Loans and allowance for credit losses - Past Due Loans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Loans	$ 4,568,038	$ 4,409,642	$ 4,409,642
Collectively evaluated:
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning of period non-accrual amortized cost	25,282
End of period non-accrual amortized cost	24,547
Non-accrual with no related allowance	7,486
Related allowance	864
Interest income on non-accrual loans	227
Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	24,547		21,062
30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans	30,010		18,477
90 Days or More and Accruing Interest
Financing Receivable, Past Due [Line Items]
Loans	6,459		5,543
Financing Receivables Current
Financing Receivable, Past Due [Line Items]
Loans	4,507,022		4,307,408
Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans	1,020,484		1,034,036
Commercial and industrial | Collectively evaluated:
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning of period non-accrual amortized cost	5,586
End of period non-accrual amortized cost	3,584
Non-accrual with no related allowance	2,496
Related allowance	185
Interest income on non-accrual loans	152
Commercial and industrial | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	3,584		5,587
Commercial and industrial | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans	5,015		1,918
Commercial and industrial | 90 Days or More and Accruing Interest
Financing Receivable, Past Due [Line Items]
Loans	728		291
Commercial and industrial | Financing Receivables Current
Financing Receivable, Past Due [Line Items]
Loans	1,011,157		1,024,556
Construction
Financing Receivable, Past Due [Line Items]
Loans	599,479		551,101
Construction | Collectively evaluated:
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning of period non-accrual amortized cost	1,254
End of period non-accrual amortized cost	1,439
Non-accrual with no related allowance	1,226
Related allowance	14
Interest income on non-accrual loans	27
Construction | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	1,439		1,087
Construction | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans	6,770		1,021
Construction | 90 Days or More and Accruing Interest
Financing Receivable, Past Due [Line Items]
Loans	183		42
Construction | Financing Receivables Current
Financing Receivable, Past Due [Line Items]
Loans	591,087		546,067
Residential real estate: | 1-to-4 family mortgage
Financing Receivable, Past Due [Line Items]
Loans	743,336		710,454
Residential real estate: | 1-to-4 family mortgage | Collectively evaluated:
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning of period non-accrual amortized cost	4,585
End of period non-accrual amortized cost	5,153
Non-accrual with no related allowance	176
Related allowance	54
Interest income on non-accrual loans	7
Residential real estate: | 1-to-4 family mortgage | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	5,153		3,332
Residential real estate: | Residential line of credit
Financing Receivable, Past Due [Line Items]
Loans	246,527		221,530
Residential real estate: | Residential line of credit | Collectively evaluated:
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning of period non-accrual amortized cost	489
End of period non-accrual amortized cost	600
Non-accrual with no related allowance	151
Related allowance	6
Interest income on non-accrual loans	1
Residential real estate: | Residential line of credit | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	600		416
Residential real estate: | Multi-family mortgage
Financing Receivable, Past Due [Line Items]
Loans	94,638		69,429
Residential real estate: | Multi-family mortgage | Collectively evaluated:
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning of period non-accrual amortized cost	0
End of period non-accrual amortized cost	0
Non-accrual with no related allowance	0
Related allowance	0
Interest income on non-accrual loans	0
Residential real estate: | Multi-family mortgage | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	0		0
Residential real estate: | 30-89 Days Past Due | 1-to-4 family mortgage
Financing Receivable, Past Due [Line Items]
Loans	9,569		10,738
Residential real estate: | 30-89 Days Past Due | Residential line of credit
Financing Receivable, Past Due [Line Items]
Loans	450		658
Residential real estate: | 30-89 Days Past Due | Multi-family mortgage
Financing Receivable, Past Due [Line Items]
Loans	415		63
Residential real estate: | 90 Days or More and Accruing Interest | 1-to-4 family mortgage
Financing Receivable, Past Due [Line Items]
Loans	3,975		3,965
Residential real estate: | 90 Days or More and Accruing Interest | Residential line of credit
Financing Receivable, Past Due [Line Items]
Loans	652		412
Residential real estate: | 90 Days or More and Accruing Interest | Multi-family mortgage
Financing Receivable, Past Due [Line Items]
Loans	0		0
Residential real estate: | Financing Receivables Current | 1-to-4 family mortgage
Financing Receivable, Past Due [Line Items]
Loans	724,639		673,081
Residential real estate: | Financing Receivables Current | Residential line of credit
Financing Receivable, Past Due [Line Items]
Loans	244,825		219,971
Residential real estate: | Financing Receivables Current | Multi-family mortgage
Financing Receivable, Past Due [Line Items]
Loans	94,223		69,366
Commercial real estate: | Owner occupied
Financing Receivable, Past Due [Line Items]
Loans	686,543		630,270
Commercial real estate: | Owner occupied | Collectively evaluated:
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning of period non-accrual amortized cost	2,285
End of period non-accrual amortized cost	1,903
Non-accrual with no related allowance	1,098
Related allowance	79
Interest income on non-accrual loans	21
Commercial real estate: | Owner occupied | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	1,903		1,793
Commercial real estate: | Non-owner occupied
Financing Receivable, Past Due [Line Items]
Loans	910,822		920,744
Commercial real estate: | Non-owner occupied | Collectively evaluated:
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning of period non-accrual amortized cost	9,460
End of period non-accrual amortized cost	9,735
Non-accrual with no related allowance	2,339
Related allowance	442
Interest income on non-accrual loans	19
Commercial real estate: | Non-owner occupied | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	9,735		7,880
Commercial real estate: | 30-89 Days Past Due | Owner occupied
Financing Receivable, Past Due [Line Items]
Loans	2,008		1,375
Commercial real estate: | 30-89 Days Past Due | Non-owner occupied
Financing Receivable, Past Due [Line Items]
Loans	2,931		327
Commercial real estate: | 90 Days or More and Accruing Interest | Owner occupied
Financing Receivable, Past Due [Line Items]
Loans	1		0
Commercial real estate: | 90 Days or More and Accruing Interest | Non-owner occupied
Financing Receivable, Past Due [Line Items]
Loans	32		0
Commercial real estate: | Financing Receivables Current | Owner occupied
Financing Receivable, Past Due [Line Items]
Loans	682,631		621,211
Commercial real estate: | Financing Receivables Current | Non-owner occupied
Financing Receivable, Past Due [Line Items]
Loans	898,124		902,340
Consumer and other
Financing Receivable, Past Due [Line Items]
Loans	266,209		272,078
Consumer and other | Collectively evaluated:
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning of period non-accrual amortized cost	1,623
End of period non-accrual amortized cost	2,133
Non-accrual with no related allowance	0
Related allowance	84
Interest income on non-accrual loans	0
Consumer and other | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	2,133		967
Consumer and other | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans	2,852		2,377
Consumer and other | 90 Days or More and Accruing Interest
Financing Receivable, Past Due [Line Items]
Loans	888		833
Consumer and other | Financing Receivables Current
Financing Receivable, Past Due [Line Items]
Loans	$ 260,336		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