COMMITMENTS AND CONTINGENCIES (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Commitments and Contingencies Disclosure [Line Items]
Commitments outstanding to extend credit	$ 0		$ 0
Letters of credit issued to guarantee performance of a client to a third party	25,000		25,300
Affordable Housing Program Obligation	33,200		35,900
Affordable housing contingent commitment	0		0
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets	2,900		3,000
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	2,900		3,000
Amortization Method Qualified Affordable Housing Project Investments, Amortization	1,300	$ 1,000
Estimated Litigation Liability	0		0
Loans and Leases Receivable, Commitments, Fixed Rates	75,600		44,300
Loans and Leases Receivable, Commitments, Variable Rates	$ 2,000,000		$ 2,000,000
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
Commitments to Extend Credit
Commitments and Contingencies Disclosure [Line Items]
Commitments outstanding to extend credit	$ 2,000,000
Affordable housing investment [Member]
Commitments and Contingencies Disclosure [Line Items]
Low-income housing tax credit	1,100	800
Historic tax credit [Member]
Commitments and Contingencies Disclosure [Line Items]
Low-income housing tax credit	$ 100	$ 100