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PENSION AND OTHER POSTRETIREMENT PLANS - 401k Plan (Details) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]      
Labor and Related Expense $ 50,458,000 $ 46,560,000 $ 41,557,000
Participants contribution $ 18,500    
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 $ 1,000,000 1,000,000 786,000
Discretionary profit sharing contribution $ 497,000 550,000 424,000
ASU 2017- 07 | Measurement Period Adjustments      
Defined Benefit Plan Disclosure [Line Items]      
Labor and Related Expense   (794,000) (644,000)
Other operating expense   $ 794,000 $ 644,000