Derivative Instruments Derivative Instruments (Balance Sheet Offsetting) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Derivative [Line Items]
Derivative assets, Gross (as reported)	$ 41.3	$ 26.7
Derivative assets, Net	32.3	23.5
Derivative liabilities, Gross (as reported)	9.0	20.8
Derivative liabilities, Net	4.9	17.6
IPL [Member]
Derivative [Line Items]
Derivative assets, Gross (as reported)	15.9	21.1
Derivative assets, Net	9.7	19.5
Derivative liabilities, Gross (as reported)	2.2	5.2
Derivative liabilities, Net	0.9	3.6
WPL [Member]
Derivative [Line Items]
Derivative assets, Gross (as reported)	25.4	5.6
Derivative assets, Net	22.6	4.0
Derivative liabilities, Gross (as reported)	6.8	15.6
Derivative liabilities, Net	$ 4.0	$ 14.0