ALLOWANCE FOR LOAN LOSSES, Allowance by Class of Loan (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
Reserve for Loan Commitments [Abstract]
Allocated reserves related to TDRs	$ 0		$ 0		$ 0
Allowance for credit losses [Roll Forward]
Provision (credit) for loan losses	11,228	$ 335	11,100	$ 1,641
Unfunded Loan Commitment [Member]
Reserve for Loan Commitments [Abstract]
Allocated reserves related to TDRs	25		25		25
Real Estate Loans [Member]
Allowance for credit losses [Roll Forward]
Beginning balance	16,850	18,230	17,818	18,993
Provision (credit) for loan losses	(824)	(129)	(1,604)	(720)
Charge-offs	(20)	(2)	(211)	(176)
Recoveries	4	13	7	15
Ending balance	16,010	18,112	16,010	18,112
Loans	5,172,609		5,172,609		5,163,122
Allowance for loan losses [Abstract]
Individually evaluated for impairment	0		0		0
Collectively evaluated for impairment	16,010		16,010		17,818
Loans [Abstract]
Individually evaluated for impairment	225		225		5,690
Collectively evaluated for impairment	5,172,384		5,172,384		5,157,432
Total ending allowance balance	5,172,609		5,172,609		5,163,122
Real Estate Loans [Member] \| Doubtful [Member]
Allowance for credit losses [Roll Forward]
Loans	0		0		0
Real Estate Loans [Member] \| One-to-Four Family Residential, Including Condominium and Cooperative Apartment [Member]
Allowance for credit losses [Roll Forward]
Beginning balance	207	123	198	116
Provision (credit) for loan losses	52	(2)	64	171
Charge-offs	(6)	(1)	(12)	(169)
Recoveries	4	10	7	12
Ending balance	257	130	257	130
Loans	134,361		134,361		96,847
Allowance for loan losses [Abstract]
Individually evaluated for impairment	0		0		0
Collectively evaluated for impairment	257		257		198
Loans [Abstract]
Individually evaluated for impairment	9		9		14
Collectively evaluated for impairment	134,352		134,352		96,833
Total ending allowance balance	134,361		134,361		96,847
Real Estate Loans [Member] \| One-to-Four Family Residential, Including Condominium and Cooperative Apartment [Member] \| Doubtful [Member]
Allowance for credit losses [Roll Forward]
Loans	0		0		0
Real Estate Loans [Member] \| Multifamily Residential and Residential Mixed-Use [Member]
Allowance for credit losses [Roll Forward]
Beginning balance	11,817	14,299	13,446	15,219
Provision (credit) for loan losses	(838)	(312)	(2,427)	(1,232)
Charge-offs	(14)	(1)	(54)	(1)
Recoveries	0	0	0	0
Ending balance	10,965	13,986	10,965	13,986
Loans	3,608,156		3,608,156		3,866,788
Allowance for loan losses [Abstract]
Individually evaluated for impairment	0		0		0
Collectively evaluated for impairment	10,965		10,965		13,446
Loans [Abstract]
Individually evaluated for impairment	153		153		551
Collectively evaluated for impairment	3,608,003		3,608,003		3,866,237
Total ending allowance balance	3,608,156		3,608,156		3,866,788
Real Estate Loans [Member] \| Multifamily Residential and Residential Mixed-Use [Member] \| Doubtful [Member]
Allowance for credit losses [Roll Forward]
Loans	0		0		0
Real Estate Loans [Member] \| Commercial Real Estate and Commercial Mixed-Use [Member]
Allowance for credit losses [Roll Forward]
Beginning balance	3,846	3,667	3,777	3,535
Provision (credit) for loan losses	(43)	177	171	315
Charge-offs	0	0	(145)	(6)
Recoveries	0	3	0	3
Ending balance	3,803	3,847	3,803	3,847
Loans	1,334,325		1,334,325		1,170,085
Allowance for loan losses [Abstract]
Individually evaluated for impairment	0		0		0
Collectively evaluated for impairment	3,803		3,803		3,777
Loans [Abstract]
Individually evaluated for impairment	63		63		5,125
Collectively evaluated for impairment	1,334,262		1,334,262		1,164,960
Total ending allowance balance	1,334,325		1,334,325		1,170,085
Real Estate Loans [Member] \| Commercial Real Estate and Commercial Mixed-Use [Member] \| Doubtful [Member]
Allowance for credit losses [Roll Forward]
Loans	0		0		0
Real Estate Loans [Member] \| ADC [Member]
Allowance for credit losses [Roll Forward]
Beginning balance	980	141	397	123
Provision (credit) for loan losses	5	8	588	26
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Ending balance	985	149	985	149
Loans	95,767		95,767		29,402
Allowance for loan losses [Abstract]
Individually evaluated for impairment	0		0		0
Collectively evaluated for impairment	985		985		397
Loans [Abstract]
Individually evaluated for impairment	0		0		0
Collectively evaluated for impairment	95,767		95,767		29,402
Total ending allowance balance	95,767		95,767		29,402
Real Estate Loans [Member] \| ADC [Member] \| Doubtful [Member]
Allowance for credit losses [Roll Forward]
Loans	0		0		0
C&I [Member]
Allowance for credit losses [Roll Forward]
Beginning balance	4,267	2,737	3,946	2,021
Provision (credit) for loan losses	12,046	463	12,698	2,358
Charge-offs	(5,046)	0	(5,378)	(1,179)
Recoveries	1	0	2	0
Ending balance	11,268	3,200	11,268	3,200
Loans	309,031		309,031		229,504
Allowance for loan losses [Abstract]
Individually evaluated for impairment	7,500		7,500		230
Collectively evaluated for impairment	3,768		3,768		3,716
Loans [Abstract]
Individually evaluated for impairment	15,000		15,000		309
Collectively evaluated for impairment	294,031		294,031		229,195
Total ending allowance balance	309,031		309,031		229,504
C&I [Member] \| Doubtful [Member]
Allowance for credit losses [Roll Forward]
Loans	15,000		15,000		0
C&I [Member] \| Non-Performing [Member]
Allowance for credit losses [Roll Forward]
Charge-offs	(5,000)
Loans	20,000		20,000
Specific reserve	7,500		7,500
C&I [Member] \| Non-Performing [Member] \| Doubtful [Member]
Allowance for credit losses [Roll Forward]
Loans	15,000		15,000
Consumer Loans [Member]
Allowance for credit losses [Roll Forward]
Beginning balance	17	17	18	19
Provision (credit) for loan losses	6	1	6	3
Charge-offs	(7)	0	(8)	(4)
Recoveries	0	0	0	0
Ending balance	16	$ 18	16	$ 18
Loans	1,389		1,389		1,192
Allowance for loan losses [Abstract]
Individually evaluated for impairment	0		0		0
Collectively evaluated for impairment	16		16		18
Loans [Abstract]
Individually evaluated for impairment	0		0		0
Collectively evaluated for impairment	1,389		1,389		1,192
Total ending allowance balance	1,389		1,389		1,192
Consumer Loans [Member] \| Non-Performing [Member]
Allowance for credit losses [Roll Forward]
Loans	$ 1		$ 1		$ 3

Amounts allocated within the allowance for loan losses related to troubled debt restructured loans at period end.

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Amount of specific reserve recognized in financial receivables.

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Amount of allowance for credit loss on financing receivable. Excludes allowance for financing receivable covered under loss sharing agreement.

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The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

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Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

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Amount of recovery of financing receivables doubtful of collection that were previously charged off.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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The balance of financing receivables that were collectively evaluated for impairment.

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Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

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Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

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