Derivative Instruments (Narrative) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012
Aggregate fair value of all derivatives with credit risk-related contingent features, net liability position	$ 83.2
Restrictive credit risk-related contingent features for derivative agreements, net liability position	82.2
IPL [Member]
Aggregate fair value of all derivatives with credit risk-related contingent features, net liability position	38.1
Restrictive credit risk-related contingent features for derivative agreements, net liability position	37.1
Cash collateral from counterparties	1.0
WPL [Member]
Aggregate fair value of all derivatives with credit risk-related contingent features, net liability position	45.1
Restrictive credit risk-related contingent features for derivative agreements, net liability position	$ 45.1