Derivative Instruments (Balance Sheet Offsetting) (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Derivative Instruments [Line Items]
Derivative assets, Gross (as reported)	$ 35.0	$ 38.6
Derivative assets, Net	32.0	33.0
Derivative liabilities, Gross (as reported)	47.0	37.6
Derivative liabilities, Net	44.0	32.0
IPL [Member]
Derivative Instruments [Line Items]
Derivative assets, Gross (as reported)	30.7	28.0
Derivative assets, Net	29.3	24.7
Derivative liabilities, Gross (as reported)	20.7	19.5
Derivative liabilities, Net	19.3	16.2
WPL [Member]
Derivative Instruments [Line Items]
Derivative assets, Gross (as reported)	4.3	10.6
Derivative assets, Net	2.7	8.3
Derivative liabilities, Gross (as reported)	26.3	18.1
Derivative liabilities, Net	$ 24.7	$ 15.8