ALLOWANCE FOR LOAN LOSSES AND CREDIT QUALITY INFORMATION - Additional Information (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) SecurityLoan loan	Mar. 31, 2017 USD ($) loan	Dec. 31, 2017 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net charge-offs	$ 3,400,000	$ 2,100,000
Percentage of average loans annualized, charged-offs	0.29%	0.19%
Reverse mortgage, fair value	$ 20,000,000	$ 22,500,000	$ 19,800,000
Performing TDRs	20,248,000	17,300,000	20,061,000
Acquired nonimpaired loans	534,841,000	675,607,000	565,500,000
Acquired impaired loans	5,600,000		5,800,000
Contractual principal balance	6,600,000		6,800,000
Acquired impaired loans	20,154,000	31,247,000	21,654,000
Interest income on impaired loans		$ 300,000
Accrued troubled debt restructured loans	14,200,000		15,300,000
Troubled debt restructuring related reserves	$ 800,000		1,000,000
Usual sustained repayment performance period	6 months
TRD defaulted | loan	2	3
Subsequent default, loan amount	$ 100,000	$ 700,000
Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired nonimpaired loans	67,722,000	85,679,000
Acquired impaired loans	$ 777,000	866,000	$ 784,000
Number of residential loans in the process of foreclosure | SecurityLoan	38		33
Total loans outstanding, residential loans	$ 3,300,000		$ 2,900,000
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired nonimpaired loans	107,183,000	141,933,000
Acquired impaired loans	$ 3,870,000	5,256,000	$ 4,156,000
Number of residential loans in the process of foreclosure | SecurityLoan	10		8
Total loans outstanding, residential loans	$ 6,100,000		$ 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 in allowance for loan losses (less than)	$ 0	300,000
Troubled debt restructurings charged off (less than)	0	$ 0
Total Residential and Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired nonimpaired loans	$ 100,900,000		108,200,000
Impairment loans, charge off period	90 days
Total Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired nonimpaired loans	$ 434,000,000		$ 457,300,000