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EMPLOYEE BENEFITS - Post-employment health care benefit plan - Assumptions (Details) - Post-employment health care benefit
Dec. 31, 2020
Dec. 31, 2019
Minimum    
Summary of assumptions used to calculate the defined benefit plans    
Average discount rate 2.80% 2.25%
Health treatment - rate assumed next year 4.99% 6.10%
Health treatment - Assumed rate of decline in the cost to achieve in the years of 2028 to 2041 3.20% 4.00%
Maximum    
Summary of assumptions used to calculate the defined benefit plans    
Average discount rate 2.90% 2.50%
Health treatment - rate assumed next year 6.20% 6.50%
Health treatment - Assumed rate of decline in the cost to achieve in the years of 2028 to 2041 4.50% 4.40%