FAIR VALUE - Quantitative Information (Details) - Significant Unobservable Inputs (Level 3) - Other real estate owned: - Non-recurring - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Information related to Level 3 non-recurring fair value measurement [Abstract]
Assets, fair value disclosure	$ 171	$ 237
Residential mortgages | Sales comparison
Information related to Level 3 non-recurring fair value measurement [Abstract]
Assets, fair value disclosure	$ 124	$ 126
Residential mortgages | Sales comparison | Minimum
Information related to Level 3 non-recurring fair value measurement [Abstract]
Discount to appraised value (in percentage)	20.80%	20.80%
Residential mortgages | Sales comparison | Maximum
Information related to Level 3 non-recurring fair value measurement [Abstract]
Discount to appraised value (in percentage)	20.80%	20.80%
Residential mortgages | Sales comparison | Weighted Average
Information related to Level 3 non-recurring fair value measurement [Abstract]
Discount to appraised value (in percentage)	20.80%	20.80%
Home equity lines and loans | Sales comparison
Information related to Level 3 non-recurring fair value measurement [Abstract]
Assets, fair value disclosure	$ 47
Home equity lines and loans | Sales comparison | Minimum
Information related to Level 3 non-recurring fair value measurement [Abstract]
Discount to appraised value (in percentage)	10.00%
Home equity lines and loans | Sales comparison | Maximum
Information related to Level 3 non-recurring fair value measurement [Abstract]
Discount to appraised value (in percentage)	10.00%
Home equity lines and loans | Sales comparison | Weighted Average
Information related to Level 3 non-recurring fair value measurement [Abstract]
Discount to appraised value (in percentage)	10.00%
Commercial mortgages, other | Sales comparison
Information related to Level 3 non-recurring fair value measurement [Abstract]
Assets, fair value disclosure		$ 111
Commercial mortgages, other | Sales comparison | Minimum
Information related to Level 3 non-recurring fair value measurement [Abstract]
Discount to appraised value (in percentage)		20.80%
Commercial mortgages, other | Sales comparison | Maximum
Information related to Level 3 non-recurring fair value measurement [Abstract]
Discount to appraised value (in percentage)		20.80%
Commercial mortgages, other | Sales comparison | Weighted Average
Information related to Level 3 non-recurring fair value measurement [Abstract]
Discount to appraised value (in percentage)		20.80%