ALLOWANCE FOR CREDIT LOSSES AND CREDIT QUALITY INFORMATION - Summary of Nonaccrual and Past Due Loans (Details) - USD ($)	Jun. 30, 2022	Dec. 31, 2021	Jun. 30, 2021
Financing Receivable, Past Due [Line Items]
Total Loans	$ 11,480,118,000	$ 7,882,055,000	$ 8,255,890,000
Nonaccrual Loans	$ 21,011,000	$ 16,609,000
Percent of nonaccrual loans	0.18%	0.21%
% of Total Loans	100.00%	100.00%
Nonaccrual with an allowance (less than)	$ 0	$ 100,000
Reverse mortgage, fair value	2,600,000	3,900,000	8,400,000
Total Past Due And Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	$ 45,377,000	$ 39,280,000
Percent past due	0.40%	0.50%
30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	$ 33,483,000	$ 29,289,000
Percent past due	0.30%	0.37%
Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	$ 11,894,000	$ 9,991,000
Percent past due	0.10%	0.13%
Accruing Current Balances
Financing Receivable, Past Due [Line Items]
Total Loans	$ 11,413,730,000	$ 7,826,166,000
Percent of accruing current balances	99.42%	99.29%
Residential
Financing Receivable, Past Due [Line Items]
Total Loans	$ 770,575,000	$ 542,733,000	606,058,000
Nonaccrual Loans	2,573,000	3,125,000
Residential | Total Past Due And Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	5,237,000	1,856,000
Residential | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	5,159,000	1,856,000
Residential | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	78,000	0
Residential | Accruing Current Balances
Financing Receivable, Past Due [Line Items]
Total Loans	762,765,000	537,752,000
Consumer
Financing Receivable, Past Due [Line Items]
Total Loans	1,522,596,000	1,158,573,000	1,105,169,000
Nonaccrual Loans	2,037,000	2,380,000
Consumer | Total Past Due And Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	22,387,000	18,861,000
Consumer | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	11,302,000	10,227,000
Consumer | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	11,085,000	8,634,000
Consumer | Accruing Current Balances
Financing Receivable, Past Due [Line Items]
Total Loans	1,498,172,000	1,137,332,000
Commercial and industrial | Commercial
Financing Receivable, Past Due [Line Items]
Total Loans	3,107,740,000	2,270,319,000	2,379,697,000
Nonaccrual Loans	6,601,000	8,211,000
Commercial and industrial | Commercial | Total Past Due And Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	7,006,000	5,554,000
Commercial and industrial | Commercial | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	6,486,000	5,007,000
Commercial and industrial | Commercial | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	520,000	547,000
Commercial and industrial | Commercial | Accruing Current Balances
Financing Receivable, Past Due [Line Items]
Total Loans	3,094,133,000	2,256,554,000
Owner-occupied commercial | Commercial
Financing Receivable, Past Due [Line Items]
Total Loans	1,823,229,000	1,341,707,000	1,360,681,000
Nonaccrual Loans	353,000	811,000
Owner-occupied commercial | Commercial | Total Past Due And Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	2,132,000	741,000
Owner-occupied commercial | Commercial | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	2,132,000	741,000
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	1,820,744,000	1,340,155,000
Commercial mortgages | Commercial
Financing Receivable, Past Due [Line Items]
Total Loans	3,321,654,000	1,881,510,000	2,024,684,000
Nonaccrual Loans	4,263,000	2,070,000
Commercial mortgages | Commercial | Total Past Due And Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	4,267,000	4,335,000
Commercial mortgages | Commercial | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	4,056,000	3,525,000
Commercial mortgages | Commercial | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	211,000	810,000
Commercial mortgages | Commercial | Accruing Current Balances
Financing Receivable, Past Due [Line Items]
Total Loans	3,313,124,000	1,875,105,000
Construction | Commercial
Financing Receivable, Past Due [Line Items]
Total Loans	934,324,000	687,213,000	$ 779,601,000
Nonaccrual Loans	5,184,000	12,000
Construction | Commercial | Total Past Due And Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	4,348,000	7,933,000
Construction | Commercial | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	4,348,000	7,933,000
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	924,792,000	679,268,000
Student loans | Consumer | Total Past Due And Still Accruing
Financing Receivable, Past Due [Line Items]
Ending balance	$ 19,600,000	$ 17,000,000