Investments (Summary Financial Information) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating revenues	$ 611	$ 575	$ 564
Operating income	326	307	307
Net income	234	218	226
Current assets	67	62
Non-current assets	3,321	3,100
Current liabilities	252	299
Non-current liabilities	1,652	1,490
WPL [Member]
Operating revenues	611	575	564
Operating income	325	308	308
Net income	239	226	221
Current assets	66	61
Non-current assets	3,292	3,071
Current liabilities	252	299
Non-current liabilities	$ 1,651	$ 1,487