Investments (Summary Financial Information) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Operating revenues	$ 575.0	$ 564.0	$ 529.0
Operating income	307.0	307.0	292.0
Net income	218.0	226.0	218.0
Current assets	62.0	64.0
Non-current assets	3,100.0	2,941.0
Current liabilities	299.0	429.0
Non-current liabilities	1,490.0	1,266.0
WPL [Member]
Operating revenues	575.0	564.0	529.0
Operating income	308.0	308.0	294.0
Net income	226.0	221.0	215.0
Current assets	61.0	63.0
Non-current assets	3,071.0	2,906.0
Current liabilities	299.0	429.0
Non-current liabilities	$ 1,487.0	$ 1,263.0