Loans and allowance for loan losses - Past Due Loans (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Past Due [Line Items]
Loans	$ 4,409,642	$ 3,667,511
Purchased Credit Impaired
Financing Receivable, Past Due [Line Items]
Loans	57,152	68,999
Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	21,062	13,685
30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans	18,477	9,208
30-89 Days Past Due | Purchased Credit Impaired
Financing Receivable, Past Due [Line Items]
Loans	2,951	3,605
90 Days or More and Accruing Interest
Financing Receivable, Past Due [Line Items]
Loans	5,543	3,041
90 Days or More and Accruing Interest | Purchased Credit Impaired
Financing Receivable, Past Due [Line Items]
Loans	751	4,076
Financing Receivables Current
Financing Receivable, Past Due [Line Items]
Loans	4,307,408	3,572,578
Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans	1,034,036	867,083
Commercial and industrial | Purchased Credit Impaired
Financing Receivable, Past Due [Line Items]
Loans	1,684	1,448
Commercial and industrial | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	5,587	438
Commercial and industrial | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans	1,918	999
Commercial and industrial | 90 Days or More and Accruing Interest
Financing Receivable, Past Due [Line Items]
Loans	291	65
Commercial and industrial | Financing Receivables Current
Financing Receivable, Past Due [Line Items]
Loans	1,024,556	864,133
Construction
Financing Receivable, Past Due [Line Items]
Loans	551,101	556,051
Construction | Purchased Credit Impaired
Financing Receivable, Past Due [Line Items]
Loans	2,884	5,755
Construction | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	1,087	283
Construction | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans	1,021	109
Construction | 90 Days or More and Accruing Interest
Financing Receivable, Past Due [Line Items]
Loans	42	0
Construction | Financing Receivables Current
Financing Receivable, Past Due [Line Items]
Loans	546,067	549,904
Residential real estate: | 1-to-4 family mortgage
Financing Receivable, Past Due [Line Items]
Loans	710,454	555,815
Residential real estate: | 1-to-4 family mortgage | Purchased Credit Impaired
Financing Receivable, Past Due [Line Items]
Loans	19,338	19,377
Residential real estate: | 1-to-4 family mortgage | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	3,332	2,704
Residential real estate: | Residential line of credit
Financing Receivable, Past Due [Line Items]
Loans	221,530	190,480
Residential real estate: | Residential line of credit | Purchased Credit Impaired
Financing Receivable, Past Due [Line Items]
Loans	73	0
Residential real estate: | Residential line of credit | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	416	804
Residential real estate: | Multi-family mortgage
Financing Receivable, Past Due [Line Items]
Loans	69,429	75,457
Residential real estate: | Multi-family mortgage | Purchased Credit Impaired
Financing Receivable, Past Due [Line Items]
Loans	0	0
Residential real estate: | Multi-family mortgage | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	0	0
Residential real estate: | 30-89 Days Past Due | 1-to-4 family mortgage
Financing Receivable, Past Due [Line Items]
Loans	10,738	4,919
Residential real estate: | 30-89 Days Past Due | Residential line of credit
Financing Receivable, Past Due [Line Items]
Loans	658	726
Residential real estate: | 30-89 Days Past Due | Multi-family mortgage
Financing Receivable, Past Due [Line Items]
Loans	63	0
Residential real estate: | 90 Days or More and Accruing Interest | 1-to-4 family mortgage
Financing Receivable, Past Due [Line Items]
Loans	3,965	737
Residential real estate: | 90 Days or More and Accruing Interest | Residential line of credit
Financing Receivable, Past Due [Line Items]
Loans	412	957
Residential real estate: | 90 Days or More and Accruing Interest | Multi-family mortgage
Financing Receivable, Past Due [Line Items]
Loans	0	0
Residential real estate: | Financing Receivables Current | 1-to-4 family mortgage
Financing Receivable, Past Due [Line Items]
Loans	673,081	528,078
Residential real estate: | Financing Receivables Current | Residential line of credit
Financing Receivable, Past Due [Line Items]
Loans	219,971	187,993
Residential real estate: | Financing Receivables Current | Multi-family mortgage
Financing Receivable, Past Due [Line Items]
Loans	69,366	75,457
Commercial real estate: | Owner occupied
Financing Receivable, Past Due [Line Items]
Loans	630,270	493,524
Commercial real estate: | Owner occupied | Purchased Credit Impaired
Financing Receivable, Past Due [Line Items]
Loans	5,891	6,016
Commercial real estate: | Owner occupied | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	1,793	2,423
Commercial real estate: | Non-owner occupied
Financing Receivable, Past Due [Line Items]
Loans	920,744	700,248
Commercial real estate: | Non-owner occupied | Purchased Credit Impaired
Financing Receivable, Past Due [Line Items]
Loans	10,197	16,266
Commercial real estate: | Non-owner occupied | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	7,880	6,885
Commercial real estate: | 30-89 Days Past Due | Owner occupied
Financing Receivable, Past Due [Line Items]
Loans	1,375	407
Commercial real estate: | 30-89 Days Past Due | Non-owner occupied
Financing Receivable, Past Due [Line Items]
Loans	327	61
Commercial real estate: | 90 Days or More and Accruing Interest | Owner occupied
Financing Receivable, Past Due [Line Items]
Loans	0	197
Commercial real estate: | 90 Days or More and Accruing Interest | Non-owner occupied
Financing Receivable, Past Due [Line Items]
Loans	0	77
Commercial real estate: | Financing Receivables Current | Owner occupied
Financing Receivable, Past Due [Line Items]
Loans	621,211	484,481
Commercial real estate: | Financing Receivables Current | Non-owner occupied
Financing Receivable, Past Due [Line Items]
Loans	902,340	676,959
Consumer and other
Financing Receivable, Past Due [Line Items]
Loans	272,078	228,853
Consumer and other | Purchased Credit Impaired
Financing Receivable, Past Due [Line Items]
Loans	17,085	20,137
Consumer and other | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	967	148
Consumer and other | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans	2,377	1,987
Consumer and other | 90 Days or More and Accruing Interest
Financing Receivable, Past Due [Line Items]
Loans	833	1,008
Consumer and other | Financing Receivables Current
Financing Receivable, Past Due [Line Items]
Loans	$ 250,816	$ 205,573