Joint Ownership of Generation, Transmission and Gas Facilities (Details) $ in Thousands	12 Months Ended
Dec. 31, 2016 USD ($) MW
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 1,550,804
Accumulated depreciation	395,013
Construction work in progress	$ 21,311
Generating capacity (in MW) \| MW	816
Electric Generation \| Hayden Unit 1
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 149,221
Accumulated depreciation	67,415
Construction work in progress	$ 97
Ownership percentage (in hundredths)	76.00%
Electric Generation \| Hayden Unit 2
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 148,795
Accumulated depreciation	64,024
Construction work in progress	$ 64
Ownership percentage (in hundredths)	37.00%
Electric Generation \| Hayden Common Facilities
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 38,230
Accumulated depreciation	18,951
Construction work in progress	$ 282
Ownership percentage (in hundredths)	53.00%
Electric Generation \| Craig Units 1 and 2
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 60,318
Accumulated depreciation	37,570
Construction work in progress	$ 15,730
Ownership percentage (in hundredths)	10.00%
Electric Generation \| Craig Common Facilities 1, 2 and 3
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 37,925
Accumulated depreciation	19,312
Construction work in progress	$ 183
Ownership percentage (in hundredths)	7.00%
Electric Generation \| Comanche Unit 3
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 892,978
Accumulated depreciation	112,254
Construction work in progress	$ 6
Ownership percentage (in hundredths)	67.00%
Electric Generation \| Comanche Common Facilities
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 24,694
Accumulated depreciation	1,821
Construction work in progress	$ 636
Ownership percentage (in hundredths)	82.00%
Electric Transmission \| Transmission and other facilities, including substations
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 166,840
Accumulated depreciation	65,619
Construction work in progress	$ 4,313
Ownership percentage of group of jointly owned facilities	Various
Gas Transportation \| Rifle to Avon
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 23,406
Accumulated depreciation	7,679
Construction work in progress	$ 0
Ownership percentage (in hundredths)	60.00%
Gas Transportation \| Gas Transportation Compressor [Member]
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 8,397
Accumulated depreciation	368
Construction work in progress	$ 0
Ownership percentage (in hundredths)	50.00%

The entity's generating capacity of the jointly owned facilities.

No definition available.

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- Definition

Jointly Owned Utility Plant Proportionate Ownership Percentage Share

+ References

No definition available.

+ Details

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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

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- References

No definition available.

+ Details

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- Definition

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

+ References

+ Details

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- Definition

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

+ References

+ Details

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- Definition

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

+ References

+ Details

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