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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 |

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