Loans and allowance for credit losses - Analysis of Aging by Class of Financing Receivable (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Loans	$ 7,198,954	$ 7,082,959
90 days or  more and accruing interest	9,098	13,696
Non-accrual loans	50,429	50,760
30-89 days past due
Financing Receivable, Past Due [Line Items]
Loans	10,746	26,995
Loans current on payments and accruing interest
Financing Receivable, Past Due [Line Items]
Loans	7,128,681	6,991,508
Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans	1,238,940	1,346,122
90 days or  more and accruing interest	104	330
Non-accrual loans	15,538	16,005
Commercial and industrial | 30-89 days past due
Financing Receivable, Past Due [Line Items]
Loans	757	3,297
Commercial and industrial | Loans current on payments and accruing interest
Financing Receivable, Past Due [Line Items]
Loans	1,222,541	1,326,490
Construction
Financing Receivable, Past Due [Line Items]
Loans	1,145,165	1,222,220
90 days or  more and accruing interest	487	573
Non-accrual loans	5,303	4,053
Construction | 30-89 days past due
Financing Receivable, Past Due [Line Items]
Loans	1,023	7,607
Construction | Loans current on payments and accruing interest
Financing Receivable, Past Due [Line Items]
Loans	1,138,352	1,209,987
Residential real estate | 1-to-4 family mortgage
Financing Receivable, Past Due [Line Items]
Loans	1,126,623	1,089,270
90 days or  more and accruing interest	8,002	10,470
Non-accrual loans	5,094	5,923
Residential real estate | 1-to-4 family mortgage | 30-89 days past due
Financing Receivable, Past Due [Line Items]
Loans	3,753	7,058
Residential real estate | 1-to-4 family mortgage | Loans current on payments and accruing interest
Financing Receivable, Past Due [Line Items]
Loans	1,109,774	1,065,819
Residential real estate | Residential line of credit
Financing Receivable, Past Due [Line Items]
Loans	401,343	408,211
90 days or  more and accruing interest	48	239
Non-accrual loans	1,189	1,757
Residential real estate | Residential line of credit | 30-89 days past due
Financing Receivable, Past Due [Line Items]
Loans	716	3,551
Residential real estate | Residential line of credit | Loans current on payments and accruing interest
Financing Receivable, Past Due [Line Items]
Loans	399,390	402,664
Residential real estate | Multi-family mortgage
Financing Receivable, Past Due [Line Items]
Loans	363,600	175,676
90 days or  more and accruing interest	0	57
Non-accrual loans	53	0
Residential real estate | Multi-family mortgage | 30-89 days past due
Financing Receivable, Past Due [Line Items]
Loans	0	0
Residential real estate | Multi-family mortgage | Loans current on payments and accruing interest
Financing Receivable, Past Due [Line Items]
Loans	363,547	175,619
Commercial real estate | Owner occupied
Financing Receivable, Past Due [Line Items]
Loans	923,605	924,841
90 days or  more and accruing interest	0	0
Non-accrual loans	8,074	7,948
Commercial real estate | Owner occupied | 30-89 days past due
Financing Receivable, Past Due [Line Items]
Loans	1,100	98
Commercial real estate | Owner occupied | Loans current on payments and accruing interest
Financing Receivable, Past Due [Line Items]
Loans	914,431	916,795
Commercial real estate | Non-owner occupied
Financing Receivable, Past Due [Line Items]
Loans	1,675,214	1,598,979
90 days or  more and accruing interest	0	0
Non-accrual loans	12,016	12,471
Commercial real estate | Non-owner occupied | 30-89 days past due
Financing Receivable, Past Due [Line Items]
Loans	737	915
Commercial real estate | Non-owner occupied | Loans current on payments and accruing interest
Financing Receivable, Past Due [Line Items]
Loans	1,662,461	1,585,593
Consumer and other
Financing Receivable, Past Due [Line Items]
Loans	324,464	317,640
90 days or  more and accruing interest	457	2,027
Non-accrual loans	3,162	2,603
Consumer and other | 30-89 days past due
Financing Receivable, Past Due [Line Items]
Loans	2,660	4,469
Consumer and other | Loans current on payments and accruing interest
Financing Receivable, Past Due [Line Items]
Loans	$ 318,185	$ 308,541