Asset Retirement Obligations (Reconciliation Of Changes In Asset Retirement Obligations) (Details) $ in Millions		3 Months Ended	6 Months Ended
		Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($)		Jun. 30, 2015 USD ($)
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance, January 1			$ 214.0		$ 114.0
Revisions in estimated cash flows	[1]		3.9		2.1
Liabilities settled			(7.3)		(6.4)
Liabilities incurred		$ 74.0	2.2	[1]	72.3	[1]
Accretion expense			3.2		2.2
Balance, June 30		184.2	216.0		184.2
IPL [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance, January 1			132.9		51.8
Revisions in estimated cash flows	[1]		4.2		0.9
Liabilities settled			(2.0)		(2.4)
Liabilities incurred		57.0	0.7	[1]	56.1	[1]
Accretion expense			1.9		0.9
Balance, June 30		107.3	$ 137.7		107.3
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			$ 71.9		52.4
Revisions in estimated cash flows	[1]		(0.3)		1.2
Liabilities settled			(5.3)		(4.0)
Liabilities incurred		17.0	1.5	[1]	16.2	[1]
Accretion expense			1.1		0.9
Balance, June 30		$ 66.7	$ 68.9		$ 66.7
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 the second quarter of 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.