LOANS RECEIVABLE AND CREDIT QUALITY, Credit Quality Indicators (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	$ 5,238,882	$ 5,163,122
C&I	266,415	229,504
Total Real Estate and C&I	5,505,297	5,392,626
Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Total Real Estate and C&I	5,450,219	5,346,180
Special Mention [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Total Real Estate and C&I	31,485	33,891
Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Total Real Estate and C&I	23,593	12,555
Doubtful [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Total Real Estate and C&I	0	0
Real Estate Loan [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	5,238,882	5,163,122
Real Estate Loan [Member] | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	5,184,698	5,117,256
Real Estate Loan [Member] | Special Mention [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	31,436	33,891
Real Estate Loan [Member] | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	22,748	11,975
Real Estate Loan [Member] | Doubtful [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	0	0
Real Estate Loan [Member] | One-to-Four Family Residential, Including Condominium and Cooperative Apartment [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	107,709	96,847
Real Estate Loan [Member] | One-to-Four Family Residential, Including Condominium and Cooperative Apartment [Member] | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	106,659	95,782
Real Estate Loan [Member] | One-to-Four Family Residential, Including Condominium and Cooperative Apartment [Member] | Special Mention [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	0	0
Real Estate Loan [Member] | One-to-Four Family Residential, Including Condominium and Cooperative Apartment [Member] | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	1,050	1,065
Real Estate Loan [Member] | One-to-Four Family Residential, Including Condominium and Cooperative Apartment [Member] | Doubtful [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	0	0
Real Estate Loan [Member] | Multifamily Residential and Residential Mixed-Use [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	3,831,145	3,866,788
Real Estate Loan [Member] | Multifamily Residential and Residential Mixed-Use [Member] | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	3,793,706	3,829,643
Real Estate Loan [Member] | Multifamily Residential and Residential Mixed-Use [Member] | Special Mention [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	25,326	32,682
Real Estate Loan [Member] | Multifamily Residential and Residential Mixed-Use [Member] | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	12,113	4,463
Real Estate Loan [Member] | Multifamily Residential and Residential Mixed-Use [Member] | Doubtful [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	0	0
Real Estate Loan [Member] | Commercial Real Estate and Commercial Mixed-Use [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	1,245,806	1,170,085
Real Estate Loan [Member] | Commercial Real Estate and Commercial Mixed-Use [Member] | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	1,230,111	1,162,429
Real Estate Loan [Member] | Commercial Real Estate and Commercial Mixed-Use [Member] | Special Mention [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	6,110	1,209
Real Estate Loan [Member] | Commercial Real Estate and Commercial Mixed-Use [Member] | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	9,585	6,447
Real Estate Loan [Member] | Commercial Real Estate and Commercial Mixed-Use [Member] | Doubtful [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	0	0
Real Estate Loan [Member] | ADC [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	54,222	29,402
Real Estate Loan [Member] | ADC [Member] | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	54,222	29,402
Real Estate Loan [Member] | ADC [Member] | Special Mention [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	0	0
Real Estate Loan [Member] | ADC [Member] | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	0	0
Real Estate Loan [Member] | ADC [Member] | Doubtful [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	0	0
C & I Loans [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
C&I	266,415	229,504
C & I Loans [Member] | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
C&I	265,521	228,924
C & I Loans [Member] | Special Mention [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
C&I	49	0
C & I Loans [Member] | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
C&I	845	580
C & I Loans [Member] | Doubtful [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
C&I	0	0
Consumer Loans [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Consumer loans	1,139	1,192
Consumer Loans [Member] | Performing [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Consumer loans	1,133	1,189
Consumer Loans [Member] | Non-Accrual [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Consumer loans	$ 6	$ 3