Common Equity (Common Share Activity) (Details)	9 Months Ended
	Sep. 30, 2012
Common Stock Oustanding [Roll Forward]
Shares outstanding, January 1 (in shares)	111,018,821
Equity incentive plans (in shares)	20,195
Other (in shares)	(51,616)	[1]
Shares outstanding, September 30 (in shares)	110,987,400

[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 incentive plans.