Benefit Plans (Schedule Of Qualified Pension Benefit Costs And Other Post Retirement Benefit Costs) (Details) (USD $) In Millions	3 Months Ended				9 Months Ended				3 Months Ended				9 Months Ended				3 Months Ended		6 Months Ended	9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended
	Sep. 30, 2011 IPL [Member] Defined Benefit Pension Plans [Member]		Sep. 30, 2010 IPL [Member] Defined Benefit Pension Plans [Member]		Sep. 30, 2011 IPL [Member] Defined Benefit Pension Plans [Member]		Sep. 30, 2010 IPL [Member] Defined Benefit Pension Plans [Member]		Sep. 30, 2011 WPL [Member] Defined Benefit Pension Plans [Member]		Sep. 30, 2010 WPL [Member] Defined Benefit Pension Plans [Member]		Sep. 30, 2011 WPL [Member] Defined Benefit Pension Plans [Member]		Sep. 30, 2010 WPL [Member] Defined Benefit Pension Plans [Member]		Sep. 30, 2011 Defined Benefit Pension Plans [Member]		Dec. 31, 2011 Defined Benefit Pension Plans [Member]	Sep. 30, 2011 Defined Benefit Pension Plans [Member]		Sep. 30, 2011 IPL [Member] Other Postretirement Benefit Plans [Member]	Sep. 30, 2010 IPL [Member] Other Postretirement Benefit Plans [Member]	Sep. 30, 2011 IPL [Member] Other Postretirement Benefit Plans [Member]	Sep. 30, 2010 IPL [Member] Other Postretirement Benefit Plans [Member]	Sep. 30, 2011 WPL [Member] Other Postretirement Benefit Plans [Member]	Sep. 30, 2010 WPL [Member] Other Postretirement Benefit Plans [Member]	Sep. 30, 2011 WPL [Member] Other Postretirement Benefit Plans [Member]	Sep. 30, 2010 WPL [Member] Other Postretirement Benefit Plans [Member]
Defined benefit plan pension benefit cost	$ 0.7	[1]		[1]	$ 6.0	[1]		[1]	$ 0.5	[1]	$ 0.1	[1]	$ 3.8	[1]	$ 0.5	[1]							$ 0.4	$ 0.4	$ 1.7	$ (0.1)	$ 0.3	$ 0.2	$ 1.1
Additional benefit costs	$ 0.8				$ 6.3				$ 0.4				$ 3.4				$ 1.3	[2]	$ 10.2	$ 10.2	[2]

[1] For the three and nine months ended Sep. 30, 2011, additional qualified pension benefits costs resulting from the second quarter 2011 amendment to the Cash Balance Pension Plan allocated to IPL were $0.8 million and $6.3 million, respectively, and to WPL were $0.4 million and $3.4 million, respectively.

[2] Alliant Energy reached an agreement with the IRS, which resulted in a favorable determination letter for the Alliant Energy Cash Balance Pension Plan (Plan) during the first quarter of 2011. The agreement with the IRS required Alliant Energy to amend the Plan, which was completed in the second quarter of 2011 resulting in aggregate additional benefits of $10.2 million to be paid to certain former participants in the Plan in the second half of 2011. Alliant Energy recognized $1.3 million and $10.2 million of additional benefits costs during the three and nine months ended Sep. 30, 2011, respectively, related to these benefits. Refer to Note 11(c) for additional information regarding the Plan.