Quarterly Information (Unaudited) - Financial Results Overview (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Sep. 30, 2018	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Effect of Fourth Quarter Events [Line Items]
Inventory adjustments			$ 30.6
Net gain (loss) on forward and derivative contracts		$ (16.1)	$ 6.3	$ (16.5)	[1]	$ 3.5
Helguvik [Member]
Effect of Fourth Quarter Events [Line Items]
Gain on extinguishment of liability	$ 4.5			$ 7.3

[1] As adjusted due to the adoption of ASU 2017-07 Compensation - Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost