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Commitments And Contingencies (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 1 Months Ended |  | 3 Months Ended |  | 9 Months Ended | 12 Months Ended | 1 Months Ended |  | 3 Months Ended
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Jul. 31, 2012 subclass | Dec. 31, 2010 | Mar. 31, 2013 | Mar. 31, 2013 IPL [Member] site | Mar. 31, 2013 WPL [Member] site | Mar. 31, 2013 Manufactured Gas Plant Sites [Member] | Mar. 31, 2013 Manufactured Gas Plant Sites [Member] IPL [Member] | Mar. 31, 2013 Manufactured Gas Plant Sites [Member] WPL [Member] | Mar. 31, 2013 Indemnifications [Member] RMT [Member] | Sep. 30, 2012 Defined benefit pension plans [Member] | Dec. 31, 2011 Defined benefit pension plans [Member] | Feb. 28, 2013 Defined benefit pension plans [Member] Cash Balance Plan [Member] | Mar. 31, 2013 Performance guarantees outstanding, 2013 [Member] | Mar. 31, 2013 Performance guarantees outstanding, 2014 [Member] | Mar. 31, 2013 Performance guarantees outstanding, 2015 [Member] | Mar. 31, 2013 Scenario, Forecast [Member] | Mar. 31, 2013 Scenario, Forecast [Member] IPL [Member] | Mar. 31, 2013 Scenario, Forecast [Member] WPL [Member] | Mar. 31, 2013 Scenario, Forecast [Member] Capital Purchase Obligation [Member] | Mar. 31, 2013 Scenario, Forecast [Member] Capital Purchase Obligation [Member] IPL [Member] | Mar. 31, 2013 Scenario, Forecast [Member] Capital Purchase Obligation [Member] WPL [Member] | Mar. 31, 2013 Civil Penalty [Member] WPL [Member] | Mar. 31, 2013 Environmental Issue [Member] WPL [Member] | Mar. 31, 2013 Minimum [Member] | Mar. 31, 2013 Maximum [Member]
Long-term Purchase Commitment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Minimum future commitments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,598 | $ 2,180 | $ 290 | $ 63 | $ 12 | $ 51 |  |  |  |
Interest crediting rate |  | 8.20% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate additional payments to certain plan participants |  |  |  |  |  |  |  |  |  |  | 10 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of new subclasses certified as a result of court opinion and order | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated damages from court opinion and order |  |  |  |  |  |  |  |  |  | 18.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss contingency, estimate of possible loss |  |  |  |  |  |  |  |  | 3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Warranty period |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 12 months | 60 months
Loss contingency, damages sought, value |  |  |  |  |  |  |  |  |  | 9.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Damages to be paid out of common fund |  |  |  |  |  |  |  |  |  | 4.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss contingency, damages awarded, value |  |  |  |  |  |  |  |  |  |  |  | 6.4 |  |  |  |  |  |  |  |  |  | 2.0 | 7.0 |  |
Performance guarantees outstanding |  |  | 617 |  |  |  |  |  |  |  |  |  | 270 | 294 | 53 |  |  |  |  |  |  |  |  |  |
Present value abandonment obligation |  |  | 30 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 20 | 17 | 3 |  |  |  |  |  |  |
Maximum remaining estimated cost incurred for investigation, remediation and monitoring |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 35 | 31 | 4 |  |  |  |  |  |  |
Current and non-current environmental liabilities |  |  |  |  |  | $ 26 | $ 23 | $ 3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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