Derivative Instruments (Narrative) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012
Derivative [Line Items]
Aggregate fair value of all derivatives with credit risk-related contingent features, net liability position	$ 57.3
Restrictive credit risk-related contingent features for derivative agreements, net liability position	57.0
IPL [Member]
Derivative [Line Items]
Aggregate fair value of all derivatives with credit risk-related contingent features, net liability position	22.8
Derivative, counterparty (in counterparty)	1
Collateral already posted, aggregate fair value	0.3
Restrictive credit risk-related contingent features for derivative agreements, net liability position	22.5
WPL [Member]
Derivative [Line Items]
Aggregate fair value of all derivatives with credit risk-related contingent features, net liability position	34.5
Restrictive credit risk-related contingent features for derivative agreements, net liability position	$ 34.5