Summary of significant accounting policies - Inventories (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023
Accounting Policies [Abstract]
Fossil fuels inventories	$ 83,705	$ 74,149
Spare parts	$ 272,580	$ 262,896

Gross amount of components of assembled products that can be sold directly or indirectly through sales of assembled products.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(6)(a))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant