Loans and allowance for loan losses - Additional Information (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Past Due [Line Items]
Carrying value of PCI loans	$ 57,152,000	$ 57,152,000	$ 68,999,000	$ 88,835,000
Accretion of interest income		8,556,000	7,608,000	5,419,000
Non-accrual loans	5,201,000	5,201,000	2,703,000
Loans	4,409,642,000	4,409,642,000	3,667,511,000
Recorded investment in troubled debt restructurings	12,206,000	12,206,000	6,794,000
Allocation to specific reserves	360,000	360,000	63,000
Payment default for loans modified as troubled debt restructurings	0	0	0	0
Purchased Credit Impaired
Financing Receivable, Past Due [Line Items]
Carrying value of PCI loans	57,152,000	57,152,000	68,999,000
Purchase accounting liquidity discount		292,000	2,436,000
Purchase accounting non accretable credit discount		3,537,000	4,355,000
Accretion of interest income		7,003,000	9,010,000	5,299,000
Total purchase accounting contribution through accretion for purchased loans		8,556,000	7,608,000	$ 5,419,000
Non-accrual loans	0	0	0
Loans	57,152,000	57,152,000	68,999,000
Non-Purchased Credit Impaired
Financing Receivable, Past Due [Line Items]
Purchase accounting liquidity discount		3,924,000	2,197,000
Purchase accounting accretable credit discount		8,964,000	7,527,000
Federal Reserve
Financing Receivable, Past Due [Line Items]
Pledged loans to the Federal Reserve Bank	1,407,662,000	1,407,662,000	1,336,092,000
Residential Mortgage Loans | Federal Home Loan Bank of Cincinnati
Financing Receivable, Past Due [Line Items]
Pledge loans for Federal Home Loan Bank Debt	412,966,000	412,966,000	618,976,000
Commercial Loan | Federal Home Loan Bank of Cincinnati
Financing Receivable, Past Due [Line Items]
Pledge loans for Federal Home Loan Bank Debt	545,540,000	545,540,000	608,735,000
30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans	18,477,000	18,477,000	9,208,000
30-89 Days Past Due | Purchased Credit Impaired
Financing Receivable, Past Due [Line Items]
Loans	2,951,000	2,951,000	3,605,000
90 Days or More and Accruing Interest
Financing Receivable, Past Due [Line Items]
Loans	5,543,000	5,543,000	3,041,000
90 Days or More and Accruing Interest | Purchased Credit Impaired
Financing Receivable, Past Due [Line Items]
Loans	$ 751,000	$ 751,000	$ 4,076,000