Loans and allowance for credit losses - Past Due Loans (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Loans	$ 4,827,023	$ 4,827,023	$ 4,409,642	$ 4,409,642
Collectively Evaluated
Financing Receivable, Past Due [Line Items]
Non-accrual amortized cost				25,282
Non-accrual with no related allowance		8,741
Non-accrual with related allowance		19,672
Related allowance		2,314
Interest income on non-accrual loans	205	432
Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	28,413	28,413		21,062
30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans	11,324	11,324		18,477
90 Days or More and Accruing Interest
Financing Receivable, Past Due [Line Items]
Loans	6,412	6,412		5,543
Financing Receivables Current
Financing Receivable, Past Due [Line Items]
Loans	4,780,874	4,780,874		4,307,408
Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans	1,289,646	1,289,646		1,034,036
Commercial and industrial | Collectively Evaluated
Financing Receivable, Past Due [Line Items]
Non-accrual amortized cost				5,586
Non-accrual with no related allowance		1,098
Non-accrual with related allowance		1,210
Related allowance		350
Interest income on non-accrual loans	17	169
Commercial and industrial | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	2,308	2,308		5,587
Commercial and industrial | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans	1,057	1,057		1,918
Commercial and industrial | 90 Days or More and Accruing Interest
Financing Receivable, Past Due [Line Items]
Loans	111	111		291
Commercial and industrial | Financing Receivables Current
Financing Receivable, Past Due [Line Items]
Loans	1,286,170	1,286,170		1,024,556
Construction
Financing Receivable, Past Due [Line Items]
Loans	553,619	553,619		551,101
Construction | Collectively Evaluated
Financing Receivable, Past Due [Line Items]
Non-accrual amortized cost				1,254
Non-accrual with no related allowance		1,218
Non-accrual with related allowance		264
Related allowance		47
Interest income on non-accrual loans	6	33
Construction | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	1,482	1,482		1,087
Construction | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans	769	769		1,021
Construction | 90 Days or More and Accruing Interest
Financing Receivable, Past Due [Line Items]
Loans	465	465		42
Construction | Financing Receivables Current
Financing Receivable, Past Due [Line Items]
Loans	550,903	550,903		546,067
Residential real estate: | 1-to-4 family mortgage
Financing Receivable, Past Due [Line Items]
Loans	741,936	741,936		710,454
Residential real estate: | 1-to-4 family mortgage | Collectively Evaluated
Financing Receivable, Past Due [Line Items]
Non-accrual amortized cost				4,585
Non-accrual with no related allowance		1,789
Non-accrual with related allowance		3,603
Related allowance		68
Interest income on non-accrual loans	6	13
Residential real estate: | 1-to-4 family mortgage | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	5,392	5,392		3,332
Residential real estate: | Residential line of credit
Financing Receivable, Past Due [Line Items]
Loans	236,974	236,974		221,530
Residential real estate: | Residential line of credit | Collectively Evaluated
Financing Receivable, Past Due [Line Items]
Non-accrual amortized cost				489
Non-accrual with no related allowance		151
Non-accrual with related allowance		553
Related allowance		12
Interest income on non-accrual loans	0	1
Residential real estate: | Residential line of credit | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	704	704		416
Residential real estate: | Multi-family mortgage
Financing Receivable, Past Due [Line Items]
Loans	115,149	115,149		69,429
Residential real estate: | Multi-family mortgage | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	0	0		0
Residential real estate: | 30-89 Days Past Due | 1-to-4 family mortgage
Financing Receivable, Past Due [Line Items]
Loans	5,658	5,658		10,738
Residential real estate: | 30-89 Days Past Due | Residential line of credit
Financing Receivable, Past Due [Line Items]
Loans	398	398		658
Residential real estate: | 30-89 Days Past Due | Multi-family mortgage
Financing Receivable, Past Due [Line Items]
Loans	58	58		63
Residential real estate: | 90 Days or More and Accruing Interest | 1-to-4 family mortgage
Financing Receivable, Past Due [Line Items]
Loans	4,712	4,712		3,965
Residential real estate: | 90 Days or More and Accruing Interest | Residential line of credit
Financing Receivable, Past Due [Line Items]
