ACCOUNTING POLICIES (Detail Textuals) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Accounting Policies [Abstract]
Average remaining life expectancy of the inactive participants in the defined benefit pension plan	19
Acquisition-related costs	$ 48.8	$ 123.4	$ 4.2
Gain in Other Operating Income due to Insurance Recoveries at Becancour		42.3
Gain in Other Operating Income due to Insurance Recoveries at McIntosh		3.7
Short-term Investments	0.0	0.0
Asset retirement obligation non-current	42.8	46.2
Asset Retirement Obligation, Revision of Estimate	7.4	3.1
Goodwill, Impairment Loss	0.0	0.0	0.0
Impairment of Intangible Assets, Finite-lived	$ 0.0	$ 0.0	$ 0.0
Average remaining life expectancy of the inactive participants in the defined benefit pension plan (in years)	19 years	19 years