ALLOWANCE FOR CREDIT LOSSES AND CREDIT QUALITY INFORMATION - Schedule of Nonaccrual and Past Due Loans (Details) - USD ($)	Sep. 30, 2023	Dec. 31, 2022	Sep. 30, 2022
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans With No Allowance	$ 47,139,000	$ 22,802,000
Nonaccrual Loans With An Allowance	10,321,000	0
Gross loans	$ 12,675,621,000	$ 11,909,436,000	$ 11,702,372,000
Percent of nonaccrual loans with no allowance	0.37%	0.19%
Percent of nonaccrual loans with allowance	0.08%
% of Total Loans	100.00%	100.00%
Reverse mortgage, fair value	$ 2,800,000	$ 2,400,000	3,000,000
Total Past Due And Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	$ 73,529,000	$ 47,765,000
Percent past due	0.58%	0.40%
30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	$ 59,172,000	$ 31,230,000
Percent past due	0.47%	0.26%
Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	$ 14,357,000	$ 16,535,000
Percent past due	0.11%	0.14%
Accruing Current Balances
Financing Receivable, Past Due [Line Items]
Total Loans	$ 12,544,632,000	$ 11,838,869,000
Percent of accruing current balances	98.97%	99.41%
Residential
Financing Receivable, Past Due [Line Items]
Total Loans	$ 858,111,000	$ 761,882,000
Nonaccrual Loans With No Allowance	2,531,000	3,199,000
Nonaccrual Loans With An Allowance	0
Gross loans	855,340,000	759,465,000	758,593,000
Residential | Total Past Due And Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	6,764,000	2,563,000
Residential | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	6,764,000	2,563,000
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	846,045,000	753,703,000
Consumer
Financing Receivable, Past Due [Line Items]
Total Loans	1,957,082,000	1,810,930,000
Nonaccrual Loans With No Allowance	1,953,000	2,145,000
Nonaccrual Loans With An Allowance	0
Gross loans	1,957,082,000	1,810,930,000	1,677,128,000
Consumer | Total Past Due And Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	26,802,000	27,776,000
Consumer | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	15,580,000	12,263,000
Consumer | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	11,222,000	15,513,000
Consumer | Accruing Current Balances
Financing Receivable, Past Due [Line Items]
Total Loans	1,928,327,000	1,781,009,000
Commercial and industrial | Commercial
Financing Receivable, Past Due [Line Items]
Total Loans	2,644,039,000	2,575,345,000
Nonaccrual Loans With No Allowance	21,138,000	6,770,000
Nonaccrual Loans With An Allowance	5,157,000
Gross loans	3,249,737,000	3,134,326,000	3,169,478,000
Commercial and industrial | Commercial | Total Past Due And Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	11,302,000	11,078,000
Commercial and industrial | Commercial | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	10,183,000	10,767,000
Commercial and industrial | Commercial | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	1,119,000	311,000
Commercial and industrial | Commercial | Accruing Current Balances
Financing Receivable, Past Due [Line Items]
Total Loans	3,212,140,000	3,116,478,000
Owner-occupied commercial | Commercial
Financing Receivable, Past Due [Line Items]
Total Loans	1,924,171,000	1,809,582,000
Nonaccrual Loans With No Allowance	14,085,000	386,000
Nonaccrual Loans With An Allowance	1,062,000
Gross loans	1,924,171,000	1,809,582,000	1,788,591,000
Owner-occupied commercial | Commercial | Total Past Due And Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	1,669,000	3,974,000
Owner-occupied commercial | Commercial | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	1,669,000	3,500,000
Owner-occupied commercial | Commercial | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	0	474,000
Owner-occupied commercial | Commercial | Accruing Current Balances
Financing Receivable, Past Due [Line Items]
Total Loans	1,907,355,000	1,805,222,000
Commercial mortgages | Commercial
Financing Receivable, Past Due [Line Items]
Total Loans	3,645,723,000	3,351,084,000
Nonaccrual Loans With No Allowance	6,706,000	5,159,000
Nonaccrual Loans With An Allowance	0
Gross loans	3,645,723,000	3,351,084,000	3,280,379,000
Commercial mortgages | Commercial | Total Past Due And Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	16,691,000	2,374,000
Commercial mortgages | Commercial | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	16,691,000	2,137,000
Commercial mortgages | Commercial | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	0	237,000
Commercial mortgages | Commercial | Accruing Current Balances
Financing Receivable, Past Due [Line Items]
Total Loans	3,622,326,000	3,343,551,000
Construction | Commercial
Financing Receivable, Past Due [Line Items]
Total Loans	1,043,568,000	1,044,049,000
Nonaccrual Loans With No Allowance	726,000	5,143,000
Nonaccrual Loans With An Allowance	4,102,000
Gross loans	1,043,568,000	1,044,049,000	$ 1,028,203,000
Construction | Commercial | Total Past Due And Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	10,301,000	0
Construction | Commercial | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	8,285,000	0
Construction | Commercial | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	2,016,000	0
Construction | Commercial | Accruing Current Balances
Financing Receivable, Past Due [Line Items]
Total Loans	1,028,439,000	1,038,906,000
Student loans | Consumer | Total Past Due And Still Accruing
Financing Receivable, Past Due [Line Items]
Total Loans	$ 16,100,000	$ 21,100,000