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Regulatory Matters (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 9 Months Ended | 12 Months Ended | 9 Months Ended | 12 Months Ended | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended | 36 Months Ended | 1 Months Ended |  |  |  | 1 Months Ended |
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Sep. 30, 2014 change | Sep. 30, 2013 | Sep. 30, 2014 WPL [Member] | Sep. 30, 2013 WPL [Member] | Sep. 30, 2014 IPL [Member] | Sep. 30, 2013 IPL [Member] | Dec. 31, 2013 IPL [Member] Revenue Requirement Adjustment [Member] | Sep. 30, 2014 Alliant Energy and IPL [Member] Tax benefit riders [Member] | Dec. 31, 2014 Scenario, Forecast [Member] IPL [Member] Revenue Requirement Adjustment [Member] | Jul. 31, 2014 2015/2016 Test Period Retail Electric And Gas [Member] WPL [Member] | Sep. 30, 2014 2013 Test Year Retail Electric [Member] IPL [Member] | Sep. 30, 2014 2013 Test Year Retail Electric [Member] IPL [Member] | Dec. 31, 2016 2013 Test Year Retail Electric [Member] Scenario, Forecast [Member] IPL [Member] | Dec. 31, 2015 2013 Test Year Retail Electric [Member] Scenario, Forecast [Member] IPL [Member] | Dec. 31, 2014 2013 Test Year Retail Electric [Member] Scenario, Forecast [Member] IPL [Member] | Dec. 31, 2016 2013 Test Year Retail Electric [Member] Scenario, Forecast [Member] IPL [Member] | Jun. 30, 2014 2015 Test Year Retail Electric Fuel Related Request [Member] WPL [Member] | Dec. 31, 2015 2015 Test Year Retail Electric Fuel Related Request [Member] Scenario, Forecast [Member] WPL [Member] | Dec. 31, 2014 2015 Test Year Retail Electric Fuel Related Request [Member] Scenario, Forecast [Member] WPL [Member] | Dec. 31, 2015 Fuel per MWh [Member] Scenario, Forecast [Member] WPL [Member] | Dec. 31, 2015 Increased sales volumes [Member] Scenario, Forecast [Member] WPL [Member] | Dec. 31, 2013 2014 Test Year Retail Electric Fuel Related Request [Member] WPL [Member] | Sep. 30, 2014 2014 Test Year Retail Electric Fuel Related Request [Member] Alliant Energy and WPL [Member]
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of tax accounting method changes (in number of changes) | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in regulatory assets and regulatory liabilities |  |  |  |  |  |  |  | $ 74 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Authorized increase (decrease) in final rates, amount |  |  |  |  |  |  |  |  |  | (5) |  |  |  |  |  |  |  |  |  |  |  | 19 |
Customer billing credits |  |  |  |  |  |  |  |  |  |  | 26 | 46 | 10 | 25 | 70 | 105 |  |  |  |  |  |  |
Increase (decrease) in regulatory liabilities | 61.1 | (74.9) | 46.3 | (6.6) | 14.8 | (68.3) | (24.0) | (73.0) | (15.0) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested increase (decrease) in rates, amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 55 |  |  |  |  |  |
Requested increase (decrease) in rates, percent |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5.00% |  |  |  |  |  |
Fuel-related costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 41 | $ 14 | $ 28 | $ 13 |  | $ 21
Authorized increase (decrease) in final rates, percent |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.00% |
Annual bandwidth for fuel-related costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.00% |

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- Definition Carrying amount as of the balance sheet date of capitalized fuel costs of a regulated entity that are expected to be recoverable through rate adjustments within one year or the normal operating cycle, if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499739&loc=d3e43603-110378 + Details
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- Definition The increase (decrease) during the reporting period in the value of assets that are created when regulatory agencies permits public utilities to defer costs (revenues) to the balance sheet. This element is a the increase (decrease) of regulatory assets and liabilities combined. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 + Details
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- Definition The increase (decrease) during the reporting period in regulated liabilities. Regulated liabilities are created when regulatory agency permits an entity to defer revenues to the balance sheet. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 + Details
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