Contractual Maturities of Marketable Debt Securities (Detail) - Truck, Parts and Other $ in Millions	Sep. 30, 2024 USD ($)
Amortized Cost Maturities:
Within one year	$ 619.1
One to five years	1,859.0
More than ten years	9.4
Marketable debt securities, Cost	2,487.5
Fair Value Maturities:
Within one year	617.7
One to five years	1,878.2
More than ten years	8.9
Fair Value	$ 2,504.8

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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