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Equity Securities (Narrative) (Detail) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2025
Jun. 30, 2024
Jun. 30, 2025
Jun. 30, 2024
Schedule Of Investments [Line Items]        
Realized and unrealized net gains (losses) on equity securities $ 26.1 $ (12.2) $ 19.1 $ (1.3)
Net unrealized gains (losses) on equity securities $ 26.0 $ (13.2) $ 19.3 $ (2.7)