ALLOWANCE FOR LOAN LOSSES AND CREDIT QUALITY INFORMATION - Additional Information (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) SecurityLoan loan	Jun. 30, 2017 USD ($) loan	Dec. 31, 2017 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net charge-offs			$ 5,700,000	$ 3,800,000
Percentage of average loans annualized, charged-offs			0.24%	0.17%
Reverse mortgage, fair value	$ 16,100,000	$ 21,600,000	$ 16,100,000	$ 21,600,000	$ 19,800,000
Performing TDRs			16,273,000	18,100,000	20,061,000
Acquired nonimpaired loans	501,057,000	629,797,000	501,057,000	629,797,000	565,500,000
Acquired impaired loans	4,800,000		4,800,000		5,800,000
Contractual principal balance	5,500,000		5,500,000		6,800,000
Interest income on impaired loans	400,000	400,000	700,000	$ 700,000
Accrued troubled debt restructured loans	14,200,000		14,200,000		15,300,000
Troubled debt restructuring related reserves	2,600,000		$ 2,600,000		$ 1,000,000
Usual sustained repayment performance period			6 months
TRD defaulted | loan			2	2
Subsequent default, loan amount			$ 100,000	$ 600,000
Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired nonimpaired loans	65,723,000	80,074,000	$ 65,723,000	80,074,000
Number of residential loans in the process of foreclosure | SecurityLoan			27		33
Total loans outstanding, residential loans			$ 2,500,000		$ 2,900,000
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired nonimpaired loans	101,532,000	$ 130,986,000	$ 101,532,000	130,986,000
Number of residential loans in the process of foreclosure | SecurityLoan			10		8
Total loans outstanding, residential loans			$ 5,800,000		$ 6,000,000
Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Period for impairment loans			90 days
Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increase (decrease) in allowance for loan losses (less than)			$ (700,000)	300,000
Troubled debt restructurings charged off (less than)			100,000	$ 0
Total Residential and Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired nonimpaired loans	95,000,000		$ 95,000,000		108,200,000
Impairment loans, charge off period			90 days
Total Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired nonimpaired loans	$ 406,100,000		$ 406,100,000		$ 457,300,000