Derivative Instruments (Balance Sheet Offsetting) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Derivative Instruments [Line Items]
Derivative assets, Gross (as reported)	$ 20.3	$ 38.6
Derivative assets, Net	18.0	33.0
Derivative liabilities, Gross (as reported)	59.1	37.6
Derivative liabilities, Net	56.8	32.0
IPL [Member]
Derivative Instruments [Line Items]
Derivative assets, Gross (as reported)	18.5	28.0
Derivative assets, Net	17.3	24.7
Derivative liabilities, Gross (as reported)	22.0	19.5
Derivative liabilities, Net	20.8	16.2
WPL [Member]
Derivative Instruments [Line Items]
Derivative assets, Gross (as reported)	1.8	10.6
Derivative assets, Net	0.7	8.3
Derivative liabilities, Gross (as reported)	37.1	18.1
Derivative liabilities, Net	$ 36.0	$ 15.8