Loans and allowance for loan losses - Past Due Loans (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	$ 3,667,511	$ 3,166,911
Purchased Credit Impaired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	68,999	88,835
Non Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	13,685	8,101
30-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	9,208	15,063
90 Days or More and Accruing Interest
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	3,041	1,996
Financing Receivables Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	3,572,578	3,052,916
Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	867,083	715,075
Commercial and industrial | Purchased Credit Impaired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	1,448	2,144
Commercial and industrial | Non Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	6,124	533
Commercial and industrial | 30-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	999	5,859
Commercial and industrial | 90 Days or More and Accruing Interest
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	65	90
Commercial and industrial | Financing Receivables Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	858,447	706,449
Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	556,051	448,326
Construction | Purchased Credit Impaired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	5,755	7,728
Construction | Non Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	283	300
Construction | 30-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	109	1,412
Construction | 90 Days or More and Accruing Interest
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	0	241
Construction | Financing Receivables Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	549,904	438,645
Residential real estate: | 1-to-4 family mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	555,815	480,989
Residential real estate: | 1-to-4 family mortgage | Purchased Credit Impaired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	19,377	23,498
Residential real estate: | 1-to-4 family mortgage | Non Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	2,704	2,548
Residential real estate: | Residential line of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	190,480	194,986
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	804	699
Residential real estate: | Multi-family mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	75,457	62,374
Residential real estate: | Multi-family mortgage | Purchased Credit Impaired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	0	20
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	4,919	4,678
Residential real estate: | 30-89 Days Past Due | Residential line of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	726	527
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	737	956
Residential real estate: | 90 Days or More and Accruing Interest | Residential line of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	957	134
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	528,078	449,309
Residential real estate: | Financing Receivables Current | Residential line of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	187,993	193,626
Residential real estate: | Financing Receivables Current | Multi-family mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	75,457	62,354
Commercial real estate: | Owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	493,524	495,872
Commercial real estate: | Owner occupied | Purchased Credit Impaired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	6,016	11,962
Commercial real estate: | Owner occupied | Non Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	2,423	2,582
Commercial real estate: | Non-owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	700,248	551,588
Commercial real estate: | Non-owner occupied | Purchased Credit Impaired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	16,266	18,164
Commercial real estate: | Non-owner occupied | Non Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	1,199	1,371
Commercial real estate: | 30-89 Days Past Due | Owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	407	521
Commercial real estate: | 30-89 Days Past Due | Non-owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	61	121
Commercial real estate: | 90 Days or More and Accruing Interest | Owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	197	358
Commercial real estate: | 90 Days or More and Accruing Interest | Non-owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	77	0
Commercial real estate: | Financing Receivables Current | Owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	484,481	480,449
Commercial real estate: | Financing Receivables Current | Non-owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	682,645	531,932
Consumer and other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	228,853	217,701
Consumer and other | Purchased Credit Impaired
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	20,137	25,319
Consumer and other | Non Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	148	68
Consumer and other | 30-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	1,987	1,945
Consumer and other | 90 Days or More and Accruing Interest
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	1,008	217
Consumer and other | Financing Receivables Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	$ 205,573	$ 190,152