Fair Value Measurements (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Assets and liabilities subject to fair value measurements
Short-term Investments	$ 69,529	$ 228,783
Commodity futures - financial assets	1,376	4,558
Commodity futures - financial liabilities	3,905	3,890
Outstanding Balance	2,500,000	2,400,000
Fair Value, Inputs, Level 2 [Member]
Assets and liabilities subject to fair value measurements
Short-term Investments	69,529	228,783
Fair value of long-term debt, including current maturities	2,500,000	2,400,000
Futures \| Recurring \| Commodity contract
Assets and liabilities subject to fair value measurements
Commodity futures - financial assets	1,376	4,558
Commodity futures - financial liabilities	3,905	3,890
Futures \| Recurring \| Fair Value, Inputs, Level 2 [Member] \| Commodity contract
Assets and liabilities subject to fair value measurements
Commodity futures - financial assets	1,376	4,558
Commodity futures - financial liabilities	$ 3,905	$ 3,890

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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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.

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