Marketable Securities (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	$ 2,806.9
Marketable debt securities, Fair Value	2,819.8
Marketable equity securities, Cost	10.0	$ 10.0
Marketable equity securities, Unrealized Losses	3.3	3.7
Marketable equity securities, Fair Value	6.7	6.3
Total marketable securities, Cost	2,816.9	2,783.5
Total marketable securities, Unrealized Gains	18.1	10.7
Total marketable securities, Unrealized Losses	8.5	15.4
Total marketable securities, Fair Value	2,826.5	2,778.8
U.S. tax-exempt securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	296.6	304.5
Marketable debt securities, Unrealized Gains	0.7	0.6
Marketable debt securities, Unrealized Losses	0.5	1.4
Marketable debt securities, Fair Value	296.8	303.7
U.S. taxable municipal / non-U.S. provincial bonds
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	400.5	381.1
Marketable debt securities, Unrealized Gains	2.7	1.2
Marketable debt securities, Unrealized Losses	1.0	2.2
Marketable debt securities, Fair Value	402.2	380.1
U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	904.0	864.3
Marketable debt securities, Unrealized Gains	6.1	3.1
Marketable debt securities, Unrealized Losses	1.5	3.2
Marketable debt securities, Fair Value	908.6	864.2
U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	241.7	287.1
Marketable debt securities, Unrealized Gains	1.1	0.1
Marketable debt securities, Unrealized Losses	0.4	1.5
Marketable debt securities, Fair Value	242.4	285.7
Non-U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	605.5	606.6
Marketable debt securities, Unrealized Gains	4.6	3.5
Marketable debt securities, Unrealized Losses	0.9	2.0
Marketable debt securities, Fair Value	609.2	608.1
Non-U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	178.0	163.5
Marketable debt securities, Unrealized Gains	1.7	1.4
Marketable debt securities, Unrealized Losses	0.3	0.6
Marketable debt securities, Fair Value	179.4	164.3
Other debt securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	180.6	166.4
Marketable debt securities, Unrealized Gains	1.2	0.8
Marketable debt securities, Unrealized Losses	0.6	0.8
Marketable debt securities, Fair Value	$ 181.2	$ 166.4

Equity securities, FV-NI, accumulated gross unrealized loss, before tax.

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Marketable securities, accumulated gross unrealized gain, before tax.

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on 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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Amount of investment in marketable security, classified as current.

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