Loans and allowance for credit losses - Additional Information (Details) - USD ($)				1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Aug. 15, 2020	Mar. 22, 2020	Feb. 14, 2020	Mar. 31, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Accrued interest receivable					$ 41,797,000		$ 41,797,000
Carrying value of PCI loans									$ 57,152,000
Accretion of interest income							3,080,000	$ 6,030,000
Recorded investment in troubled debt restructurings					16,681,000		16,681,000		12,206,000
Allocation to specific reserves					854,000		854,000		360,000
Non-accrual loans					12,243,000		12,243,000		5,201,000
Payment default for loans modified as troubled debt restructurings					0	$ 0	0	0
Classification of TDR, exemption period, CARES ACT		60 days
Deferred loans that were transfered to normal payment status					1,181,232,000		1,181,232,000
Minimum
Financing Receivable, Past Due [Line Items]
Payment deferral period				60 days
Maximum
Financing Receivable, Past Due [Line Items]
Payment deferral period				90 days
Purchased Credit Impaired
Financing Receivable, Past Due [Line Items]
Carrying value of PCI loans									57,152,000
Purchase accounting liquidity discount									292,000
Purchase accounting non accretable credit discount									3,537,000
Accretion of interest income						1,583,000		5,471,000
Total accretion excluded interest income						$ 2,102,000		$ 6,030,000
Non-Purchased Credit Impaired
Financing Receivable, Past Due [Line Items]
Purchase accounting liquidity discount									3,924,000
Purchase accounting accretable credit discount									8,964,000
Federal Reserve
Financing Receivable, Past Due [Line Items]
Pledged loans to the Federal Reserve Bank					1,378,968,000		1,378,968,000		1,407,662,000
Residential Mortgage Loans | Federal Home Loan Bank of Cincinnati
Financing Receivable, Past Due [Line Items]
Pledge loans for Federal Home Loan Bank Debt					1,083,393,000		1,083,393,000		412,966,000
Commercial Loan | Federal Home Loan Bank of Cincinnati
Financing Receivable, Past Due [Line Items]
Pledge loans for Federal Home Loan Bank Debt					996,582,000		$ 996,582,000		$ 545,540,000
Farmers National
Financing Receivable, Past Due [Line Items]
Financing receivable, allowance for credit loss, acquisition			$ 4,494,000
Franklin Financial Network, Inc.
Financing Receivable, Past Due [Line Items]
Financing receivable, allowance for credit loss, acquisition	$ 77,653,000
Financing receivable, allowance for credit loss, period reduction					$ 7,336,000