Regulatory Matters (Tax Benefit Riders) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	$ 14.2	$ (28.7)
Alliant Energy and IPL [Member] | Electric tax benefit rider [Member]
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	(17.0)
Alliant Energy and IPL [Member] | Gas tax benefit rider [Member]
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	(2.0)
Alliant Energy and IPL [Member] | Tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	$ (19.0)