Derivative Instruments (Fair Value Of Financial Instruments) (Details) - Commodity Contracts [Member] - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Current derivative assets	$ 29.4	$ 15.1
Non-current derivative assets	12.0	3.3
Current derivative liabilities	13.3	47.3
Non-current derivative liabilities	15.3	17.3
IPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	19.1	13.8
Non-current derivative assets	1.7	1.7
Current derivative liabilities	2.7	18.5
Non-current derivative liabilities	5.6	4.9
WPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	10.3	1.3
Non-current derivative assets	10.3	1.6
Current derivative liabilities	10.6	28.8
Non-current derivative liabilities	$ 9.7	$ 12.4