Derivative Instruments (Fair Value Of Financial Instruments) (Details) - Commodity Contracts [Member] - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Current derivative assets	$ 28.0	$ 21.1
Non-current derivative assets	3.6	4.0
Current derivative liabilities	11.1	18.7
Non-current derivative liabilities	31.6	23.0
IPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	17.8	15.8
Non-current derivative assets	1.2	1.3
Current derivative liabilities	3.5	5.0
Non-current derivative liabilities	19.5	14.4
WPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	10.2	5.3
Non-current derivative assets	2.4	2.7
Current derivative liabilities	7.6	13.7
Non-current derivative liabilities	$ 12.1	$ 8.6