Investments (Summary Financial Information) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating revenues	$ 658	$ 624	$ 643
Operating income	331	299	330
Net income	232	186	240
Current assets	82	88
Non-current assets	4,340	3,987
Current liabilities	498	332
Non-current liabilities	2,144	2,052
WPL [Member]
Operating revenues	658	624	643
Operating income	331	299	330
Net income	234	202	$ 240
Current assets	6	87
Non-current assets	19	3,977
Current liabilities	3	332
Non-current liabilities	$ 7	$ 2,052