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Investments (Summary Financial Information) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Operating revenues $ 741 $ 658 $ 624
Operating income 374 331 299
Net income 267 232 186
Current assets 104 82  
Non-current assets 5,041 4,340  
Current liabilities 770 498  
Non-current liabilities 2,038 2,144  
Minority interest 255 0  
WPL [Member]      
Operating revenues 8 658 624
Operating income 4 331 299
Net income 2 234 $ 202
Current assets 7 6  
Non-current assets 20 19  
Current liabilities 2 3  
Non-current liabilities 8 7  
Minority interest $ 0 $ 0