Summary Of Significant Accounting Policies (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Public Utility, Property, Plant and Equipment [Line Items]
Regulatory liabilities	$ 20.4	$ (26.5)
Alliant Energy and IPL [Member] | IPL's tax benefit riders [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Regulatory liabilities	20.7
IPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Regulatory liabilities	14.7	(33.6)
IPL [Member] | Electric tax benefit rider [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Regulatory liabilities	18.3
IPL [Member] | Gas tax benefit rider [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Regulatory liabilities	$ 2.4