Derivative Instruments (Fair Value Of Financial Instruments) (Details) - Commodity Contracts [Member] - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Current derivative assets	$ 39.7	$ 21.1
Non-current derivative assets	8.3	4.0
Current derivative liabilities	7.2	18.7
Non-current derivative liabilities	14.8	23.0
IPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	26.2	15.8
Non-current derivative assets	2.9	1.3
Current derivative liabilities	3.2	5.0
Non-current derivative liabilities	6.5	14.4
WPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	13.5	5.3
Non-current derivative assets	5.4	2.7
Current derivative liabilities	4.0	13.7
Non-current derivative liabilities	$ 8.3	$ 8.6