Regulatory Matters (Tax Benefit Riders) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Regulatory Liabilities [Line Items]
Decrease in regulatory liabilities	$ (40.0)	$ 45.2
Alliant Energy and IPL [Member] | Electric tax benefit rider [Member]
Regulatory Liabilities [Line Items]
Decrease in regulatory liabilities	35.0
Alliant Energy and IPL [Member] | Gas tax benefit rider [Member]
Regulatory Liabilities [Line Items]
Decrease in regulatory liabilities	6.0
Alliant Energy and IPL [Member] | Tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Decrease in regulatory liabilities	$ 41.0