Summary of significant accounting policies: - Restricted investments and Inventories (Details) - USD ($)		12 Months Ended
Oct. 01, 2020	Dec. 31, 2018	Dec. 31, 2017
Restricted investments
Guaranteed interest rate on deposit (as a percent)		5.00%	5.00%
Restricted investments		$ 653,158,000	$ 882,909,000
Restricted investments, long-term		503,158,000	653,585,000
Inventories
Fossil fuels inventories		48,709,000	54,050,000
Spare parts		$ 210,379,000	$ 212,169,000
Forecast
Restricted investments
Guaranteed interest rate on deposit (as a percent)	4.00%

The interest rate for investments that have an interest rate. For fixed rate investments, this indicates the fixed interest rate. If the investment has a variable interest rate, the rate stated here may be the rate that is currently relevant. In this case the "Investment, Interest Rate is Current Rate for Variable Rate Investment Flag", "Investment, Interest Rate is Discount Rate at Purchase Flag", "Investment, Interest Rate Reflects Current Yield Flag" should be used. If one of these flags are not appropriate to explain the interest rate that is indicated, then a new flag should be added or an explanation should be included in "Investment, Additional Information".

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 12
-Sentence Column A

+ Details

Name:	invest_InvestmentInterestRate
Namespace Prefix:	invest_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of fossil fuel included in inventory classified as other.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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

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

- References

No definition available.

+ Details

Name:	us-gaap_InventoryDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
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://www.xbrl.org/2003/role/presentationRef
-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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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_RestrictedCashAndInvestmentsAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- 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

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

- 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

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

- Details

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