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SEGMENT INFORMATION (Details) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2018 USD ($) | Jun. 30, 2017 USD ($) | Jun. 30, 2018 USD ($) segment | Jun. 30, 2017 USD ($)
Segment information |  |  |  |
Number of reportable segments | segment |  |  | 6 |
Revenues | $ 1,672.5 | $ 1,631.5 | $ 3,959.0 | $ 3,936.0
Other operation and maintenance | 537.7 | 479.8 | 1,049.6 | 984.3
Depreciation and amortization | 206.7 | 197.7 | 415.3 | 392.3
Operating income (loss) | 330.8 | 362.2 | 875.9 | 976.9
Equity in earnings of transmission affiliates | 28.7 | 41.8 | 61.5 | 83.7
Interest expense | 108.5 | 101.9 | 215.2 | 206.6
Intersegment revenues |  |  |  |
Segment information |  |  |  |
Revenues | 0.0 | 0.0 | 0.0 | 0.0
External Revenues |  |  |  |
Segment information |  |  |  |
Revenues | 1,672.5 | 1,631.5 | 3,959.0 | 3,936.0
Electric Transmission |  |  |  |
Segment information |  |  |  |
Revenues | 0.0 |  | 0.0 |
Other operation and maintenance | 0.0 | 0.0 | 0.0 | 0.0
Depreciation and amortization | 0.0 | 0.0 | 0.0 | 0.0
Operating income (loss) | 0.0 | 0.0 | 0.0 | 0.0
Equity in earnings of transmission affiliates | 28.7 | 41.8 | 61.5 | 83.7
Interest expense | 0.0 | 0.0 | 0.0 | 0.0
Electric Transmission | Intersegment revenues |  |  |  |
Segment information |  |  |  |
Revenues | 0.0 | 0.0 | 0.0 | 0.0
Electric Transmission | External Revenues |  |  |  |
Segment information |  |  |  |
Revenues | 0.0 | 0.0 | 0.0 | 0.0
Non-Utility Energy Infrastructure |  |  |  |
Segment information |  |  |  |
Revenues | 117.0 |  | 235.1 |
Other operation and maintenance | 4.5 | 2.7 | 6.2 | 3.1
Depreciation and amortization | 18.3 | 17.4 | 36.6 | 34.9
Operating income (loss) | 92.4 | 98.7 | 185.4 | 196.1
Equity in earnings of transmission affiliates | 0.0 | 0.0 | 0.0 | 0.0
Interest expense | 16.0 | 15.2 | 32.1 | 30.5
Non-Utility Energy Infrastructure | Intersegment revenues |  |  |  |
Segment information |  |  |  |
Revenues | 113.5 | 112.6 | 212.8 | 221.6
Non-Utility Energy Infrastructure | External Revenues |  |  |  |
Segment information |  |  |  |
Revenues | 3.5 | 6.2 | 22.3 | 12.5
Corporate and Other |  |  |  |
Segment information |  |  |  |
Revenues | 3.1 |  | 4.5 |
Other operation and maintenance | 2.2 | 4.3 | 1.9 | 3.9
Depreciation and amortization | 7.5 | 6.4 | 15.2 | 12.0
Operating income (loss) | (6.5) | (7.7) | (11.9) | (10.1)
Equity in earnings of transmission affiliates | 0.0 | 0.0 | 0.0 | 0.0
Interest expense | 30.3 | 26.8 | 58.3 | 55.9
Corporate and Other | Intersegment revenues |  |  |  |
Segment information |  |  |  |
Revenues | 0.0 | 0.0 | 0.0 | 0.0
Corporate and Other | External Revenues |  |  |  |
Segment information |  |  |  |
Revenues | 3.1 | 3.2 | 4.5 | 6.1
Reconciling Eliminations |  |  |  |
Segment information |  |  |  |
Other operation and maintenance | (100.4) | (112.6) | (197.2) | (221.6)
Depreciation and amortization | 0.0 | 0.0 | 0.0 | 0.0
Operating income (loss) | 0.0 | 0.0 | 0.0 | 0.0
Equity in earnings of transmission affiliates | 0.0 | 0.0 | 0.0 | 0.0
Interest expense | (0.7) | (1.1) | (1.9) | (2.9)
Reconciling Eliminations | Intersegment revenues |  |  |  |
Segment information |  |  |  |
Revenues | (113.5) | (112.6) | (212.8) | (221.6)
