-service branches | branch	82
Provision for credit losses on available-for-sale securities	$ 0	$ 0
Transfer from loans held for sale to loan portfolio	86,315,000	55,766,000	$ 12,259,000
Transfer of from loan portfolio to held for sale	$ 10,408,000	11,483,000	7,981,000
Percent of remaining principal allowed to buy back under GNMA optional repurchase programs	100.00%
Other noninterest income	$ 60,690,000	46,527,000	34,481,000
Goodwill impairment	0	0
Amounts related to uncertain tax positions	0	0	0
Impairment of long-lived assets	$ 0	0
Number of methods to deliver mortgage loans | method	2
Deposits with Federal Home Loan Banks	$ 17,130,000	12,729,000
FHLB lender risk account guaranty	$ 8,372,000	6,183,000
Lender risk account, contribution percentage	1
Lender risk account, period to release portion of funds not used to cover losses	30 years
Lender risk account, initial fill-up period	5 years
Residential mortgage
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Transfer of from loan portfolio to held for sale	$ 1,188,000	$ 2,116,000
PSUs
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Criteria period		3 years
Franklin Financial Network, Inc.
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other noninterest income	11,172,000	$ 3,228,000
Mortgage Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net (losses) gains from fair value changes of mortgage loans	(16,976,000)	24,233,000	(2,861,000)
GNMA
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Transfer of from loan portfolio to held for sale	9,220,000	9,367,000	7,891,000
Delinquent GNMA loans that had been previously sold	94,648,000	151,184,000
Residential Mortgage Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Transfer from loans held for sale to loan portfolio	86,315,000	55,766,000	$ 12,259,000
FHLB Cincinnati
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deposits with Federal Home Loan Banks	$ 17,130,000	$ 12,729,000
Leasehold improvements | Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, plant and equipment, useful life	20 years
Premises
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, plant and equipment, useful life	40 years
Furniture and fixtures | Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, plant and equipment, useful life	7 years
Furniture and fixtures | Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, plant and equipment, useful life	10 years
Equipment | Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, plant and equipment, useful life	3 years
Equipment | Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, plant and equipment, useful life	7 years