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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, 2017
Dec. 31, 2016
Salary [Member] | Pension Benefits [Member] Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý
Rate of compensation increase 3.00% 3.00%
Hourly [Member] | Pension Benefits [Member] Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý
Rate of compensation increase 2.00% 2.00%
Iron Hourly [Member] | Pension Benefits [Member] Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý
Discount rate 3.60% 4.02%
Salaried [Member] | Pension Benefits [Member] Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý
Discount rate 3.52% 3.92%
Salaried [Member] | Other Benefits [Member] Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý
Discount rate 3.57% 3.99%
Ore Mining [Member] | Pension Benefits [Member] Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý
Discount rate 3.61% 4.04%
Supplemental Executive Retirement Plan S E R P [Member] | Pension Benefits [Member] Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý
Discount rate 3.50% 3.90%
Hourly [Member] | Other Benefits [Member] Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý
Discount rate 3.60% 4.02%