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PENSION AND OTHER POSTRETIREMENT PLANS - 401k Plan (Details) - USD ($)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
PENSION AND OTHER POSTRETIREMENT PLANS.      
Labor and Related Expense $ 56,244,000 $ 50,458,000 $ 46,560,000
Participants contribution $ 19,000    
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 $ 1,100,000 1,000,000 1,000,000
Discretionary profit sharing contribution $ 583,000 $ 497,000 $ 550,000