Loans and allowance for loan losses - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Financing Receivable, Recorded Investment, Past Due [Line Items]
Carrying value of PCI loans	$ 63,069,000		$ 63,069,000		$ 68,999,000
Accretion of interest income			6,030,000	$ 5,745,000
Non-accrual loans	3,211,000		3,211,000		2,703,000
Loans	4,345,344,000		4,345,344,000		3,667,511,000
Recorded investment in troubled debt restructurings	11,460,000		11,460,000		6,794,000
Allocation to specific reserves	171,000		171,000		63,000
Payment default for loans modified as troubled debt restructurings	0	$ 0	0	0
Purchased Credit Impaired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Carrying value of PCI loans	63,069,000		63,069,000		68,999,000
Purchase accounting liquidity discount			781,000		2,436,000
Purchase accounting non accretable credit discount			4,331,000		4,355,000
Accretion of interest income	1,583,000	2,103,000	5,471,000	6,943,000
Total purchase accounting contribution through accretion for purchased loans	2,102,000	$ 2,130,000	6,030,000	$ 5,745,000
Non-accrual loans	0		0		0
Loans	63,069,000		63,069,000		68,999,000
Non-Purchased Credit Impaired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Purchase accounting liquidity discount			4,483,000		2,197,000
Purchase accounting accretable credit discount			10,075,000		7,527,000
Federal Reserve
Financing Receivable, Recorded Investment, Past Due [Line Items]
Pledged loans to the Federal Reserve Bank	1,403,511,000		1,403,511,000		1,336,092,000
Residential Mortgage Loans | Federal Home Loan Bank of Cincinnati
Financing Receivable, Recorded Investment, Past Due [Line Items]
Pledge loans for Federal Home Loan Bank Debt	527,351,000		527,351,000		618,976,000
Commercial Loan | Federal Home Loan Bank of Cincinnati
Financing Receivable, Recorded Investment, Past Due [Line Items]
Pledge loans for Federal Home Loan Bank Debt	521,754,000		521,754,000		608,735,000
30-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	15,450,000		15,450,000		9,208,000
30-89 Days Past Due | Purchased Credit Impaired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	3,598,000		3,598,000		3,605,000
90 Days or More and Accruing Interest
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	2,452,000		2,452,000		3,041,000
90 Days or More and Accruing Interest | Purchased Credit Impaired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	$ 754,000		$ 754,000		$ 4,076,000