Regulatory Matters (Tax Benefit Riders) (Details) (Alliant Energy and IPL [Member], USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015
Electric tax benefit rider [Member]
Regulatory Liabilities [Line Items]
Decrease in regulatory liabilities	$ 18.0
Gas tax benefit rider [Member]
Regulatory Liabilities [Line Items]
Decrease in regulatory liabilities	3.0
Tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Decrease in regulatory liabilities	$ 21.0