LOANS RECEIVABLE AND CREDIT QUALITY, Credit Quality Indicators (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	$ 5,163,122	$ 5,464,067
C&I	229,504	136,671
Total Real Estate and C&I	5,392,626	5,600,738
Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Total Real Estate and C&I	5,346,180	5,581,385
Special Mention [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Total Real Estate and C&I	25,871	8,401
Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Total Real Estate and C&I	12,555	10,952
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	5,163,122	5,464,067
Real Estate Loan [Member] | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	5,117,256	5,444,714
Real Estate Loan [Member] | Special Mention [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	25,871	8,401
Real Estate Loan [Member] | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	11,975	10,952
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	96,847	63,095
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	95,782	62,042
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	178
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	1,065	875
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	3,866,788	4,381,180
Real Estate Loan [Member] | Multifamily Residential and Residential Mixed-Use [Member] | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	3,829,643	4,374,388
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	25,378	6,326
Real Estate Loan [Member] | Multifamily Residential and Residential Mixed-Use [Member] | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	4,463	466
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	1,170,085	1,010,603
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	1,162,429	999,095
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	493	1,897
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	6,447	9,611
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	29,402	9,189
Real Estate Loan [Member] | ADC [Member] | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Real estate loans	29,402	9,189
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	229,504	136,671
C & I Loans [Member] | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
C&I	228,924	136,671
C & I Loans [Member] | Special Mention [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
C&I	0	0
C & I Loans [Member] | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
C&I	580	0
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	1,192	1,379
Consumer Loans [Member] | Performing [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Consumer loans	1,189	1,375
Consumer Loans [Member] | Non-Accrual [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Consumer loans	$ 3	$ 4