v2.4.0.6

Common Equity (Narrative) (Details) (USD $) | 9 Months Ended
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Sep. 30, 2012 | Sep. 30, 2011
IPL [Member] |  |
Common Equity [Line Items] |  |
Retained earnings free of dividend restrictions | $ 375,000,000 |
Restricted net assets of subsidiaries | 1,100,000,000 |
Capital contributions from parent | 80,000,000 | 0
Common stock dividends to Alliant Energy | 91,600,000 |
WPL [Member] |  |
Common Equity [Line Items] |  |
Retained earnings free of dividend restrictions | 209,000,000 |
Restricted net assets of subsidiaries | 1,300,000,000 |
Capital contributions from parent | 0 | 25,000,000
Common stock dividends to Alliant Energy | $ 84,100,000 |

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