ALLOWANCE FOR CREDIT LOSSES AND CREDIT QUALITY INFORMATION - Summary of Nonaccrual and Past Due Loans (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021
Financing Receivable, Past Due [Line Items]
Total Loans	$ 11,329,528	$ 7,882,055	$ 8,569,142
Nonaccrual Loans	$ 23,087	$ 16,609
Percent of nonaccrual loans	0.20%	0.21%
% of Total Loans	100.00%	100.00%
Nonaccrual with an allowance (prior year less than)	$ 100	$ 100
Reverse mortgage, fair value	4,300	3,900	9,400
Total Past Due And Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	$ 40,245	$ 39,280
Percent past due	0.36%	0.50%
30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	$ 28,622	$ 29,289
Percent past due	0.26%	0.37%
Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	$ 11,623	$ 9,991
Percent past due	0.10%	0.13%
Accruing Current Balances
Financing Receivable, Past Due [Line Items]
Total Loans	$ 11,266,196	$ 7,826,166
Percent of accruing current balances	99.44%	99.29%
Residential
Financing Receivable, Past Due [Line Items]
Total Loans	$ 805,326	$ 542,733	671,580
Nonaccrual Loans	3,175	3,125
Residential | Total Past Due And Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	1,925	1,856
Residential | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	1,925	1,856
Residential | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	0	0
Residential | Accruing Current Balances
Financing Receivable, Past Due [Line Items]
Total Loans	800,226	537,752
Consumer
Financing Receivable, Past Due [Line Items]
Total Loans	1,382,116	1,158,573	1,140,034
Nonaccrual Loans	2,311	2,380
Consumer | Total Past Due And Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	25,648	18,861
Consumer | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	14,067	10,227
Consumer | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	11,581	8,634
Consumer | Accruing Current Balances
Financing Receivable, Past Due [Line Items]
Total Loans	1,354,157	1,137,332
Commercial and industrial | Commercial
Financing Receivable, Past Due [Line Items]
Total Loans	2,984,127	2,270,319	2,663,472
Nonaccrual Loans	7,932	8,211
Commercial and industrial | Commercial | Total Past Due And Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	4,820	5,554
Commercial and industrial | Commercial | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	4,778	5,007
Commercial and industrial | Commercial | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	42	547
Commercial and industrial | Commercial | Accruing Current Balances
Financing Receivable, Past Due [Line Items]
Total Loans	2,971,375	2,256,554
Owner-occupied commercial | Commercial
Financing Receivable, Past Due [Line Items]
Total Loans	1,872,827	1,341,707	1,333,989
Nonaccrual Loans	497	811
Owner-occupied commercial | Commercial | Total Past Due And Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	779	741
Owner-occupied commercial | Commercial | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	779	741
Owner-occupied commercial | Commercial | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	0	0
Owner-occupied commercial | Commercial | Accruing Current Balances
Financing Receivable, Past Due [Line Items]
Total Loans	1,871,551	1,340,155
Commercial mortgages | Commercial
Financing Receivable, Past Due [Line Items]
Total Loans	3,361,242	1,881,510	1,975,966
Nonaccrual Loans	3,616	2,070
Commercial mortgages | Commercial | Total Past Due And Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	3,288	4,335
Commercial mortgages | Commercial | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	3,288	3,525
Commercial mortgages | Commercial | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	0	810
Commercial mortgages | Commercial | Accruing Current Balances
Financing Receivable, Past Due [Line Items]
Total Loans	3,354,338	1,875,105
Construction | Commercial
Financing Receivable, Past Due [Line Items]
Total Loans	923,890	687,213	$ 784,101
Nonaccrual Loans	5,556	12
Construction | Commercial | Total Past Due And Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	3,785	7,933
Construction | Commercial | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	3,785	7,933
Construction | Commercial | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	0	0
Construction | Commercial | Accruing Current Balances
Financing Receivable, Past Due [Line Items]
Total Loans	914,549	679,268
Student loans | Consumer | Total Past Due And Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	$ 23,500	$ 17,000