XML 507 R104.htm IDEA: XBRL DOCUMENT v3.24.1
Investment Income-Schedule of Investment Income (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Analysis of income and expense [abstract]      
Dividends received $ 0.3 $ 0.1 $ 0.1
Interest received – environmental trust funds 1.2 1.1 0.8
Interest received – cash balances 23.4 12.1 7.4
Total investment income $ 24.9 $ 13.3 $ 8.3