Loans and allowance for loan losses - Credit Quality Indicators by Portfolio Class (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	$ 4,352,490	$ 3,598,512
Purchased credit impaired loans	57,152	68,999	$ 88,835
Loans	4,409,642	3,667,511	3,166,911
Pass
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	4,143,116	3,458,495
Purchased credit impaired loans	0	0
Loans	4,143,116	3,458,495
Watch
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	142,511	95,857
Purchased credit impaired loans	43,669	46,672
Loans	186,180	142,529
Substandard
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	66,863	44,160
Purchased credit impaired loans	13,483	22,327
Loans	80,346	66,487
Commercial and industrial
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	1,032,352	865,635
Purchased credit impaired loans	1,684	1,448	2,144
Loans	1,034,036	867,083	715,075
Commercial and industrial | Pass
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	946,247	804,447
Purchased credit impaired loans	0	0
Commercial and industrial | Watch
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	66,910	52,624
Purchased credit impaired loans	1,224	964
Commercial and industrial | Substandard
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	19,195	8,564
Purchased credit impaired loans	460	484
Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	548,217	550,296
Purchased credit impaired loans	2,884	5,755	7,728
Loans	551,101	556,051	448,326
Construction | Pass
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	541,201	543,953
Purchased credit impaired loans	0	0
Construction | Watch
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	4,790	5,012
Purchased credit impaired loans	2,681	3,229
Construction | Substandard
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	2,226	1,331
Purchased credit impaired loans	203	2,526
Residential real estate: | 1-to-4 family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	691,116	536,438
Purchased credit impaired loans	19,338	19,377	23,498
Loans	710,454	555,815	480,989
Residential real estate: | Residential line of credit
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	221,457	190,480
Purchased credit impaired loans	73	0	0
Loans	221,530	190,480	194,986
Residential real estate: | Multi-family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	69,429	75,457
Purchased credit impaired loans	0	0	20
Loans	69,429	75,457	62,374
Residential real estate: | Pass | 1-to-4 family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	666,177	519,541
Purchased credit impaired loans	0	0
Residential real estate: | Pass | Residential line of credit
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	218,086	186,753
Purchased credit impaired loans	0	0
Residential real estate: | Pass | Multi-family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	69,366	75,381
Purchased credit impaired loans	0	0
Residential real estate: | Watch | 1-to-4 family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	11,380	8,697
Purchased credit impaired loans	15,091	14,681
Residential real estate: | Watch | Residential line of credit
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	1,343	1,039
Purchased credit impaired loans	0	0
Residential real estate: | Watch | Multi-family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	63	76
Purchased credit impaired loans	0	0
Residential real estate: | Substandard | 1-to-4 family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	13,559	8,200
Purchased credit impaired loans	4,247	4,696
Residential real estate: | Substandard | Residential line of credit
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	2,028	2,688
Purchased credit impaired loans	73	0
Residential real estate: | Substandard | Multi-family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	0	0
Purchased credit impaired loans	0	0
Commercial real estate: | Owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	624,379	487,508
Purchased credit impaired loans	5,891	6,016	11,962
Loans	630,270	493,524	495,872
Commercial real estate: | Non-owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	910,547	683,982
Purchased credit impaired loans	10,197	16,266	18,164
Loans	920,744	700,248	551,588
Commercial real estate: | Pass | Owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	576,737	456,694
Purchased credit impaired loans	0	0
Commercial real estate: | Pass | Non-owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	876,670	667,447
Purchased credit impaired loans	0	0
Commercial real estate: | Watch | Owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	30,379	16,765
Purchased credit impaired loans	4,535	4,110
Commercial real estate: | Watch | Non-owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	24,342	8,881
Purchased credit impaired loans	6,617	8,266
Commercial real estate: | Substandard | Owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	17,263	14,049
Purchased credit impaired loans	1,356	1,906
Commercial real estate: | Substandard | Non-owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	9,535	7,654
Purchased credit impaired loans	3,580	8,000
Consumer and other
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	254,993	208,716
Purchased credit impaired loans	17,085	20,137	25,319
Loans	272,078	228,853	$ 217,701
Consumer and other | Pass
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	248,632	204,279
Purchased credit impaired loans	0	0
Consumer and other | Watch
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	3,304	2,763
Purchased credit impaired loans	13,521	15,422
Consumer and other | Substandard
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans, excluding purchased credit impaired loans	3,057	1,674
Purchased credit impaired loans	$ 3,564	$ 4,715