Jointly Owned Facilities (Details) $ in Thousands	12 Months Ended
Dec. 31, 2015 USD ($) MW
Wyodak Plant [Member]
Jointly Owned Utility Plant Interests [Line Items]
Proportionate Ownership Percentage	20.00%
Plant in Service	$ 111,532
Construction Work in Progress	1,039
Accumulated Depreciation	$ 56,812
Transmission Tie [Member]
Jointly Owned Utility Plant Interests [Line Items]
Proportionate Ownership Percentage	35.00%
Plant in Service	$ 19,648
Construction Work in Progress	0
Accumulated Depreciation	$ 5,390
Wygen I I I Generating Facility [Member]
Jointly Owned Utility Plant Interests [Line Items]
Proportionate Ownership Percentage	52.00%
Plant in Service	$ 137,860
Construction Work in Progress	446
Accumulated Depreciation	$ 16,217
Cheyenne Prairie [Member]
Jointly Owned Utility Plant Interests [Line Items]
Electric Generation Capacity, Megawatts \| MW	55
Plant in Service	$ 91,081
Construction Work in Progress	0
Accumulated Depreciation	$ 3,301
Cheyenne Prairie [Member] \| Cheyenne Light [Member]
Jointly Owned Utility Plant Interests [Line Items]
Electric Generation Capacity, Megawatts \| MW	40

Electric Generation Capacity, Megawatts

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- Definition

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

+ References

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

+ Details

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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.

+ References

No definition available.

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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.

+ References

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- Definition

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

+ References

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