Loans and allowance for loan losses - Past Due Loans (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	$ 4,345,344	$ 3,667,511
Purchased Credit Impaired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	63,069	68,999
Non Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	17,911	13,685
30-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	15,450	9,208
30-89 Days Past Due | Purchased Credit Impaired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	3,598	3,605
90 Days or More and Accruing Interest
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	2,452	3,041
90 Days or More and Accruing Interest | Purchased Credit Impaired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	754	4,076
Financing Receivables Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	4,246,462	3,572,578
Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	997,921	867,083
Commercial and industrial | Purchased Credit Impaired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	1,624	1,448
Commercial and industrial | Non Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	383	438
Commercial and industrial | 30-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	3,892	999
Commercial and industrial | 90 Days or More and Accruing Interest
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	4	65
Commercial and industrial | Financing Receivables Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	992,018	864,133
Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	537,784	556,051
Construction | Purchased Credit Impaired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	3,202	5,755
Construction | Non Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	1,107	283
Construction | 30-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	327	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	533,148	549,904
Residential real estate: | 1-to-4 family mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	710,077	555,815
Residential real estate: | 1-to-4 family mortgage | Purchased Credit Impaired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	20,983	19,377
Residential real estate: | 1-to-4 family mortgage | Non Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	7,260	2,704
Residential real estate: | Residential line of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	215,493	190,480
Residential real estate: | Residential line of credit | Purchased Credit Impaired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	73	0
Residential real estate: | Residential line of credit | Non Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	421	804
Residential real estate: | Multi-family mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	80,352	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	5,552	4,919
Residential real estate: | 30-89 Days Past Due | Residential line of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	475	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	1,316	737
Residential real estate: | 90 Days or More and Accruing Interest | Residential line of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	298	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	674,966	528,078
Residential real estate: | Financing Receivables Current | Residential line of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	214,226	187,993
Residential real estate: | Financing Receivables Current | Multi-family mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	80,352	75,457
Commercial real estate: | Owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	620,635	493,524
Commercial real estate: | Owner occupied | Purchased Credit Impaired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	6,047	6,016
Commercial real estate: | Owner occupied | Non Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	1,274	2,423
Commercial real estate: | Non-owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	914,502	700,248
Commercial real estate: | Non-owner occupied | Purchased Credit Impaired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	12,188	16,266
Commercial real estate: | Non-owner occupied | Non Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	6,596	6,885
Commercial real estate: | 30-89 Days Past Due | Owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	153	407
Commercial real estate: | 30-89 Days Past Due | Non-owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	2,605	61
Commercial real estate: | 90 Days or More and Accruing Interest | Owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	0	197
Commercial real estate: | 90 Days or More and Accruing Interest | Non-owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	5	77
Commercial real estate: | Financing Receivables Current | Owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	613,161	484,481
Commercial real estate: | Financing Receivables Current | Non-owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	893,108	676,959
Consumer and other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	268,580	228,853
Consumer and other | Purchased Credit Impaired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	18,952	20,137
Consumer and other | Non Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	870	148
Consumer and other | 30-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	2,446	1,987
Consumer and other | 90 Days or More and Accruing Interest
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	829	1,008
Consumer and other | Financing Receivables Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	$ 245,483	$ 205,573