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Debt (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Mar. 31, 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 | 256.1 | 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 $1.7 million and $1.8 million, respectively, interest payable semiannually | $ 248.3 | $ 248.2 |
Maturity date | Jun. 01, 2021 | Jun. 01, 2021 |
Stated interest rate, percentage | 7.50% | 7.50% | 7.50%
Debt discount | $ 1.7 | $ 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 | 1.78% |  |
Short-term Debt [Member] | Industrial revenue bonds due 2028 [Member] | Maximum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Effective interest rate, maximum | 12.00% | 12.00% |

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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/presentationRef -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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399 + Details
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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -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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- Definition Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(2)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Amount, after accumulated amortization, of debt discount. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28541-108399 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400 + Details
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- Definition Carrying amount of collateralized debt obligations with maturities initially due after one year or beyond the operating cycle, if longer, excluding the current portion. Obligations include, but not limited to, mortgage loans, chattel loans, and other borrowings secured by assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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