Regulatory Matters Regulatory Matters (Tax Benefit Riders) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	$ 61.1	$ (74.9)
Alliant Energy and IPL [Member] | Electric tax benefit rider [Member]
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	(64.0)
Alliant Energy and IPL [Member] | Gas tax benefit rider [Member]
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	(9.0)
Alliant Energy and IPL [Member] | Tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	$ (73.0)