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SEGMENT INFORMATION (Details) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 USD ($) | Sep. 30, 2017 USD ($) | Sep. 30, 2018 USD ($) segment | Sep. 30, 2017 USD ($)
Segment information |  |  |  |
Number of reportable segments | segment |  |  | 6 |
Operating revenues | $ 1,643.7 | $ 1,657.5 | $ 5,602.7 | $ 5,593.5
Other operation and maintenance | 553.1 | 473.1 | 1,602.7 | 1,457.4
Depreciation and amortization | 212.8 | 201.2 | 628.1 | 593.5
Operating income (loss) | 302.7 | 392.2 | 1,178.6 | 1,369.1
Equity in earnings of transmission affiliates | 33.7 | 39.2 | 95.2 | 122.9
Interest expense | 112.0 | 103.8 | 327.2 | 310.4
Wisconsin |  |  |  |
Segment information |  |  |  |
Operating revenues | 1,388.7 |  | 4,303.3 |
Illinois |  |  |  |
Segment information |  |  |  |
Operating revenues | 197.9 |  | 973.2 |
Other States |  |  |  |
Segment information |  |  |  |
Operating revenues | 50.2 |  | 292.5 |
Electric Transmission |  |  |  |
Segment information |  |  |  |
Operating 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 | 33.7 | 39.2 | 95.2 | 122.9
Interest expense | 0.0 | 0.0 | 0.0 | 0.0
Non-Utility Energy Infrastructure |  |  |  |
Segment information |  |  |  |
Operating revenues | 115.2 |  | 350.3 |
Other operation and maintenance | 3.0 | 1.5 | 9.2 | 4.6
Depreciation and amortization | 19.1 | 18.2 | 55.7 | 53.1
Operating income (loss) | 91.6 | 103.4 | 277.0 | 299.5
Equity in earnings of transmission affiliates | 0.0 | 0.0 | 0.0 | 0.0
Interest expense | 15.9 | 16.2 | 48.0 | 46.7
Corporate and Other |  |  |  |
Segment information |  |  |  |
Operating revenues | 2.4 |  | 6.9 |
Other operation and maintenance | (4.4) | (0.4) | (2.5) | 3.5
Depreciation and amortization | 7.1 | 6.3 | 22.3 | 18.3
Operating income (loss) | (0.4) | 0.0 | (12.3) | (10.1)
Equity in earnings of transmission affiliates | 0.0 | 0.0 | 0.0 | 0.0
Interest expense | 32.9 | 25.4 | 91.2 | 81.3
Reconciling Eliminations |  |  |  |
Segment information |  |  |  |
Other operation and maintenance | (98.0) | (109.0) | (295.2) | (330.6)
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 | $ (1.3) | 0.4 | $ (3.2) | (2.5)
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 | $ (0.9) | (1.7) | $ (2.6) | (8.5)
ATC Holdco | Electric Transmission |  |  |  |
Segment information |  |  |  |
Equity method investment, ownership interest (as a percent) | 75.00% |  | 75.00% |
Bishop Hill III Wind Energy Center | Non-Utility Energy Infrastructure |  |  |  |
Segment information |  |  |  |
WEC's ownership interest in Bishop Hill III Wind Energy Center | 80.00% |  | 80.00% |
Total utility revenues |  |  |  |
Segment information |  |  |  |
Other operation and maintenance | $ 652.5 | 581.0 | $ 1,891.2 | 1,779.9
Depreciation and amortization | 186.6 | 176.7 | 550.1 | 522.1
Operating income (loss) | 211.5 | 288.8 | 913.9 | 1,079.7
Equity in earnings of transmission affiliates | 0.0 | 0.0 | 0.0 | 0.0
Interest expense | 64.5 | 61.8 | 191.2 | 184.9
Total utility revenues | Wisconsin |  |  |  |
Segment information |  |  |  |
Other operation and maintenance | 525.0 | 460.5 | 1,495.9 | 1,385.9
Depreciation and amortization | 137.2 | 131.5 | 406.9 | 391.1
Operating income (loss) | 201.4 | 277.5 | 670.2 | 829.6
Equity in earnings of transmission affiliates | 0.0 | 0.0 | 0.0 | 0.0
Interest expense | 49.6 | 48.5 | 147.5 | 145.4
