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Investment in Transmission Affiliates - ATC Summarized Financial Data (Details) - ATC - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Income statement data |  |  |
Revenues | $ 721.7 | $ 650.8 | $ 615.8
Operating expenses | 345.0 | 322.5 | 319.3
Other expense | 104.1 | 95.5 | 96.1
Net income | 272.6 | 232.8 | $ 200.4
Balance sheet data |  |  |
Current assets | 87.7 | 75.8 |
Noncurrent assets | 4,598.9 | 4,312.9 |
Total assets | 4,686.6 | 4,388.7 |
Current liabilities | 767.2 | 495.1 |
Long-term debt | 1,790.6 | 1,865.3 |
Other noncurrent liabilities | 240.3 | 271.5 |
Shareholders' equity | 1,888.5 | 1,756.8 |
Total liabilities and shareholders' equity | $ 4,686.6 | $ 4,388.7 |

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