Loans and allowance for credit losses - Amortized Cost Basis by Origination Year (Details) $ in Thousands	Sep. 30, 2020 USD ($)
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	$ 1,251,350
2019	1,363,764
2018	961,417
2017	768,142
2016	680,590
Prior	968,015
Revolving Loans Amortized Cost Basis	1,220,260
Total	7,213,538
Commercial and industrial
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	403,183
2019	192,294
2018	93,221
2017	63,417
2016	45,955
Prior	58,974
Revolving Loans Amortized Cost Basis	560,627
Total	1,417,671
Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	312,399
2019	450,222
2018	120,550
2017	63,799
2016	43,935
Prior	84,079
Revolving Loans Amortized Cost Basis	115,894
Total	1,190,878
Residential real estate: \| 1-to-4 family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	234,037
2019	205,439
2018	195,063
2017	187,433
2016	125,791
Prior	192,848
Revolving Loans Amortized Cost Basis	0
Total	1,140,611
Residential real estate: \| Residential line of credit
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	363
2019	1
2018	87
2017	3
2016	208
Prior	3,577
Revolving Loans Amortized Cost Basis	416,079
Total	420,318
Residential real estate: \| Multi-family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	23,747
2019	15,055
2018	12,629
2017	54,660
2016	11,718
Prior	36,869
Revolving Loans Amortized Cost Basis	11,259
Total	165,937
Residential real estate: \| Non-owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	108,229
2019	255,182
2018	378,336
2017	245,958
2016	315,094
Prior	298,551
Revolving Loans Amortized Cost Basis	43,050
Total	1,644,400
Commercial real estate:
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	103,851
2019	186,859
2018	116,870
2017	122,085
2016	92,172
Prior	249,038
Revolving Loans Amortized Cost Basis	54,112
Total	924,987
Consumer and other
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	65,541
2019	58,712
2018	44,661
2017	30,787
2016	45,717
Prior	44,079
Revolving Loans Amortized Cost Basis	19,239
Total	308,736
Pass
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	1,240,589
2019	1,340,084
2018	890,485
2017	672,770
2016	622,002
Prior	835,442
Revolving Loans Amortized Cost Basis	1,154,858
Total	6,756,230
Pass \| Commercial and industrial
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	399,906
2019	186,100
2018	69,916
2017	53,213
2016	33,976
Prior	45,860
Revolving Loans Amortized Cost Basis	511,111
Total	1,300,082
Pass \| Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	311,117
2019	448,019
2018	110,473
2017	49,845
2016	41,750
Prior	75,570
Revolving Loans Amortized Cost Basis	115,894
Total	1,152,668
Pass \| Residential real estate: \| 1-to-4 family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	232,248
2019	203,167
2018	188,471
2017	170,725
2016	119,327
Prior	168,732
Revolving Loans Amortized Cost Basis	0
Total	1,082,670
Pass \| Residential real estate: \| Residential line of credit
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	254
2019	0
2018	87
2017	0
2016	208
Prior	3,577
Revolving Loans Amortized Cost Basis	405,767
Total	409,893
Pass \| Residential real estate: \| Multi-family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	23,747
2019	15,055
2018	12,629
2017	54,660
2016	11,718
Prior	36,812
Revolving Loans Amortized Cost Basis	11,259
Total	165,880
Pass \| Residential real estate: \| Non-owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	108,229
2019	248,852
2018	360,309
2017	218,431
2016	291,626
Prior	250,806
Revolving Loans Amortized Cost Basis	43,005
Total	1,521,258
Pass \| Commercial real estate:
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	99,827
2019	181,214
2018	106,212
2017	97,689
2016	81,383
Prior	221,365
Revolving Loans Amortized Cost Basis	49,382
Total	837,072
Pass \| Consumer and other
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	65,261
2019	57,677
2018	42,388
2017	28,207
2016	42,014
Prior	32,720
Revolving Loans Amortized Cost Basis	18,440
Total	286,707
Watch
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	9,530
2019	16,282
2018	33,490
2017	80,772
2016	49,430
Prior	95,905
Revolving Loans Amortized Cost Basis	44,913
Total	330,322
Watch \| Commercial and industrial
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	3,236
2019	3,034
2018	6,283
2017	3,652
2016	11,293
Prior	6,245
Revolving Loans Amortized Cost Basis	36,083
Total	69,826
Watch \| Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	1,282
2019	1,231
2018	9,435
2017	13,822
2016	1,305
Prior	3,585
Revolving Loans Amortized Cost Basis	0
Total	30,660
