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Asset Retirement Obligations (AROs) (Reconciliation Of Changes In Asset Retirement Obligations) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2013 | Mar. 31, 2012
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] |  |  |  |
Balance, January 1 | $ 101.5 |  | $ 91.1 |
Revisions in estimated cash flows | 0 | [1] | (8.2) | [1]
Liabilities settled | (0.1) |  | (1.0) |
Accretion expense | 1.0 |  | 0.9 |
Balance, March 31 | 102.4 |  | 82.8 |
IPL [Member] |  |  |  |
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] |  |  |  |
Balance, January 1 | 45.5 |  | 56.2 |
Revisions in estimated cash flows | 0 | [1] | (8.2) | [1]
Liabilities settled | 0 |  | (0.9) |
Accretion expense | 0.5 |  | 0.5 |
Balance, March 31 | 46.0 |  | 47.6 |
WPL [Member] |  |  |  |
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] |  |  |  |
Balance, January 1 | 46.9 |  | 34.9 |
Revisions in estimated cash flows | 0 | [1] | 0 | [1]
Liabilities settled | (0.1) |  | (0.1) |
Accretion expense | 0.4 |  | 0.4 |
Balance, March 31 | 47.2 |  | 35.2 |
Sixth Street [Member] | IPL [Member] |  |  |  |
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] |  |  |  |
Revisions in estimated cash flows |  |  | $ (8.2) | [1]
[1] | For the three months ended March 31, 2012, IPL recorded revisions in estimated cash flows of ($8.2) million based on revised remediation timing and cost information for asbestos remediation at its Sixth Street Generating Station.

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- Definition The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 143 -Paragraph 3, 10, 22 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Asset Retirement Obligation -URI http://asc.fasb.org/extlink&oid=6505190 + Details
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- Definition Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 143 -Paragraph 13, 14 -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. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6392676&loc=d3e7480-110848 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 143 -Paragraph 22 -Subparagraph c(3) -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. + Details
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- Definition Amount of asset retirement obligations settled, or otherwise disposed of, during the period. This may include asset retirement obligations transferred to third parties associated with the sale of a long-lived asset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 143 -Paragraph 22 -Subparagraph c(2) -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. + Details
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- Definition Amount of the Increase or Decrease in the amount of the asset retirement obligation during the current period for changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c)(4) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 143 -Paragraph 22 -Subparagraph c(4) -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. + Details
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- Details
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