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Components of Accrued Revenue (Detail) - USD ($)
$ in Millions
Sep. 30, 2020
Dec. 31, 2019
Sep. 30, 2019
Deferred Revenue Arrangement [Line Items]      
Regulatory Assets – Current $ 31.7 $ 35.8 $ 24.9
Total Accrued Revenue 39.1 50.0 35.3
Unbilled Revenues      
Deferred Revenue Arrangement [Line Items]      
Regulatory Assets – Current 7.4 14.2 10.4
Regulatory Assets      
Deferred Revenue Arrangement [Line Items]      
Regulatory Assets – Current $ 31.7 $ 35.8 $ 24.9