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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% |  |

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- Definition Carrying value as of the balance sheet date of the long-term debt associated with industrial revenue bonds. For classified balance sheets, these bonds are considered current assets because they are remarketed weekly and the bonds could be required to be repaid upon demand if there is a failed remarketing. + References No definition available. + Details
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- Definition Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 20 -Section 50 -Paragraph 6 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6036836-161870 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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