Investments (Summary Financial Information) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating revenues	$ 643	$ 634	$ 611
Operating income	330	334	326
Net income	240	248	234
Current assets	72	86
Non-current assets	3,773	3,553
Current liabilities	315	383
Non-current liabilities	1,871	1,682
WPL [Member]
Operating revenues	643	634	611
Operating income	330	334	325
Net income	240	250	239
Current assets	70	84
Non-current assets	3,747	3,527
Current liabilities	315	383
Non-current liabilities	$ 1,870	$ 1,681