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, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	$ 23,810	$ 13,716	$ 9,373
Collectively evaluated for impairment	4,328,680	3,584,796	3,068,703
Fair value	57,152	68,999	88,835
Loans	4,409,642	3,667,511	3,166,911
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	9,026	1,847	1,579
Collectively evaluated for impairment	1,023,326	863,788	711,352
Fair value	1,684	1,448	2,144
Loans	1,034,036	867,083	715,075
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	2,061	1,221	1,289
Collectively evaluated for impairment	546,156	549,075	439,309
Fair value	2,884	5,755	7,728
Loans	551,101	556,051	448,326
Residential real estate: | 1-to-4 family mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	1,347	987	1,262
Collectively evaluated for impairment	689,769	535,451	456,229
Fair value	19,338	19,377	23,498
Loans	710,454	555,815	480,989
Residential real estate: | Residential line of credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	579	245	0
Collectively evaluated for impairment	220,878	190,235	194,986
Fair value	73	0	0
Loans	221,530	190,480	194,986
Residential real estate: | Multi-family mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	0	0	978
Collectively evaluated for impairment	69,429	75,457	61,376
Fair value	0	0	20
Loans	69,429	75,457	62,374
Commercial real estate: | Owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	2,993	2,608	2,520
Collectively evaluated for impairment	621,386	484,900	481,390
Fair value	5,891	6,016	11,962
Loans	630,270	493,524	495,872
Commercial real estate: | Non-owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	7,755	6,735	1,720
Collectively evaluated for impairment	902,792	677,247	531,704
Fair value	10,197	16,266	18,164
Loans	920,744	700,248	551,588
Consumer and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	49	73	25
Collectively evaluated for impairment	254,944	208,643	192,357
Fair value	17,085	20,137	25,319
Loans	$ 272,078	$ 228,853	$ 217,701