Summary Of Significant Accounting Policies (Carrying Value Of Assets Purchased) (Details) (USD $) In Millions	9 Months Ended		3 Months Ended	1 Months Ended	9 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010	Mar. 31, 2011 WPL [Member] WPL's Edgewater Unit 5 Purchase [Member]	Mar. 31, 2011 WPL [Member]	Sep. 30, 2011 WPL [Member]	Sep. 30, 2010 WPL [Member]	Dec. 31, 2010 WPL [Member]
Electric plant in service	$ 8,103.9		$ 7,676.8	$ 84.0		$ 3,452.0		$ 3,114.6
Accumulated depreciation	(3,176.5)		(2,982.2)	(50.0)		(1,348.2)		(1,243.8)
Construction work in progress				2
Production fuel	96.8		122.8	1.0		36.0		42.7
Materials and supplies	61.1		61.6	1.0		26.7		25.7
Carrying value of assets purchased	$ 480.2	$ 587.6		$ 38.0	$ 38.0	$ 247.9	$ 315.1

The cash outflow for purchases of and capital improvements on property, plant and equipment (capital expenditures), software, and other intangible assets for the utility business.

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- Definition

Carrying amount as of the balance sheet date of long-lived, depreciable electric utility plant.

+ Details

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- Definition

Carrying amount as of the balance sheet date of other types of fuel derived from living matter of a previous geologic time, not otherwise itemized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 6

-Subparagraph a

-Article 5

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.6(a))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Aggregated amount of unprocessed materials to be used in manufacturing or production process and supplies that will be consumed.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 6

-Subparagraph a

-Article 5

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.6(a)(4),(5))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Period end book value of accumulated depreciation on property, plant and equipment (PPE) that is owned by the regulated operations of the public utility.

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- Definition

Period end amount of construction work in progress in public utility.

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