Summary of significant accounting policies - Inventories (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Accounting Policies [Abstract]
Fossil fuels inventories	$ 44,601	$ 83,547
Spare parts	$ 215,925	$ 196,742

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.6)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

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