LOANS RECEIVABLE AND CREDIT QUALITY, Past Due (Details) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 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	3		1
Real Estate Loans [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Non-accrual	$ 1,377	[1]	$ 2,033	[2]
Total past due	1,774		2,445
Current	5,170,835		5,160,677
Loans	5,172,609		5,163,122
Real Estate Loans [Member] | 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	17		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	380		100
Real Estate Loans [Member] | One-to-Four Family Residential, Including Condominium and Cooperative Apartment [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Non-accrual	1,161	[1]	712	[2]
Total past due	1,217		1,024
Current	133,144		95,823
Loans	134,361		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	17		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	39		0
Real Estate Loans [Member] | Multifamily Residential and Residential Mixed-Use [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Non-accrual	153	[1]	280	[2]
Total past due	494		380
Current	3,607,662		3,866,408
Loans	3,608,156		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	341		100
Real Estate Loans [Member] | Commercial Real Estate and Commercial Mixed-Use [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Non-accrual	63	[1]	1,041	[2]
Total past due	63		1,041
Current	1,334,262		1,169,044
Loans	1,334,325		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	0		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	95,767		29,402
Loans	95,767		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	15,000	[1]	309	[2]
Total past due	15,116		408
Current	293,915		229,096
Loans	309,031		229,504
C&I [Member] | 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	116		50
C&I [Member] | 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total past due	0		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	0		0
Consumer [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Non-accrual	1	[1]	3	[2]
Total past due	7		16
Current	1,382		1,176
Loans	1,389		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	2		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	5,481,640		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	$ 380		$ 100

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