Joint Ownership of Generation, Transmission and Gas Facilities (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2013
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 1,416,114
Accumulated depreciation	319,241
Construction work in progress	9,818
Generating capacity (in MW)	820
Electric Generation \| Hayden Unit 1
Jointly Owned Utility Plant [Abstract]
Plant in service	97,879
Accumulated depreciation	63,474
Construction work in progress	53
Ownership percentage (in hundredths)	75.50%
Electric Generation \| Hayden Unit 2
Jointly Owned Utility Plant [Abstract]
Plant in service	119,972
Accumulated depreciation	57,875
Construction work in progress	5,563
Ownership percentage (in hundredths)	37.40%
Electric Generation \| Hayden Common Facilities
Jointly Owned Utility Plant [Abstract]
Plant in service	36,916
Accumulated depreciation	16,055
Construction work in progress	2
Ownership percentage (in hundredths)	53.10%
Electric Generation \| Craig Units 1 and 2
Jointly Owned Utility Plant [Abstract]
Plant in service	60,089
Accumulated depreciation	34,754
Construction work in progress	537
Ownership percentage (in hundredths)	9.70%
Electric Generation \| Craig Common Facilities 1, 2 and 3
Jointly Owned Utility Plant [Abstract]
Plant in service	37,177
Accumulated depreciation	17,247
Construction work in progress	0
Ownership percentage (in hundredths)	6.50%
Electric Generation \| Comanche Unit 3
Jointly Owned Utility Plant [Abstract]
Plant in service	877,489
Accumulated depreciation	63,963
Construction work in progress	581
Ownership percentage (in hundredths)	66.70%
Electric Generation \| Comanche Common Facilities
Jointly Owned Utility Plant [Abstract]
Plant in service	19,812
Accumulated depreciation	711
Construction work in progress	2,255
Ownership percentage (in hundredths)	82.00%
Electric Transmission \| Transmission and other facilities, including substations
Jointly Owned Utility Plant [Abstract]
Plant in service	150,502
Accumulated depreciation	59,118
Construction work in progress	827
Ownership percentage of group of jointly owned facilities	Various
Gas Transportation \| Rifle to Avon
Jointly Owned Utility Plant [Abstract]
Plant in service	16,278
Accumulated depreciation	6,044
Construction work in progress	$ 0
Ownership percentage (in hundredths)	60.00%

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

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

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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