Derivative Instruments (Narrative) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013
Derivative [Line Items]
Aggregate fair value of all derivatives with credit risk-related contingent features, net liability position	$ 20.8
Restrictive credit risk-related contingent features for derivative agreements, net liability position	20.8
IPL [Member]
Derivative [Line Items]
Aggregate fair value of all derivatives with credit risk-related contingent features, net liability position	5.2
Restrictive credit risk-related contingent features for derivative agreements, net liability position	5.2
WPL [Member]
Derivative [Line Items]
Aggregate fair value of all derivatives with credit risk-related contingent features, net liability position	15.6
Restrictive credit risk-related contingent features for derivative agreements, net liability position	$ 15.6