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Schedule of Other Operating Revenues and Operating Expenses (Detail) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Component of Operating Other Cost and Expense [Abstract] |  |  |  |
Revenue | $ 87.1 | $ 82.6 | $ 315.0 | $ 286.3
Less: Revenue Sharing Payments | 0.3 | 0.3 | 0.8 | 0.8
Total Other Operating Revenues | 1.1 |  | 3.5 |
Operation and Maintenance Expense | 16.4 | 16.9 | 51.5 | 49.4
Usource Contract Revenues [Member] |  |  |  |
Component of Operating Other Cost and Expense [Abstract] |  |  |  |
Revenue | $ 1.4 |  | $ 4.3 |
ASU 2014-09 |  |  |  |
Component of Operating Other Cost and Expense [Abstract] |  |  |  |
Less: Revenue Sharing Payments |  | 0.3 |  | 0.8
Total Other Operating Revenues |  | 1.1 |  | 3.7
Operation and Maintenance Expense |  | 16.6 |  | 48.6
ASU 2014-09 | Usource Contract Revenues [Member] |  |  |  |
Component of Operating Other Cost and Expense [Abstract] |  |  |  |
Revenue |  | $ 1.4 |  | $ 4.5

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