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, 2026 |
Jun. 30, 2025 |
Jun. 30, 2026 |
Jun. 30, 2025 |
|
| Equity Securities Without Readily Determinable Fair Value [Line Items] | ||||
| Carrying amount, beginning of period | $ 253.1 | |||
| Carrying amount, end of period | $ 214.4 | 214.4 | ||
| Non Marketable Equity Securities | ||||
| Equity Securities Without Readily Determinable Fair Value [Line Items] | ||||
| Carrying amount, beginning of period | 257.6 | $ 216.6 | 253.1 | $ 202.4 |
| Net additions (transfers) | (0.6) | 0.2 | 4.4 | 15.2 |
| Gross unrealized gains | 0.4 | 1.5 | 0.4 | 1.5 |
| Gross unrealized losses and impairments | (43.0) | 0.0 | (43.5) | (0.8) |
| Carrying amount, end of period | $ 214.4 | $ 218.3 | $ 214.4 | $ 218.3 |