Derivative instruments - Gas hedges (Details) - Natural gas swaps $ in Thousands, MMBTU in Millions	12 Months Ended
Dec. 31, 2024 USD ($) MMBTU	Dec. 31, 2023 USD ($)
Derivative Instruments
Net Carrying Value Presented on the Balance Sheet \| $	$ 28,624	$ 13,445
Credit collateral posted \| $	$ 500	$ 0
Notional volume of natural gas derivatives (in MMBTUs)	63.4
2025
Derivative Instruments
Notional volume of natural gas derivatives (in MMBTUs)	26.1
2026
Derivative Instruments
Notional volume of natural gas derivatives (in MMBTUs)	22.8
2027
Derivative Instruments
Notional volume of natural gas derivatives (in MMBTUs)	11.8
2028
Derivative Instruments
Notional volume of natural gas derivatives (in MMBTUs)	1.2
2029
Derivative Instruments
Notional volume of natural gas derivatives (in MMBTUs)	1.5

The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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