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Components of Accrued Revenue (Detail) - USD ($)
$ in Millions
Jun. 30, 2020
Dec. 31, 2019
Jun. 30, 2019
Deferred Revenue Arrangement [Line Items]      
Regulatory Assets – Current $ 26.8 $ 35.8 $ 23.1
Total Accrued Revenue 34.9 50.0 32.1
Unbilled Revenues      
Deferred Revenue Arrangement [Line Items]      
Regulatory Assets – Current 8.1 14.2 9.0
Regulatory Assets      
Deferred Revenue Arrangement [Line Items]      
Regulatory Assets – Current $ 26.8 $ 35.8 $ 23.1