Derivative Instruments (Narrative) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivative [Line Items]
Aggregate fair value of all derivatives with credit risk-related contingent features, net liability position	$ 17.1
Restrictive credit risk-related contingent features for derivative agreements, net liability position	17.1
IPL [Member]
Derivative [Line Items]
Aggregate fair value of all derivatives with credit risk-related contingent features, net liability position	6.1
Restrictive credit risk-related contingent features for derivative agreements, net liability position	6.1
Cash collateral posted	0	0.5
WPL [Member]
Derivative [Line Items]
Aggregate fair value of all derivatives with credit risk-related contingent features, net liability position	11.0
Restrictive credit risk-related contingent features for derivative agreements, net liability position	$ 11.0