Benefit Plans (Estimated Future Funding) (Details) $ in Millions	Dec. 31, 2018 USD ($)
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated funding for calendar year 2019	$ 33.8	[1]
Defined Benefit Pension Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated funding for calendar year 2019	16.5	[1]
Defined Benefit Pension Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated funding for calendar year 2019	15.6	[1]
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated funding for calendar year 2019	9.6
Other Postretirement Benefits Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated funding for calendar year 2019	1.9
Other Postretirement Benefits Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated funding for calendar year 2019	$ 7.4

[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.