Quarterly report pursuant to Section 13 or 15(d)

ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Asset Retirement Obligation Disclosure) (Details)

v3.10.0.1
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Asset Retirement Obligation Disclosure) (Details) - USD ($)
$ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Asset Retirement Obligation [Roll Forward]    
Asset retirement obligation at beginning of period $ 168.4 $ 187.8
Accretion expense 7.1 13.9
Remediation payments (0.9) (5.6)
Revision in estimated cash flows 0.0 (27.7)
Asset retirement obligation at end of period $ 174.6 $ 168.4