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Debt (Details) - USD ($)
$ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Jun. 04, 2013
Debt Instrument [Line Items]      
Hancock County industrial revenue bonds (IRBs) due 2028, interest payable quarterly (variable interest rates (not to exceed 12%)) (1) $ 7.8 $ 7.8  
Total debt 270.6 256.0  
Long-term Debt [Member] | Senior secured notes due June 01, 2021 [Member]      
Debt Instrument [Line Items]      
7.5% senior secured notes due June 1, 2021, net of debt discount of $1.5 million and $1.8 million, respectively, interest payable semiannually $ 248.5 $ 248.2  
Maturity date Jun. 01, 2021 Jun. 01, 2021  
Stated interest rate, percentage 7.50% 7.50% 7.50%
Debt discount $ 1.5 $ 1.8  
Short-term Debt [Member] | Industrial revenue bonds due 2028 [Member]      
Debt Instrument [Line Items]      
Hancock County industrial revenue bonds (IRBs) due 2028, interest payable quarterly (variable interest rates (not to exceed 12%)) (1) $ 7.8 $ 7.8  
Maturity date Apr. 01, 2028 Apr. 01, 2028  
Stated interest rate, percentage 1.76%    
Short-term Debt [Member] | Industrial revenue bonds due 2028 [Member] | Maximum [Member]      
Debt Instrument [Line Items]      
Effective interest rate, maximum 12.00% 12.00%  
Short-term Debt [Member] | U.S. revolving credit facility borrowings [Member]      
Debt Instrument [Line Items]      
Secured Debt, Current $ 14.3 $ 0.0  
Stated interest rate, percentage 5.50%