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Equity Securities - (Summary of Changes in Carrying Amounts of Non Marketable Equity Securities) (Detail) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2024 | Mar. 31, 2023
Equity Securities Without Readily Determinable Fair Value [Line Items] |  |
Carrying amount, beginning of period | $ 224.1 |
Carrying amount, end of period | 226.1 |
Non Marketable Equity Securities |  |
Equity Securities Without Readily Determinable Fair Value [Line Items] |  |
Carrying amount, beginning of period | 224.1 | $ 395.8
Net additions | 1.8 | 6.1
Gross unrealized gains | 0.2 | 0.0
Gross unrealized losses and impairments | 0.0 | (17.2)
Carrying amount, end of period | $ 226.1 | $ 384.7

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