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Income Taxes (Narrative) (Details) (USD $) In Millions, unless otherwise specified |  | 3 Months Ended | 12 Months Ended |  | 3 Months Ended | 12 Months Ended |  | 3 Months Ended | 12 Months Ended
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Dec. 31, 2013 wind_projects | Dec. 31, 2013 After 2024 [Member] | Mar. 31, 2012 State Apportionment [Member] | Dec. 31, 2011 Wisconsin Tax Legislation [Member] | Dec. 31, 2013 Bonus Depreciation 2013 Return [Member] Scenario, Forecast [Member] | Dec. 31, 2013 IPL [Member] After 2024 [Member] | Mar. 31, 2012 IPL [Member] State Apportionment [Member] | Dec. 31, 2013 IPL [Member] Bonus Depreciation 2013 Return [Member] Scenario, Forecast [Member] | Dec. 31, 2013 IPL [Member] Whispering Willow - East Wind Project [Member] MW | Dec. 31, 2013 WPL [Member] After 2024 [Member] | Mar. 31, 2012 WPL [Member] State Apportionment [Member] | Dec. 31, 2013 WPL [Member] Bonus Depreciation 2013 Return [Member] Scenario, Forecast [Member] | Dec. 31, 2013 WPL [Member] Bent Tree - Phase I Wind Project [Member] MW | Dec. 31, 2013 WPL [Member] Cedar Ridge Wind Project [Member] MW
Income Tax [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Wind projects generating production tax credits (in wind projects) | 3 |  |  |  |  |  |  |  |  |  |  |  |  |
Wind EGU capacity (in megawatts) |  |  |  |  |  |  |  |  | 200 |  |  |  | 201 | 68
Deferred income tax expense |  |  | $ 15 |  |  |  | $ 8 |  |  |  | $ 7 |  |  |
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 |  |  |  |  | $ 130 |  |  | $ 70 |  |  |  | $ 45 |  |
Net operating loss carryforwards, percentage expiring |  | 98.00% |  |  |  | 95.00% |  |  |  | 98.00% |  |  |  |

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