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FAIR VALUE - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
FAIR VALUE - Additional Information (Details) - USD ($)	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2018
Assets Pledged As Collateral For Loans Receivable [Line Items]
Mortgage servicing rights	$ 4,200,000		$ 4,200,000
Servicing Asset at Fair Value, Amount	4,200,000		$ 4,200,000
Percentage of discount from appraised value (in hundredths)			3.72%
Weighted average maturity of residential mortgages			20 years
Prepayment Speed used in unobservable assumptions			209		118
Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value, Qualitative Information			209		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	4,603,243,000		$ 4,603,243,000		$ 4,067,385,000
Valuation allowance	60,747,000	$ 50,628,000	60,747,000	$ 50,628,000	50,652,000	$ 59,019,000	$ 50,564,000	$ 48,453,000
Provision for Loan and Lease Losses	1,750,000	1,000,000	13,850,000	2,985,000
Recoveries	283,000	285,000	810,000	779,000
Loans and Leases Receivable, Gross	4,603,243,000		4,603,243,000		4,067,385,000
Loans and Leases Receivable, Allowance	60,747,000	50,628,000	60,747,000	50,628,000	$ 50,652,000	$ 59,019,000	$ 50,564,000	$ 48,453,000
Mortgage servicing rights, fair value	4,200,000		4,200,000
Mortgage servicing rights, impairment			628,000
Mortgage servicing rights, at amortized cost	3,600,000		3,600,000
Impaired Loans
Assets Pledged As Collateral For Loans Receivable [Line Items]
Subtotal	17,700,000	20,400,000	17,700,000	20,400,000
Valuation allowance	7,800,000	10,100,000	7,800,000	10,100,000
Provision for Loan and Lease Losses	100,000		542,000
Recoveries		500,000		600,000
Loans and Leases Receivable, Gross	17,700,000	20,400,000	17,700,000	20,400,000
Loans and Leases Receivable, Allowance	$ 7,800,000	$ 10,100,000	7,800,000	$ 10,100,000
Minimum
Assets Pledged As Collateral For Loans Receivable [Line Items]
Amount of variance			$ 100,000
Minimum \| Commercial Real Estates \| Level 3
Assets Pledged As Collateral For Loans Receivable [Line Items]
Percentage of discount from appraised value (in hundredths)			0.00%
Minimum \| Inventory Finished Goods \| Level 3
Assets Pledged As Collateral For Loans Receivable [Line Items]
Percentage of discount from appraised value (in hundredths)			35.00%
Minimum \| Finished Goods \| Level 3
Assets Pledged As Collateral For Loans Receivable [Line Items]
Percentage of discount from appraised value (in hundredths)			30.00%
Minimum \| Inventory Work In Process \| Level 3
Assets Pledged As Collateral For Loans Receivable [Line Items]
Percentage of discount from appraised value (in hundredths)			50.00%
Minimum \| Equipment \| Level 3
Assets Pledged As Collateral For Loans Receivable [Line Items]
Percentage of discount from appraised value (in hundredths)			30.00%
Minimum \| Marketable Securities \| Level 3
Assets Pledged As Collateral For Loans Receivable [Line Items]
Percentage of discount from appraised value (in hundredths)			10.00%
Maximum \| Commercial Real Estates \| Level 3
Assets Pledged As Collateral For Loans Receivable [Line Items]
Percentage of discount from appraised value (in hundredths)			50.00%
Maximum \| Inventory Finished Goods \| Level 3
Assets Pledged As Collateral For Loans Receivable [Line Items]
Percentage of discount from appraised value (in hundredths)			65.00%
Maximum \| Finished Goods \| Level 3
Assets Pledged As Collateral For Loans Receivable [Line Items]
Percentage of discount from appraised value (in hundredths)			60.00%
Maximum \| Inventory Work In Process \| Level 3
Assets Pledged As Collateral For Loans Receivable [Line Items]
Percentage of discount from appraised value (in hundredths)			100.00%
Maximum \| Equipment \| Level 3
Assets Pledged As Collateral For Loans Receivable [Line Items]
Percentage of discount from appraised value (in hundredths)			70.00%
Maximum \| Marketable Securities \| Level 3
Assets Pledged As Collateral For Loans Receivable [Line Items]
Percentage of discount from appraised value (in hundredths)			30.00%
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%)
US Government Agencies Short-term Debt Securities \| Maximum
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%