Benefit Plans (Employees Participate In Defined Contribution Retirement Plans) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Alliant Energy [Member]
401(k) costs	$ 4.4		$ 4.4		$ 14.1		$ 14.3
IPL [Member]
401(k) costs	2.4	[1]	2.2	[1]	7.4	[1]	7.1	[1]
WPL [Member]
401(k) costs	$ 2.0	[1]	$ 2.0	[1]	$ 6.2	[1]	$ 6.6	[1]

[1] IPL’s and WPL’s amounts include allocated costs associated with Corporate Services employees.