Loans and allowance for credit losses - Allocation of Allowance for Loan Losses by Loan Category Broken Out Between Loans Individually Evaluated for Impairment and Collectively Evaluated for Impairment (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment			$ 895
Collectively evaluated for impairment			28,571
Acquired with deteriorated credit quality			1,673
Ending balance	$ 89,141	$ 31,139	31,139	$ 29,814	$ 28,932
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment			241
Collectively evaluated for impairment			4,457
Acquired with deteriorated credit quality			107
Ending balance	10,881		4,805	5,514	5,348
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment			0
Collectively evaluated for impairment			10,192
Acquired with deteriorated credit quality			2
Ending balance	22,842		10,194	9,758	9,729
Residential real estate: | 1-to-4 family mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment			8
Collectively evaluated for impairment			2,940
Acquired with deteriorated credit quality			164
Ending balance	13,006		3,112	3,295	3,428
Residential real estate: | Residential line of credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment			9
Collectively evaluated for impairment			743
Acquired with deteriorated credit quality			0
Ending balance	6,213		752	731	811
Residential real estate: | Multi-family mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment			0
Collectively evaluated for impairment			544
Acquired with deteriorated credit quality			0
Ending balance	2,328		544	539	566
Commercial real estate: | Owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment			238
Collectively evaluated for impairment			3,853
Acquired with deteriorated credit quality			18
Ending balance	9,047		4,109	3,098	3,132
Commercial real estate: | Non-owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment			399
Collectively evaluated for impairment			3,909
Acquired with deteriorated credit quality			313
Ending balance	18,005		4,621	4,583	4,149
Consumer and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment			0
Collectively evaluated for impairment			1,933
Acquired with deteriorated credit quality			1,069
Ending balance	$ 6,819		$ 3,002	$ 2,296	$ 1,769