Derivative Instruments (Fair Value Of Financial Instruments) (Details) - Commodity Contracts [Member] - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Current derivative assets	$ 6.1	$ 15.1
Non-current derivative assets	1.6	3.3
Current derivative liabilities	56.3	47.3
Non-current derivative liabilities	27.7	17.3
IPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	4.8	13.8
Non-current derivative assets	1.0	1.7
Current derivative liabilities	18.7	18.5
Non-current derivative liabilities	4.6	4.9
WPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	1.3	1.3
Non-current derivative assets	0.6	1.6
Current derivative liabilities	37.6	28.8
Non-current derivative liabilities	$ 23.1	$ 12.4