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, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Equity Securities Without Readily Determinable Fair Value [Line Items]
Carrying amount, beginning of period			$ 224.1
Carrying amount, end of period	$ 225.8		225.8
Non Marketable Equity Securities
Equity Securities Without Readily Determinable Fair Value [Line Items]
Carrying amount, beginning of period	226.1	$ 384.7	224.1	$ 395.8
Net additions	1.3	0.8	3.1	6.8
Gross unrealized gains	1.2	0.0	1.4	0.0
Gross unrealized losses and impairments	(2.8)	(22.6)	(2.8)	(39.7)
Carrying amount, end of period	$ 225.8	$ 362.9	$ 225.8	$ 362.9