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Investment Income-Schedule of Investment Income (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Analysis of income and expense [abstract]      
Dividends received $ 0.1 $ 0.1 $ 0.4
Interest received – environmental trust funds 1.1 0.8 0.7
Interest received – cash balances 12.1 7.4 7.6
Total investment income $ 13.3 $ 8.3 $ 8.7