Watch \| Residential real estate: \| 1-to-4 family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	1,257
2019	786
2018	3,318
2017	12,580
2016	4,155
Prior	14,506
Revolving Loans Amortized Cost Basis	0
Total	36,602
Watch \| Residential real estate: \| Residential line of credit
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans Amortized Cost Basis	5,205
Total	5,205
Watch \| Residential real estate: \| Multi-family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans Amortized Cost Basis	0
Total	0
Watch \| Residential real estate: \| Non-owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	0
2019	6,181
2018	5,897
2017	26,361
2016	21,775
Prior	43,984
Revolving Loans Amortized Cost Basis	45
Total	104,243
Watch \| Commercial real estate:
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	3,596
2019	4,343
2018	6,843
2017	22,939
2016	7,858
Prior	19,233
Revolving Loans Amortized Cost Basis	3,188
Total	68,000
Watch \| Consumer and other
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	159
2019	707
2018	1,714
2017	1,418
2016	3,044
Prior	8,352
Revolving Loans Amortized Cost Basis	392
Total	15,786
Substandard
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	1,196
2019	7,104
2018	37,147
2017	14,056
2016	8,692
Prior	35,819
Revolving Loans Amortized Cost Basis	20,066
Total	124,080
Substandard \| Commercial and industrial
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	6
2019	3,160
2018	17,022
2017	6,552
2016	686
Prior	6,869
Revolving Loans Amortized Cost Basis	13,269
Total	47,564
Substandard \| Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	0
2019	972
2018	642
2017	132
2016	880
Prior	4,919
Revolving Loans Amortized Cost Basis	0
Total	7,545
Substandard \| Residential real estate: \| 1-to-4 family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	532
2019	1,452
2018	3,274
2017	4,128
2016	2,039
Prior	9,120
Revolving Loans Amortized Cost Basis	0
Total	20,545
Substandard \| Residential real estate: \| Residential line of credit
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	109
2019	1
2018	0
2017	3
2016	0
Prior	0
Revolving Loans Amortized Cost Basis	4,848
Total	4,961
Substandard \| Residential real estate: \| Multi-family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	57
Revolving Loans Amortized Cost Basis	0
Total	57
Substandard \| Residential real estate: \| Non-owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	0
2019	149
2018	12,130
2017	1,166
2016	1,693
Prior	3,761
Revolving Loans Amortized Cost Basis	0
Total	18,899
Substandard \| Commercial real estate:
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	428
2019	1,302
2018	3,815
2017	1,457
2016	2,931
Prior	8,440
Revolving Loans Amortized Cost Basis	1,542
Total	19,915
Substandard \| Consumer and other
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	121
2019	68
2018	264
2017	618
2016	463
Prior	2,653
Revolving Loans Amortized Cost Basis	407
Total	4,594
Doubtful
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	35
2019	294
2018	295
2017	544
2016	466
Prior	849
Revolving Loans Amortized Cost Basis	423
Total	2,906
Doubtful \| Commercial and industrial
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	35
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans Amortized Cost Basis	164
Total	199
Doubtful \| Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	5
Revolving Loans Amortized Cost Basis	0
Total	5
Doubtful \| Residential real estate: \| 1-to-4 family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	0
2019	34
2018	0
2017	0
2016	270
Prior	490
Revolving Loans Amortized Cost Basis	0
Total	794
Doubtful \| Residential real estate: \| Residential line of credit
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans Amortized Cost Basis	259
Total	259
Doubtful \| Residential real estate: \| Multi-family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans Amortized Cost Basis	0
Total	0
Doubtful \| Residential real estate: \| Non-owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans Amortized Cost Basis	0
Total	0
Doubtful \| Commercial real estate:
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans Amortized Cost Basis	0
Total	0
Doubtful \| Consumer and other
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	0
2019	260
2018	295
2017	544
2016	196
Prior	354
Revolving Loans Amortized Cost Basis	0
Total	$ 1,649

Loans and allowance for credit losses - Amortized Cost Basis by Origination Year (Details)
$ in Thousands

Sep. 30, 2020

USD ($)

Financing Receivable, Allowance for Credit Loss [Line Items]

$ 1,251,350

1,363,764

961,417

768,142