<?xml version="1.0" encoding="us-ascii"?><InstanceReport xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"><Version>2.2.0.25</Version><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios><ReportLongName>006070 - Disclosure - Long-Term Debt</ReportLongName><DisplayLabelColumn>true</DisplayLabelColumn><ShowElementNames>false</ShowElementNames><RoundingOption /><HasEmbeddedReports>false</HasEmbeddedReports><Columns><Column><Id>1</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelColumn>false</LabelColumn><CurrencyCode>USD</CurrencyCode><FootnoteIndexer /><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios><MCU><KeyName>1/1/2011 - 3/31/2011
USD ($)
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font-size: 12pt;"&gt;&lt;tr valign="top"&gt;&lt;td style="width: 40pt;"&gt;&lt;div style="text-indent: 0pt; margin-left: 0pt; margin-right: 0pt;"&gt;&lt;font style="display: inline; font-family: Times New Roman; font-size: 12pt; font-weight: bold;"&gt;7.&lt;/font&gt;&lt;/div&gt;&lt;/td&gt;&lt;td&gt;&lt;div align="justify"&gt;&lt;font style="display: inline; font-family: Times New Roman; font-size: 12pt; font-weight: bold;"&gt;Long-Term Debt&lt;/font&gt;&lt;/div&gt;&lt;/td&gt;&lt;/tr&gt;&lt;/table&gt;&lt;/div&gt;&lt;div align="justify" style="text-indent: 0pt; display: block; margin-left: 0pt; margin-right: 0pt;"&gt;&amp;#160;&lt;/div&gt;&lt;div align="justify" style="text-indent: 0pt; display: block; margin-left: 40pt; margin-right: 0pt;"&gt;&lt;font style="display: inline; font-family: Times New Roman; font-size: 12pt;"&gt;At March 31, 2011, the Company was in compliance with all of its debt agreements.&lt;/font&gt;&lt;/div&gt;&lt;div align="justify" style="text-indent: 0pt; display: block; margin-left: 0pt; margin-right: 0pt;"&gt;&amp;#160;&lt;/div&gt;&lt;div align="justify" style="text-indent: 40pt; display: block; margin-left: 0pt; margin-right: 0pt;"&gt;&lt;font style="display: inline; font-family: Times New Roman; font-size: 12pt;"&gt;The Company has tax-exempt pollution control bonds with optional redemption provisions that allow the holders to request repayment of the bonds at various dates prior to the maturity.&amp;#160;&amp;#160;The bonds, which can be tendered at the option of the holder during the next 12 months, are as follows:&lt;/font&gt;&lt;/div&gt;&lt;div align="justify" style="text-indent: 0pt; display: block; margin-left: 0pt; margin-right: 0pt;"&gt;&amp;#160;&lt;/div&gt;&lt;div&gt;&lt;table cellpadding="0" cellspacing="0" width="100%" style="font-family: times new roman; font-size: 12pt;"&gt;&lt;tr&gt;&lt;td align="left" valign="top" width="22%" style="border-bottom: black 2px solid;"&gt;&lt;div align="left" style="text-indent: 0pt; display: block; margin-left: 32pt; margin-right: 0pt;"&gt;&lt;font style="display: inline; font-family: times new roman; font-size: 12pt; font-weight: bold;"&gt;SERIES&lt;/font&gt;&lt;/div&gt;&lt;/td&gt;&lt;td align="left" valign="top" width="66%" style="border-bottom: black 2px solid;"&gt;&lt;div align="left" style="text-indent: 0pt; display: block; margin-left: 40pt; margin-right: 0pt;"&gt;&lt;font style="display: inline; font-family: times new roman; font-size: 12pt; font-weight: bold;"&gt;DATE DUE&lt;/font&gt;&lt;/div&gt;&lt;/td&gt;&lt;td colspan="3" valign="top" width="12%" style="border-bottom: black 2px solid;"&gt;&lt;div align="center" style="text-indent: 0pt; display: block; margin-left: 0pt; margin-right: 0pt;"&gt;&lt;font style="display: inline; font-family: times new roman; font-size: 12pt; font-weight: bold;"&gt;AMOUNT&lt;/font&gt;&lt;/div&gt;&lt;/td&gt;&lt;/tr&gt;&lt;tr&gt;&lt;td valign="top" width="22%"&gt;&lt;font style="display: inline; font-family: times new roman; font-size: 12pt;"&gt;&amp;#160; &lt;/font&gt;&lt;/td&gt;&lt;td valign="top" width="66%"&gt;&lt;font style="display: inline; font-family: times new roman; font-size: 12pt;"&gt;&amp;#160; &lt;/font&gt;&lt;/td&gt;&lt;td colspan="3" valign="top" width="12%"&gt;&lt;div align="center" style="text-indent: 0pt; display: block; margin-left: 0pt; margin-right: 0pt;"&gt;&lt;font style="font-style: italic; display: inline; font-family: times new roman; font-size: 12pt;"&gt;(In