Loans and allowance for loan losses - Credit Quality Indicators by Portfolio Class (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	$ 3,724,227	$ 3,598,512
Purchased credit impaired loans	62,564	68,999
Loans	3,786,791	3,667,511
Pass
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	3,591,133	3,458,495
Purchased credit impaired loans	0	0
Loans	3,591,133	3,458,495
Watch
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	90,911	95,857
Purchased credit impaired loans	44,001	46,672
Loans	134,912	142,529
Substandard
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	42,183	44,160
Purchased credit impaired loans	18,563	22,327
Loans	60,746	66,487
Commercial and industrial
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	887,034	865,635
Purchased credit impaired loans	1,311	1,448
Loans	888,345	867,083
Commercial and industrial | Pass
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	830,017	804,447
Purchased credit impaired loans	0	0
Commercial and industrial | Watch
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	49,599	52,624
Purchased credit impaired loans	828	964
Commercial and industrial | Substandard
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	7,418	8,564
Purchased credit impaired loans	483	484
Construction
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	534,458	550,296
Purchased credit impaired loans	4,607	5,755
Loans	539,065	556,051
Construction | Pass
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	528,421	543,953
Purchased credit impaired loans	0	0
Construction | Watch
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	4,717	5,012
Purchased credit impaired loans	3,719	3,229
Construction | Substandard
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	1,320	1,331
Purchased credit impaired loans	888	2,526
Residential real estate: | 1-to-4 family mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	534,167	536,438
Purchased credit impaired loans	18,072	19,377
Loans	552,239	555,815
Residential real estate: | Residential line of credit
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	187,415	190,480
Purchased credit impaired loans	0	0
Loans	187,415	190,480
Residential real estate: | Multi-family mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	71,532	75,457
Purchased credit impaired loans	0	0
Loans	71,532	75,457
Residential real estate: | Pass | 1-to-4 family mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	519,688	519,541
Purchased credit impaired loans	0	0
Residential real estate: | Pass | Residential line of credit
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	183,448	186,753
Purchased credit impaired loans	0	0
Residential real estate: | Pass | Multi-family mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	71,460	75,381
Purchased credit impaired loans	0	0
Residential real estate: | Watch | 1-to-4 family mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	6,046	8,697
Purchased credit impaired loans	13,644	14,681
Residential real estate: | Watch | Residential line of credit
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	1,337	1,039
Purchased credit impaired loans	0	0
Residential real estate: | Watch | Multi-family mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	72	76
Purchased credit impaired loans	0	0
Residential real estate: | Substandard | 1-to-4 family mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	8,433	8,200
Purchased credit impaired loans	4,428	4,696
Residential real estate: | Substandard | Residential line of credit
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	2,630	2,688
Purchased credit impaired loans	0	0
Residential real estate: | Substandard | Multi-family mortgage
Financing Receivable, Allowance for Credit Losses [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 Losses [Line Items]
Loans, excluding purchased credit impaired loans	493,147	487,508
Purchased credit impaired loans	5,976	6,016
Loans	499,123	493,524
Commercial real estate: | Non-owner occupied
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	803,343	683,982
Purchased credit impaired loans	13,537	16,266
Loans	816,880	700,248
Commercial real estate: | Pass | Owner occupied
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	463,724	456,694
Purchased credit impaired loans	0	0
Commercial real estate: | Pass | Non-owner occupied
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	785,933	667,447
Purchased credit impaired loans	0	0
Commercial real estate: | Watch | Owner occupied
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	15,857	16,765
Purchased credit impaired loans	4,080	4,110
Commercial real estate: | Watch | Non-owner occupied
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	10,301	8,881
Purchased credit impaired loans	5,608	8,266
Commercial real estate: | Substandard | Owner occupied
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	13,566	14,049
Purchased credit impaired loans	1,896	1,906
Commercial real estate: | Substandard | Non-owner occupied
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	7,109	7,654
Purchased credit impaired loans	7,929	8,000
Consumer and other
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	213,131	208,716
Purchased credit impaired loans	19,061	20,137
Loans	232,192	228,853
Consumer and other | Pass
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	208,442	204,279
Purchased credit impaired loans	0	0
Consumer and other | Watch
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	2,982	2,763
Purchased credit impaired loans	16,122	15,422
Consumer and other | Substandard
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	1,707	1,674
Purchased credit impaired loans	$ 2,939	$ 4,715