Summary of significant accounting policies - Restricted investments and Inventories (Details) $ in Thousands		12 Months Ended
Oct. 01, 2020	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Restricted investments
Guaranteed interest rate on deposit		0.05
Restricted investments		$ 533,590	$ 653,158
Restricted investments, long-term		461,757	503,158
Inventories
Fossil fuels inventories		74,257	48,709
Spare parts		$ 203,472	$ 210,379
Forecast
Restricted investments
Guaranteed interest rate on deposit	0.04

Investment Interest Rate

No definition available.

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- Definition

Amount of fossil fuel included in inventory classified as other.

+ References

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(a))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

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Period Type:	duration

- Definition

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

+ References

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=6877327&loc=d3e13212-122682

+ Details

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Period Type:	instant

- References

No definition available.

+ Details

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- Definition

Investments which are not defined as or included in marketable (debt, equity, or other) securities whose use is restricted in whole or in part, generally by contractual agreements or regulatory requirements. For use in an unclassified balance sheet.

+ References

No definition available.

+ Details

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- Definition

This element represents the noncurrent portion of investments which are not defined as or included in marketable (debt, equity, or other) securities that are pledged or subject to withdrawal restrictions.

+ References

No definition available.

+ Details

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- Details

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