Asset Retirement Obligations (Reconciliation Of Changes In Asset Retirement Obligations) (Details) $ in Millions	3 Months Ended	9 Months Ended
	Jun. 30, 2015 USD ($)	Sep. 30, 2015 USD ($)		Sep. 30, 2014 USD ($)
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance, January 1		$ 114.0		$ 109.7
Revisions in estimated cash flows		8.9		0.0
Liabilities settled		(7.1)		(1.0)
Liabilities incurred	$ 74.0	76.1	[1]	16.5	[1]
Accretion expense		3.4		3.3
Balance, September 30		195.3		128.5
IPL [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance, January 1		51.8		47.9
Revisions in estimated cash flows		11.9		0.0
Liabilities settled		(3.1)		(0.5)
Liabilities incurred	57.0	59.9	[1]	16.3	[1]
Accretion expense		1.6		1.6
Balance, September 30		$ 122.1		65.3
Electric generating units with coal ash ponds (in number of electric generating units)		9
Active coal combustion residuals landfills (in number of landfills)		4
WPL [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance, January 1		$ 52.4		52.4
Revisions in estimated cash flows		(1.9)		0.0
Liabilities settled		(4.0)		(0.5)
Liabilities incurred	$ 17.0	16.2	[1]	0.2	[1]
Accretion expense		1.4		1.3
Balance, September 30		$ 64.1		$ 53.4
Electric generating units with coal ash ponds (in number of electric generating units)		3
Active coal combustion residuals landfills (in number of landfills)		2

[1] In April 2015, the EPA published the final CCR Rule, which regulates CCR as a non-hazardous waste and is effective October 2015. IPL and WPL have nine and three coal-fired EGUs, respectively, with coal ash ponds that are impacted by this rule. In addition, IPL and WPL have four and two active CCR landfills, respectively, that are impacted by this rule. During the nine months ended September 30, 2015, Alliant Energy, IPL and WPL recognized additional AROs of $74 million, $57 million and $17 million, respectively, as a result of the final CCR Rule. The increases in AROs resulted in corresponding increases in “Property, plant and equipment, net” on the respective balance sheets. Actual costs resulting from the CCR rule may be different than the amounts recorded during the nine months ended September 30, 2015 due to potential changes in compliance strategies that will be used, as well as other potential cost estimate changes. Expenditures incurred by IPL and WPL to comply with the CCR Rule are anticipated to be recovered in rates from their customers.