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Equity Securities - (Summary of Investments in Marketable Equity Securities) (Detail) - USD ($)
$ in Millions
Jun. 30, 2025
Dec. 31, 2024
Schedule Of Trading Securities And Other Trading Assets [Line Items]    
Equity securities, Cost $ 405.1 $ 409.3
Equity securities, Unrealized losses (9.1) (22.5)
Equity securities, Estimated fair value 396.0 386.8
Common Stock    
Schedule Of Trading Securities And Other Trading Assets [Line Items]    
Equity securities, Cost 393.7 397.3
Equity securities, Unrealized losses (9.5) (22.6)
Equity securities, Estimated fair value 384.2 374.7
Preferred Stock    
Schedule Of Trading Securities And Other Trading Assets [Line Items]    
Equity securities, Cost 11.4 12.0
Equity securities, Unrealized losses 0.4 0.1
Equity securities, Estimated fair value $ 11.8 $ 12.1