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Income Taxes (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 6 Months Ended | 3 Months Ended | 6 Months Ended | 3 Months Ended | 6 Months Ended | 3 Months Ended | 6 Months Ended | 3 Months Ended | 6 Months Ended | 3 Months Ended | 6 Months Ended
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Jun. 30, 2013 wind_projects | Jun. 30, 2012 | Jun. 30, 2013 wind_projects | Jun. 30, 2012 | Mar. 31, 2012 State Apportionment [Member] | Jun. 30, 2012 State Apportionment [Member] | Jun. 30, 2013 Deferred income tax assets [Member] | Jun. 30, 2013 IPL [Member] | Jun. 30, 2012 IPL [Member] | Jun. 30, 2013 IPL [Member] | Jun. 30, 2012 IPL [Member] | Mar. 31, 2012 IPL [Member] State Apportionment [Member] | Jun. 30, 2012 IPL [Member] State Apportionment [Member] | Jun. 30, 2013 IPL [Member] Whispering Willow - East Wind Project [Member] MW | Jun. 30, 2013 WPL [Member] | Jun. 30, 2012 WPL [Member] | Jun. 30, 2013 WPL [Member] | Jun. 30, 2012 WPL [Member] | Mar. 31, 2012 WPL [Member] State Apportionment [Member] | Jun. 30, 2012 WPL [Member] State Apportionment [Member] | Jun. 30, 2013 WPL [Member] Cedar Ridge Wind Project [Member] MW | Jun. 30, 2013 WPL [Member] Bent Tree - Phase I Wind Project [Member] MW | Jun. 30, 2013 WPL [Member] Deferred income tax assets [Member]
Income Tax [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Statutory federal income tax rate |  |  | 35.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred income tax expense |  |  |  |  | $ 15 |  |  |  |  |  |  | $ 8 |  |  |  |  |  |  | $ 7 |  |  |  |
Income tax rates for continuing operations | 13.30% | 19.50% | 13.00% | 28.30% |  | 9.70% |  | (24.70%) | (12.00%) | (24.30%) | 22.60% |  | 34.60% |  | 33.10% | 34.60% | 32.50% | 39.40% |  | 6.20% |  |  |
Wind projects generating production tax credits (in wind projects) | 3 |  | 3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Electric capacity of wind projects generating production tax credits (in megawatts) |  |  |  |  |  |  |  |  |  |  |  |  |  | 200 |  |  |  |  |  |  | 68 | 200 |
Increase (decrease) current deferred tax assets |  |  | (43.4) | (26.7) |  |  | (27.9) |  |  | (43.1) | (25.7) |  |  |  |  |  |  |  |  |  |  |  | (42.6)
Increase (decrease) non-current deferred tax liabilities |  |  | $ 56.9 |  |  |  |  |  |  | $ 44.3 |  |  |  |  |  |  |  |  |  |  |  |  |

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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 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 2: 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 3: 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 4: 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 Reference 5: 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 6: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph h -Article 4 Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 6 -Section I -Subsection 7 Reference 8: 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 9: 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 10: 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 + Details
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