Fair Value - Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Assets
Total assets	$ 28,071	$ 235,902
Liabilities
Contingent consideration	3,623
Total liabilities	22,662	24,104
Money market funds
Assets
Money market funds	28,071	235,902
Natural gas fixed price forward contracts
Liabilities
Derivatives		2,574
Embedded EPP derivatives
Liabilities
Derivatives	7,186	5,541
Interest rate swap agreements
Liabilities
Derivatives	11,853	15,989
Level 1
Assets
Total assets	28,071	235,902
Liabilities
Contingent consideration	0
Total liabilities	0	0
Level 1 \| Money market funds
Assets
Money market funds	28,071	235,902
Level 1 \| Natural gas fixed price forward contracts
Liabilities
Derivatives		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
Contingent consideration	0
Total liabilities	11,853	15,989
Level 2 \| Money market funds
Assets
Money market funds	0	0
Level 2 \| Natural gas fixed price forward contracts
Liabilities
Derivatives		0
Level 2 \| Embedded EPP derivatives
Liabilities
Derivatives	0	0
Level 2 \| Interest rate swap agreements
Liabilities
Derivatives	11,853	15,989
Level 3
Assets
Total assets	0	0
Liabilities
Contingent consideration	3,623
Total liabilities	10,809	8,115
Level 3 \| Money market funds
Assets
Money market funds	0	0
Level 3 \| Natural gas fixed price forward contracts
Liabilities
Derivatives		2,574
Level 3 \| Embedded EPP derivatives
Liabilities
Derivatives	7,186	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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Amount of liability recognized arising from contingent consideration in a business combination.

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