Regulatory Matters (Gas Tax Benefit Rider) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	$ 10.8	$ (90.8)	$ 16.4
Income tax expense (benefit)	44.3	53.9	89.4
Gas [Member] | IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	(12.0)
Alliant Energy and IPL [Member] | Gas [Member] | IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	(12.0)	(11.0)
Income tax expense (benefit)	(7.0)	(7.0)
Alliant Energy and IPL [Member] | Gas [Member] | IPL's Tax Benefit Rider Decrease In Taxable Income [Member]
Regulatory Liabilities [Line Items]
Income tax expense (benefit)	$ (5.0)	$ (4.0)