Loans and allowance for loan losses - Credit Quality Indicators by Portfolio Class (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	$ 4,282,275	$ 3,598,512
Purchased credit impaired loans	63,069	68,999
Loans	4,345,344	3,667,511
Pass
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	4,104,319	3,458,495
Purchased credit impaired loans	0	0
Loans	4,104,319	3,458,495
Watch
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	117,649	95,857
Purchased credit impaired loans	44,495	46,672
Loans	162,144	142,529
Substandard
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	60,307	44,160
Purchased credit impaired loans	18,574	22,327
Loans	78,881	66,487
Commercial and industrial
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	996,297	865,635
Purchased credit impaired loans	1,624	1,448
Loans	997,921	867,083
Commercial and industrial | Pass
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	930,094	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	49,209	52,624
Purchased credit impaired loans	1,023	964
Commercial and industrial | Substandard
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	16,994	8,564
Purchased credit impaired loans	601	484
Construction
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	534,582	550,296
Purchased credit impaired loans	3,202	5,755
Loans	537,784	556,051
Construction | Pass
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	528,358	543,953
Purchased credit impaired loans	0	0
Construction | Watch
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	4,074	5,012
Purchased credit impaired loans	2,994	3,229
Construction | Substandard
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	2,150	1,331
Purchased credit impaired loans	208	2,526
Residential real estate: | 1-to-4 family mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	689,094	536,438
Purchased credit impaired loans	20,983	19,377
Loans	710,077	555,815
Residential real estate: | Residential line of credit
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	215,420	190,480
Purchased credit impaired loans	73	0
Loans	215,493	190,480
Residential real estate: | Multi-family mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	80,352	75,457
Purchased credit impaired loans	0	0
Loans	80,352	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	666,942	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	212,556	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	80,285	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	8,254	8,697
Purchased credit impaired loans	16,540	14,681
Residential real estate: | Watch | Residential line of credit
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	1,171	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	67	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	13,898	8,200
Purchased credit impaired loans	4,443	4,696
Residential real estate: | Substandard | Residential line of credit
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	1,693	2,688
Purchased credit impaired loans	73	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	614,588	487,508
Purchased credit impaired loans	6,047	6,016
Loans	620,635	493,524
Commercial real estate: | Non-owner occupied
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	902,314	683,982
Purchased credit impaired loans	12,188	16,266
Loans	914,502	700,248
Commercial real estate: | Pass | Owner occupied
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	574,582	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	868,007	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	25,962	16,765
Purchased credit impaired loans	4,244	4,110
Commercial real estate: | Watch | Non-owner occupied
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	25,597	8,881
Purchased credit impaired loans	5,620	8,266
Commercial real estate: | Substandard | Owner occupied
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	14,044	14,049
Purchased credit impaired loans	1,803	1,906
Commercial real estate: | Substandard | Non-owner occupied
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	8,710	7,654
Purchased credit impaired loans	6,568	8,000
Consumer and other
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	249,628	208,716
Purchased credit impaired loans	18,952	20,137
Loans	268,580	228,853
Consumer and other | Pass
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	243,495	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	3,315	2,763
Purchased credit impaired loans	14,074	15,422
Consumer and other | Substandard
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	2,818	1,674
Purchased credit impaired loans	$ 4,878	$ 4,715