Jointly-Owned Electric Facility (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2011 MW
Jointly Owned Electric Facility [Line Items]
Plant in Service	$ 418.8
Accumulated Depreciation	177.4
Construction Work in Progress	24.7
Boswell Unit 4 [Member]
Jointly Owned Electric Facility [Line Items]
Plant in Service	406.9
Accumulated Depreciation	177.4
Construction Work in Progress	8.8
% Ownership	80.00%
Coal-fired generating unit capacity	585
Jointly-Owned Electric Facility Percentage Not Owned	20.00%
CapX2020 [Member]
Jointly Owned Electric Facility [Line Items]
Plant in Service	11.9
Accumulated Depreciation	0
Construction Work in Progress	$ 15.9
% Ownership Range	9.3 - 14.7

Represents MW capacity for coal-fired generating unit near Center, ND.

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- Definition

Jointly-Owned Electric Facility Percentage Not Owned

+ References

No definition available.

+ Details

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- Definition

Jointly Owned Utility Plant, Proportionate Ownership Share Range

+ References

No definition available.

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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 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

+ 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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