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SEGMENT INFORMATION (Details) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 USD ($) segment | Mar. 31, 2017 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Segment information |  |  |  |  |
Number of reportable segments | segment | 6 |  |  |  |
Revenues | $ 2,286.5 | $ 2,304.5 |  |  |
Other operation and maintenance | 511.9 | 504.5 | $ 2,056.1 | $ 2,171.3 | $ 1,710.7
Depreciation and amortization | 208.6 | 194.6 |  |  |
Operating income (loss) | 545.1 | 614.7 |  |  |
Equity in earnings of transmission affiliates | 32.8 | 41.9 |  |  |
Interest expense | 106.7 | 104.7 |  |  |
Intersegment revenues |  |  |  |  |
Segment information |  |  |  |  |
Revenues | 0.0 | 0.0 |  |  |
External Revenues |  |  |  |  |
Segment information |  |  |  |  |
Revenues | 2,286.5 | 2,304.5 |  |  |
Electric Transmission |  |  |  |  |
Segment information |  |  |  |  |
Revenues | 0.0 |  |  |  |
Other operation and maintenance | 0.0 | 0.0 |  |  |
Depreciation and amortization | 0.0 | 0.0 |  |  |
Operating income (loss) | 0.0 | 0.0 |  |  |
Equity in earnings of transmission affiliates | 32.8 | 41.9 |  |  |
Interest expense | 0.0 | 0.0 |  |  |
Electric Transmission | Intersegment revenues |  |  |  |  |
Segment information |  |  |  |  |
Revenues | 0.0 | 0.0 |  |  |
Electric Transmission | External Revenues |  |  |  |  |
Segment information |  |  |  |  |
Revenues | 0.0 | 0.0 |  |  |
Non-Utility Energy Infrastructure |  |  |  |  |
Segment information |  |  |  |  |
Revenues | 118.1 |  |  |  |
Other operation and maintenance | 1.7 | 0.4 |  |  |
Depreciation and amortization | 18.3 | 17.5 |  |  |
Operating income (loss) | 93.0 | 97.4 |  |  |
Equity in earnings of transmission affiliates | 0.0 | 0.0 |  |  |
Interest expense | 16.1 | 15.3 |  |  |
Non-Utility Energy Infrastructure | Intersegment revenues |  |  |  |  |
Segment information |  |  |  |  |
Revenues | 99.3 | 109.0 |  |  |
Non-Utility Energy Infrastructure | External Revenues |  |  |  |  |
Segment information |  |  |  |  |
Revenues | 18.8 | 6.3 |  |  |
Corporate and Other |  |  |  |  |
Segment information |  |  |  |  |
Revenues | 1.4 |  |  |  |
Other operation and maintenance | (0.3) | (0.4) |  |  |
Depreciation and amortization | 7.7 | 5.6 |  |  |
Operating income (loss) | (5.4) | (2.4) |  |  |
Equity in earnings of transmission affiliates | 0.0 | 0.0 |  |  |
Interest expense | 28.0 | 29.1 |  |  |
Corporate and Other | Intersegment revenues |  |  |  |  |
Segment information |  |  |  |  |
Revenues | 0.0 | 0.0 |  |  |
Corporate and Other | External Revenues |  |  |  |  |
Segment information |  |  |  |  |
Revenues | 1.4 | 2.9 |  |  |
Reconciling Eliminations |  |  |  |  |
Segment information |  |  |  |  |
Other operation and maintenance | (96.8) | (109.0) |  |  |
Depreciation and amortization | 0.0 | 0.0 |  |  |
Operating income (loss) | 0.0 | 0.0 |  |  |
Equity in earnings of transmission affiliates | 0.0 | 0.0 |  |  |
Interest expense | (1.2) | (1.8) |  |  |
Reconciling Eliminations | Intersegment revenues |  |  |  |  |
Segment information |  |  |  |  |
Revenues | (99.3) | (109.0) |  |  |
