Equity Securities - (Summary of Investments in Marketable Equity Securities) (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Equity securities, Cost	$ 445.1	$ 339.0
Equity securities, Unrealized losses	(8.2)	(59.5)
Equity securities, Estimated fair value	436.9	279.5
Common Stock
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Equity securities, Cost	429.4	323.7
Equity securities, Unrealized losses	(4.9)	(55.6)
Equity securities, Estimated fair value	424.5	268.1
Preferred Stock
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Equity securities, Cost	15.7	15.3
Equity securities, Unrealized losses	(3.3)	(3.9)
Equity securities, Estimated fair value	$ 12.4	$ 11.4