LOANS RECEIVABLE AND CREDIT QUALITY, Credit Quality Indicators (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	$ 5,360,555	$ 5,464,067
C&I	145,818	136,671
Total Real Estate and C&I	5,506,373	5,600,738
Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Real Estate and C&I	5,486,236	5,581,385
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Real Estate and C&I	5,807	8,401
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Real Estate and C&I	14,330	10,952
Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Real Estate and C&I	0	0
Real Estate Loan [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	5,360,555	5,464,067
Real Estate Loan [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	5,341,597	5,444,714
Real Estate Loan [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	5,807	8,401
Real Estate Loan [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	13,151	10,952
Real Estate Loan [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	0	0
Real Estate Loan [Member] | One- to Four-Family Residential, Including Condominium and Cooperative Apartment [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	62,596	63,095
Real Estate Loan [Member] | One- to Four-Family Residential, Including Condominium and Cooperative Apartment [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	61,586	62,042
Real Estate Loan [Member] | One- to Four-Family Residential, Including Condominium and Cooperative Apartment [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	176	178
Real Estate Loan [Member] | One- to Four-Family Residential, Including Condominium and Cooperative Apartment [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	834	875
Real Estate Loan [Member] | One- to Four-Family Residential, Including Condominium and Cooperative Apartment [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	0	0
Real Estate Loan [Member] | Multifamily Residential and Residential Mixed Use [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	4,280,951	4,381,180
Real Estate Loan [Member] | Multifamily Residential and Residential Mixed Use [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	4,275,143	4,374,388
Real Estate Loan [Member] | Multifamily Residential and Residential Mixed Use [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	2,409	6,326
Real Estate Loan [Member] | Multifamily Residential and Residential Mixed Use [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	3,399	466
Real Estate Loan [Member] | Multifamily Residential and Residential Mixed Use [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	0	0
Real Estate Loan [Member] | Commercial Mixed Use Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	402,169	401,555
Real Estate Loan [Member] | Commercial Mixed Use Real Estate [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	396,584	396,647
Real Estate Loan [Member] | Commercial Mixed Use Real Estate [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	1,343	0
Real Estate Loan [Member] | Commercial Mixed Use Real Estate [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	4,242	4,908
Real Estate Loan [Member] | Commercial Mixed Use Real Estate [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	0	0
Real Estate Loan [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	605,426	609,048
Real Estate Loan [Member] | Commercial Real Estate [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	598,871	602,448
Real Estate Loan [Member] | Commercial Real Estate [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	1,879	1,897
Real Estate Loan [Member] | Commercial Real Estate [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	4,676	4,703
Real Estate Loan [Member] | Commercial Real Estate [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	0	0
Real Estate Loan [Member] | ADC [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	9,413	9,189
Real Estate Loan [Member] | ADC [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	9,413	9,189
Real Estate Loan [Member] | ADC [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	0	0
Real Estate Loan [Member] | ADC [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	0	0
Real Estate Loan [Member] | ADC [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	0	0
C & I Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
C&I	145,818	136,671
C & I Loans [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
C&I	144,639	136,671
C & I Loans [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
C&I	0	0
C & I Loans [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
C&I	1,179	0
C & I Loans [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
C&I	0	0
Consumer Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Consumer loans	1,053	1,379
Consumer Loans [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Consumer loans	1,052	1,375
Consumer Loans [Member] | Non-Accrual [Member]
Financing Receivable, Recorded Investment [Line Items]
Consumer loans	$ 1	$ 4