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Schedule of Other Operating Revenues and Operating Expenses (Detail) - USD ($)
$ in Millions
 12 Months Ended
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