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Common Equity (Common Share Activity) (Details) | 3 Months Ended
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Mar. 31, 2013
Common Stock Oustanding [Roll Forward] |  |
Shares outstanding, January 1 (in shares) | 110,987,400 |
Equity-based compensation plans (in shares) | (45,028) |
Other (in shares) | (20,357) | [1]
Shares outstanding, March 31 (in shares) | 110,922,015 |
[1] | Includes shares transferred from employees to Alliant Energy to satisfy tax withholding requirements in connection with the vesting of certain restricted stock under the equity-based compensation plans.

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