Total utility revenues | Illinois |  |  |  |
Segment information |  |  |  |
Other operation and maintenance | 104.5 | 99.0 | 320.8 | 321.0
Depreciation and amortization | 43.0 | 38.9 | 125.7 | 112.6
Operating income (loss) | 15.5 | 14.3 | 204.8 | 214.9
Equity in earnings of transmission affiliates | 0.0 | 0.0 | 0.0 | 0.0
Interest expense | 12.8 | 11.0 | 37.4 | 33.0
Total utility revenues | Other States |  |  |  |
Segment information |  |  |  |
Other operation and maintenance | 23.0 | 21.5 | 74.5 | 73.0
Depreciation and amortization | 6.4 | 6.3 | 17.5 | 18.4
Operating income (loss) | (5.4) | (3.0) | 38.9 | 35.2
Equity in earnings of transmission affiliates | 0.0 | 0.0 | 0.0 | 0.0
Interest expense | 2.1 | 2.3 | 6.3 | 6.5
External Revenues |  |  |  |
Segment information |  |  |  |
Operating revenues | 1,643.7 | 1,657.5 | 5,602.7 | 5,593.5
External Revenues | Electric Transmission |  |  |  |
Segment information |  |  |  |
Operating revenues | 0.0 | 0.0 | 0.0 | 0.0
External Revenues | Non-Utility Energy Infrastructure |  |  |  |
Segment information |  |  |  |
Operating revenues | 4.5 | 13.6 | 26.8 | 26.1
External Revenues | Corporate and Other |  |  |  |
Segment information |  |  |  |
Operating revenues | 2.4 | 5.6 | 6.9 | 11.7
External Revenues | Reconciling Eliminations |  |  |  |
Segment information |  |  |  |
Operating revenues | 0.0 | 0.0 | 0.0 | 0.0
External Revenues | Total utility revenues |  |  |  |
Segment information |  |  |  |
Operating revenues | 1,636.8 | 1,638.3 | 5,569.0 | 5,555.7
External Revenues | Total utility revenues | Wisconsin |  |  |  |
Segment information |  |  |  |
Operating revenues | 1,388.7 | 1,401.3 | 4,303.3 | 4,316.6
External Revenues | Total utility revenues | Illinois |  |  |  |
Segment information |  |  |  |
Operating revenues | 197.9 | 187.2 | 973.2 | 965.7
External Revenues | Total utility revenues | Other States |  |  |  |
Segment information |  |  |  |
Operating revenues | 50.2 | 49.8 | 292.5 | 273.4
Intersegment Transactions [Member] |  |  |  |
Segment information |  |  |  |
Operating revenues | 0.0 | 0.0 | 0.0 | 0.0
Intersegment Transactions [Member] | Electric Transmission |  |  |  |
Segment information |  |  |  |
Operating revenues | 0.0 | 0.0 | 0.0 | 0.0
Intersegment Transactions [Member] | Non-Utility Energy Infrastructure |  |  |  |
Segment information |  |  |  |
Operating revenues | 110.7 | 111.6 | 323.5 | 333.2
Intersegment Transactions [Member] | Corporate and Other |  |  |  |
Segment information |  |  |  |
Operating revenues | 0.0 | 0.0 | 0.0 | 0.0
Intersegment Transactions [Member] | Reconciling Eliminations |  |  |  |
Segment information |  |  |  |
Operating revenues | (110.7) | (111.6) | (323.5) | (333.2)
Intersegment Transactions [Member] | Total utility revenues |  |  |  |
Segment information |  |  |  |
Operating revenues | 0.0 | 0.0 | 0.0 | 0.0
Intersegment Transactions [Member] | Total utility revenues | Wisconsin |  |  |  |
Segment information |  |  |  |
Operating revenues | 0.0 | 0.0 | 0.0 | 0.0
Intersegment Transactions [Member] | Total utility revenues | Illinois |  |  |  |
Segment information |  |  |  |
Operating revenues | 0.0 | 0.0 | 0.0 | 0.0
Intersegment Transactions [Member] | Total utility revenues | Other States |  |  |  |
Segment information |  |  |  |
Operating revenues | $ 0.0 | $ 0.0 | $ 0.0 | $ 0.0

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