Derivative Instruments (Narrative) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012
Derivative [Line Items]
Aggregate fair value of all derivatives with credit risk-related contingent features, net liability position	$ 65.3
Restrictive credit risk-related contingent features for derivative agreements, net liability position	65.3
IPL [Member]
Derivative [Line Items]
Aggregate fair value of all derivatives with credit risk-related contingent features, net liability position	29.3
Restrictive credit risk-related contingent features for derivative agreements, net liability position	29.3
WPL [Member]
Derivative [Line Items]
Aggregate fair value of all derivatives with credit risk-related contingent features, net liability position	36.0
Restrictive credit risk-related contingent features for derivative agreements, net liability position	$ 36.0