Jointly owned facilities (Details) - Jointly owned electricity generation plant - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Big Stone Station [Member]
Jointly Owned Utility Plant Interests [Line Items]
Proportionate ownership share	22.70%
Utility plant in service	$ 156,534	$ 158,084
Less accumulated depreciation	49,345	51,740
Utility plant in services net	$ 107,189	106,344
Coyote Station [Member]
Jointly Owned Utility Plant Interests [Line Items]
Proportionate ownership share	25.00%
Utility plant in service	$ 155,236	155,287
Less accumulated depreciation	105,565	103,897
Utility plant in services net	$ 49,671	51,390
Wygen III [Member]
Jointly Owned Utility Plant Interests [Line Items]
Proportionate ownership share	25.00%
Utility plant in service	$ 65,382	65,065
Less accumulated depreciation	9,174	7,652
Utility plant in services net	$ 56,208	$ 57,413

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=115995737&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 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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