Derivative Instruments (Fair Value Of Financial Instruments) (Details) - Commodity Contracts [Member] - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Current derivative assets	$ 15.1	$ 30.5
Non-current derivative assets	3.3	8.1
Current derivative liabilities	47.3	28.1
Non-current derivative liabilities	17.3	9.5
IPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	13.8	27.4
Non-current derivative assets	1.7	0.6
Current derivative liabilities	18.5	16.4
Non-current derivative liabilities	4.9	3.1
WPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	1.3	3.1
Non-current derivative assets	1.6	7.5
Current derivative liabilities	28.8	11.7
Non-current derivative liabilities	$ 12.4	$ 6.4