Annual report pursuant to Section 13 and 15(d)

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

v3.19.3.a.u2
PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Weighted-Average Assumptions Used to Determine Net Benefit Cost) (Details)
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Pension Plan [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Interest crediting rate 6.00% 6.00% 6.00%
Expected return on plan assets 8.25% 8.25% 8.25%
Other Postretirement Benefits Plan [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Expected return on plan assets 7.00% 7.00% 7.00%
Iron Hourly [Member] | Pension Plan [Member] | Obligation [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 4.31% 3.61% 4.02%
Iron Hourly [Member] | Pension Plan [Member] | Service Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 4.49% 3.76% 4.30%
Iron Hourly [Member] | Pension Plan [Member] | Interest Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.96% 3.21% 3.38%
Salaried [Member] | Pension Plan [Member] | Obligation [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 4.22% 3.52% 3.91%
Salaried [Member] | Pension Plan [Member] | Service Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 4.23% 3.53% 3.93%
Salaried [Member] | Pension Plan [Member] | Interest Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.85% 3.08% 3.21%
Ore Mining [Domain] | Pension Plan [Member] | Obligation [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate   3.61% 4.04%
Ore Mining [Domain] | Pension Plan [Member] | Service Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate   3.75% 4.27%
Ore Mining [Domain] | Pension Plan [Member] | Interest Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate   3.22% 3.41%
SERP [Domain] | Pension Plan [Member] | Obligation [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 4.21% 3.54% 3.90%
SERP [Domain] | Pension Plan [Member] | Service Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 4.11% 3.43% 3.69%
SERP [Domain] | Pension Plan [Member] | Interest Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.89% 3.16% 3.36%
Hourly [Member] | Other Postretirement Benefits Plan [Member] | Obligation [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 4.29% 3.60% 4.03%
Hourly [Member] | Other Postretirement Benefits Plan [Member] | Service Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 4.48% 3.73% 4.23%
Hourly [Member] | Other Postretirement Benefits Plan [Member] | Interest Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.95% 3.10% 3.24%
Salary [Member] | Other Postretirement Benefits Plan [Member] | Obligation [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 4.28% 3.57% 3.98%
Salary [Member] | Other Postretirement Benefits Plan [Member] | Service Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 4.54% 3.76% 4.30%
Salary [Member] | Other Postretirement Benefits Plan [Member] | Interest Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.91% 3.15% 3.28%
Hourly [Member] | Pension Plan [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Rate of compensation increase 2.00% 2.00% 2.00%
Salary [Member] | Pension Plan [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Rate of compensation increase 3.00% 3.00% 3.00%
Salary [Member] | Other Postretirement Benefits Plan [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Rate of compensation increase 3.00% 3.00% 3.00%