Marketable Securities (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	$ 1,793.0
Marketable debt securities, Fair Value	1,740.7
Marketable equity securities, Cost	10.0	$ 10.0
Marketable equity securities, Unrealized Losses	7.6	8.8
Marketable equity securities, Fair Value	2.4	1.2
Total marketable securities, Cost	1,803.0	1,681.1
Total marketable securities, Unrealized Gains	0.2	0.9
Total marketable securities, Unrealized Losses	60.1	67.8
Total marketable securities, Fair Value	1,743.1	1,614.2
U.S. tax-exempt securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	293.0	452.8
Marketable debt securities, Unrealized Gains		0.5
Marketable debt securities, Unrealized Losses	8.1	8.2
Marketable debt securities, Fair Value	284.9	445.1
U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	354.4	262.5
Marketable debt securities, Unrealized Gains		0.1
Marketable debt securities, Unrealized Losses	10.2	11.6
Marketable debt securities, Fair Value	344.2	251.0
U.S. taxable municipal / non-U.S. provincial bonds
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	249.3	191.6
Marketable debt securities, Unrealized Losses	9.0	10.8
Marketable debt securities, Fair Value	240.3	180.8
U.S. government and agency securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	151.6	118.0
Marketable debt securities, Unrealized Gains		0.1
Marketable debt securities, Unrealized Losses	4.4	3.1
Marketable debt securities, Fair Value	147.2	115.0
Non-U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	510.4	467.9
Marketable debt securities, Unrealized Losses	14.3	17.9
Marketable debt securities, Fair Value	496.1	450.0
Non-U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	134.2	78.9
Marketable debt securities, Unrealized Gains	0.2	0.2
Marketable debt securities, Unrealized Losses	2.2	2.7
Marketable debt securities, Fair Value	132.2	76.4
Other debt securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	100.1	99.4
Marketable debt securities, Unrealized Losses	4.3	4.7
Marketable debt securities, Fair Value	$ 95.8	$ 94.7

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Marketable securities, accumulated gross unrealized gain, before tax.

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