Common Equity (Common Share Activity) (Details)	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Common Stock Oustanding [Roll Forward]
Shares outstanding, January 1 (in shares)	111,018,821		110,893,901		110,656,498
Equity-based compensation plans (in shares)	20,195		164,400		260,316
Other (in shares)	(51,616)	[1]	(39,480)	[1]	(22,913)	[1]
Shares outstanding, December 31 (in shares)	110,987,400		111,018,821		110,893,901

[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.