Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets and liabilities subject to fair value measurements
Short-term Investments	$ 228,783
Commodity futures - financial assets	4,558	$ 2,790
Commodity futures - financial liabilities	3,890	10,506
Outstanding Balance	2,376,723	2,381,940
Level 2
Assets and liabilities subject to fair value measurements
Short-term Investments	228,783
Fair value of long-term debt, including current maturities	2,400,000	2,500,000
Recurring \| Commodity contract \| Futures
Assets and liabilities subject to fair value measurements
Commodity futures - financial assets	4,558	2,790
Commodity futures - financial liabilities	3,890	10,506
Recurring \| Level 2 \| Commodity contract \| Futures
Assets and liabilities subject to fair value measurements
Commodity futures - financial assets	4,558	2,790
Commodity futures - financial liabilities	$ 3,890	$ 10,506

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