LOANS RECEIVABLE AND CREDIT QUALITY, Troubled Debt Restructurings (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) Loan	Mar. 31, 2017 Loan	Dec. 31, 2017 USD ($) Loan
Troubled Debt Restructuring on Receivables [Abstract]
Number of TDRs on non-accrual status	0		0
Number of loans modified	0	0
Number of loan commitments to borrowers with outstanding TDRs	0		0
TDRs which defaulted within twelve months following the modification	0	0
Real Estate Loans [Member]
Troubled Debt Restructuring on Receivables [Abstract]
Number of loans	6		6
Balance | $	$ 8,055		$ 8,111
Real Estate Loans [Member] | One- to Four-Family Residential, Including Condominium and Cooperative Apartment [Member]
Troubled Debt Restructuring on Receivables [Abstract]
Number of loans	1		1
Balance | $	$ 20		$ 22
Real Estate Loans [Member] | Multifamily Residential and Residential Mixed Use [Member]
Troubled Debt Restructuring on Receivables [Abstract]
Number of loans	3		3
Balance | $	$ 604		$ 619
Real Estate Loans [Member] | Commercial Mixed Use Real Estate [Member]
Troubled Debt Restructuring on Receivables [Abstract]
Number of loans	1		1
Balance | $	$ 4,152		$ 4,174
Real Estate Loans [Member] | Commercial Real Estate [Member]
Troubled Debt Restructuring on Receivables [Abstract]
Number of loans	1		1
Balance | $	$ 3,279		$ 3,296