Regulatory Matters (Electric Tax Benefit Rider) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	$ (83.8)	$ (63.0)	$ (67.8)
Income tax expense (benefit)	66.7	59.4	70.4
Alliant Energy and IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	(17.0)
Alliant Energy and IPL [Member] | Electric [Member] | IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	(65.0)	(64.0)	(72.0)
Income tax expense (benefit)	(38.0)	(37.0)	(42.0)
Alliant Energy and IPL [Member] | Electric [Member] | Decrease In taxable income from IPL's tax benefit rider [Member]
Regulatory Liabilities [Line Items]
Income tax expense (benefit)	$ (27.0)	$ (27.0)	$ (30.0)