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Schedule of Other Operating Revenues and Operating Expenses (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Component of Operating Other Cost and Expense [Line Items]      
Total Operating Revenues $ 437.3 $ 439.4 $ 400.2
Less: Revenue Sharing Payments (0.2) (1.0) (1.1)
Operation and Maintenance Expense 67.2 69.5 64.5
Usource Contract Revenues [Member]      
Component of Operating Other Cost and Expense [Line Items]      
Total Operating Revenues 1.1 5.7  
Other      
Component of Operating Other Cost and Expense [Line Items]      
Total Operating Revenues $ 0.9 $ 4.7 4.9
ASU 2014-09      
Component of Operating Other Cost and Expense [Line Items]      
Less: Revenue Sharing Payments     (1.1)
Operation and Maintenance Expense     63.4
ASU 2014-09 | Usource Contract Revenues [Member]      
Component of Operating Other Cost and Expense [Line Items]      
Total Operating Revenues     $ 6.0