millions)&lt;/font&gt;&lt;/div&gt;&lt;/td&gt;&lt;/tr&gt;&lt;tr&gt;&lt;td align="left" valign="top" width="22%"&gt;&lt;div align="left" style="text-indent: 0pt; display: block; margin-left: 24pt; margin-right: 0pt;"&gt;&lt;font style="display: inline; font-family: times new roman; font-size: 12pt;"&gt;0.39% - 0.44%&lt;/font&gt;&lt;/div&gt;&lt;/td&gt;&lt;td align="left" valign="top" width="66%"&gt;&lt;div align="left" style="text-indent: 0pt; display: block; margin-left: 0pt; margin-right: 0pt;"&gt;&lt;font style="display: inline; font-family: times new roman; font-size: 12pt;"&gt;Garfield Industrial Authority, January 1, 2025&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&lt;/font&gt;&lt;/div&gt;&lt;/td&gt;&lt;td align="right" valign="top" width="3%"&gt;&lt;div align="right" style="text-indent: 0pt; display: block; margin-left: 0pt; margin-right: 0pt;"&gt;&lt;font style="display: inline; font-family: times new roman; font-size: 12pt; font-weight: bold;"&gt;$&lt;/font&gt;&lt;/div&gt;&lt;/td&gt;&lt;td align="right" valign="top" width="6%"&gt;&lt;div align="right" style="text-indent: 0pt; display: block; margin-left: 0pt; margin-right: 0pt;"&gt;&lt;font style="display: inline; font-family: times new roman; font-size: 12pt; font-weight: bold;"&gt;47.0&lt;/font&gt;&lt;/div&gt;&lt;/td&gt;&lt;td valign="top" width="3%"&gt;&lt;font style="display: inline; font-family: times new roman; font-size: 12pt;"&gt;&amp;#160; &lt;/font&gt;&lt;/td&gt;&lt;/tr&gt;&lt;tr&gt;&lt;td align="left" valign="top" width="22%"&gt;&lt;div align="left" style="text-indent: 0pt; display: block; margin-left: 24pt; margin-right: 0pt;"&gt;&lt;font style="display: inline; font-family: times new roman; font-size: 12pt;"&gt;0.38% - 0.44%&lt;/font&gt;&lt;/div&gt;&lt;/td&gt;&lt;td align="left" valign="top" width="66%"&gt;&lt;div align="left" style="text-indent: 0pt; display: block; margin-left: 0pt; margin-right: 0pt;"&gt;&lt;font style="display: inline; font-family: times new roman; font-size: 12pt;"&gt;Muskogee Industrial Authority, January 1, 2025&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&lt;/font&gt;&lt;/div&gt;&lt;/td&gt;&lt;td valign="top" width="3%"&gt;&lt;font style="display: inline; font-family: times new roman; font-size: 12pt;"&gt;&amp;#160; &lt;/font&gt;&lt;/td&gt;&lt;td align="right" valign="top" width="6%"&gt;&lt;div align="right" style="text-indent: 0pt; display: block; margin-left: 0pt; margin-right: 0pt;"&gt;&lt;font style="display: inline; font-family: times new roman; font-size: 12pt; font-weight: bold;"&gt;32.4&lt;/font&gt;&lt;/div&gt;&lt;/td&gt;&lt;td valign="top" width="3%"&gt;&lt;font style="display: inline; font-family: times new roman; font-size: 12pt;"&gt;&amp;#160; &lt;/font&gt;&lt;/td&gt;&lt;/tr&gt;&lt;tr&gt;&lt;td align="left" valign="top" width="22%" style="border-bottom: black 2px solid;"&gt;&lt;div align="left" style="text-indent: 0pt; display: block; margin-left: 24pt; margin-right: 0pt;"&gt;&lt;font style="display: inline; font-family: times new roman; font-size: 12pt;"&gt;0.50% - 0.50%&lt;/font&gt;&lt;/div&gt;&lt;/td&gt;&lt;td align="left" valign="top" width="66%" style="border-bottom: black 2px solid;"&gt;&lt;div align="left" style="text-indent: 0pt; display: block; margin-left: 0pt; margin-right: 0pt;"&gt;&lt;font style="display: inline; font-family: times new roman; font-size: 12pt;"&gt;Muskogee Industrial Authority, June 1, 2027&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&lt;/font&gt;&lt;/div&gt;&lt;/td&gt;&lt;td valign="top" width="3%" style="border-bottom: black 2px solid;"&gt;&lt;font style="display: inline; font-family: times new roman; font-size: 12pt;"&gt;&amp;#160; &lt;/font&gt;&lt;/td&gt;&lt;td align="right" valign="top" width="6%" style="border-bottom: black 2px solid;"&gt;&lt;div align="right" style="text-indent: 0pt; display: block; margin-left: 0pt; margin-right: 0pt;"&gt;&lt;font style="display: inline; font-family: times new roman; font-size: 12pt; font-weight: bold;"&gt;56.0&lt;/font&gt;&lt;/div&gt;&lt;/td&gt;&lt;td valign="top" width="3%" style="border-bottom: black 2px solid;"&gt;&lt;font style="display: inline; font-family: times new roman; font-size: 