INVESTMENT AND MORTGAGE-BACKED SECURITIES - Amortized Cost and Fair Value By Contractual Maturity (Details) - USD ($)	Sep. 30, 2021	Dec. 31, 2020
Available for sale, Amortized Cost
Within one year	$ 1,346,000
One to five years	270,069,000
Five to ten years	209,938,000
Beyond ten years	12,929,000
Pass-through MBS issued by GSEs and Agency CMO	1,212,610,000
Amortized Cost	1,706,892,000	$ 520,261,000
Available for sale, Fair Value
Within one year	1,351,000
One to five years	269,665,000
Five to ten years	212,881,000
Beyond ten years	12,783,000
Pass-through MBS issued by GSEs and Agency CMO	1,212,383,000
Total	1,709,063,000	538,861,000
Held-to-maturity, Amortized Cost
Pass-through MBS issued by GSEs and agency CMO	40,303,000
Total	40,303,000
Held-to-maturity, Fair Value
Pass-through MBS issued by GSEs and agency CMO	40,303,000
Fair Value	$ 40,303,000	$ 0