Regulatory Matters (IPL's Tax Benefit Riders) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Regulatory Assets [Line Items]
Increase (decrease) in regulatory liabilities	$ 83.8	$ 63.0	$ 67.8
Alliant Energy and IPL [Member] | Electric tax benefit rider credits [Member]
Regulatory Assets [Line Items]
Increase (decrease) in regulatory liabilities	(65.0)
Alliant Energy and IPL [Member] | Gas tax benefit rider credits [Member]
Regulatory Assets [Line Items]
Increase (decrease) in regulatory liabilities	(6.0)
Alliant Energy and IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Assets [Line Items]
Increase (decrease) in regulatory liabilities	17.0
Alliant Energy and IPL [Member] | Tax Reform adjustment [Member]
Regulatory Assets [Line Items]
Increase (decrease) in regulatory liabilities	(5.0)
Alliant Energy and IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Assets [Line Items]
Increase (decrease) in regulatory liabilities	$ (59.0)