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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, 2019
Dec. 31, 2018
Dec. 31, 2017
Pension Plan [Member] Ìý Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý Ìý
Interest crediting rate 6.00% 6.00% 6.00%
Salary [Member] | Pension Plan [Member] Ìý Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý Ìý
Rate of compensation increase 3.00% 3.00% Ìý
Salary [Member] | Other Postretirement Benefits Plan [Member] Ìý Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý Ìý
Rate of compensation increase 3.00% 3.00% Ìý
Hourly [Member] | Pension Plan [Member] Ìý Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý Ìý
Rate of compensation increase 2.00% 2.00% Ìý
Iron Hourly [Member] | Pension Plan [Member] Ìý Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý Ìý
Discount rate 3.34% 4.31% Ìý
Salaried [Member] | Pension Plan [Member] Ìý Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý Ìý
Discount rate 3.18% 4.21% Ìý
Salaried [Member] | Other Postretirement Benefits Plan [Member] Ìý Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý Ìý
Discount rate 3.29% 4.27% Ìý
Ore Mining [Member] | Pension Plan [Member] Ìý Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý Ìý
Discount rate Ìý 4.33% Ìý
SERP [Domain] | Pension Plan [Member] Ìý Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý Ìý
Discount rate 3.05% 4.22% Ìý
Hourly [Member] | Other Postretirement Benefits Plan [Member] Ìý Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý Ìý
Discount rate 3.28% 4.29% Ìý