Regulatory Matters (Tax Benefit Riders) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	$ (29.2)	$ (40.0)
Alliant Energy and IPL [Member] | Electric tax benefit rider [Member]
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	(30.0)
Alliant Energy and IPL [Member] | Gas tax benefit rider [Member]
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	(6.0)
Alliant Energy and IPL [Member] | Tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	$ (36.0)