Asset Retirement Obligations (Reconciliation Of Changes In Asset Retirement Obligations) (Details) $ in Millions		3 Months Ended	12 Months Ended
		Jun. 30, 2015 USD ($)	Dec. 31, 2016 USD ($)		Dec. 31, 2015 USD ($)
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance, January 1			$ 214.0		$ 114.0
Revisions in estimated cash flows	[1]		(13.3)		17.3
Liabilities settled			(14.0)		(8.8)
Liabilities incurred		$ 87.0	2.6	[1]	86.6	[1]
Accretion expense			6.4		4.9
Balance, December 31			195.7		214.0
IPL [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance, January 1			132.9		51.8
Revisions in estimated cash flows	[1]		(5.8)		15.1
Liabilities settled			(6.8)		(4.3)
Liabilities incurred		67.0	0.7	[1]	67.8	[1]
Accretion expense			3.7		2.5
Balance, December 31			$ 124.7		132.9
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			$ 71.9		52.4
Revisions in estimated cash flows	[1]		(7.5)		3.2
Liabilities settled			(7.2)		(4.5)
Liabilities incurred		$ 20.0	1.9	[1]	18.8	[1]
Accretion expense			2.3		2.0
Balance, December 31			$ 61.4		$ 71.9
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 was 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. In 2015, Alliant Energy, IPL and WPL recognized additional AROs of $87 million, $67 million and $20 million, respectively, as a result of the final CCR Rule. These 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 in 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.