LOANS RECEIVABLE AND CREDIT QUALITY, Past Due (Details) $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2019 USD ($) Loan		Dec. 31, 2018 USD ($) Loan
Financing Receivable, Recorded Investment, Aging [Abstract]
Number of real estate loans more than 90 days past due on contractual balloon payment | Loan	6		1
Real Estate Loans [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Non-accrual	$ 2,534	[1]	$ 2,033	[2]
Total past due	3,967		2,445
Current	5,209,723		5,160,677
Loans	5,213,690		5,163,122
Real Estate Loans [Member] | 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	51		312
Real Estate Loans [Member] | 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	0		0
Real Estate Loans [Member] | Loans 90 Days or More Past Due and Still Accruing Interest [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	1,382		100
Real Estate Loans [Member] | One-to-Four Family Residential, Including Condominium and Cooperative Apartment [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Non-accrual	832	[1]	712	[2]
Total past due	1,404		1,024
Current	119,119		95,823
Loans	120,523		96,847
Real Estate Loans [Member] | One-to-Four Family Residential, Including Condominium and Cooperative Apartment [Member] | 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	51		312
Real Estate Loans [Member] | One-to-Four Family Residential, Including Condominium and Cooperative Apartment [Member] | 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	0		0
Real Estate Loans [Member] | One-to-Four Family Residential, Including Condominium and Cooperative Apartment [Member] | Loans 90 Days or More Past Due and Still Accruing Interest [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	521		0
Real Estate Loans [Member] | Multifamily Residential and Residential Mixed-Use [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Non-accrual	847	[1]	280	[2]
Total past due	1,224		380
Current	3,735,276		3,866,408
Loans	3,736,500		3,866,788
Real Estate Loans [Member] | Multifamily Residential and Residential Mixed-Use [Member] | 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	0		0
Real Estate Loans [Member] | Multifamily Residential and Residential Mixed-Use [Member] | 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	0		0
Real Estate Loans [Member] | Multifamily Residential and Residential Mixed-Use [Member] | Loans 90 Days or More Past Due and Still Accruing Interest [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	377		100
Real Estate Loans [Member] | Commercial Real Estate and Commercial Mixed-Use [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Non-accrual	855	[1]	1,041	[2]
Total past due	1,339		1,041
Current	1,277,849		1,169,044
Loans	1,279,188		1,170,085
Real Estate Loans [Member] | Commercial Real Estate and Commercial Mixed-Use [Member] | 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	0		0
Real Estate Loans [Member] | Commercial Real Estate and Commercial Mixed-Use [Member] | 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	0		0
Real Estate Loans [Member] | Commercial Real Estate and Commercial Mixed-Use [Member] | Loans 90 Days or More Past Due and Still Accruing Interest [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	484		0
Real Estate Loans [Member] | ADC [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Non-accrual	0	[1]	0	[2]
Total past due	0		0
Current	77,479		29,402
Loans	77,479		29,402
Real Estate Loans [Member] | ADC [Member] | 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	0		0
Real Estate Loans [Member] | ADC [Member] | 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	0		0
Real Estate Loans [Member] | ADC [Member] | Loans 90 Days or More Past Due and Still Accruing Interest [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	0		0
C&I [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Non-accrual	0	[1]	309	[2]
Total past due	196		408
Current	315,865		229,096
Loans	316,061		229,504
C&I [Member] | 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	0		50
C&I [Member] | 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	47		49
C&I [Member] | Loans 90 Days or More Past Due and Still Accruing Interest [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	149		0
Consumer [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Non-accrual	4	[1]	3	[2]
Total past due	11		16
Current	1,769		1,176
Loans	1,780		1,192
Consumer [Member] | 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	4		12
Consumer [Member] | 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	3		1
Consumer [Member] | Loans 90 Days or More Past Due and Still Accruing Interest [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	0		0
Real Estate Loans and C & I Loans [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Loans	5,529,751		5,392,626
Real Estate Loans and C & I Loans [Member] | Loans 90 Days or More Past Due and Still Accruing Interest [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	$ 1,531		$ 100

[1] Includes all loans on non-accrual status regardless of the number of days such loans were delinquent as of June 30, 2019.

[2] Includes all loans on non-accrual status regardless of the number of days such loans were delinquent as of December 31, 2018.