Marketable Securities (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	$ 2,487.5
Marketable debt securities, Fair Value	2,504.8
Marketable equity securities, Cost	10.0	$ 10.0
Marketable equity securities, Unrealized Losses	4.1	5.6
Marketable equity securities, Fair Value	5.9	4.4
Total marketable securities, Cost	2,497.5	1,846.9
Total marketable securities, Unrealized Gains	25.7	8.1
Total marketable securities, Unrealized Losses	12.5	32.4
Total marketable securities, Fair Value	2,510.7	1,822.6
U.S. tax-exempt securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	303.9	312.5
Marketable debt securities, Unrealized Gains	2.3	1.2
Marketable debt securities, Unrealized Losses	0.7	3.0
Marketable debt securities, Fair Value	305.5	310.7
U.S. taxable municipal / non-U.S. provincial bonds
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	321.5	244.9
Marketable debt securities, Unrealized Gains	3.0	0.8
Marketable debt securities, Unrealized Losses	2.3	5.6
Marketable debt securities, Fair Value	322.2	240.1
U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	744.5	357.1
Marketable debt securities, Unrealized Gains	9.3	1.4
Marketable debt securities, Unrealized Losses	1.8	5.2
Marketable debt securities, Fair Value	752.0	353.3
U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	214.1	159.2
Marketable debt securities, Unrealized Gains	1.5	0.6
Marketable debt securities, Unrealized Losses	0.3	1.7
Marketable debt securities, Fair Value	215.3	158.1
Non-U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	617.8	529.4
Marketable debt securities, Unrealized Gains	6.1	2.3
Marketable debt securities, Unrealized Losses	2.1	7.5
Marketable debt securities, Fair Value	621.8	524.2
Non-U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	168.5	141.0
Marketable debt securities, Unrealized Gains	2.2	1.5
Marketable debt securities, Unrealized Losses	0.5	1.3
Marketable debt securities, Fair Value	170.2	141.2
Other debt securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	117.2	92.8
Marketable debt securities, Unrealized Gains	1.3	0.3
Marketable debt securities, Unrealized Losses	0.7	2.5
Marketable debt securities, Fair Value	$ 117.8	$ 90.6

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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