Derivative Instruments Derivative Instruments (Balance Sheet Offsetting) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Derivative [Line Items]
Derivative assets, Gross (as reported)	$ 119.0	$ 26.7
Derivative assets, Net	103.9	23.5
Derivative liabilities, Gross (as reported)	13.3	20.8
Derivative liabilities, Net	6.6	17.6
IPL [Member]
Derivative [Line Items]
Derivative assets, Gross (as reported)	73.0	21.1
Derivative assets, Net	68.8	19.5
Derivative liabilities, Gross (as reported)	6.3	5.2
Derivative liabilities, Net	2.4	3.6
WPL [Member]
Derivative [Line Items]
Derivative assets, Gross (as reported)	46.0	5.6
Derivative assets, Net	35.1	4.0
Derivative liabilities, Gross (as reported)	7.0	15.6
Derivative liabilities, Net	$ 4.2	$ 14.0