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Components of Accrued Revenue (Detail) - USD ($)
$ in Millions
Sep. 30, 2021
Dec. 31, 2020
Sep. 30, 2020
Deferred Revenue Arrangement [Line Items]      
Regulatory Assets – Current [1] $ 30.6 $ 37.3 $ 31.7
Total Accrued Revenue 38.6 50.9 39.1
Unbilled Revenues      
Deferred Revenue Arrangement [Line Items]      
Regulatory Assets – Current 8.0 13.6 7.4
Regulatory Assets      
Deferred Revenue Arrangement [Line Items]      
Regulatory Assets – Current $ 30.6 $ 37.3 $ 31.7
[1] Reflects amounts included in Accrued Revenue, discussed above, on the Company’s Consolidated Balance Sheets.