Annual report pursuant to Section 13 and 15(d)

PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Weighted-Average Assumptions Used to Determine Benefit Obligations) (Details)

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PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Weighted-Average Assumptions Used to Determine Benefit Obligations) (Details)
Dec. 31, 2015
Dec. 31, 2014
Pension Benefits [Member] | United States [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Discount rate   3.83%
Salary [Member] | Pension Benefits [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Rate of compensation increase 3.00% 3.00%
Hourly [Member] | Pension Benefits [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Rate of compensation increase 2.00% 2.50%
Iron Hourly [Member] | Pension Benefits [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Discount rate 4.27% 3.83%
Salaried [Member] | Pension Benefits [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Discount rate 4.12% 3.83%
Salaried [Member] | Other Benefits [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Discount rate 4.22% 3.83%
Ore Mining [Member] | Pension Benefits [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Discount rate 4.28% 3.83%
Supplemental Executive Retirement Plan S E R P [Member] | Pension Benefits [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Discount rate 4.22% 3.83%
Hourly [Member] | Other Benefits [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Discount rate 4.32% 3.83%