Reconciling Eliminations | External Revenues |  |  |  |  |
Segment information |  |  |  |  |
Revenues | 0.0 | 0.0 |  |  |
Public Utility |  |  |  |  |
Segment information |  |  |  |  |
Revenues | 2,266.3 |  |  |  |
Other operation and maintenance | 607.3 | 613.5 |  |  |
Depreciation and amortization | 182.6 | 171.5 |  |  |
Operating income (loss) | 457.5 | 519.7 |  |  |
Equity in earnings of transmission affiliates | 0.0 | 0.0 |  |  |
Interest expense | 63.8 | 62.1 |  |  |
Public Utility | Intersegment revenues |  |  |  |  |
Segment information |  |  |  |  |
Revenues | 0.0 | 0.0 |  |  |
Public Utility | External Revenues |  |  |  |  |
Segment information |  |  |  |  |
Revenues | 2,266.3 | 2,295.3 |  |  |
Public Utility | Wisconsin |  |  |  |  |
Segment information |  |  |  |  |
Revenues | 1,589.1 |  |  |  |
Other operation and maintenance | 468.5 | 465.7 |  |  |
Depreciation and amortization | 135.1 | 129.3 |  |  |
Operating income (loss) | 273.7 | 329.5 |  |  |
Equity in earnings of transmission affiliates | 0.0 | 0.0 |  |  |
Interest expense | 49.4 | 48.7 |  |  |
Public Utility | Wisconsin | Intersegment revenues |  |  |  |  |
Segment information |  |  |  |  |
Revenues | 0.0 | 0.0 |  |  |
Public Utility | Wisconsin | External Revenues |  |  |  |  |
Segment information |  |  |  |  |
Revenues | 1,589.1 | 1,612.1 |  |  |
Public Utility | Illinois |  |  |  |  |
Segment information |  |  |  |  |
Revenues | 507.3 |  |  |  |
Other operation and maintenance | 112.2 | 119.6 |  |  |
Depreciation and amortization | 40.9 | 36.2 |  |  |
Operating income (loss) | 147.6 | 156.7 |  |  |
Equity in earnings of transmission affiliates | 0.0 | 0.0 |  |  |
Interest expense | 12.3 | 11.1 |  |  |
Public Utility | Illinois | Intersegment revenues |  |  |  |  |
Segment information |  |  |  |  |
Revenues | 0.0 | 0.0 |  |  |
Public Utility | Illinois | External Revenues |  |  |  |  |
Segment information |  |  |  |  |
Revenues | 507.3 | 525.3 |  |  |
Public Utility | Other States |  |  |  |  |
Segment information |  |  |  |  |
Revenues | 169.9 |  |  |  |
Other operation and maintenance | 26.6 | 28.2 |  |  |
Depreciation and amortization | 6.6 | 6.0 |  |  |
Operating income (loss) | 36.2 | 33.5 |  |  |
Equity in earnings of transmission affiliates | 0.0 | 0.0 |  |  |
Interest expense | 2.1 | 2.3 |  |  |
Public Utility | Other States | Intersegment revenues |  |  |  |  |
Segment information |  |  |  |  |
Revenues | 0.0 | 0.0 |  |  |
Public Utility | Other States | External Revenues |  |  |  |  |
Segment information |  |  |  |  |
Revenues | $ 169.9 | 157.9 |  |  |
ATC | Electric Transmission |  |  |  |  |
Segment information |  |  |  |  |
Equity method investment, ownership interest (as a percent) | 60.00% |  |  |  |
ATC Holdco |  |  |  |  |
Segment information |  |  |  |  |
Equity method investment, ownership interest (as a percent) | 75.00% |  |  |  |
Equity in earnings of transmission affiliates | $ (0.6) | $ (5.8) |  |  |
ATC Holdco | Electric Transmission |  |  |  |  |
Segment information |  |  |  |  |
Equity method investment, ownership interest (as a percent) | 75.00% |  |  |  |

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