12pt;"&gt;&amp;#160; &lt;/font&gt;&lt;/td&gt;&lt;/tr&gt;&lt;tr&gt;&lt;td align="left" colspan="2" valign="top" width="88%" style="border-bottom: black 4px double;"&gt;&lt;div align="left" style="text-indent: 0pt; display: block; margin-left: 56pt; margin-right: 0pt;"&gt;&lt;font style="display: inline; font-family: times new roman; font-size: 12pt;"&gt;Total (redeemable during next 12 months)&lt;/font&gt;&lt;/div&gt;&lt;/td&gt;&lt;td align="right" valign="top" width="3%" style="border-bottom: black 4px double;"&gt;&lt;div align="right" style="text-indent: 0pt; display: block; margin-left: 0pt; margin-right: 0pt;"&gt;&lt;font style="display: inline; font-family: times new roman; font-size: 12pt; font-weight: bold;"&gt;$&lt;/font&gt;&lt;/div&gt;&lt;/td&gt;&lt;td align="right" valign="top" width="6%" style="border-bottom: black 4px double;"&gt;&lt;div align="right" style="text-indent: 0pt; display: block; margin-left: 0pt; margin-right: 0pt;"&gt;&lt;font style="display: inline; font-family: times new roman; font-size: 12pt; font-weight: bold;"&gt;135.4&lt;/font&gt;&lt;/div&gt;&lt;/td&gt;&lt;td valign="top" width="3%" style="border-bottom: black 4px double;"&gt;&lt;font style="display: inline; font-family: times new roman; font-size: 12pt;"&gt;&amp;#160; &lt;/font&gt;&lt;/td&gt;&lt;/tr&gt;&lt;/table&gt;&lt;/div&gt;&lt;div style="text-indent: 0pt; display: block;"&gt;&lt;br /&gt;&lt;/div&gt;&lt;div align="justify" style="text-indent: 40pt; display: block; margin-left: 0pt; margin-right: 0pt;"&gt;&lt;font style="display: inline; font-family: Times New Roman; font-size: 12pt;"&gt;All of these bonds are subject to an optional tender at the request of the holders, at 100 percent of the principal amount, together with accrued and unpaid interest to the date of purchase.&amp;#160;&amp;#160;The bond holders, on any business day, can request repayment of the bond by delivering an irrevocable notice to the tender agent stating the principal amount of the bond, payment instructions for the purchase price and the business day the bond is to be purchased.&amp;#160;&amp;#160;The repayment option may only be exercised by the holder of a bond for the principal amount.&amp;#160;&amp;#160;When a tender notice has been received by the trustee, a third party remarketing agent for the bonds will attempt to remarket any bonds tendered for purchase.&amp;#160;&amp;#160;This process occurs once per week.&amp;#160;&amp;#160;Since the original issuance of these series of bonds in 1995 and 1997, the remarketing agent has &lt;/font&gt;&lt;font style="display: inline; font-family: Times New Roman; font-size: 12pt;"&gt;successfully remarketed all tendered bonds.&amp;#160;&amp;#160;If the remarketing agent is unable to remarket any such bonds, the Company is obligated to repurchase such unremarketed bonds.&amp;#160;&amp;#160;As the Company has both the intent and ability to refinance the bonds on a long-term basis and such ability is supported by an ability to consummate the refinancing, the bonds are classified as long-term debt in the Company's Condensed Financial Statements. The Company believes that it has sufficient liquidity to meet these obligations.&lt;/font&gt;&lt;/div&gt;&lt;div align="justify" style="text-indent: 0pt; display: block; margin-left: 0pt; margin-right: 0pt;"&gt;&amp;#160;&lt;/div&gt;&lt;/div&gt;&lt;/div&gt;</NonNumbericText><NonNumericTextHeader>7.Long-Term Debt&amp;#160;At March 31, 2011, the Company was in compliance with all of its debt agreements.&amp;#160;The Company has tax-exempt pollution control bonds</NonNumericTextHeader><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios></Cell></Cells><OriginalInstanceReportColumns /><Unit>Other</Unit><ElementDataType>us-types:textBlockItemType</ElementDataType><SimpleDataType>string</SimpleDataType><ElementDefenition>This element may be used as a single block of text to encapsulate the entire disclosure for long-term borrowings including data and tables.</ElementDefenition><ElementReferences>Reference 1: http://www.xbrl.org/2003/role/presentationRef
 -Publisher SEC
 -Name Regulation S-X (SX)
 -Number 210
 -Section 02
 -Paragraph 22
 -Article 5

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