Jointly owned facilities (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Jointly owned electricity generation plant \| Big Stone Station
Jointly Owned Utility Plant Interests [Line Items]
Proportionate ownership share	22.70%
Utility plant in service	$ 157,259	$ 155,967
Construction work in progress	571	104
Less accumulated depreciation	47,293	45,435
Utility plant in services net	$ 110,537	110,636
Jointly owned electricity generation plant \| Coyote Station
Jointly Owned Utility Plant Interests [Line Items]
Proportionate ownership share	25.00%
Utility plant in service	$ 157,764	159,784
Construction work in progress	784	323
Less accumulated depreciation	109,202	108,852
Utility plant in services net	$ 49,346	51,255
Jointly owned electricity generation plant \| Wygen III
Jointly Owned Utility Plant Interests [Line Items]
Proportionate ownership share	25.00%
Utility plant in service	$ 66,357	66,101
Construction work in progress	108	232
Less accumulated depreciation	11,383	10,038
Utility plant in services net	$ 55,082	56,295
Jointly Owned Electricity Transmission and Distribution System \| BSSE
Jointly Owned Utility Plant Interests [Line Items]
Proportionate ownership share	50.00%
Utility plant in service	$ 107,424	107,442
Construction work in progress	0	0
Less accumulated depreciation	4,506	2,682
Utility plant in services net	$ 102,918	$ 104,760

The period end amount owned by the reporting utility of a jointly owned utility plant in service.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 360
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 10.C)
-URI http://asc.fasb.org/extlink&oid=122136288&loc=d3e659983-123033

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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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- Definition

The total period end amount of plant in service and construction in progress net of accumulated depreciation for the utility's share of jointly owned electricity generation plant.

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- Definition

The period end amount of construction work in progress at a jointly owned electricity generation plant.

+ References

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- Definition

The period end amount of accumulated depreciation in a jointly owned electricity generation plant.

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- Definition

The proportionate share of interest (percent) in a jointly owned utility plant.

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