LOANS RECEIVABLE AND CREDIT QUALITY, Troubled Debt Restructurings (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Loan	Dec. 31, 2016 USD ($) Loan	Dec. 31, 2015 Loan
Troubled Debt Restructuring on Receivables [Abstract]
Number of TDRs on non-accrual status	0	0
Number of loans modified	0		0
Allocated allowance on TDRs | $	$ 0	$ 0
Number of loan commitments to borrowers with outstanding TDRs	0	0
TDRs which defaulted within twelve months following the modification	0	0	0
Minimum [Member]
Troubled Debt Restructuring on Receivables [Abstract]
Short term extensions grant period	6 months
Maximum [Member]
Troubled Debt Restructuring on Receivables [Abstract]
Short term extensions grant period	12 months
Real Estate Loans [Member]
Troubled Debt Restructuring on Receivables [Abstract]
Number of loans	6	7
Balance | $	$ 8,111	$ 8,689
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	2
Balance | $	$ 22	$ 407
Number of loans modified		1
Pre-modification outstanding recorded investment | $		$ 33
Post-modification outstanding recorded investment | $		$ 33
Real Estate Loans [Member] | Multifamily Residential and Residential Mixed Use [Member]
Troubled Debt Restructuring on Receivables [Abstract]
Number of loans	3	3
Balance | $	$ 619	$ 658
Real Estate Loans [Member] | Commercial Mixed Use Real Estate [Member]
Troubled Debt Restructuring on Receivables [Abstract]
Number of loans	1	1
Balance | $	$ 4,174	$ 4,261
Real Estate Loans [Member] | Commercial Real Estate [Member]
Troubled Debt Restructuring on Receivables [Abstract]
Number of loans	1	1
Balance | $	$ 3,296	$ 3,363