Related Parties (Narrative) (Details) (USD $) In Millions, unless otherwise specified			6 Months Ended			3 Months Ended
	Jun. 30, 2012 WPL Owed ATC [Member]	Dec. 31, 2011 WPL Owed ATC [Member]	Jun. 30, 2012 IPL [Member]	Jun. 30, 2011 IPL [Member]	Dec. 31, 2011 IPL [Member]	Jun. 30, 2011 IPL [Member] Franklin County Wind Project [Member]
Related Party Transaction [Line Items]
Net amounts owed	$ 7	$ 6	$ 79		$ 82
Proceeds from sale of wind project assets to affiliate			$ 0	$ 115.3		$ 115.3