Marketable Securities (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	$ 2,886.0
Marketable debt securities, Fair Value	2,908.5
Marketable equity securities, Cost	10.0	$ 10.0
Marketable equity securities, Unrealized Losses	4.8	3.7
Marketable equity securities, Fair Value	5.2	6.3
Total marketable securities, Cost	2,896.0	2,783.5
Total marketable securities, Unrealized Gains	25.2	10.7
Total marketable securities, Unrealized Losses	7.5	15.4
Total marketable securities, Fair Value	2,913.7	2,778.8
U.S. tax-exempt securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	379.7	304.5
Marketable debt securities, Unrealized Gains	2.3	0.6
Marketable debt securities, Unrealized Losses	0.1	1.4
Marketable debt securities, Fair Value	381.9	303.7
U.S. taxable municipal / non-U.S. provincial bonds
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	403.3	381.1
Marketable debt securities, Unrealized Gains	3.6	1.2
Marketable debt securities, Unrealized Losses	0.6	2.2
Marketable debt securities, Fair Value	406.3	380.1
U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	892.9	864.3
Marketable debt securities, Unrealized Gains	7.4	3.1
Marketable debt securities, Unrealized Losses	0.8	3.2
Marketable debt securities, Fair Value	899.5	864.2
U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	208.7	287.1
Marketable debt securities, Unrealized Gains	1.8	0.1
Marketable debt securities, Unrealized Losses	0.2	1.5
Marketable debt securities, Fair Value	210.3	285.7
Non-U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	613.0	606.6
Marketable debt securities, Unrealized Gains	5.7	3.5
Marketable debt securities, Unrealized Losses	0.4	2.0
Marketable debt securities, Fair Value	618.3	608.1
Non-U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	195.8	163.5
Marketable debt securities, Unrealized Gains	3.0	1.4
Marketable debt securities, Unrealized Losses	0.1	0.6
Marketable debt securities, Fair Value	198.7	164.3
Other debt securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	192.6	166.4
Marketable debt securities, Unrealized Gains	1.4	0.8
Marketable debt securities, Unrealized Losses	0.5	0.8
Marketable debt securities, Fair Value	$ 193.5	$ 166.4

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

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