Benefit Plans (Estimated Future Employer Contributions) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Qualified Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated Future and Actual Employer Contributions
Qualified Defined Benefit Pension Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated Future and Actual Employer Contributions
Qualified Defined Benefit Pension Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated Future and Actual Employer Contributions
Non-Qualified Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated Future and Actual Employer Contributions	17.0
Non-Qualified Defined Benefit Pension Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated Future and Actual Employer Contributions		[1]
Non-Qualified Defined Benefit Pension Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated Future and Actual Employer Contributions		[1]
Directly Assigned Defined Benefit Pension Benefits [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated Future and Actual Employer Contributions	0.8	[2]
Directly Assigned Defined Benefit Pension Benefits [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated Future and Actual Employer Contributions	0.2	[2]
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated Future and Actual Employer Contributions	4.9
Other Postretirement Benefits Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated Future and Actual Employer Contributions	0.6
Other Postretirement Benefits Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated Future and Actual Employer Contributions	$ 3.9

[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.