Fair Value - Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Assets
Total assets	$ 158,151	$ 235,902
Liabilities
Total liabilities	19,879	24,104
Money market funds
Assets
Money market funds	158,151	235,902
Natural gas fixed price forward contracts
Liabilities
Derivatives	226	2,574
Embedded EPP derivatives
Liabilities
Derivatives	7,002	5,541
Interest rate swap agreements
Liabilities
Derivatives	12,651	15,989
Level 1
Assets
Total assets	158,151	235,902
Liabilities
Total liabilities	0	0
Level 1 \| Money market funds
Assets
Money market funds	158,151	235,902
Level 1 \| Natural gas fixed price forward contracts
Liabilities
Derivatives	0	0
Level 1 \| Embedded EPP derivatives
Liabilities
Derivatives	0	0
Level 1 \| Interest rate swap agreements
Liabilities
Derivatives	0	0
Level 2
Assets
Total assets	0	0
Liabilities
Total liabilities	12,877	15,989
Level 2 \| Money market funds
Assets
Money market funds	0	0
Level 2 \| Natural gas fixed price forward contracts
Liabilities
Derivatives	226	0
Level 2 \| Embedded EPP derivatives
Liabilities
Derivatives	0	0
Level 2 \| Interest rate swap agreements
Liabilities
Derivatives	12,651	15,989
Level 3
Assets
Total assets	0	0
Liabilities
Total liabilities	7,002	8,115
Level 3 \| Money market funds
Assets
Money market funds	0	0
Level 3 \| Natural gas fixed price forward contracts
Liabilities
Derivatives	0	2,574
Level 3 \| Embedded EPP derivatives
Liabilities
Derivatives	7,002	5,541
Level 3 \| Interest rate swap agreements
Liabilities
Derivatives	$ 0	$ 0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Fair value, after the 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 not subject to a master netting arrangement and not elected to be offset.

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Fair value of financial and nonfinancial obligations.

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