Common Equity (Common Share Activity) (Details)	3 Months Ended
	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.