Derivative Instruments (Fair Value Of Financial Instruments) (Details) - Commodity Contracts [Member] - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Current derivative assets	$ 12.2	$ 29.4
Non-current derivative assets	1.1	12.0
Current derivative liabilities	13.4	13.3
Non-current derivative liabilities	31.9	15.3
IPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	8.2	19.1
Non-current derivative assets	0.6	1.7
Current derivative liabilities	2.9	2.7
Non-current derivative liabilities	13.4	5.6
WPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	4.0	10.3
Non-current derivative assets	0.5	10.3
Current derivative liabilities	10.5	10.6
Non-current derivative liabilities	$ 18.5	$ 9.7