Annual report pursuant to Section 13 and 15(d)

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

v3.10.0.1
PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Weighted-Average Assumptions Used to Determine Net Benefit Cost) (Details)
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Pension Benefits [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Expected return on plan assets 8.25% 8.25% 8.25%
Other Benefits [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%
SERP [Domain] | Pension Benefits [Member] | Obligation [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.54% 3.90% 4.01%
SERP [Domain] | Pension Benefits [Member] | Service Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.43% 3.69% 3.87%
SERP [Domain] | Pension Benefits [Member] | Interest Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.16% 3.36% 3.30%
Ore Mining [Domain] | Pension Benefits [Member] | Obligation [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.61% 4.04% 4.28%
Ore Mining [Domain] | Pension Benefits [Member] | Service Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.75% 4.27% 4.60%
Ore Mining [Domain] | Pension Benefits [Member] | Interest Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.22% 3.41% 3.48%
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% 3.00%
Salary [Member] | Pension Benefits [Member] | Obligation [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.52% 3.91% 4.13%
Salary [Member] | Pension Benefits [Member] | Service Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.53% 3.93% 4.14%
Salary [Member] | Pension Benefits [Member] | Interest Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.08% 3.21% 3.21%
Salary [Member] | Other Benefits [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 Benefits [Member] | Obligation [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.57% 3.98% 4.22%
Salary [Member] | Other Benefits [Member] | Service Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.76% 4.30% 4.63%
Salary [Member] | Other Benefits [Member] | Interest Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.15% 3.28% 3.31%
Hourly [Member] | Pension Benefits [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Expected return on plan assets 2.00% 2.00% 2.00%
Hourly [Member] | Pension Benefits [Member] | Obligation [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.61% 4.02% 4.27%
Hourly [Member] | Pension Benefits [Member] | Service Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.76% 4.30% 4.66%
Hourly [Member] | Pension Benefits [Member] | Interest Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.21% 3.38% 3.46%
Hourly [Member] | Other Benefits [Member] | Obligation [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.60% 4.03% 4.32%
Hourly [Member] | Other Benefits [Member] | Service Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.73% 4.23% 4.56%
Hourly [Member] | Other Benefits [Member] | Interest Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.10% 3.24% 3.48%