Derivative Instruments (Narrative) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Derivative [Line Items]
Aggregate fair value of all derivatives with credit risk-related contingent features, net liability position	$ 40.4
Restrictive credit risk-related contingent features for derivative agreements, net liability position	39.9
IPL [Member]
Derivative [Line Items]
Aggregate fair value of all derivatives with credit risk-related contingent features, net liability position	16.1
Number of counterparties exceeding derivative liability position	1
Cash collateral from counterparties	0.5
Restrictive credit risk-related contingent features for derivative agreements, net liability position	15.6
WPL [Member]
Derivative [Line Items]
Aggregate fair value of all derivatives with credit risk-related contingent features, net liability position	24.3
Restrictive credit risk-related contingent features for derivative agreements, net liability position	$ 24.3