Loans	277	277		412
Residential real estate: | 90 Days or More and Accruing Interest | Multi-family mortgage
Financing Receivable, Past Due [Line Items]
Loans	0	0		0
Residential real estate: | Financing Receivables Current | 1-to-4 family mortgage
Financing Receivable, Past Due [Line Items]
Loans	726,174	726,174		673,081
Residential real estate: | Financing Receivables Current | Residential line of credit
Financing Receivable, Past Due [Line Items]
Loans	235,595	235,595		219,971
Residential real estate: | Financing Receivables Current | Multi-family mortgage
Financing Receivable, Past Due [Line Items]
Loans	115,091	115,091		69,366
Commercial real estate: | Owner occupied
Financing Receivable, Past Due [Line Items]
Loans	683,245	683,245		630,270
Commercial real estate: | Owner occupied | Collectively Evaluated
Financing Receivable, Past Due [Line Items]
Non-accrual amortized cost				2,285
Non-accrual with no related allowance		1,723
Non-accrual with related allowance		921
Related allowance		78
Interest income on non-accrual loans	43	64
Commercial real estate: | Owner occupied | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	2,644	2,644		1,793
Commercial real estate: | Non-owner occupied
Financing Receivable, Past Due [Line Items]
Loans	923,192	923,192		920,744
Commercial real estate: | Non-owner occupied | Collectively Evaluated
Financing Receivable, Past Due [Line Items]
Non-accrual amortized cost				9,460
Non-accrual with no related allowance		2,762
Non-accrual with related allowance		10,351
Related allowance		1,625
Interest income on non-accrual loans	109	128
Commercial real estate: | Non-owner occupied | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	13,113	13,113		7,880
Commercial real estate: | 30-89 Days Past Due | Owner occupied
Financing Receivable, Past Due [Line Items]
Loans	43	43		1,375
Commercial real estate: | 30-89 Days Past Due | Non-owner occupied
Financing Receivable, Past Due [Line Items]
Loans	169	169		327
Commercial real estate: | 90 Days or More and Accruing Interest | Owner occupied
Financing Receivable, Past Due [Line Items]
Loans	0	0		0
Commercial real estate: | 90 Days or More and Accruing Interest | Non-owner occupied
Financing Receivable, Past Due [Line Items]
Loans	226	226		0
Commercial real estate: | Financing Receivables Current | Owner occupied
Financing Receivable, Past Due [Line Items]
Loans	680,558	680,558		621,211
Commercial real estate: | Financing Receivables Current | Non-owner occupied
Financing Receivable, Past Due [Line Items]
Loans	909,684	909,684		902,340
Consumer and other
Financing Receivable, Past Due [Line Items]
Loans	283,262	283,262		272,078
Consumer and other | Collectively Evaluated
Financing Receivable, Past Due [Line Items]
Non-accrual amortized cost				1,623
Non-accrual with no related allowance		0
Non-accrual with related allowance		2,770
Related allowance		134
Interest income on non-accrual loans	24	24
Consumer and other | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	2,770	2,770		967
Consumer and other | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans	3,172	3,172		2,377
Consumer and other | 90 Days or More and Accruing Interest
Financing Receivable, Past Due [Line Items]
Loans	621	621		833
Consumer and other | Financing Receivables Current
Financing Receivable, Past Due [Line Items]
Loans	$ 276,699	$ 276,699		250,816
Purchased Credit Impaired
Financing Receivable, Past Due [Line Items]
Loans				57,152
Purchased Credit Impaired | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans				1,684
Purchased Credit Impaired | Construction
Financing Receivable, Past Due [Line Items]
Loans				2,884
Purchased Credit Impaired | Residential real estate: | 1-to-4 family mortgage
Financing Receivable, Past Due [Line Items]
Loans				19,338
Purchased Credit Impaired | Residential real estate: | Residential line of credit
Financing Receivable, Past Due [Line Items]
Loans				73
Purchased Credit Impaired | Residential real estate: | Multi-family mortgage
Financing Receivable, Past Due [Line Items]
Loans				0
Purchased Credit Impaired | Commercial real estate: | Owner occupied
Financing Receivable, Past Due [Line Items]
Loans				5,891
Purchased Credit Impaired | Commercial real estate: | Non-owner occupied
Financing Receivable, Past Due [Line Items]
Loans				10,197
Purchased Credit Impaired | Consumer and other
Financing Receivable, Past Due [Line Items]
Loans				$ 17,085