Fair Value Measurements (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Assets and liabilities subject to fair value measurements
Commodity futures - financial assets	$ 4,149	$ 4,060
Commodity futures - financial liabilities	893	2,083
Fair value of long-term debt, including current maturities	2,600,000	2,500,000
Outstanding Balance	2,500,000	2,400,000
Futures \| Recurring \| Commodity contract
Assets and liabilities subject to fair value measurements
Commodity futures - financial assets	4,149	4,060
Commodity futures - financial liabilities	893	2,083
Futures \| Recurring \| Fair Value, Inputs, Level 2 [Member] \| Commodity contract
Assets and liabilities subject to fair value measurements
Commodity futures - financial assets	4,149	4,060
Commodity futures - financial liabilities	$ 893	$ 2,083

Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
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-Publisher FASB
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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

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The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

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