Loans and allowance for loan losses - Amount of Loans by Loan Category Broken Out Between Loans Individually Evaluated for Impairment and Collectively Evaluated for Impairment (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	$ 15,678	$ 13,716
Collectively evaluated for impairment	4,206,388	3,584,796
Fair value	67,450	68,999
Loans	4,289,516	3,667,511
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	3,914	1,847
Collectively evaluated for impairment	983,542	863,788
Fair value	1,832	1,448
Loans	989,288	867,083
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	1,216	1,221
Collectively evaluated for impairment	520,062	549,075
Fair value	4,676	5,755
Loans	525,954	556,051
Residential real estate: | 1-to-4 family mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	853	987
Collectively evaluated for impairment	665,730	535,451
Fair value	22,401	19,377
Loans	688,984	555,815
Residential real estate: | Residential line of credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	609	245
Collectively evaluated for impairment	217,323	190,235
Fair value	74	0
Loans	218,006	190,480
Residential real estate: | Multi-family mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	82,945	75,457
Fair value	0	0
Loans	82,945	75,457
Commercial real estate: | Owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	1,980	2,608
Collectively evaluated for impairment	594,120	484,900
Fair value	6,623	6,016
Loans	602,723	493,524
Commercial real estate: | Non-owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	6,539	6,735
Collectively evaluated for impairment	902,221	677,247
Fair value	13,390	16,266
Loans	922,150	700,248
Consumer and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	567	73
Collectively evaluated for impairment	240,445	208,643
Fair value	18,454	20,137
Loans	$ 259,466	$ 228,853