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Asset Retirement Obligations (Reconciliation Of Changes In Asset Retirement Obligations) (Details) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Jun. 30, 2015 USD ($) | Sep. 30, 2015 USD ($) | Sep. 30, 2014 USD ($)
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] |  |  |  |  |
Balance, January 1 |  | $ 114.0 |  | $ 109.7 |
Revisions in estimated cash flows |  | 8.9 |  | 0.0 |
Liabilities settled |  | (7.1) |  | (1.0) |
Liabilities incurred | $ 74.0 | 76.1 | [1] | 16.5 | [1]
Accretion expense |  | 3.4 |  | 3.3 |
Balance, September 30 |  | 195.3 |  | 128.5 |
IPL [Member] |  |  |  |  |
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] |  |  |  |  |
Balance, January 1 |  | 51.8 |  | 47.9 |
Revisions in estimated cash flows |  | 11.9 |  | 0.0 |
Liabilities settled |  | (3.1) |  | (0.5) |
Liabilities incurred | 57.0 | 59.9 | [1] | 16.3 | [1]
Accretion expense |  | 1.6 |  | 1.6 |
Balance, September 30 |  | $ 122.1 |  | 65.3 |
Electric generating units with coal ash ponds (in number of electric generating units) |  | 9 |  |  |
Active coal combustion residuals landfills (in number of landfills) |  | 4 |  |  |
WPL [Member] |  |  |  |  |
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] |  |  |  |  |
Balance, January 1 |  | $ 52.4 |  | 52.4 |
Revisions in estimated cash flows |  | (1.9) |  | 0.0 |
Liabilities settled |  | (4.0) |  | (0.5) |
Liabilities incurred | $ 17.0 | 16.2 | [1] | 0.2 | [1]
Accretion expense |  | 1.4 |  | 1.3 |
Balance, September 30 |  | $ 64.1 |  | $ 53.4 |
Electric generating units with coal ash ponds (in number of electric generating units) |  | 3 |  |  |
Active coal combustion residuals landfills (in number of landfills) |  | 2 |  |  |
[1] | In April 2015, the EPA published the final CCR Rule, which regulates CCR as a non-hazardous waste and is effective October 2015. IPL and WPL have nine and three coal-fired EGUs, respectively, with coal ash ponds that are impacted by this rule. In addition, IPL and WPL have four and two active CCR landfills, respectively, that are impacted by this rule. During the nine months ended September 30, 2015, Alliant Energy, IPL and WPL recognized additional AROs of $74 million, $57 million and $17 million, respectively, as a result of the final CCR Rule. The increases in AROs resulted in corresponding increases in “Property, plant and equipment, net” on the respective balance sheets. Actual costs resulting from the CCR rule may be different than the amounts recorded during the nine months ended September 30, 2015 due to potential changes in compliance strategies that will be used, as well as other potential cost estimate changes. Expenditures incurred by IPL and WPL to comply with the CCR Rule are anticipated to be recovered in rates from their customers.

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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 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 2: 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 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 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 + Details
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- Definition Amount of asset retirement obligations incurred during the period. + 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)(1) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + 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 + 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 + Details
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