Loans and allowance for loan losses - Credit Quality Indicators by Portfolio Class (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	$ 4,222,066	$ 3,598,512
Purchased credit impaired loans	67,450	68,999
Loans	4,289,516	3,667,511
Pass
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	4,061,141	3,458,495
Purchased credit impaired loans	0	0
Loans	4,061,141	3,458,495
Watch
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	111,624	95,857
Purchased credit impaired loans	47,923	46,672
Loans	159,547	142,529
Substandard
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	49,301	44,160
Purchased credit impaired loans	19,527	22,327
Loans	68,828	66,487
Commercial and industrial
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	987,456	865,635
Purchased credit impaired loans	1,832	1,448
Loans	989,288	867,083
Commercial and industrial | Pass
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	919,900	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	57,599	52,624
Purchased credit impaired loans	1,101	964
Commercial and industrial | Substandard
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	9,957	8,564
Purchased credit impaired loans	731	484
Construction
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	521,278	550,296
Purchased credit impaired loans	4,676	5,755
Loans	525,954	556,051
Construction | Pass
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	512,525	543,953
Purchased credit impaired loans	0	0
Construction | Watch
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	7,426	5,012
Purchased credit impaired loans	3,747	3,229
Construction | Substandard
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	1,327	1,331
Purchased credit impaired loans	929	2,526
Residential real estate: | 1-to-4 family mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	666,583	536,438
Purchased credit impaired loans	22,401	19,377
Loans	688,984	555,815
Residential real estate: | Residential line of credit
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	217,932	190,480
Purchased credit impaired loans	74	0
Loans	218,006	190,480
Residential real estate: | Multi-family mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	82,945	75,457
Purchased credit impaired loans	0	0
Loans	82,945	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	646,636	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	213,362	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	82,876	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	7,341	8,697
Purchased credit impaired loans	17,365	14,681
Residential real estate: | Watch | Residential line of credit
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	1,864	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	69	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	12,606	8,200
Purchased credit impaired loans	5,036	4,696
Residential real estate: | Substandard | Residential line of credit
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	2,706	2,688
Purchased credit impaired loans	74	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	596,100	487,508
Purchased credit impaired loans	6,623	6,016
Loans	602,723	493,524
Commercial real estate: | Non-owner occupied
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	908,760	683,982
Purchased credit impaired loans	13,390	16,266
Loans	922,150	700,248
Commercial real estate: | Pass | Owner occupied
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	559,544	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	890,293	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	23,251	16,765
Purchased credit impaired loans	4,613	4,110
Commercial real estate: | Watch | Non-owner occupied
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	11,418	8,881
Purchased credit impaired loans	5,520	8,266
Commercial real estate: | Substandard | Owner occupied
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	13,305	14,049
Purchased credit impaired loans	2,010	1,906
Commercial real estate: | Substandard | Non-owner occupied
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	7,049	7,654
Purchased credit impaired loans	7,870	8,000
Consumer and other
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	241,012	208,716
Purchased credit impaired loans	18,454	20,137
Loans	259,466	228,853
Consumer and other | Pass
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	236,005	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	2,656	2,763
Purchased credit impaired loans	15,577	15,422
Consumer and other | Substandard
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	2,351	1,674
Purchased credit impaired loans	$ 2,877	$ 4,715