Benefit Plans (Estimated Future And Actual Employer Contributions) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012 Non-Qualified Defined Benefit Pension Plans [Member] Alliant Energy Corporation [Member]		Mar. 31, 2012 Directly Assigned Defined Benefit Pension Plans [Member] IPL [Member]		Mar. 31, 2012 Other Postretirement Benefits Costs [Member] Alliant Energy Corporation [Member]	Mar. 31, 2012 Other Postretirement Benefits Costs [Member] IPL [Member]	Mar. 31, 2012 Other Postretirement Benefits Costs [Member] WPL [Member]	Dec. 31, 2012 Scenario, Forecast [Member] Non-Qualified Defined Benefit Pension Plans [Member] Alliant Energy Corporation [Member]		Dec. 31, 2012 Scenario, Forecast [Member] Non-Qualified Defined Benefit Pension Plans [Member] IPL [Member]		Dec. 31, 2012 Scenario, Forecast [Member] Non-Qualified Defined Benefit Pension Plans [Member] WPL [Member]		Dec. 31, 2012 Scenario, Forecast [Member] Directly Assigned Defined Benefit Pension Plans [Member] IPL [Member]		Dec. 31, 2012 Scenario, Forecast [Member] Directly Assigned Defined Benefit Pension Plans [Member] WPL [Member]		Dec. 31, 2012 Scenario, Forecast [Member] Other Postretirement Benefits Costs [Member] Alliant Energy Corporation [Member]	Dec. 31, 2012 Scenario, Forecast [Member] Other Postretirement Benefits Costs [Member] IPL [Member]	Dec. 31, 2012 Scenario, Forecast [Member] Other Postretirement Benefits Costs [Member] WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated Future Employer Contributions								$ 17.0	[1]		[1]		[1]	$ 0.8	[2]	$ 0.2	[2]	$ 5.4	$ 1.1	$ 3.9
Actual Employer Contributions	$ 2.1	[1]	$ 0.2	[2]	$ 2.2	$ 1.1	$ 1.0

[1] Alliant Energy sponsors several non-qualified defined benefit pension plans that cover certain current and former key employees of IPL and WPL. Alliant Energy allocates pension costs to IPL and WPL for these plans.

[2] Amounts directly assigned to IPL and WPL for non-bargaining employees who are participants in Alliant Energy and Corporate Services sponsored qualified and non-qualified defined benefit pension plans.