FAIR VALUE - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Assets Pledged As Collateral For Loans Receivable [Line Items]
Mortgage servicing rights	$ 4,200,000
Valuation reserve	715,000	$ 0	$ 0
Amortized cost	3,500,000
Servicing Asset at Fair Value, Amount	$ 4,200,000
Percentage of discount from appraised value (in hundredths)	3.70%
Weighted average maturity of residential mortgages	20 years
Prepayment Speed used in unobservable assumptions	204	118
Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value, Qualitative Information	204	118
Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value, Discount Rate	9.40%	9.40%
Discount rate used to estimate fair value (in hundredths)	9.40%	9.40%
Subtotal	$ 4,657,183,000	$ 4,067,385,000
Valuation allowance	61,408,000	50,652,000	48,453,000	$ 47,121,000
Provision for Loan and Lease Losses	14,770,000	3,235,000	6,400,000
Recoveries	1,239,000	874,000	1,043,000
Loans and Leases Receivable, Gross	4,657,183,000	4,067,385,000
Loans and Leases Receivable, Allowance	61,408,000	50,652,000	$ 48,453,000	$ 47,121,000
Ten Percent Adverse Changes In Psa
Assets Pledged As Collateral For Loans Receivable [Line Items]
Mortgage servicing rights	146,000
Servicing Asset at Fair Value, Amount	146,000
Twenty Percent Adverse Changes In Psa
Assets Pledged As Collateral For Loans Receivable [Line Items]
Mortgage servicing rights	285,000
Servicing Asset at Fair Value, Amount	285,000
Ten Percent Adverse Changes In Discount Rate
Assets Pledged As Collateral For Loans Receivable [Line Items]
Mortgage servicing rights	115,000
Servicing Asset at Fair Value, Amount	115,000
Twenty Percent Adverse Changes In Discount Rate
Assets Pledged As Collateral For Loans Receivable [Line Items]
Mortgage servicing rights	224,000
Servicing Asset at Fair Value, Amount	224,000
Impaired Loans
Assets Pledged As Collateral For Loans Receivable [Line Items]
Subtotal	14,600,000	22,200,000
Valuation allowance	7,800,000	10,100,000
Recoveries	2,300,000	600,000
Loans and Leases Receivable, Gross	14,600,000	22,200,000
Loans and Leases Receivable, Allowance	7,800,000	$ 10,100,000
Minimum
Assets Pledged As Collateral For Loans Receivable [Line Items]
Amount of variance	$ 100,000
Municipal Notes | Minimum
Assets Pledged As Collateral For Loans Receivable [Line Items]
Percentage of Variance	(5.00%)
Municipal Notes | Maximum
Assets Pledged As Collateral For Loans Receivable [Line Items]
Percentage of Variance	5.00%
US Government Agencies Short-term Debt Securities | Minimum
Assets Pledged As Collateral For Loans Receivable [Line Items]
Percentage of Variance	(3.00%)
U.S. Treasury securities | Minimum
Assets Pledged As Collateral For Loans Receivable [Line Items]
Percentage of Variance	(1.00%)
U.S. Treasury securities | Maximum
Assets Pledged As Collateral For Loans Receivable [Line Items]
Percentage of Variance	1.00%