Jointly owned facilities (Details) - Jointly owned electricity generation plant - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Big Stone Station [Member]
Jointly Owned Utility Plant Interests [Line Items]
Utility plant in service	$ 158,084	$ 157,144
Less accumulated depreciation	51,740	49,568
Utility plant in services net	106,344	107,576
Coyote Station [Member]
Jointly Owned Utility Plant Interests [Line Items]
Utility plant in service	155,287	156,334
Less accumulated depreciation	103,897	105,928
Utility plant in services net	51,390	50,406
Wygen III [Member]
Jointly Owned Utility Plant Interests [Line Items]
Utility plant in service	65,065	66,251
Less accumulated depreciation	7,652	7,550
Utility plant in services net	$ 57,413	$ 58,701

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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=6500719&loc=d3e659983-123033

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- Definition

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.

+ References

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- Definition

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

+ References

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