Reconciling Eliminations | External Revenues |  |  |  |
Segment information |  |  |  |
Revenues | 0.0 | 0.0 | 0.0 | 0.0
Public Utility |  |  |  |
Segment information |  |  |  |
Revenues | 1,665.9 |  | 3,932.2 |
Other operation and maintenance | 631.4 | 585.4 | 1,238.7 | 1,198.9
Depreciation and amortization | 180.9 | 173.9 | 363.5 | 345.4
Operating income (loss) | 244.9 | 271.2 | 702.4 | 790.9
Equity in earnings of transmission affiliates | 0.0 | 0.0 | 0.0 | 0.0
Interest expense | 62.9 | 61.0 | 126.7 | 123.1
Public Utility | Intersegment revenues |  |  |  |
Segment information |  |  |  |
Revenues | 0.0 | 0.0 | 0.0 | 0.0
Public Utility | External Revenues |  |  |  |
Segment information |  |  |  |
Revenues | 1,665.9 | 1,622.1 | 3,932.2 | 3,917.4
Public Utility | Wisconsin |  |  |  |
Segment information |  |  |  |
Revenues | 1,325.5 |  | 2,914.6 |
Other operation and maintenance | 502.4 | 459.7 | 970.9 | 925.4
Depreciation and amortization | 134.6 | 130.3 | 269.7 | 259.6
Operating income (loss) | 195.1 | 222.6 | 468.8 | 552.1
Equity in earnings of transmission affiliates | 0.0 | 0.0 | 0.0 | 0.0
Interest expense | 48.5 | 48.2 | 97.9 | 96.9
Public Utility | Wisconsin | Intersegment revenues |  |  |  |
Segment information |  |  |  |
Revenues | 0.0 | 0.0 | 0.0 | 0.0
Public Utility | Wisconsin | External Revenues |  |  |  |
Segment information |  |  |  |
Revenues | 1,325.5 | 1,303.2 | 2,914.6 | 2,915.3
Public Utility | Illinois |  |  |  |
Segment information |  |  |  |
Revenues | 268.0 |  | 775.3 |
Other operation and maintenance | 104.1 | 102.4 | 216.3 | 222.0
Depreciation and amortization | 41.8 | 37.5 | 82.7 | 73.7
Operating income (loss) | 41.7 | 43.9 | 189.3 | 200.6
Equity in earnings of transmission affiliates | 0.0 | 0.0 | 0.0 | 0.0
Interest expense | 12.3 | 10.9 | 24.6 | 22.0
Public Utility | Illinois | Intersegment revenues |  |  |  |
Segment information |  |  |  |
Revenues | 0.0 | 0.0 | 0.0 | 0.0
Public Utility | Illinois | External Revenues |  |  |  |
Segment information |  |  |  |
Revenues | 268.0 | 253.2 | 775.3 | 778.5
Public Utility | Other States |  |  |  |
Segment information |  |  |  |
Revenues | 72.4 |  | 242.3 |
Other operation and maintenance | 24.9 | 23.3 | 51.5 | 51.5
Depreciation and amortization | 4.5 | 6.1 | 11.1 | 12.1
Operating income (loss) | 8.1 | 4.7 | 44.3 | 38.2
Equity in earnings of transmission affiliates | 0.0 | 0.0 | 0.0 | 0.0
Interest expense | 2.1 | 1.9 | 4.2 | 4.2
Public Utility | Other States | Intersegment revenues |  |  |  |
Segment information |  |  |  |
Revenues | 0.0 | 0.0 | 0.0 | 0.0
Public Utility | Other States | External Revenues |  |  |  |
Segment information |  |  |  |
Revenues | $ 72.4 | 65.7 | $ 242.3 | 223.6
ATC | Electric Transmission |  |  |  |
Segment information |  |  |  |
Equity method investment, ownership interest (as a percent) | 60.00% |  | 60.00% |
ATC Holdco |  |  |  |
Segment information |  |  |  |
Equity method investment, ownership interest (as a percent) | 75.00% |  | 75.00% |
Equity in earnings of transmission affiliates | $ (1.1) | $ (1.0) | $ (1.7) | $ (6.8)
ATC Holdco | Electric Transmission |  |  |  |
Segment information |  |  |  |
Equity method investment, ownership interest (as a percent) | 75.00% |  | 75.00% |

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