Equity Securities - (Summary of Changes in Carrying Amounts of Non Marketable Equity Securities) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Equity Securities Without Readily Determinable Fair Value [Line Items]
Carrying amount, beginning of period			$ 202.4
Carrying amount, end of period	$ 218.3		218.3
Non Marketable Equity Securities
Equity Securities Without Readily Determinable Fair Value [Line Items]
Carrying amount, beginning of period	216.6	$ 226.1	202.4	$ 224.1
Net additions	0.2	1.3	15.2	3.1
Gross unrealized gains	1.5	1.2	1.5	1.4
Gross unrealized losses and impairments	0.0	(2.8)	(0.8)	(2.8)
Carrying amount, end of period	$ 218.3	$ 225.8	$ 218.3	$ 225.8