LOANS - Loans Receivable (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	$ 6,148,501		$ 6,230,977
Allowance for loan losses	(46,063)	$ (45,985)	(36,001)	$ (34,923)	$ (35,221)	$ (34,314)
Loans held for investment, net	6,102,438		6,194,976
Multifamily residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	4,086,059		3,985,981
Allowance for loan losses	(31,673)	(30,837)	(23,372)	(23,153)	(22,745)	(21,326)
Multifamily residential | Permanent mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	4,086,059		3,985,981
Single family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	1,839,156		2,021,320
Allowance for loan losses	(10,617)	(11,628)	(10,076)	(9,026)	(9,439)	(10,125)
Single family residential | Permanent mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	1,839,156		2,021,320
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	203,920		203,134
Allowance for loan losses	(3,552)	(3,305)	(2,341)	(2,527)	(2,412)	(2,441)
Commercial real estate | Permanent mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	203,920		203,134
Construction and land loans | Permanent mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	19,266		20,442
Land, Construction and NM
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	19,366		20,542
Allowance for loan losses	(221)	$ (215)	(212)	$ (217)	$ (625)	$ (422)
Land, Construction and NM | Non-Mortgage loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	$ 100		$ 100