Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Assets and liabilities subject to fair value measurements
Commodity futures - financial assets	$ 1,276	$ 4,558
Commodity futures - financial liabilities	1,732	3,890
Outstanding principal balance	2,764,472	2,401,393
Level 2
Assets and liabilities subject to fair value measurements
Fair value of long-term debt, including current maturities	2,800,000	2,400,000
Recurring
Assets and liabilities subject to fair value measurements
Short-term Investments	262,174	228,783
Recurring \| Level 1
Assets and liabilities subject to fair value measurements
Short-term Investments
Recurring \| Level 2
Assets and liabilities subject to fair value measurements
Short-term Investments	262,174	228,783
Recurring \| Level 3
Assets and liabilities subject to fair value measurements
Short-term Investments
Futures \| Recurring \| Commodity contract
Assets and liabilities subject to fair value measurements
Commodity futures - financial assets	1,276	4,558
Commodity futures - financial liabilities	1,732	3,890
Futures \| Recurring \| Level 1 \| Commodity contract
Assets and liabilities subject to fair value measurements
Commodity futures - financial assets
Commodity futures - financial liabilities
Futures \| Recurring \| Level 2 \| Commodity contract
Assets and liabilities subject to fair value measurements
Commodity futures - financial assets	1,276	4,558
Commodity futures - financial liabilities	1,732	3,890
Futures \| Recurring \| Level 3 \| Commodity contract
Assets and liabilities subject to fair value measurements
Commodity futures - financial assets
Commodity futures - financial liabilities

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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