INVESTMENT AND MORTGAGE-BACKED SECURITIES - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Feb. 01, 2021
INVESTMENT AND MORTGAGE-BACKED SECURITIES
Acquired securities available-for-sale on the Merger Date						$ 652,000,000.0
Investment securities held-to-maturity, fair value	$ 40,303,000		$ 40,303,000		$ 0
Securities pledged as collateral, carrying amount	$ 684,000,000.0		$ 684,000,000.0		$ 99,400,000
Number of holdings of securities of any one issuer in an amount greater than 10% of stockholders equity	0		0		0
Threshold for disclosure percentage	10.00%				10.00%
Net gain on equity securities	$ 0	$ 175,000	$ 131,000	$ 139,000