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EMPLOYEE BENEFIT PLANS - Projected benefit obligation and net benefit cost (Details) - Retirement Income Plan
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
PROJECTED BENEFIT OBLIGATION:      
Discount rate 4.05% 4.50% 4.75%
Rate of compensation increase
NET BENEFIT COST:      
Discount rate 4.50% 4.75% 4.25%
Expected return on plan assets 7.00% 7.00% 7.00%
Rate of compensation increase