Derivative Instruments - Fair value of derivative instruments (Details) - Natural gas swaps - USD ($)	Sep. 30, 2023	Dec. 31, 2022
Assets:
Fair value of assets	$ 47,054,000	$ 131,804,000
Other current assets
Assets:
Fair value of assets	10,495,000	35,285,000
Other deferred charges
Assets:
Fair value of assets	41,366,000	99,725,000
Other current liabilities
Liabilities:
Fair value of liabilities	3,940,000	3,206,000
Other deferred credits
Liabilities:
Fair value of liabilities	$ 867,000	$ 0