Benefit Plans (Employees Participate In Defined Contribution Retirement Plans) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011
Alliant Energy [Member]
401(k) costs	$ 4.5		$ 4.2		$ 9.7		$ 9.9
IPL [Member]
401(k) costs	2.3	[1]	2.0	[1]	5.0	[1]	4.9	[1]
WPL [Member]
401(k) costs	$ 1.9	[1]	$ 2.0	[1]	$ 4.2	[1]	$ 4.6	[1]

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