Benefit Plans (Regulatory Assets) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets	$ 33.7	$ 28.2
Regulatory liabilities	0.3
WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets	22.3	18.6
Regulatory liabilities	$ 0.2