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Commitments And Contingencies (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 1 Months Ended | 9 Months Ended |  | 9 Months Ended | 12 Months Ended | 3 Months Ended | 9 Months Ended |  | 9 Months Ended
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Dec. 31, 2010 | Sep. 30, 2012 | Sep. 30, 2012 IPL [Member] site | Sep. 30, 2012 WPL [Member] site | Sep. 30, 2012 RMT [Member] Vendor | Sep. 30, 2012 Manufactured Gas Plant Sites [Member] Alliant Energy [Member] | Sep. 30, 2012 Manufactured Gas Plant Sites [Member] IPL [Member] | Sep. 30, 2012 Manufactured Gas Plant Sites [Member] WPL [Member] | Sep. 30, 2012 Defined Benefit Pension Plans [Member] | Dec. 31, 2011 Defined Benefit Pension Plans [Member] | Sep. 30, 2012 Defined Benefit Pension Plans [Member] Alliant Energy [Member] | Sep. 30, 2011 Defined Benefit Pension Plans [Member] Alliant Energy [Member] | Sep. 30, 2012 Defined Benefit Pension Plans [Member] Alliant Energy [Member] | Sep. 30, 2011 Defined Benefit Pension Plans [Member] Alliant Energy [Member] | Sep. 30, 2012 Minimum [Member] | Sep. 30, 2012 Maximum [Member] | Sep. 30, 2012 Performance guarantees outstanding, 2012 [Member] | Sep. 30, 2012 Performance guarantees outstanding, 2013 [Member] | Sep. 30, 2012 Performance guarantees outstanding, 2014 [Member] | Sep. 30, 2012 Performance guarantees outstanding, 2015 [Member] | Sep. 30, 2012 Scenario, Forecast [Member] | Sep. 30, 2012 Scenario, Forecast [Member] Alliant Energy [Member] | Sep. 30, 2012 Scenario, Forecast [Member] IPL [Member] | Sep. 30, 2012 Scenario, Forecast [Member] WPL [Member] | Sep. 30, 2012 Scenario, Forecast [Member] Capital Purchase Obligation [Member] Alliant Energy and WPL [Member] | Sep. 30, 2012 EPA new source performance standards for GHG [Member] lb MW
Long-term Purchase Commitment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Minimum future commitments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,131 | $ 605 | $ 355 | $ 34 |
Interest crediting rate | 8.20% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate additional payments to certain plan participants |  |  |  |  |  |  |  |  |  | 10.0 | 0 | [1] | 1.3 | [1] | 0 | [1] | 10.2 | [1] |  |  |  |  |  |  |  |  |  |  |  |
Estimated damages from court opinion and order |  |  |  |  |  |  |  |  | 18.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss contingency, damages sought, value |  |  |  |  |  |  |  |  | 9.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Damages to be paid out of common fund |  |  |  |  |  |  |  |  | 4.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of vendors who filed lawsuits (in vendors) |  |  |  |  | 3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss contingency accrual, carrying value, payments |  |  |  |  | 16 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Performance guarantees outstanding |  | 598 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 262 | 289 | 47 |  |  |  |  |  |
Surety bonds outstanding |  | 79 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 70 | 9 |  |  |  |  |  |  |  |
Present value abandonment obligation |  | 29 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of sites with ownership interest (in sites) |  |  | 40 | 14 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of sites with no further action (in sites) |  |  | 13 | 9 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Sites with no further action for soil remediation (in sites) |  |  | 3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Minimum remaining estimated cost incurred for investigation, remediation and monitoring |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 19 |  | 17 | 2 |  |
Maximum remaining estimated cost incurred for investigation, remediation and monitoring |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 44 |  | 40 | 4 |  |
Current and non-current environmental liabilities |  |  |  |  |  | $ 30 | $ 27 | $ 3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual standard fine particle NAAQS, current (in micrograms/cubic meter) |  | 15 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual standard fine particle NAAQS (in micrograms/cubic meter) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 12 | 13 |  |  |  |  |  |  |  |  |  |
Fossil fueled energy generating units capacity (in megawatts) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 25
Emissions rate limitation (in pounds of carbon dioxide per Mwh) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,000
[1] | Alliant Energy reached an agreement with the IRS, which resulted in a favorable determination letter for the Cash Balance Plan during the first quarter of 2011. The agreement with the IRS required Alliant Energy to amend the Cash Balance Plan, which was completed in the second quarter of 2011, resulting in aggregate additional benefits of $10.2 million paid by Alliant Energy to certain former participants in the Cash Balance Plan in the second half of 2011. Alliant Energy recognized $1.3 million and $10.2 million of additional benefits costs during the three and nine months ended September 30, 2011, respectively, related to these benefits. IPL recognized $0.8 million ($0.6 million directly assigned and $0.2 million allocated by Corporate Services) and $6.3 million ($2.8 million directly assigned and $3.5 million allocated by Corporate Services) of additional benefits costs during the three and nine months ended September 30, 2011, respectively, related to these benefits. WPL recognized $0.4 million ($0.1 million directly assigned and $0.3 million allocated by Corporate Services) and $3.4 million ($0.7 million directly assigned and $2.7 million allocated by Corporate Services) of additional benefits costs during the three and nine months ended September 30, 2011, respectively, related to these benefits. Refer to Note 11(c) for additional information regarding the Cash Balance Plan.

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