Equity Securities - (Summary of Changes in Carrying Amounts of Non Marketable Equity Securities) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Equity Securities Without Readily Determinable Fair Value [Line Items]
Carrying amount, beginning of period	$ 224.1
Carrying amount, end of period	202.4	$ 224.1
Non Marketable Equity Securities
Equity Securities Without Readily Determinable Fair Value [Line Items]
Carrying amount, beginning of period	224.1	395.8
Net additions (disposals)	6.0	(4.0)
Gross unrealized gains	1.5	1.5
Gross unrealized losses and impairments	(29.2)	(169.2)
Carrying amount, end of period	$ 202.4	$ 224.1