Derivative Instruments Derivative Instruments (Balance Sheet Offsetting) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Derivative Instruments [Line Items]
Derivative assets, Gross (as reported)	$ 83.1	$ 26.7
Derivative assets, Net	72.2	23.5
Derivative liabilities, Gross (as reported)	14.1	20.8
Derivative liabilities, Net	7.9	17.6
IPL [Member]
Derivative Instruments [Line Items]
Derivative assets, Gross (as reported)	44.7	21.1
Derivative assets, Net	39.8	19.5
Derivative liabilities, Gross (as reported)	8.2	5.2
Derivative liabilities, Net	3.6	3.6
WPL [Member]
Derivative Instruments [Line Items]
Derivative assets, Gross (as reported)	38.4	5.6
Derivative assets, Net	32.4	4.0
Derivative liabilities, Gross (as reported)	5.9	15.6
Derivative liabilities, Net	$ 4.3	$ 14.0