Derivative Instruments (Fair Value Of Financial Instruments) (Details) - Commodity Contracts [Member] - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Current derivative assets	$ 31.9	$ 15.1
Non-current derivative assets	5.0	3.3
Current derivative liabilities	23.7	47.3
Non-current derivative liabilities	13.4	17.3
IPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	25.8	13.8
Non-current derivative assets	2.3	1.7
Current derivative liabilities	7.2	18.5
Non-current derivative liabilities	2.6	4.9
WPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	6.1	1.3
Non-current derivative assets	2.7	1.6
Current derivative liabilities	16.5	28.8
Non-current derivative liabilities	$ 10.8	$ 12.4