LOANS RECEIVABLE AND CREDIT QUALITY, Credit Quality Indicators (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Real Estate Loan [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	$ 5,172,609		$ 5,172,609		$ 5,163,122
Charge-offs	20	$ 2	211	$ 176
Real Estate Loan [Member] | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	5,126,848		5,126,848		5,117,256
Real Estate Loan [Member] | Special Mention [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	14,868		14,868		33,891
Real Estate Loan [Member] | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	30,893		30,893		11,975
Real Estate Loan [Member] | Doubtful [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	0		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	134,361		134,361		96,847
Charge-offs	6	1	12	169
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	133,200		133,200		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		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	1,161		1,161		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		0
Real Estate Loan [Member] | Multifamily Residential and Residential Mixed-Use [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	3,608,156		3,608,156		3,866,788
Charge-offs	14	1	54	1
Real Estate Loan [Member] | Multifamily Residential and Residential Mixed-Use [Member] | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	3,567,254		3,567,254		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	14,388		14,388		32,682
Real Estate Loan [Member] | Multifamily Residential and Residential Mixed-Use [Member] | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	26,514		26,514		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		0
Real Estate Loan [Member] | Commercial Real Estate and Commercial Mixed-Use [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	1,334,325		1,334,325		1,170,085
Charge-offs	0	0	145	6
Real Estate Loan [Member] | Commercial Real Estate and Commercial Mixed-Use [Member] | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	1,330,627		1,330,627		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	480		480		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	3,218		3,218		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		0
Real Estate Loan [Member] | ADC [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	95,767		95,767		29,402
Charge-offs	0	0	0	0
Real Estate Loan [Member] | ADC [Member] | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	95,767		95,767		29,402
Real Estate Loan [Member] | ADC [Member] | Special Mention [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	0		0		0
Real Estate Loan [Member] | ADC [Member] | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	0		0		0
Real Estate Loan [Member] | ADC [Member] | Doubtful [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	0		0		0
C&I [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	309,031		309,031		229,504
Charge-offs	5,046	0	5,378	1,179
C&I [Member] | Non-Accrual [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	20,000		20,000
Charge-offs	5,000
Specific reserve	7,500		7,500
C&I [Member] | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	292,865		292,865		228,924
C&I [Member] | Special Mention [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	1,166		1,166		0
C&I [Member] | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	0		0		580
C&I [Member] | Doubtful [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	15,000		15,000		0
C&I [Member] | Doubtful [Member] | Non-Accrual [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	15,000		15,000
Real Estate Loans and C & I Loans [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	5,481,640		5,481,640		5,392,626
Real Estate Loans and C & I Loans [Member] | Pass [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	5,419,713		5,419,713		5,346,180
Real Estate Loans and C & I Loans [Member] | Special Mention [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	16,034		16,034		33,891
Real Estate Loans and C & I Loans [Member] | Substandard [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	30,893		30,893		12,555
Real Estate Loans and C & I Loans [Member] | Doubtful [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	15,000		15,000		0
Consumer Loans [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	1,389		1,389		1,192
Charge-offs	7	$ 0	8	$ 4
Consumer Loans [Member] | Performing [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	1,388		1,388		1,189
Consumer Loans [Member] | Non-Accrual [Member]
Credit Risk Profile of Real Estate Loans [Abstract]
Loans	$ 1		$ 1		$ 3