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Investment in Transmission Affiliates - ATC Summarized Financial Data (Details) - ATC - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Income statement data |  |  |
Revenues | $ 690.5 | $ 721.7 | $ 650.8
Operating expenses | 358.7 | 345.0 | 322.5
Other expense | 108.3 | 104.1 | 95.5
Net income | 223.5 | 272.6 | $ 232.8
Balance sheet data |  |  |
Current assets | 87.2 | 87.7 |
Noncurrent assets | 4,928.8 | 4,598.9 |
Total assets | 5,016.0 | 4,686.6 |
Current liabilities | 640.0 | 767.2 |
Long-term debt | 2,014.0 | 1,790.6 |
Other noncurrent liabilities | 295.3 | 240.3 |
Shareholders' equity | 2,066.7 | 1,888.5 |
Total liabilities and shareholders' equity | $ 5,016.0 | $ 4,686.6 |

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