Regulatory Matters (Tax Benefit Riders) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	$ 64.8	$ 66.5
Alliant Energy and IPL [Member] | Electric tax benefit rider [Member]
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	(51.0)
Alliant Energy and IPL [Member] | Gas tax benefit rider [Member]
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	(5.0)
Alliant Energy and IPL [Member] | Rate-making accounting change [Member]
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	17.0
Alliant Energy and IPL [Member] | Tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	$ (39.0)