Derivative Instruments - Fair value of derivative instruments (Details) - Natural gas swaps - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Prepayments and other current assets
Assets:
Fair value of assets	$ 6,356	$ 0
Other deferred charges
Assets:
Fair value of assets	19,995	25,459
Other current liabilities
Liabilities:
Fair value of liabilities	3,869	10,370
Other deferred credits
Liabilities:
Fair value of liabilities	$ 392	$ 1,644