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Equity Securities - (Summary of Investments in Marketable Equity Securities) (Detail) - USD ($)
$ in Millions
Jun. 30, 2024
Dec. 31, 2023
Schedule Of Trading Securities And Other Trading Assets [Line Items]    
Equity securities, Cost $ 416.2 $ 445.1
Equity securities, Unrealized losses (26.3) (8.2)
Equity securities, Estimated fair value 389.9 436.9
Common Stock    
Schedule Of Trading Securities And Other Trading Assets [Line Items]    
Equity securities, Cost 402.4 429.4
Equity securities, Unrealized losses (25.7) (4.9)
Equity securities, Estimated fair value 376.7 424.5
Preferred Stock    
Schedule Of Trading Securities And Other Trading Assets [Line Items]    
Equity securities, Cost 13.8 15.7
Equity securities, Unrealized losses (0.6) (3.3)
Equity securities, Estimated fair value $ 13.2 $ 12.4