Loans and allowance for loan losses - Credit Quality Indicators by Portfolio Class (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	$ 3,598,512	$ 3,078,076
Purchased credit impaired loans	68,999	88,835	$ 16,058
Total loans	3,667,511	3,166,911	1,848,784
Pass
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	3,458,495	2,940,174
Purchased credit impaired loans	0	0
Total loans	3,458,495	2,940,174
Watch
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	95,857	114,401
Purchased credit impaired loans	46,672	56,848
Total loans	142,529	171,249
Substandard
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	44,160	23,501
Purchased credit impaired loans	22,327	31,987
Total loans	66,487	55,488
Commercial and industrial
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	865,635	712,931
Purchased credit impaired loans	1,448	2,144	478
Total loans	867,083	715,075	386,233
Commercial and industrial | Pass
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	804,447	657,595
Purchased credit impaired loans	0	0
Commercial and industrial | Watch
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	52,624	50,946
Purchased credit impaired loans	964	1,499
Commercial and industrial | Substandard
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	8,564	4,390
Purchased credit impaired loans	484	645
Construction
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	550,296	440,598
Purchased credit impaired loans	5,755	7,728	4,319
Total loans	556,051	448,326	245,905
Construction | Pass
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	543,953	431,242
Purchased credit impaired loans	0	0
Construction | Watch
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	5,012	7,388
Purchased credit impaired loans	3,229	3,324
Construction | Substandard
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	1,331	1,968
Purchased credit impaired loans	2,526	4,404
Residential real estate: | 1-to-4 family mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	536,438	457,491
Purchased credit impaired loans	19,377	23,498	2,107
Total loans	555,815	480,989	294,924
Residential real estate: | Residential line of credit
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	190,480	194,986
Purchased credit impaired loans	0	0	0
Total loans	190,480	194,986	177,190
Residential real estate: | Multi-family mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	75,457	62,354
Purchased credit impaired loans	0	20	28
Total loans	75,457	62,374	44,977
Residential real estate: | Pass | 1-to-4 family mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	519,541	440,202
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	186,753	192,427
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	75,381	61,234
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	8,697	9,522
Purchased credit impaired loans	14,681	20,284
Residential real estate: | Watch | Residential line of credit
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	1,039	1,184
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	76	142
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	8,200	7,767
Purchased credit impaired loans	4,696	3,214
Residential real estate: | Substandard | Residential line of credit
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	2,688	1,375
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	978
Purchased credit impaired loans	0	20
Commercial real estate: | Owner occupied
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	487,508	483,910
Purchased credit impaired loans	6,016	11,962	3,782
Total loans	493,524	495,872	357,346
Commercial real estate: | Non-owner occupied
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	683,982	533,424
Purchased credit impaired loans	16,266	18,164	5,340
Total loans	700,248	551,588	267,902
Commercial real estate: | Pass | Owner occupied
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	456,694	451,140
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	667,447	517,253
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	16,765	28,308
Purchased credit impaired loans	4,110	4,631
Commercial real estate: | Watch | Non-owner occupied
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	8,881	14,199
Purchased credit impaired loans	8,266	7,359
Commercial real estate: | Substandard | Owner occupied
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	14,049	4,462
Purchased credit impaired loans	1,906	7,331
Commercial real estate: | Substandard | Non-owner occupied
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	7,654	1,972
Purchased credit impaired loans	8,000	10,805
Consumer and other
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	208,716	192,382
Purchased credit impaired loans	20,137	25,319	4
Total loans	228,853	217,701	$ 74,307
Consumer and other | Pass
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	204,279	189,081
Purchased credit impaired loans	0	0
Consumer and other | Watch
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	2,763	2,712
Purchased credit impaired loans	15,422	19,751
Consumer and other | Substandard
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, excluding purchased credit impaired loans	1,674	589
Purchased credit impaired loans	$ 4,715	$ 5,568