INVESTMENT AND MORTGAGE-BACKED SECURITIES - Securities Held-to-maturity Major Categories (Details) - USD ($)	Sep. 30, 2021	Dec. 31, 2020
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 40,303,000
Fair Value	40,303,000	$ 0
Pass-through MBS issued by GSEs
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	25,325,000
Fair Value	25,325,000
Agency Collateralized Mortgage Obligations ("CMOs")
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	14,978,000
Fair Value	$ 14,978,000