Derivative Instruments - Fair value of derivative instruments (Details) - Natural gas swaps - USD ($)	Mar. 31, 2024	Dec. 31, 2023
Assets:
Fair value of assets	$ 23,698,000	$ 25,459,000
Derivatives From Counterparties
Assets:
Fair value of assets	10,381,000	13,445,000
Other current liabilities
Liabilities:
Fair value of liabilities	12,688,000	10,370,000
Other deferred credits
Liabilities:
Fair value of liabilities	$ 629,000	$ 1,644,000