Derivative Instruments (Fair Value Of Financial Instruments) (Details) - Commodity Contracts [Member] - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Current derivative assets	$ 24.6	$ 21.1
Non-current derivative assets	3.7	4.0
Current derivative liabilities	5.6	18.7
Non-current derivative liabilities	17.7	23.0
IPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	16.1	15.8
Non-current derivative assets	1.6	1.3
Current derivative liabilities	3.1	5.0
Non-current derivative liabilities	8.1	14.4
WPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	8.5	5.3
Non-current derivative assets	2.1	2.7
Current derivative liabilities	2.5	13.7
Non-current derivative liabilities	$ 9.6	$ 8.6