Derivative Instruments Derivative Instruments (Credit Risk-related Contingent Features) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Derivative Instruments [Line Items]
Aggregate fair value	$ 14.1	$ 20.8
Credit support to be posted if triggered	13.9	20.8
IPL [Member]
Derivative Instruments [Line Items]
Aggregate fair value	8.2	5.2
Credit support to be posted if triggered	8.2	5.2
WPL [Member]
Derivative Instruments [Line Items]
Aggregate fair value	5.9	15.6
Credit support to be posted if triggered	$ 5.7	$ 15.6