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Investments (Summary Financial Information) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Revenues | $ 724 | $ 741 | $ 658
Operating income | 325 | 374 | 331
Net income | 217 | 267 | 232
Current assets | 144 | 104 |
Non-current assets | 5,498 | 5,041 |
Current liabilities | 644 | 770 |
Non-current liabilities | 2,315 | 2,038 |
Noncontrolling interest | 223 | 255 |
WPL [Member] |  |  |
Revenues | 8 | 8 | 658
Operating income | 2 | 4 | 331
Net income | 1 | 2 | $ 234
Current assets | 4 | 7 |
Non-current assets | 19 | 20 |
Current liabilities | 1 | 2 |
Non-current liabilities | 6 | 8 |
Noncontrolling interest | $ 0 | $ 0 |

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