Summary of significant accounting policies - Nuclear fuel cost (Details) - USD ($)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Nuclear fuel cost
Nuclear fuel expense	$ 75,968,000	$ 79,893,000	$ 85,949,000
Nuclear fuel disposal cost litigation for period from January 1, 2015 through December 31, 2017 \| Pending litigation \| Plant Hatch and Plant Vogtle
Nuclear fuel cost
Damages receivable	$ 0	$ 0

Amount of receivable related to a loss contingency accrual. For example, an insurance recovery receivable.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=118943654&loc=d3e12053-110248

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