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	Sep. 30, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	$ 18,267	$ 13,716
Collectively evaluated for impairment	4,264,008	3,584,796
Fair value	63,069	68,999
Loans	4,345,344	3,667,511
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	3,834	1,847
Collectively evaluated for impairment	992,463	863,788
Fair value	1,624	1,448
Loans	997,921	867,083
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	2,061	1,221
Collectively evaluated for impairment	532,521	549,075
Fair value	3,202	5,755
Loans	537,784	556,051
Residential real estate: | 1-to-4 family mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	1,346	987
Collectively evaluated for impairment	687,748	535,451
Fair value	20,983	19,377
Loans	710,077	555,815
Residential real estate: | Residential line of credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	245	245
Collectively evaluated for impairment	215,175	190,235
Fair value	73	0
Loans	215,493	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	80,352	75,457
Fair value	0	0
Loans	80,352	75,457
Commercial real estate: | Owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	2,476	2,608
Collectively evaluated for impairment	612,112	484,900
Fair value	6,047	6,016
Loans	620,635	493,524
Commercial real estate: | Non-owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	7,846	6,735
Collectively evaluated for impairment	894,468	677,247
Fair value	12,188	16,266
Loans	914,502	700,248
Consumer and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	459	73
Collectively evaluated for impairment	249,169	208,643
Fair value	18,952	20,137
Loans	$ 268,580	$ 228,853