Derivative Instruments - Fair value of derivative instruments (Details) - Natural gas swaps - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Assets:
Fair value of assets	$ 49,235	$ 131,804
Other current assets
Assets:
Fair value of assets	9,688	35,285
Other deferred charges
Assets:
Fair value of assets	44,681	99,725
Other current liabilities
Liabilities:
Fair value of liabilities	5,115	3,206
Other deferred credits
Liabilities:
Fair value of liabilities	$ 19	$ 0