Property Plant and Equipment Joint Ownership (Details) $ in Millions	12 Months Ended
Dec. 31, 2018 USD ($)
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 1,589.5
Accumulated Depreciation	430.8
CWIP	0.9
Electric Generation \| Hayden Unit 1
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	152.8
Accumulated Depreciation	76.5
CWIP	$ 0.0
Ownership percentage (in hundredths)	76.00%
Electric Generation \| Hayden Unit 2
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 148.9
Accumulated Depreciation	68.0
CWIP	$ 0.0
Ownership percentage (in hundredths)	37.00%
Electric Generation \| Hayden Common Facilities
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 40.8
Accumulated Depreciation	20.9
CWIP	$ 0.0
Ownership percentage (in hundredths)	53.00%
Electric Generation \| Craig Units 1 and 2
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 81.0
Accumulated Depreciation	40.0
CWIP	$ 0.0
Ownership percentage (in hundredths)	10.00%
Electric Generation \| Craig Common Facilities 1, 2 and 3
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 39.1
Accumulated Depreciation	20.9
CWIP	$ 0.0
Ownership percentage (in hundredths)	7.00%
Electric Generation \| Comanche Unit 3
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 886.3
Accumulated Depreciation	130.7
CWIP	$ 0.0
Ownership percentage (in hundredths)	67.00%
Electric Generation \| Comanche Common Facilities
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 27.9
Accumulated Depreciation	2.5
CWIP	$ 0.1
Ownership percentage (in hundredths)	82.00%
Electric Transmission \| Transmission and Other Facilities, including Substations
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 182.8
Accumulated Depreciation	63.2
CWIP	$ 0.7
Ownership percentage of group of jointly owned facilities	Various
Gas Transportation \| Rifle to Avon
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 21.5
Accumulated Depreciation	7.2
CWIP	$ 0.1
Ownership percentage (in hundredths)	60.00%
Gas Transportation \| Gas Transportation Compressor
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 8.4
Accumulated Depreciation	0.9
CWIP	$ 0.0
Ownership percentage (in hundredths)	50.00%

Jointly Owned Utility Plant Proportionate Ownership Percentage Share

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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://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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- 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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