Benefit Plans (Regulatory Assets and Regulatory Liabilities) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets	$ 38.2	$ 26.5
Regulatory liabilities	0	1.7
WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets	28.0	19.8
Regulatory liabilities	$ 0	$ 1.3