Regulatory Matters (IPL's Tax Benefit Riders) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Regulatory Assets [Line Items]
Increase (decrease) in regulatory liabilities	$ (63.0)	$ (67.8)	$ 10.8
Alliant Energy and IPL [Member] | Electric tax benefit rider credits [Member]
Regulatory Assets [Line Items]
Increase (decrease) in regulatory liabilities	(64.0)
Alliant Energy and IPL [Member] | Gas tax benefit rider credits [Member]
Regulatory Assets [Line Items]
Increase (decrease) in regulatory liabilities	(12.0)
Alliant Energy and IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Assets [Line Items]
Increase (decrease) in regulatory liabilities	$ (76.0)