Derivative Instruments (Fair Value Of Financial Instruments) (Details) - Commodity Contracts [Member] - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Current derivative assets	$ 25.1	$ 15.1
Non-current derivative assets	2.7	3.3
Current derivative liabilities	21.2	47.3
Non-current derivative liabilities	15.7	17.3
IPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	20.5	13.8
Non-current derivative assets	1.7	1.7
Current derivative liabilities	5.4	18.5
Non-current derivative liabilities	3.6	4.9
WPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	4.6	1.3
Non-current derivative assets	1.0	1.6
Current derivative liabilities	15.8	28.8
Non-current derivative liabilities	$ 12.1	$ 12.4