Marketable Securities (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	$ 1,836.9
Marketable debt securities, Fair Value	1,818.2
Marketable equity securities, Cost	10.0	$ 10.0
Marketable equity securities, Unrealized Losses	5.6	8.8
Marketable equity securities, Fair Value	4.4	1.2
Total marketable securities, Cost	1,846.9	1,681.1
Total marketable securities, Unrealized Gains	8.1	0.9
Total marketable securities, Unrealized Losses	32.4	67.8
Total marketable securities, Fair Value	1,822.6	1,614.2
U.S. tax-exempt securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	312.5	452.8
Marketable debt securities, Unrealized Gains	1.2	0.5
Marketable debt securities, Unrealized Losses	3.0	8.2
Marketable debt securities, Fair Value	310.7	445.1
U.S. taxable municipal / non-U.S. provincial bonds
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	244.9	191.6
Marketable debt securities, Unrealized Gains	0.8
Marketable debt securities, Unrealized Losses	5.6	10.8
Marketable debt securities, Fair Value	240.1	180.8
U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	357.1	262.5
Marketable debt securities, Unrealized Gains	1.4	0.1
Marketable debt securities, Unrealized Losses	5.2	11.6
Marketable debt securities, Fair Value	353.3	251.0
U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	159.2	118.0
Marketable debt securities, Unrealized Gains	0.6	0.1
Marketable debt securities, Unrealized Losses	1.7	3.1
Marketable debt securities, Fair Value	158.1	115.0
Non-U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	529.4	467.9
Marketable debt securities, Unrealized Gains	2.3
Marketable debt securities, Unrealized Losses	7.5	17.9
Marketable debt securities, Fair Value	524.2	450.0
Non-U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	141.0	78.9
Marketable debt securities, Unrealized Gains	1.5	0.2
Marketable debt securities, Unrealized Losses	1.3	2.7
Marketable debt securities, Fair Value	141.2	76.4
Other debt securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	92.8	99.4
Marketable debt securities, Unrealized Gains	0.3
Marketable debt securities, Unrealized Losses	2.5	4.7
Marketable debt securities, Fair Value	$ 90.6	$ 94.7

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Marketable securities, accumulated gross unrealized gain, before tax.

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