Marketable Securities (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	$ 2,161.2
Marketable debt securities, Fair Value	2,142.7
Marketable equity securities, Cost	10.0	$ 10.0
Marketable equity securities, Unrealized Losses	7.2	5.6
Marketable equity securities, Fair Value	2.8	4.4
Total marketable securities, Cost	2,171.2	1,846.9
Total marketable securities, Unrealized Gains	3.5	8.1
Total marketable securities, Unrealized Losses	29.2	32.4
Total marketable securities, Fair Value	2,145.5	1,822.6
U.S. tax-exempt securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	302.6	312.5
Marketable debt securities, Unrealized Gains	0.3	1.2
Marketable debt securities, Unrealized Losses	2.7	3.0
Marketable debt securities, Fair Value	300.2	310.7
U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	566.0	357.1
Marketable debt securities, Unrealized Gains	0.9	1.4
Marketable debt securities, Unrealized Losses	4.8	5.2
Marketable debt securities, Fair Value	562.1	353.3
U.S. taxable municipal / non-U.S. provincial bonds
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	323.2	244.9
Marketable debt securities, Unrealized Gains	0.2	0.8
Marketable debt securities, Unrealized Losses	5.0	5.6
Marketable debt securities, Fair Value	318.4	240.1
U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	168.5	159.2
Marketable debt securities, Unrealized Gains		0.6
Marketable debt securities, Unrealized Losses	1.9	1.7
Marketable debt securities, Fair Value	166.6	158.1
Non-U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	543.6	529.4
Marketable debt securities, Unrealized Gains	1.1	2.3
Marketable debt securities, Unrealized Losses	5.0	7.5
Marketable debt securities, Fair Value	539.7	524.2
Non-U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	158.8	141.0
Marketable debt securities, Unrealized Gains	0.7	1.5
Marketable debt securities, Unrealized Losses	1.1	1.3
Marketable debt securities, Fair Value	158.4	141.2
Other debt securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	98.5	92.8
Marketable debt securities, Unrealized Gains	0.3	0.3
Marketable debt securities, Unrealized Losses	1.5	2.5
Marketable debt securities, Fair Value	$ 97.3	$ 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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