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Quarterly Information (Unaudited) - Financial Results Overview (Details) $ in Millions	3 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($)	Jun. 30, 2018 pot_line	Dec. 31, 2019 USD ($)
Effect of Fourth Quarter Events [Line Items]
Inventory adjustments			$ (30.6)
Number of potlines, temporary curtailment | pot_line		1
BHH [Member]
Effect of Fourth Quarter Events [Line Items]
Loss on sale of equity investments			$ 4.3
Helguvik [Member]
Effect of Fourth Quarter Events [Line Items]
Gain on extinguishment of liability	$ 4.5

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