Annual report pursuant to Section 13 and 15(d)

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

v3.8.0.1
PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Weighted-Average Assumptions Used to Determine Net Benefit Cost) (Details)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
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 3.90% 4.01% 3.83%
SERP [Domain] | Pension Benefits [Member] | Service Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.69% 3.87% 3.83%
SERP [Domain] | Pension Benefits [Member] | Interest Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.36% 3.30% 3.83%
Ore Mining [Domain] | Pension Benefits [Member] | Obligation [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 4.04% 4.28% 3.83%
Ore Mining [Domain] | Pension Benefits [Member] | Service Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 4.27% 4.60% 3.83%
Ore Mining [Domain] | Pension Benefits [Member] | Interest Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.41% 3.48% 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% 3.00%
Salary [Member] | Pension Benefits [Member] | Obligation [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.91% 4.13% 3.83%
Salary [Member] | Pension Benefits [Member] | Service Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.93% 4.14% 3.83%
Salary [Member] | Pension Benefits [Member] | Interest Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.21% 3.21% 3.83%
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 3.98% 4.22% 3.83%
Salary [Member] | Other Benefits [Member] | Service Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 4.30% 4.63% 3.83%
Salary [Member] | Other Benefits [Member] | Interest Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.28% 3.31% 3.83%
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% 2.50%
Hourly [Member] | Pension Benefits [Member] | Obligation [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 4.02% 4.27% 3.83%
Hourly [Member] | Pension Benefits [Member] | Service Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 4.30% 4.66% 3.83%
Hourly [Member] | Pension Benefits [Member] | Interest Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.38% 3.46% 3.83%
Hourly [Member] | Other Benefits [Member] | Obligation [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 4.03% 4.32% 3.83%
Hourly [Member] | Other Benefits [Member] | Service Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 4.23% 4.56% 3.83%
Hourly [Member] | Other Benefits [Member] | Interest Cost [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate 3.24% 3.48% 3.83%