COMMITMENTS AND CONTINGENCIES - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Commitments and Contingencies Disclosure [Line Items]
Reserves for unfunded commitments	$ 13,000		$ 12,500
Loans and Leases Receivable, Commitments, Fixed Rates	127,300		123,600
Loans and Leases Receivable, Commitments, Variable Rates	$ 3,600,000		$ 3,300,000
Loan Commitments, Fixed Interest Rate Range, Minimum	0.00%		0.00%
Loan Commitments, Fixed Interest Rate Range, Maximum	21.00%		21.00%
Loan Commitments, Fixed Rate, Maturities, Minimum	1 year		1 year
Loan Commitments, Fixed Rate, Maturities, Maximum	30 years		30 years 9 months 18 days
Letters of credit issued to guarantee performance of a client to a third party	$ 36,400		$ 36,100
Affordable Housing Program Obligation	44,800		47,500
Amortization Method Qualified Affordable Housing Project Investments, Amortization	2,000	$ 2,100
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	3,900		4,000
Estimated Litigation Liability	0		0
Renewable Energy Maximum Exposure to Loss	14,300		7,700
Renewable Energy Recorded Investment	15,200		5,200
Affordable housing contingent commitment	0		0
Variable Interest Entity, Not Primary Beneficiary [Member]
Commitments and Contingencies Disclosure [Line Items]
Historic Tax Credit Recorded Investment	3,500		3,600
Commitments to Extend Credit
Commitments and Contingencies Disclosure [Line Items]
Commitments outstanding to extend credit	3,700,000		$ 3,400,000
Affordable housing investment [Member]
Commitments and Contingencies Disclosure [Line Items]
Low-income housing tax credit	2,200	1,900
Historic tax credit [Member]
Commitments and Contingencies Disclosure [Line Items]
Low-income housing tax credit	$ 100	$ 82