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Schedule of Other Operating Revenues and Operating Expenses (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Component of Operating Other Cost and Expense [Abstract] |  |  |
Total Operating Revenues | $ 439.4 | $ 400.2 | $ 377.3
Less: Revenue Sharing Payments | (1.0) | (1.1) | (1.0)
Operation and Maintenance Expense | 69.5 | 64.5 | 61.4
Usource Contract Revenues [Member] |  |  |
Component of Operating Other Cost and Expense [Abstract] |  |  |
Total Operating Revenues | 5.7 |  |
Other |  |  |
Component of Operating Other Cost and Expense [Abstract] |  |  |
Total Operating Revenues | $ 4.7 |  |
ASU 2014-09 |  |  |
Component of Operating Other Cost and Expense [Abstract] |  |  |
Less: Revenue Sharing Payments |  | (1.1) | (1.0)
Operation and Maintenance Expense |  | 63.4 | 60.4
ASU 2014-09 | Usource Contract Revenues [Member] |  |  |
Component of Operating Other Cost and Expense [Abstract] |  |  |
Total Operating Revenues |  | 6.0 | 6.1
ASU 2014-09 | Other |  |  |
Component of Operating Other Cost and Expense [Abstract] |  |  |
Total Operating Revenues |  | $ 4.9 | $ 5.1

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