Derivative Instruments (Fair Value Of Financial Instruments) (Details) - Commodity Contracts [Member] - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Current derivative assets	$ 10.8	$ 21.1
Non-current derivative assets	3.3	4.0
Current derivative liabilities	15.9	18.7
Non-current derivative liabilities	30.8	23.0
IPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	7.0	15.8
Non-current derivative assets	0.9	1.3
Current derivative liabilities	4.8	5.0
Non-current derivative liabilities	19.9	14.4
WPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	3.8	5.3
Non-current derivative assets	2.4	2.7
Current derivative liabilities	11.1	13.7
Non-current derivative liabilities	$ 10.9	$ 8.6