PENSION AND OTHER POSTRETIREMENT PLANS - 401k Plan (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
PENSION AND OTHER POSTRETIREMENT PLANS.
Participants contribution	$ 19,500
Description of Defined Contribution Pension and Other Postretirement Plans	100% of each employee’s contributions up to 1% of each employee’s compensation plus 50% of each employee’s contributions over 1% but not in excess of 6% of each employee’s compensation for a maximum contribution of 3.5% of a participating employee’s compensation
Minimum employee contribution percentage	1.00%
Percentage of employer matching contribution	50.00%
Threshold limit percentage of employee compensation	6.00%
Maximum percentage of annual contribution per employee	3.50%
Cash contributions by the Bank	$ 1,400,000	$ 1,100,000	$ 1,000,000.0
Discretionary profit sharing contribution	$ 546,000	$ 583,000	$ 497,000