Regulatory Matters (Tax Benefit Riders) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	$ 28.7	$ 6.6
Alliant Energy and IPL [Member] | Electric tax benefit rider [Member]
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	(15.0)
Alliant Energy and IPL [Member] | Gas tax benefit rider [Member]
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	(3.0)
Alliant Energy and IPL [Member] | Tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	$ (18.0)