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	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	$ 13,716	$ 9,373	$ 12,951
Collectively evaluated for impairment	3,584,796	3,068,703	1,819,775
Acquired with deteriorated credit quality	68,999	88,835	16,058
Gross loans	3,667,511	3,166,911	1,848,784
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	1,847	1,579	1,476
Collectively evaluated for impairment	863,788	711,352	384,279
Acquired with deteriorated credit quality	1,448	2,144	478
Gross loans	867,083	715,075	386,233
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	1,221	1,289	2,686
Collectively evaluated for impairment	549,075	439,309	238,900
Acquired with deteriorated credit quality	5,755	7,728	4,319
Gross loans	556,051	448,326	245,905
Residential real estate: | 1-to-4 family mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	987	1,262	2,471
Collectively evaluated for impairment	535,451	456,229	290,346
Acquired with deteriorated credit quality	19,377	23,498	2,107
Gross loans	555,815	480,989	294,924
Residential real estate: | Residential line of credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	245	0	311
Collectively evaluated for impairment	190,235	194,986	176,879
Acquired with deteriorated credit quality	0	0	0
Gross loans	190,480	194,986	177,190
Residential real estate: | Multi-family mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	0	978	1,027
Collectively evaluated for impairment	75,457	61,376	43,922
Acquired with deteriorated credit quality	0	20	28
Gross loans	75,457	62,374	44,977
Commercial real estate: | Owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	2,608	2,520	2,752
Collectively evaluated for impairment	484,900	481,390	350,812
Acquired with deteriorated credit quality	6,016	11,962	3,782
Gross loans	493,524	495,872	357,346
Commercial real estate: | Non-owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	6,735	1,720	2,201
Collectively evaluated for impairment	677,247	531,704	260,361
Acquired with deteriorated credit quality	16,266	18,164	5,340
Gross loans	700,248	551,588	267,902
Consumer and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	73	25	27
Collectively evaluated for impairment	208,643	192,357	74,276
Acquired with deteriorated credit quality	20,137	25,319	4
Gross loans	$ 228,853	$ 217,701	$ 74,307