Loans and allowance for loan losses - Past Due Loans (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	$ 3,786,791	$ 3,667,511
Purchased Credit Impaired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	62,564	68,999
Non Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	13,721	13,685
30-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	9,044	9,208
90 Days or More and Accruing Interest
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	1,885	3,041
Financing Receivables Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	3,699,577	3,572,578
Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	888,345	867,083
Commercial and industrial | Purchased Credit Impaired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	1,311	1,448
Commercial and industrial | Non Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	342	438
Commercial and industrial | 30-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	2,998	999
Commercial and industrial | 90 Days or More and Accruing Interest
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	37	65
Commercial and industrial | Financing Receivables Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	883,657	864,133
Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	539,065	556,051
Construction | Purchased Credit Impaired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	4,607	5,755
Construction | Non Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	275	283
Construction | 30-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	1,014	109
Construction | 90 Days or More and Accruing Interest
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	0	0
Construction | Financing Receivables Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	533,169	549,904
Residential real estate: | 1-to-4 family mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	552,239	555,815
Residential real estate: | 1-to-4 family mortgage | Purchased Credit Impaired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	18,072	19,377
Residential real estate: | 1-to-4 family mortgage | Non Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	3,107	2,704
Residential real estate: | Residential line of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	187,415	190,480
Residential real estate: | Residential line of credit | Purchased Credit Impaired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	0	0
Residential real estate: | Residential line of credit | Non Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	973	804
Residential real estate: | Multi-family mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	71,532	75,457
Residential real estate: | Multi-family mortgage | Purchased Credit Impaired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	0	0
Residential real estate: | Multi-family mortgage | Non Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	0	0
Residential real estate: | 30-89 Days Past Due | 1-to-4 family mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	1,470	4,919
Residential real estate: | 30-89 Days Past Due | Residential line of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	730	726
Residential real estate: | 30-89 Days Past Due | Multi-family mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	0	0
Residential real estate: | 90 Days or More and Accruing Interest | 1-to-4 family mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	648	737
Residential real estate: | 90 Days or More and Accruing Interest | Residential line of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	487	957
Residential real estate: | 90 Days or More and Accruing Interest | Multi-family mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	0	0
Residential real estate: | Financing Receivables Current | 1-to-4 family mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	528,942	528,078
Residential real estate: | Financing Receivables Current | Residential line of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	185,225	187,993
Residential real estate: | Financing Receivables Current | Multi-family mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	71,532	75,457
Commercial real estate: | Owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	499,123	493,524
Commercial real estate: | Owner occupied | Purchased Credit Impaired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	5,976	6,016
Commercial real estate: | Owner occupied | Non Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	1,671	2,423
Commercial real estate: | Non-owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	816,880	700,248
Commercial real estate: | Non-owner occupied | Purchased Credit Impaired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	13,537	16,266
Commercial real estate: | Non-owner occupied | Non Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	6,973	6,885
Commercial real estate: | 30-89 Days Past Due | Owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	162	407
Commercial real estate: | 30-89 Days Past Due | Non-owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	795	61
Commercial real estate: | 90 Days or More and Accruing Interest | Owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	117	197
Commercial real estate: | 90 Days or More and Accruing Interest | Non-owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	57	77
Commercial real estate: | Financing Receivables Current | Owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	491,197	484,481
Commercial real estate: | Financing Receivables Current | Non-owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	795,518	676,959
Consumer and other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	232,192	228,853
Consumer and other | Purchased Credit Impaired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	19,061	20,137
Consumer and other | Non Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	380	148
Consumer and other | 30-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	1,875	1,987
Consumer and other | 90 Days or More and Accruing Interest
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	539	1,008
Consumer and other | Financing Receivables Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	$ 210,337	$ 205,573