Investments (Summary Financial Information) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating revenues	$ 624	$ 643	$ 634
Operating income	299	330	334
Net income	186	240	248
Current assets	88	72
Non-current assets	3,987	3,773
Current liabilities	332	315
Non-current liabilities	2,052	1,871
WPL [Member]
Operating revenues	624	643	634
Operating income	299	330	334
Net income	202	240	$ 250
Current assets	87	70
Non-current assets	3,977	3,747
Current liabilities	332	315
Non-current liabilities	$ 2,052	$ 1,870