Derivative Instruments (Fair Value Of Financial Instruments) (Details) - Commodity Contracts [Member] - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Current derivative assets	$ 15.8	$ 24.6
Non-current derivative assets	5.9	3.7
Current derivative liabilities	3.6	5.6
Non-current derivative liabilities	12.1	17.7
IPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	9.0	16.1
Non-current derivative assets	2.9	1.6
Current derivative liabilities	2.1	3.1
Non-current derivative liabilities	5.0	8.1
WPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	6.8	8.5
Non-current derivative assets	3.0	2.1
Current derivative liabilities	1.5	2.5
Non-current derivative liabilities	$ 7.1	$ 9.6