Joint Ownership of Generation, Transmission and Gas Facilities (Details) (USD $)	Dec. 31, 2011 MW
NSP-Minnesota [Member]
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 778,472,000
Accumulated depreciation	453,511,000
Construction work in progress	78,666,000
Generating capacity (in MW)	500
NSP-Minnesota [Member] \| Electric Generation [Member] \| Sherco Unit 3 [Member]
Jointly Owned Utility Plant [Abstract]
Plant in service	565,832,000
Accumulated depreciation	358,907,000
Construction work in progress	3,731,000
Ownership % (in hundredths)	59.00%
Generating capacity (in MW)	860
Deductible for insurance policy	1,500,000
NSP-Minnesota [Member] \| Electric Generation [Member] \| Sherco Common Facilities Units 1, 2 and 3 [Member]
Jointly Owned Utility Plant [Abstract]
Plant in service	138,790,000
Accumulated depreciation	82,229,000
Construction work in progress	531,000
Ownership % (in hundredths)	80.00%
NSP-Minnesota [Member] \| Electric Generation [Member] \| Sherco Substation [Member]
Jointly Owned Utility Plant [Abstract]
Plant in service	4,790,000
Accumulated depreciation	2,621,000
Construction work in progress	0
Ownership % (in hundredths)	59.00%
NSP-Minnesota [Member] \| Electric Transmission [Member] \| Grand Meadow Line and Substation [Member]
Jointly Owned Utility Plant [Abstract]
Plant in service	11,204,000
Accumulated depreciation	855,000
Construction work in progress	0
Ownership % (in hundredths)	50.00%
NSP-Minnesota [Member] \| Electric Transmission [Member] \| CapX2020 Transmission [Member]
Jointly Owned Utility Plant [Abstract]
Plant in service	57,856,000
Accumulated depreciation	8,899,000
Construction work in progress	74,404,000
Ownership % (in hundredths)	49.60%
PSCo [Member]
Jointly Owned Utility Plant [Abstract]
Plant in service	1,379,117,000
Accumulated depreciation	271,130,000
Construction work in progress	6,241,000
Generating capacity (in MW)	820
PSCo [Member] \| Electric Generation [Member] \| Hayden Unit 1 [Member]
Jointly Owned Utility Plant [Abstract]
Plant in service	88,337,000
Accumulated depreciation	60,549,000
Construction work in progress	830,000
Ownership % (in hundredths)	75.50%
PSCo [Member] \| Electric Generation [Member] \| Hayden Unit 2 [Member]
Jointly Owned Utility Plant [Abstract]
Plant in service	119,621,000
Accumulated depreciation	55,126,000
Construction work in progress	722,000
Ownership % (in hundredths)	37.40%
PSCo [Member] \| Electric Generation [Member] \| Hayden Common Facilities [Member]
Jointly Owned Utility Plant [Abstract]
Plant in service	34,558,000
Accumulated depreciation	14,155,000
Construction work in progress	1,000
Ownership % (in hundredths)	53.10%
PSCo [Member] \| Electric Generation [Member] \| Craig Units 1 and 2 [Member]
Jointly Owned Utility Plant [Abstract]
Plant in service	54,058,000
Accumulated depreciation	33,225,000
Construction work in progress	193,000
Ownership % (in hundredths)	9.70%
PSCo [Member] \| Electric Generation [Member] \| Craig Common Facilities 1, 2 and 3 [Member]
Jointly Owned Utility Plant [Abstract]
Plant in service	35,241,000
Accumulated depreciation	15,896,000
Construction work in progress	2,863,000
Ownership %, lower range (in hundredths)	6.50%
Ownership %, lower range (in hundredths)	9.70%
PSCo [Member] \| Electric Generation [Member] \| Comanche Unit 3 [Member]
Jointly Owned Utility Plant [Abstract]
Plant in service	867,976,000
Accumulated depreciation	28,973,000
Construction work in progress	1,014,000
Ownership % (in hundredths)	66.70%
PSCo [Member] \| Electric Generation [Member] \| Comanche Common Facilities [Member]
Jointly Owned Utility Plant [Abstract]
Plant in service	12,628,000
Accumulated depreciation	219,000
Construction work in progress	169,000
Ownership % (in hundredths)	82.00%
PSCo [Member] \| Electric Transmission [Member] \| Trasmission and Other Facilities, including Substations [Member]
Jointly Owned Utility Plant [Abstract]
Plant in service	150,420,000
Accumulated depreciation	56,654,000
Construction work in progress	449,000
Ownership %	Various
PSCo [Member] \| Gas Transportation [Member] \| Rifle to Avon [Member]
Jointly Owned Utility Plant [Abstract]
Plant in service	16,278,000
Accumulated depreciation	6,333,000
Construction work in progress	$ 0
Ownership % (in hundredths)	60.00%

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 SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 10

-Section C

Reference 2: 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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- 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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- Definition

Deductible amount included in insurance policy for the electric generation unit.

+ References

No definition available.

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- Definition

Generating capacity of the facility.

+ References

No definition available.

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- Definition

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

+ References

No definition available.

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- Definition

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

+ References

No definition available.

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- Definition

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

+ References

No definition available.

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- Definition

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

+ References

No definition available.

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