Derivative Instruments Derivative Instruments (Credit Risk-related Contingent Features) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Derivative [Line Items]
Aggregate fair value	$ 13.3	$ 20.8
Credit support to be posted if triggered	13.3	20.8
IPL [Member]
Derivative [Line Items]
Aggregate fair value	6.3	5.2
Credit support to be posted if triggered	6.3	5.2
WPL [Member]
Derivative [Line Items]
Aggregate fair value	7.0	15.6
Credit support to be posted if triggered	$ 7.0	$ 15.6