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Schedule of Other Operating Revenues and Operating Expenses (Detail) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2018 | Mar. 31, 2017
Component of Operating Other Cost and Expense [Abstract] |  |
Usource Contract Revenue | $ 1.6 |
Less: Revenue Sharing Payments | 0.3 | $ 0.3
Total Other Operating Revenues | 1.3 |
Operation and Maintenance Expense | $ 17.3 | 16.0
ASU 2014-09 |  |
Component of Operating Other Cost and Expense [Abstract] |  |
Usource Contract Revenue |  | 1.7
Less: Revenue Sharing Payments |  | 0.3
Total Other Operating Revenues |  | 1.4
Operation and Maintenance Expense |  | $ 15.7

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- Definition Amount of revenue earned from products sold or services provided under the terms of a contract, including, but not limited to, government contracts and construction contracts, classified as other. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.1) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 + Details
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