Marketable Securities (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	$ 2,773.5
Marketable debt securities, Fair Value	2,772.5
Marketable equity securities, Cost	10.0	$ 10.0
Marketable equity securities, Unrealized Losses	3.7	5.6
Marketable equity securities, Fair Value	6.3	4.4
Total marketable securities, Cost	2,783.5	1,846.9
Total marketable securities, Unrealized Gains	10.7	8.1
Total marketable securities, Unrealized Losses	15.4	32.4
Total marketable securities, Fair Value	2,778.8	1,822.6
U.S. tax-exempt securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	304.5	312.5
Marketable debt securities, Unrealized Gains	0.6	1.2
Marketable debt securities, Unrealized Losses	1.4	3.0
Marketable debt securities, Fair Value	303.7	310.7
U.S. taxable municipal / non-U.S. provincial bonds
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	381.1	244.9
Marketable debt securities, Unrealized Gains	1.2	0.8
Marketable debt securities, Unrealized Losses	2.2	5.6
Marketable debt securities, Fair Value	380.1	240.1
U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	864.3	357.1
Marketable debt securities, Unrealized Gains	3.1	1.4
Marketable debt securities, Unrealized Losses	3.2	5.2
Marketable debt securities, Fair Value	864.2	353.3
U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	287.1	159.2
Marketable debt securities, Unrealized Gains	0.1	0.6
Marketable debt securities, Unrealized Losses	1.5	1.7
Marketable debt securities, Fair Value	285.7	158.1
Non-U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	606.6	529.4
Marketable debt securities, Unrealized Gains	3.5	2.3
Marketable debt securities, Unrealized Losses	2.0	7.5
Marketable debt securities, Fair Value	608.1	524.2
Non-U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	163.5	141.0
Marketable debt securities, Unrealized Gains	1.4	1.5
Marketable debt securities, Unrealized Losses	0.6	1.3
Marketable debt securities, Fair Value	164.3	141.2
Other debt securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	166.4	92.8
Marketable debt securities, Unrealized Gains	0.8	0.3
Marketable debt securities, Unrealized Losses	0.8	2.5
Marketable debt securities, Fair Value	$ 166.4	$ 90.6

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

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