v2.4.0.6

Income Taxes (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 9 Months Ended | 3 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended | 9 Months Ended | 12 Months Ended | 9 Months Ended
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Sep. 30, 2012 wind_projects | Sep. 30, 2011 | Sep. 30, 2012 Alliant Energy [Member] | Sep. 30, 2011 Alliant Energy [Member] | Sep. 30, 2012 Alliant Energy [Member] | Sep. 30, 2011 Alliant Energy [Member] | Sep. 30, 2012 IPL [Member] | Sep. 30, 2011 IPL [Member] | Sep. 30, 2012 IPL [Member] | Sep. 30, 2011 IPL [Member] | Sep. 30, 2012 WPL [Member] | Sep. 30, 2011 WPL [Member] | Sep. 30, 2012 WPL [Member] | Sep. 30, 2011 WPL [Member] | Mar. 31, 2012 State Apportionment [Member] Alliant Energy [Member] | Sep. 30, 2012 State Apportionment [Member] Alliant Energy [Member] | Mar. 31, 2012 State Apportionment [Member] IPL [Member] | Sep. 30, 2012 State Apportionment [Member] IPL [Member] | Mar. 31, 2012 State Apportionment [Member] WPL [Member] | Sep. 30, 2012 State Apportionment [Member] WPL [Member] | Sep. 30, 2012 Tax Benefit Rider [Member] | Sep. 30, 2011 Tax Benefit Rider [Member] Alliant Energy [Member] | Sep. 30, 2011 Tax Benefit Rider [Member] Alliant Energy [Member] | Sep. 30, 2012 Tax Benefit Rider [Member] IPL [Member] | Sep. 30, 2011 Tax Benefit Rider [Member] IPL [Member] | Sep. 30, 2012 Tax Benefit Rider [Member] IPL [Member] | Sep. 30, 2011 Tax Benefit Rider [Member] IPL [Member] | Sep. 30, 2011 Wisconsin Tax Legislation [Member] | Sep. 30, 2011 Wisconsin Tax Legislation [Member] | Dec. 31, 2011 Wisconsin Tax Legislation [Member] | Sep. 30, 2012 Cedar Ridge Wind Project [Member] WPL [Member] MW | Sep. 30, 2012 Bent Tree - Phase I Wind Project [Member] WPL [Member] MW | Sep. 30, 2012 Whispering Willow - East Wind Project [Member] IPL [Member] MW | Dec. 31, 2012 Scenario, Forecast [Member] Bonus Depreciation 2012 Return [Member] Alliant Energy [Member] | Dec. 31, 2012 Scenario, Forecast [Member] Bonus Depreciation 2012 Return [Member] IPL [Member] | Dec. 31, 2012 Scenario, Forecast [Member] Bonus Depreciation 2012 Return [Member] WPL [Member] | Dec. 31, 2012 Scenario, Forecast [Member] Bonus Depreciation 2012 Return [Member] Resources [Member] | Sep. 30, 2012 Scenario, Forecast [Member] Tax Benefit Rider [Member] Alliant Energy [Member] | Sep. 30, 2012 Scenario, Forecast [Member] Tax Benefit Rider [Member] IPL [Member] | Sep. 30, 2012 Deferred income tax assets [Member] | Sep. 30, 2011 Deferred income tax assets [Member] | Sep. 30, 2012 Deferred income tax assets [Member] IPL [Member] | Sep. 30, 2011 Deferred income tax assets [Member] IPL [Member] | Sep. 30, 2012 Deferred income tax assets [Member] WPL [Member] | Sep. 30, 2011 Deferred income tax assets [Member] WPL [Member]
Income Tax [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Statutory federal income tax rate | 35.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred income tax expense |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 15.2 |  | $ 8.1 |  | $ 7.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income tax rates for continuing operations |  |  | 20.40% | 23.50% | 24.00% | 16.60% | (12.60%) | (3.30%) | (5.60%) | (1.70%) | 35.50% | 34.40% | 37.70% | 33.00% |  | 4.30% |  | 6.90% |  | 3.50% |  | 9.20% | 8.90% | 26.50% |  |  | 25.40% |  | 5.80% |  |  |  |  |  |  |  |  | 11.00% | 34.50% |  |  |  |  |  |
Regulatory liabilities | 5.0 | 158.5 |  |  |  |  |  |  | (27.5) | 145.1 |  |  | 32.5 | 13.4 |  |  |  |  |  |  | 63.0 |  |  | 23.0 | 20.0 | 63.0 | 44.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Wind projects generating production tax credits (in wind projects) | 3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Electric capacity of wind projects generating production tax credits (in megawatts) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 68 | 200 | 200 |  |  |  |  |  |  |  |  |  |  |  |
Operating losses utilized |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 368 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation allowance, deferred tax asset, change in amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 19 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred income taxes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 67.2 | 0.6 | 22.4 | (3.1) | 44.3 | 1.3
Increase in deferred income taxes noncurrent liabilities |  |  |  |  | 244.5 |  |  |  | 102.5 |  |  |  | 136.8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deductions in income tax return |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 415 | $ 114 | $ 203 | $ 98 |  |  |  |  |  |  |  |

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- Definition The component of income tax expense for the period representing the increase (decrease) in the entity's deferred tax assets and liabilities pertaining to continuing operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Deferred Tax Expense (or Benefit) -URI http://asc.fasb.org/extlink&oid=6510177 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph h -Article 4 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 6 -Section I -Subsection 7 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 109 -Paragraph 45 -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. Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 109 -Paragraph 289 -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 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 28 -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 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32639-109319 Reference 8: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(h)) -URI http://asc.fasb.org/extlink&oid=6881521&loc=d3e23780-122690 Reference 9: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585 Reference 10: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 6.I.7) -URI http://asc.fasb.org/extlink&oid=6889476&loc=d3e330036-122817 + Details
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