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Components of Accrued Revenue (Detail) - USD ($)
$ in Millions
Mar. 31, 2021
Dec. 31, 2020
Mar. 31, 2020
Deferred Revenue Arrangement [Line Items]      
Regulatory Assets – Current [1] $ 24.0 $ 37.3 $ 28.4
Total Accrued Revenue 34.7 50.9 38.6
Unbilled Revenues      
Deferred Revenue Arrangement [Line Items]      
Regulatory Assets – Current 10.7 13.6 10.2
Regulatory Assets      
Deferred Revenue Arrangement [Line Items]      
Regulatory Assets – Current $ 24.0 $ 37.3 $ 28.4
[1] Reflects amounts included in the Accrued Revenue on the Company’s Consolidated Balance Sheets.