COMMITMENTS AND CONTINGENCIES (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Commitments and Contingencies Disclosure [Line Items]
Reserves for unfunded commitments	$ 800,000		$ 700,000
Commitments outstanding to extend credit	0		0
Loans and Leases Receivable, Commitments, Fixed Rates	182,500,000		174,000,000.0
Loans and Leases Receivable, Commitments, Variable Rates	$ 2,900,000,000		$ 2,900,000,000
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		1 year
Loan Commitments, Fixed Rate, Maturities, Maximum	30 years		30 years
Letters of credit issued to guarantee performance of a client to a third party	$ 33,400,000		$ 32,700,000
Affordable Housing Program Obligation	36,200,000		39,400,000
Amortization Method Qualified Affordable Housing Project Investments, Amortization	1,600,000	$ 1,300,000
Affordable housing contingent commitment	0		0
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets	3,700,000		3,900,000
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	3,700,000		3,900,000
Estimated Litigation Liability	0		0
Commitments to Extend Credit
Commitments and Contingencies Disclosure [Line Items]
Commitments outstanding to extend credit	3,100,000,000		3,000,000,000.0
Affordable housing investment [Member]
Commitments and Contingencies Disclosure [Line Items]
Low-income housing tax credit	1,600,000	1,100,000
Historic tax credit [Member]
Commitments and Contingencies Disclosure [Line Items]
Low-income housing tax credit	$ 36,000	$ 100,000
Maximum [Member]
Commitments and Contingencies Disclosure [Line Items]
Loans and Leases Receivable, Commitments, Variable Rates			0.210
Minimum [Member]
Commitments and Contingencies Disclosure [Line Items]
Loans and Leases Receivable, Commitments, Variable Rates			$ 0.000