LOANS RECEIVABLE AND CREDIT QUALITY, Credit Quality Indicators (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	$ 5,866,567	$ 5,633,007
C&I	111,099	2,058
Total Real Estate and C&I	5,977,666
Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Real Estate and C&I	5,959,541
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Real Estate and C&I	4,431
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Real Estate and C&I	13,694
Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Real Estate and C&I	0
Real Estate Loan [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	5,866,567	5,633,007
Real Estate Loan [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	5,848,442	5,606,870
Real Estate Loan [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	4,431	4,760
Real Estate Loan [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	13,694	21,377
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	66,519	74,022
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	65,182	72,325
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	180	212
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	1,157	1,485
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	4,787,291	4,600,526
Real Estate Loan [Member] | Multifamily Residential and Residential Mixed Use [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	4,781,016	4,589,838
Real Estate Loan [Member] | Multifamily Residential and Residential Mixed Use [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	3,403	3,488
Real Estate Loan [Member] | Multifamily Residential and Residential Mixed Use [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	2,872	7,200
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	415,299	404,139
Real Estate Loan [Member] | Commercial Mixed Use Real Estate [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	410,363	398,139
Real Estate Loan [Member] | Commercial Mixed Use Real Estate [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	0	535
Real Estate Loan [Member] | Commercial Mixed Use Real Estate [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	4,936	5,465
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	588,343	554,320
Real Estate Loan [Member] | Commercial Real Estate [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	582,766	546,568
Real Estate Loan [Member] | Commercial Real Estate [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	848	525
Real Estate Loan [Member] | Commercial Real Estate [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	4,729	7,227
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	9,115
Real Estate Loan [Member] | ADC [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	9,115
Real Estate Loan [Member] | ADC [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	0
Real Estate Loan [Member] | ADC [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	0
Real Estate Loan [Member] | ADC [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Real estate loans	0
C & I Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
C&I	111,099	2,058
C & I Loans [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
C&I	111,099
C & I Loans [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
C&I	0
C & I Loans [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
C&I	0
C & I Loans [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
C&I	0
Consumer Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Consumer loans	1,092	3,415	[1]
Consumer Loans [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Consumer loans	1,090	3,414	[1]
Consumer Loans [Member] | Non-Accrual [Member]
Financing Receivable, Recorded Investment [Line Items]
Consumer loans	$ 2	$ 1	[1]

[1] Included in the balance of consumer loans at December 31, 2016 are $2,058 of C&I loans. Subsequent to December 31, 2016, C&I loans were evaluated based on risk ratings and included in the preceding credit risk profile table.