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Common Equity (Narrative) (Details) (USD $)
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9 Months Ended | |
|---|---|---|
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Sep. 30, 2012
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Sep. 30, 2011
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IPL [Member]
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| 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]
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| 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 | |