Jointly owned facilities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Big Stone Station:
Jointly owned facilities [Line Items]
Utility plant in service	$ 64,283	$ 63,890
Less accumulated depreciation	43,043	41,323
Utility plant in services net	21,240	22,567
Coyote Station:
Jointly owned facilities [Line Items]
Utility plant in service	138,810	138,261
Less accumulated depreciation	94,443	89,528
Utility plant in services net	44,367	48,733
Wygen III:
Jointly owned facilities [Line Items]
Utility plant in service	65,597	64,332
Less accumulated depreciation	5,928	4,639
Utility plant in services net	$ 59,669	$ 59,693

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

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 10

-Section C

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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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