Benefit Plans (Schedule Of Qualified And Non-Qualified Pension Benefit Costs And Other Post Retirement Benefit Costs) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Additional benefit costs	$ 10.2
Defined Benefit Pension Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan pension benefits costs	5.8	[1]	2.9	[1]	5.1	[1]
Additional benefit costs	6.3
Defined Benefit Pension Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan pension benefits costs	4.2	[1]	1.9	[1]	3.2	[1]
Additional benefit costs	3.4
Other Postretirement Benefits Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan pension benefits costs	0.3		2.0		1.8
Other Postretirement Benefits Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan pension benefits costs	0.2		1.3		1.2
Allocated From Corporate Services [Member] | Defined Benefit Pension Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Additional benefit costs	3.5
Allocated From Corporate Services [Member] | Defined Benefit Pension Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Additional benefit costs	$ 2.7

[1] In 2011, additional qualified pension benefits costs resulting from the 2011 amendment to the Cash Balance Plan allocated to IPL and WPL by Corporate Services were $3.5 million and $2.7 million, respectively.