Benefit Plans (Regulatory Assets and Regulatory Liabilities) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets	$ 26.5	$ 38.1
Regulatory liabilities	1.7	0.6
WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets	19.8	25.5
Regulatory liabilities	$ 1.3	$ 0.4