Regulatory Matters (IPL's Tax Benefit Riders) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Regulatory Assets [Line Items]
Increase (decrease) in regulatory liabilities	$ (1.3)	$ 83.8	$ 63.0
Alliant Energy and IPL [Member] | Electric tax benefit rider credits [Member]
Regulatory Assets [Line Items]
Increase (decrease) in regulatory liabilities	17.0
Alliant Energy and IPL [Member] | Gas tax benefit rider credits [Member]
Regulatory Assets [Line Items]
Increase (decrease) in regulatory liabilities	2.0
Alliant Energy and IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Assets [Line Items]
Increase (decrease) in regulatory liabilities	$ 19.0