Derivative Instruments (Narrative) (Details) (USD $) In Millions	Sep. 30, 2011
Aggregate fair value of all derivatives with credit risk-related contingent features, net liability position	$ 52.4
Restrictive credit risk-related contingent features for derivative agreements, net liability position	50.4
IPL [Member]
Aggregate fair value of all derivatives with credit risk-related contingent features, net liability position	20.7
Restrictive credit risk-related contingent features for derivative agreements, net liability position	18.7
Cash collateral from counterparties	2.0
WPL [Member]
Aggregate fair value of all derivatives with credit risk-related contingent features, net liability position	31.7
Restrictive credit risk-related contingent features for derivative agreements, net liability position	$ 31.7

The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a net liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4H

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6935481&loc=SL5624258-113959

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 44D

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant