Commitments and Contingencies (Details) $ in Millions			1 Months Ended			3 Months Ended
	Mar. 31, 2016 company	Mar. 13, 2013 company mi	Sep. 30, 2017 company mi	Sep. 30, 2016 mi	Mar. 31, 2016 mi	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 01, 2017 USD ($)	Mar. 04, 2016 USD ($)
Schedule of Equity Method Investments [Line Items]
Accrued probable future environmental expenditures						$ 116.5		$ 117.1
Cost of Sales
Schedule of Equity Method Investments [Line Items]
Expense related to environmental activities						3.1	$ 2.2
Proceeds from insurance recoveries						0.7
Calvert City
Schedule of Equity Method Investments [Line Items]
Accrued probable future environmental expenditures						$ 107.0
Total estimated cost of remedy									$ 244.0
Contamination of Passaic River Study Area
Schedule of Equity Method Investments [Line Items]
Approximate number of other companies assuming responsibility | company		70
Contamination of Passaic River Study Area | Lower Passaic River
Schedule of Equity Method Investments [Line Items]
Total estimated cost of remedy										$ 1,400.0
Length of portion of river | mi		17	8	8	8
Number of companies receiving notice of potential liability | company	100
Number of companies receiving notice of process to allocate remedial costs | company			80