v2.4.0.6

Income Taxes (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |  | 3 Months Ended | 12 Months Ended |  | 3 Months Ended | 12 Months Ended |  | 3 Months Ended | 12 Months Ended
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Dec. 31, 2012 wind_projects | Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2012 2009 through 2011 [Member] | Dec. 31, 2012 After 2020 [Member] | Mar. 31, 2012 State Apportionment [Member] | Dec. 31, 2010 Audit Completion Income Tax Benefits [Member] | Dec. 31, 2012 Bonus Depreciation 2012 Return [Member] Scenario, Forecast [Member] | Dec. 31, 2011 Wisconsin Tax Legislation [Member] | Dec. 31, 2010 Federal Healthcare Legislation [Member] | Dec. 31, 2012 IPL [Member] | Dec. 31, 2011 IPL [Member] | Dec. 31, 2010 IPL [Member] | Dec. 31, 2012 IPL [Member] After 2021 [Member] | Mar. 31, 2012 IPL [Member] State Apportionment [Member] | Dec. 31, 2010 IPL [Member] Audit Completion Income Tax Benefits [Member] | Dec. 31, 2012 IPL [Member] Bonus Depreciation 2012 Return [Member] Scenario, Forecast [Member] | Dec. 31, 2010 IPL [Member] Federal Healthcare Legislation [Member] | Dec. 31, 2012 IPL [Member] Whispering Willow - East Wind Project [Member] MW | Dec. 31, 2012 WPL [Member] | Dec. 31, 2011 WPL [Member] | Dec. 31, 2010 WPL [Member] | Dec. 31, 2012 WPL [Member] After 2023 [Member] | Mar. 31, 2012 WPL [Member] State Apportionment [Member] | Dec. 31, 2012 WPL [Member] Bonus Depreciation 2012 Return [Member] Scenario, Forecast [Member] | Dec. 31, 2010 WPL [Member] Federal Healthcare Legislation [Member] | Dec. 31, 2012 WPL [Member] Bent Tree - Phase I Wind Project [Member] MW | Dec. 31, 2012 WPL [Member] Cedar Ridge Wind Project [Member] MW | Dec. 31, 2012 Alliant Energy And IPL [Member] | Dec. 31, 2007 Alliant Energy And IPL [Member] | Dec. 31, 2012 Alliant Energy And IPL [Member] Internal Revenue Service (IRS) [Member] | Dec. 31, 2012 Alliant Energy And IPL [Member] Whispering Willow - East Wind Project [Member] State Income Taxes [Member] | Dec. 31, 2011 Alliant Energy And IPL [Member] Whispering Willow - East Wind Project [Member] State Income Taxes [Member] | Dec. 31, 2010 Alliant Energy And IPL [Member] Whispering Willow - East Wind Project [Member] State Income Taxes [Member] | Dec. 31, 2012 Resources [Member] Bonus Depreciation 2012 Return [Member] Scenario, Forecast [Member]
Income Tax [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred income tax expense |  |  |  |  |  | $ 15 |  |  |  | $ 7 |  |  |  |  | $ 8 |  |  | $ 4 |  |  |  |  |  | $ 7 |  | $ 3 |  |  |  |  |  |  |  |  |
Wind projects generating production tax credits (in wind projects) | 3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Wattage capacity of projects generating production tax credits (in megawatts) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 200 |  |  |  |  |  |  |  | 200 | 68 |  |  |  |  |  |  |
Increase in income tax benefits related to change in accounting method |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 13 |  |  |  |
Net income tax benefits related to tax depreciation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3 | 6 | 12 |
Income tax benefit from changes in federal income tax returns |  |  |  |  |  |  | 7 |  |  |  |  |  |  |  |  | 5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income tax rates for continuing operations | 20.80% | 16.90% | 32.30% |  |  |  | 1.50% |  |  |  | (15.20%) | (2.70%) | 22.80% |  |  | 2.70% |  |  |  | 36.30% | 33.40% | 39.20% |  |  |  |  |  |  |  |  |  |  |  |  |
Operating loss carryforwards, utilization period |  |  |  |  |  |  |  |  | 20 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Operating losses utilized |  |  |  |  |  |  |  |  | 368 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
State net operating loss carryforwards, deferred tax asset, valuation allowances |  |  |  |  |  |  |  |  | 19 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deductions in income tax return |  |  |  |  |  |  |  | 284 |  |  |  |  |  |  |  |  | 67 |  |  |  |  |  |  |  | 117 |  |  |  |  |  |  |  |  |  | 100
Taxable gain on sale of electric transmission assets |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 527 |  |  |  |  |
Gain on sale of assets, income tax obligation deferral period |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 8 years |  |  |  |  |  |
Gain on sale of assets, percent recognized during each year of deferral period | 12.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net operating loss carryforwards, percentage expiring |  |  |  |  | 99.00% |  |  |  |  |  |  |  |  | 99.00% |  |  |  |  |  |  |  |  | 99.00% |  |  |  |  |  |  |  |  |  |  |  |
Statute of limitations, expiration period from extended due date of federal tax return |  |  |  | 3 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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