Benefit Plans (Regulatory Assets) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets	$ 38.1	$ 33.7
Regulatory liabilities	0.6	0.3
WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets	25.5	22.3
Regulatory liabilities	$ 0.4	$ 0.2