Loans and allowance for credit losses - Amortized Cost Basis by Origination Year (Details) $ in Thousands	Mar. 31, 2020 USD ($)
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	$ 181,241
2019	935,361
2018	714,960
2017	516,247
2016	458,166
Prior	784,782
Revolving Loans Amortized Cost Basis	977,281
Total	4,568,038
Commercial and industrial
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	29,008
2019	191,042
2018	116,034
2017	55,063
2016	45,875
Prior	42,543
Revolving Loans Amortized Cost Basis	540,919
Total	1,020,484
Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	22,601
2019	182,354
2018	104,556
2017	69,976
2016	33,867
Prior	87,331
Revolving Loans Amortized Cost Basis	98,794
Total	599,479
Residential real estate: | 1-to-4 family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	46,436
2019	188,104
2018	152,386
2017	103,394
2016	74,501
Prior	178,515
Revolving Loans Amortized Cost Basis	0
Total	743,336
Residential real estate: | Residential line of credit
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	88
2019	586
2018	426
2017	347
2016	608
Prior	4,483
Revolving Loans Amortized Cost Basis	239,989
Total	246,527
Residential real estate: | Multi-family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	18,706
2019	13,848
2018	6,819
2017	23,602
2016	2,972
Prior	28,691
Revolving Loans Amortized Cost Basis	0
Total	94,638
Residential real estate: | Non-owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	27,593
2019	144,232
2018	194,852
2017	131,762
2016	179,472
Prior	207,874
Revolving Loans Amortized Cost Basis	25,037
Total	910,822
Commercial real estate:
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	23,592
2019	148,514
2018	88,405
2017	98,206
2016	72,427
Prior	191,388
Revolving Loans Amortized Cost Basis	64,011
Total	686,543
Consumer and other
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	13,217
2019	66,681
2018	51,482
2017	33,897
2016	48,444
Prior	43,957
Revolving Loans Amortized Cost Basis	8,531
Total	266,209
Pass
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	180,896
2019	910,904
2018	671,690
2017	464,811
2016	435,953
Prior	689,886
Revolving Loans Amortized Cost Basis	929,451
Total	4,283,591
Pass | Commercial and industrial
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	29,008
2019	178,014
2018	82,142
2017	46,942
2016	38,723
Prior	34,087
Revolving Loans Amortized Cost Basis	502,092
Total	911,008
Pass | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	22,601
2019	180,870
2018	103,731
2017	59,843
2016	33,080
Prior	81,213
Revolving Loans Amortized Cost Basis	98,582
Total	579,920
Pass | Residential real estate: | 1-to-4 family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	46,111
2019	183,701
2018	149,607
2017	97,244
2016	68,876
Prior	156,697
Revolving Loans Amortized Cost Basis	0
Total	702,236
Pass | Residential real estate: | Residential line of credit
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	88
2019	586
2018	426
2017	333
2016	608
Prior	4,404
Revolving Loans Amortized Cost Basis	236,775
Total	243,220
Pass | Residential real estate: | Multi-family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	18,706
2019	13,848
2018	6,819
2017	23,602
2016	2,972
Prior	28,629
Revolving Loans Amortized Cost Basis	0
Total	94,576
Pass | Residential real estate: | Non-owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	27,593
2019	144,200
2018	192,928
2017	131,450
2016	178,873
Prior	182,983
Revolving Loans Amortized Cost Basis	24,904
Total	882,931
Pass | Commercial real estate:
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	23,592
2019	143,780
2018	86,554
2017	74,223
2016	68,452
Prior	169,360
Revolving Loans Amortized Cost Basis	59,507
Total	625,468
Pass | Consumer and other
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	13,197
2019	65,905
2018	49,483
2017	31,174
2016	44,369
Prior	32,513
Revolving Loans Amortized Cost Basis	7,591
Total	244,232
Watch
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	325
2019	18,078
2018	35,543
2017	43,970
2016	17,906
Prior	57,228
Revolving Loans Amortized Cost Basis	37,033
Total	210,083
Watch | Commercial and industrial
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	0
2019	10,643
2018	29,243
2017	6,647
2016	5,766
Prior	4,691
Revolving Loans Amortized Cost Basis	32,191
Total	89,181
Watch | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	0
2019	529
2018	825
2017	10,099
2016	769
Prior	2,877
Revolving Loans Amortized Cost Basis	0
Total	15,099
Watch | Residential real estate: | 1-to-4 family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	325
2019	3,425
2018	1,195
2017	2,286
2016	3,921
Prior	13,358
Revolving Loans Amortized Cost Basis	0
Total	24,510
Watch | Residential real estate: | Residential line of credit
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	0
2019	0
2018	0
2017	14
2016	0
Prior	0
Revolving Loans Amortized Cost Basis	858
Total	872
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	62
Revolving Loans Amortized Cost Basis	0
Total	62
Watch | Residential real estate: | Non-owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	0
2019	0
2018	1,716
2017	312
2016	214
Prior	11,705
Revolving Loans Amortized Cost Basis	133
Total	14,080
Watch | Commercial real estate:
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	0
2019	2,930
2018	1,530
2017	23,001
2016	3,915
Prior	15,473
Revolving Loans Amortized Cost Basis	3,263
Total	50,112
Watch | Consumer and other
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	0
2019	551
2018	1,034
2017	1,611
2016	3,321
Prior	9,062
Revolving Loans Amortized Cost Basis	588
Total	16,167
Substandard
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	20
2019	6,132
2018	7,354
2017	7,029
2016	4,117
Prior	36,882
Revolving Loans Amortized Cost Basis	10,277
Total	71,811
Substandard | Commercial and industrial
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	0
2019	2,385
2018	4,649
2017	1,474
2016	1,386
Prior	3,765
Revolving Loans Amortized Cost Basis	6,636
Total	20,295
Substandard | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	0
2019	854
2018	0
2017	34
2016	0
Prior	3,241
Revolving Loans Amortized Cost Basis	212
Total	4,341
Substandard | Residential real estate: | 1-to-4 family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	0
2019	978
2018	1,584
2017	3,848
2016	1,636
Prior	8,020
Revolving Loans Amortized Cost Basis	0
Total	16,066
Substandard | 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	79
Revolving Loans Amortized Cost Basis	1,836
Total	1,915
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	0
Revolving Loans Amortized Cost Basis	0
Total	0
Substandard | Residential real estate: | Non-owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	0
2019	32
2018	208
2017	0
2016	385
Prior	13,186
Revolving Loans Amortized Cost Basis	0
Total	13,811
Substandard | Commercial real estate:
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	0
2019	1,804
2018	321
2017	982
2016	60
Prior	6,555
Revolving Loans Amortized Cost Basis	1,241
Total	10,963
Substandard | Consumer and other
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	20
2019	79
2018	592
2017	691
2016	650
Prior	2,036
Revolving Loans Amortized Cost Basis	352
Total	4,420
Doubtful
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	0
2019	247
2018	373
2017	437
2016	190
Prior	786
Revolving Loans Amortized Cost Basis	520
Total	2,553
Doubtful | Commercial and industrial
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 | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	0
2019	101
2018	0
2017	0
2016	18
Prior	0
Revolving Loans Amortized Cost Basis	0
Total	119
Doubtful | Residential real estate: | 1-to-4 family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
2020	0
2019	0
2018	0
2017	16
2016	68
Prior	440
Revolving Loans Amortized Cost Basis	0
Total	524
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	520
Total	520
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	146
2018	373
2017	421
2016	104
Prior	346
Revolving Loans Amortized Cost Basis	0
Total	$ 1,390