Benefit Plans (Estimated Future And Actual Employer Contributions) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 Defined benefit pension plans [Member]		Mar. 31, 2013 Defined benefit pension plans [Member] IPL [Member]		Mar. 31, 2013 Defined benefit pension plans [Member] WPL [Member]		Mar. 31, 2013 Other postretirement benefits plans [Member]	Mar. 31, 2013 Other postretirement benefits plans [Member] IPL [Member]	Mar. 31, 2013 Other postretirement benefits plans [Member] WPL [Member]	Dec. 31, 2012 Scenario, Forecast [Member] Defined benefit pension plans [Member]		Dec. 31, 2012 Scenario, Forecast [Member] Defined benefit pension plans [Member] IPL [Member]		Dec. 31, 2012 Scenario, Forecast [Member] Defined benefit pension plans [Member] WPL [Member]		Dec. 31, 2012 Scenario, Forecast [Member] Other postretirement benefits plans [Member]	Dec. 31, 2012 Scenario, Forecast [Member] Other postretirement benefits plans [Member] IPL [Member]	Dec. 31, 2012 Scenario, Forecast [Member] Other postretirement benefits plans [Member] WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated for Calendar Year 2013										$ 2.4	[1]	$ 0.8	[1]	$ 0.2	[1]	$ 3.0	$ 0	$ 3.0
Actual Through March 31, 2013	$ 0.9	[1]	$ 0.3	[1]	$ 0.1	[1]	$ 0	$ 0	$ 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. In addition, IPL and WPL amounts reflect funding for their non-bargaining employees who are participants in the Alliant Energy and Corporate Services sponsored qualified and non-qualified defined benefit pension plans.