Regulatory liabilities (Details 2) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020
Regulatory Liabilities [Line Items]
Regulatory liabilities due within one year	$ 23,454	$ 31,450	$ 48,000
Regulatory liabilities	428,467	428,075	438,652
Total regulatory liabilities	451,921	459,525	486,652
Net regulatory position	$ 34,573	(11,617)	(75,070)
Natural gas costs refundable through rate adjustments
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of June 30, 2021	Up to 1 year
Regulatory liabilities due within one year	$ 8,935	18,565	29,557
Electric fuel and purchased power deferral
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of June 30, 2021	Up to 1 year
Regulatory liabilities due within one year	$ 5,431	3,667	7,799
Deferred Income Tax Charge [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities due within one year	3,434	3,557	4,012
Regulatory liabilities	$ 222,098	227,850	236,142
Deferred Income Tax Charge [Member] \| Minimum [Member]
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of June 30, 2021	Up to 1 year
Deferred Income Tax Charge [Member] \| Maximum [Member]
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of June 30, 2021	Over plant lives
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities due within one year	$ 5,654	5,661	6,632
Regulatory liabilities	$ 18,048	16,065	11,096
Other Regulatory Liabilities \| Minimum [Member]
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of June 30, 2021	Up to 1 year
Other Regulatory Liabilities \| Maximum [Member]
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of June 30, 2021	Up to 21 years
Plant retirement
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of June 30, 2021	Over plant lives
Regulatory liabilities	$ 171,381	167,171	173,399
Pension and postretirement benefits
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of June 30, 2021	**
Regulatory liabilities	$ 16,940	$ 16,989	$ 18,015

The net amount of all regulatory assets less all regulatory liabilities as of the end of the period.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

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Describes the continual or periodic assessment of the probability of future recovery of regulatory assets.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 40
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6499898&loc=d3e44174-110380

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 405
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-Paragraph 1
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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The amount for the individual regulatory current liability as itemized in a table of regulatory current liabilities as of the end of the period.

+ References

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The amount for the individual regulatory noncurrent liability as itemized in a table of regulatory noncurrent liabilities as of the end of the period.

+ 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 210.5-02.24)
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