LOANS RECEIVABLE AND CREDIT QUALITY, Credit Quality Indicators (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Real Estate Loan [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	$ 5,213,690	$ 5,163,122
Real Estate Loan [Member] | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	5,157,856	5,117,256
Real Estate Loan [Member] | Special Mention [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	17,441	33,891
Real Estate Loan [Member] | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	38,393	11,975
Real Estate Loan [Member] | Doubtful [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
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]
Loans	120,523	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]
Loans	119,351	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]
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]
Loans	1,172	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]
Loans	0	0
Real Estate Loan [Member] | Multifamily Residential and Residential Mixed-Use [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	3,736,500	3,866,788
Real Estate Loan [Member] | Multifamily Residential and Residential Mixed-Use [Member] | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	3,690,388	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]
Loans	17,441	32,682
Real Estate Loan [Member] | Multifamily Residential and Residential Mixed-Use [Member] | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	28,671	4,463
Real Estate Loan [Member] | Multifamily Residential and Residential Mixed-Use [Member] | Doubtful [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	0	0
Real Estate Loan [Member] | Commercial Real Estate and Commercial Mixed-Use [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	1,279,188	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]
Loans	1,270,638	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]
Loans	0	1,209
Real Estate Loan [Member] | Commercial Real Estate and Commercial Mixed-Use [Member] | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	8,550	6,447
Real Estate Loan [Member] | Commercial Real Estate and Commercial Mixed-Use [Member] | Doubtful [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	0	0
Real Estate Loan [Member] | ADC [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	77,479	29,402
Real Estate Loan [Member] | ADC [Member] | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	77,479	29,402
Real Estate Loan [Member] | ADC [Member] | Special Mention [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	0	0
Real Estate Loan [Member] | ADC [Member] | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	0	0
Real Estate Loan [Member] | ADC [Member] | Doubtful [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	0	0
C&I [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	316,061	229,504
C&I [Member] | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	315,912	228,924
C&I [Member] | Special Mention [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	0	0
C&I [Member] | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	149	580
C&I [Member] | Doubtful [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	0	0
Real Estate Loans and C & I Loans [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	5,529,751	5,392,626
Real Estate Loans and C & I Loans [Member] | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	5,473,768	5,346,180
Real Estate Loans and C & I Loans [Member] | Special Mention [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	17,441	33,891
Real Estate Loans and C & I Loans [Member] | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	38,542	12,555
Real Estate Loans and C & I Loans [Member] | Doubtful [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	0	0
Consumer Loans [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	1,780	1,192
Consumer Loans [Member] | Performing [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	1,776	1,189
Consumer Loans [Member] | Non-Accrual [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	$ 4	$ 3