COMMITMENTS AND CONTINGENCIES - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Commitments and Contingencies Disclosure [Line Items]
Reserves for unfunded commitments	$ 14,300,000		$ 600,000
Commitments outstanding to extend credit	0		0
Loans and Leases Receivable, Commitments, Fixed Rates	154,400,000	$ 123,700,000
Loans and Leases Receivable, Commitments, Variable Rates	$ 3,100,000,000	$ 3,200,000,000
Loan Commitments, Fixed Interest Rate Range, Minimum	0.00%
Loan Commitments, Fixed Interest Rate Range, Maximum	21.00%
Loan Commitments, Fixed Rate, Maturities, Minimum		1 year
Loan Commitments, Fixed Rate, Maturities, Maximum	30 years 10 months 24 days	31 years 7 months 6 days
Letters of credit issued to guarantee performance of a client to a third party	$ 32,700,000		33,400,000
Affordable Housing Program Obligation	35,600,000		38,500,000
Amortization Method Qualified Affordable Housing Project Investments, Amortization	2,100,000	$ 1,600,000
Affordable housing contingent commitment	0		0
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets	3,800,000		3,100,000
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	6,700,000		5,100,000
Estimated Litigation Liability	0		0
Commitments to Extend Credit
Commitments and Contingencies Disclosure [Line Items]
Commitments outstanding to extend credit	3,300,000,000		$ 3,300,000,000
Affordable housing investment [Member]
Commitments and Contingencies Disclosure [Line Items]
Low-income housing tax credit	1,900,000	1,600,000
Historic tax credit [Member]
Commitments and Contingencies Disclosure [Line Items]
Low-income housing tax credit	$ 100,000	0
Minimum [Member]
Commitments and Contingencies Disclosure [Line Items]
Loans and Leases Receivable, Commitments, Variable Rates		0.000
Maximum [Member]
Commitments and Contingencies Disclosure [Line Items]
Loans and Leases Receivable, Commitments, Variable Rates		$ 0.210