Equity Securities - (Summary of Investments in Marketable Equity Securities) (Detail) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Equity securities, Cost	$ 425.1	$ 445.1
Equity securities, Unrealized losses	(10.1)	(8.2)
Equity securities, Estimated fair value	415.0	436.9
Common Stock
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Equity securities, Cost	409.8	429.4
Equity securities, Unrealized losses	(8.8)	(4.9)
Equity securities, Estimated fair value	401.0	424.5
Preferred Stock
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Equity securities, Cost	15.3	15.7
Equity securities, Unrealized losses	(1.3)	(3.3)
Equity securities, Estimated fair value	$ 14.0	$ 12.4