Derivative Instruments (Fair Value Of Financial Instruments) (Details) - Commodity Contracts [Member] - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Current derivative assets	$ 34.2	$ 30.5
Non-current derivative assets	0.8	8.1
Current derivative liabilities	27.1	28.1
Non-current derivative liabilities	19.9	9.5
IPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	30.0	27.4
Non-current derivative assets	0.7	0.6
Current derivative liabilities	13.8	16.4
Non-current derivative liabilities	6.9	3.1
WPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	4.2	3.1
Non-current derivative assets	0.1	7.5
Current derivative liabilities	13.3	11.7
Non-current derivative liabilities	$ 13.0	$ 6.4