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	Mar. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	$ 15,568	$ 13,716
Collectively evaluated for impairment	3,708,659	3,584,796
Acquired with deteriorated credit quality	62,564	68,999
Loans	3,786,791	3,667,511
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	4,045	1,847
Collectively evaluated for impairment	882,989	863,788
Acquired with deteriorated credit quality	1,311	1,448
Loans	888,345	867,083
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	1,220	1,221
Collectively evaluated for impairment	533,238	549,075
Acquired with deteriorated credit quality	4,607	5,755
Loans	539,065	556,051
Residential real estate: | 1-to-4 family mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	874	987
Collectively evaluated for impairment	533,293	535,451
Acquired with deteriorated credit quality	18,072	19,377
Loans	552,239	555,815
Residential real estate: | Residential line of credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	605	245
Collectively evaluated for impairment	186,810	190,235
Acquired with deteriorated credit quality	0	0
Loans	187,415	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	71,532	75,457
Acquired with deteriorated credit quality	0	0
Loans	71,532	75,457
Commercial real estate: | Owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	2,054	2,608
Collectively evaluated for impairment	491,093	484,900
Acquired with deteriorated credit quality	5,976	6,016
Loans	499,123	493,524
Commercial real estate: | Non-owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	6,699	6,735
Collectively evaluated for impairment	796,644	677,247
Acquired with deteriorated credit quality	13,537	16,266
Loans	816,880	700,248
Consumer and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	71	73
Collectively evaluated for impairment	213,060	208,643
Acquired with deteriorated credit quality	19,061	20,137
Loans	$ 232,192	$ 228,853