Annual report pursuant to Section 13 and 15(d)

PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Additional Information) (Details)

v3.3.1.900
PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Additional Information) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Pension Benefits [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Effect of change in mine ownership & noncontrolling interest $ 48.4 $ 51.2 $ 46.3
Actual return on plan assets (6.4) 59.1 80.3
Other Benefits [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Effect of change in mine ownership & noncontrolling interest 5.5 5.9 4.8
Actual return on plan assets $ (3.9) $ 31.9 $ 11.0